Pablo Giménez Gavarrell
Transcript of Pablo Giménez Gavarrell
Pablo Giménez Gavarrell
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Departamento de Ingeniería Energética
Escuela Superior de Ingeniería
Universidad de Sevilla
Thermal Energy Storage for High
Temperature Applications
Autor:
Pablo Giménez Gavarrell
Director:
Sonia Fereres Rapoport
Investigador Senior Abengoa Research
Sevilla, 2017
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Tesis doctoral: Thermal Energy Storage for High Temperature Applications
Autor: Pablo Giménez Gavarrell
Tutor: José Julio Guerra Macho
Programa de doctorado en Ingeniería Energética, Química y Ambiental (2011)
Línea de investigación: Eficiencia Energética e Integración de Energías Renovables
en la Edificación y en la Industria
El tribunal nombrado para juzgar el Proyecto arriba indicado, compuesto por los
siguientes miembros:
Presidente:
Dr. Servando Álvarez Domínguez (Universidad de Sevilla)
Vocales:
Dr. Ignacio González Loscertales (Universidad de Málaga)
Dr. Luis Allan Pérez Maqueda (Consejo Superior de Inv. Científicas)
Dr. Yulong Ding (Universidad de Birmingham)
Secretario:
Dra. Luisa F. Cabeza Fabra (Universidad de Lleida)
Acuerdan otorgarle la calificación de:
Sevilla, 2017
El Secretario del Tribunal
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"La página escrita nunca recuerda todo lo que se ha intentado, sino lo poco que
se ha conseguido."
Antonio Machado
A mis padres, familia y Lorena
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Acknowledgements
First of all, I am sincerely thankful to my director Sonia Fereres for demonstrating
immense faith in my work and for being the only one who has believed in this
research from the beginning of this thesis. She has always tried to motivate me and
without her support I could not have completed the thesis. I would like to thank
Abengoa Research for economically funding this PhD thesis, Y-Flow for helping in
the development of the idea of PCM-borosilicate capsules, my supervisor Prof.
José Guerra and the Energetic Engineering department for accepting me as PhD
student as well as Prof. D. Shin for accepting me in his lab at UTA.
We were 21 students when we started this adventure more than 4 years ago.
Unfortunately, only few of them finished it and nobody knows how many will be
able to continue in this exciting world of research. Despite the fact that nothing was
what we were promised while pursuing this endeavor, I have personally enjoyed it
and I would not change the valuable experience I gained. I like to further thank all
my colleagues from Abengoa, especially who started with me: Ramos, my favorite
cellist Bea, Sol, Irene and Elyas, and some still working on it like Javi and Jacobo.
I would also like to thank Eva, Vince, Kike and Maria for sharing not only work
place but also our life in Seville. Everything changes when you have the best
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housemates. From my visit to the States, I would like to thank my friend Tu Price
who, together with Vince, made my stay in America awesome.
Last but not least, I especially dedicated my thesis to my parents, Paqui and Juan
Ramón, my brothers JuanRa and Franc and my family, Ambrosio, Elena, Adrian,
Amparin, Adri, Amparo, Roberto, Chritine, María, Morgane, David and Sylvan.
Lastly, I want to thank Lorena, for being with me these lovely last years in Seville.
Pau Giménez Gavarrell
Sevilla, 2017
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Resumen
La producción de energía eléctrica a gran escala a partir de energía solar ha
recibido en las últimas décadas un gran impulso debido a las diferentes ventajas
que ofrece, no sólo medioambientales sino también en temas geopolíticos como son
su independencia de los combustibles fósiles y su alta disponibilidad geográfica.
Sin embargo, antes de que la energía solar sea capaz de penetrar significativamente
en el mix de energía que abastece un país, todavía quedan algunos retos por
resolver.
El desafío principal de la energía solar es su intermitencia intrínseca, lo que a
menudo dificulta la coincidencia entre la disponibilidad de la fuente de energía con
la necesidad de satisfacer la demanda eléctrica. Por tanto, el almacenamiento de
energía es la clave para conseguir desacoplar la producción de energía eléctrica de
la radiación solar.
En este aspecto, la tecnología solar térmica está un paso por delante otras
tecnologías como la fotovoltaica. Tal vez debido a la histórica utilización del calor
para diferentes propósitos, el almacenamiento de energía térmica está bastante
desarrollado y plantea hoy en día pequeños problemas tecnológicos en
comparación con otras aún incipientes -pero muy prometedoras- tecnologías de
almacenamiento como las baterías. Gracias a esta ventaja, junto con las mayores
eficiencias en la producción a gran escala en comparación con la energía
fotovoltaica, la energía termosolar promete desempeñar un papel relevante en un
futuro próximo. Aunque menor que otras tecnologías, la elevada inversión
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necesaria para la implementación de sistemas de almacenamiento térmico es una
gran desventaja. Por tanto, el desarrollo de nuevos sistemas de almacenamiento
térmico más densos energéticamente y sobre todo de menor coste específico,
además de la mejora de los sistemas existentes, es crucial para la implantación de la
energía solar como vector energético a gran escala.
Esta tesis se plantea, dentro de este contexto, con el objetivo principal de
explorar diferentes estrategias para incrementar la densidad energética de los
sistemas de almacenamiento térmico que se utilizan actualmente en plantas
solares térmicas de concentración, específicamente las de torre, por su mayor
potencial debido a su mayor temperatura de operación y por tanto mayor
eficiencia en la transformación de la energía térmica en eléctrica. Las dos
tecnologías de almacenamiento térmico que se utilizan actualmente a nivel
comercial en este tipo de plantas son: los acumuladores de vapor y los tanques
de sal fundida. Ambos sistemas utilizan almacenamiento de calor sensible: el
primero en agua líquida saturada a alta temperatura y presión, y el segundo
sistema almacena energía mediante el incremento de la temperatura de sales
fundidas.
Como alternativas a las actuales tecnologías en esta tesis se ha investigado el
uso de a) materiales de cambio de fase (PCM) que utilizan el calor latente de
fusión como mecanismo de almacenamiento térmico complementario a los
acumuladores de vapor, y b) la modificación de las propiedades termofísicas de
las sales fundidas a través de la adición de nanopartículas con el objetivo de
incrementar su densidad energética del sistema basado en tanques de sales.
El uso de materiales de cambio de fase como sistema de almacenamiento ha
requerido la realización de una selección de materiales basada en valores de
temperatura de fusión y calores latentes de la literatura en el rango de temperatura
de interés (~300 ºC) validando dichos valores experimentalmente mediante el uso
de calorimetría diferencial barrido. El intercambio de calor entre el material de
almacenamiento y el fluido caloportador por medio de un sistema de lecho fijo de
bolas ha requerido el diseño, desarrollo y prueba del sistema PCM-capsula. Se ha
identificado el borosilicato como material encapsulante de PCM utilizándolo para
el desarrollo de diferentes pruebas de concepto.
Las cápsulas de PCM, una vez fabricadas, se han probado individualmente en una
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instalación experimental, donde el objetivo principal era el ciclado térmico con un
flujo convectivo de aire a temperaturas entre 200 y 400ºC con el fin de fundir y
congelar el material. Se ha implementado un modelo numérico, intentando
aproximarse en la medida de lo posible a la instalación experimental, para ayudar a
entender la influencia de parámetros como la naturaleza del material de cambio de
fase (sal - metal), material de cápsula, espesor, etc., en los tiempos de inicio y fin del
proceso de cambio de fase así como perfiles de temperatura dentro de la cápsula. El
modelo es capaz de capturar la física principal que tiene lugar a pesar de su
simplicidad y se utiliza para ayudar a entender los resultados experimentales. Las
simulaciones se han comparado con los experimentos cualitativa y
cuantitativamente identificando algunas fuentes de incertidumbre que podrían
explicar el desajuste entre ellos. En conclusión, el uso de PCM como
almacenamiento térmico tiene sentido desde un punto de vista de eficiencia
energética. Con este análisis se demuestra que una cápsula aislada funciona
debidamente, intercambiando calor con al aire caliente circundante. El siguiente
paso sería probar un lecho fijo de cápsulas en condiciones no sólo de alta
temperatura sino también de alta presión. Sin embargo, hasta el momento los
costes asociados al contenedor del lecho fijo (tanque de vapor) superan los
beneficios de estas cápsulas de PCM, llevando a pensar en diseños de
intercambiadores de calor alternativos.
Finalmente, se ha investigado la adición de nanopartículas para mejorar la
capacidad de almacenamiento de los sistemas basados en sales fundidas. El
objetivo inicial era reproducir el aumento de calor específico reportado en la
literatura por varios grupos de investigación. Desafortunadamente, las diferentes
concentraciones de nanopartículas, tamaños, tipos y diferentes composiciones de
fluido base ensayadas no mostraron ninguna mejora sustancial ni estadísticamente
significativa respecto al fluido base. Se introdujeron ligeras modificaciones del
proceso de síntesis, descubriendo que la etapa de evaporación de la disolución
parece separar la sal en diferentes regiones con composiciones ligeramente
diferentes, lo que podría explicar ligeras modificaciones en el calor específico y el
calor latente.
Durante esta investigación se han cuestionado algunas suposiciones que se daban
por validas en el proceso de síntesis por los diferentes grupos de investigación. El
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protocolo de síntesis basado en la disolución de la sal en agua, sonicación y
vaporación, el más ampliamente utilizado, no parece apropiado ya que es posible
encontrar muestras con Cp superior o inferior a la media incluso sin
nanopartículas. También se ha cuestionado la capacidad de los nanofluidos de sal
fundida para mantener las nanopartículas homogéneamente distribuidas en
suspensión. Las imágenes de microscopía electrónica de barrido no mostraron la
presencia de nanoestructuras "especiales" o "anómalas" en la sal.
También se ha investigado el calor latente de los nanofluidos de la sal fundida. Se
ha observado que el calor latente disminuye en una cantidad mayor que la prevista
teóricamente, a diferencia de las limitadas investigaciones existentes con sales
fundidas, pero alineadas con un gran número de estudios de materiales de cambio
de fase con nanopartículas de menor temperatura. Por último, hemos planteado la
hipótesis de la existencia de una capa líquida en la interfase partícula-fluido como
responsable de esta mayor reducción en el calor latente del cálculo del espesor de la
capa en cada una de las composiciones del líquido base analizada.
Aunque la adición de nanopartículas a fluidos caloportadores podría resultar muy
interesante para mejorar las propiedades termofísicas de los mismos, actualmente
la tecnología está muy lejos de poder ser implementada industrialmente en una
planta termosolar. Con este análisis detallado se han identificado muchos de los
problemas por resolver: la dificultad en realizar medidas a muy altas temperaturas
y con nanopartículas en suspensión, la falta de consenso en la mejora de
propiedades a obtener, la falta de un método de síntesis que no influya en los
resultados y, sobre todo, donde las partículas se mantengan uniformemente
distribuidas durante los ciclos de trabajo y a las temperaturas requeridas. Hasta el
momento, ninguno de los autores que han evaluado estas sales con nanopartículas
han reportado detalles de la estabilidad de sus dispersiones en estado líquido, paso
previo esencial a cualquier trabajo futuro.
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Abstract
Large scale electricity production from solar energy has been gaining attention in
recent years due to its advantages to solve environmental problems as well as
issues such as fossil fuel dependence. However, there are some challenges to be
solved before solar power becomes a feasible alternative representing a significant
portion of the energy consumption of a country.
The main challenge of solar energy is its intrinsic intermittence, which makes it
difficult to match the energy resource availability with the electricity demand.
Therefore, energy storage is the key element to decouple electricity production
from solar radiation.
In this aspect, solar thermal technology is one step ahead other technologies such as
photovoltaic. Perhaps due to the historical use of heat for different purposes, the
storage of thermal energy is a more cost-effective solution compared to other
storage technologies such as batteries. Thanks to this advantage, together with
higher efficiencies in electricity production compared to photovoltaics, solar
thermal energy is playing and will continue to play an important role in the near
future. On the other hand, the high investment cost for the implementation of
thermal storage systems is its main disadvantage. Therefore, the development of
newer, higher energy density and, especially, lower specific cost storage
technologies, in addition to improving the existing systems, is crucial for the
deployment of the solar thermal energy.
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In this context, the main objective of this thesis is to explore different strategies to
increase the energy density of the thermal storage systems currently used in
concentrating solar power plants, specifically those of tower plants. Tower plants
have a greater potential due to their higher operating temperatures and greater
efficiency in the transformation of thermal energy into electricity. The two thermal
storage technologies currently used commercially in this type of plants are: vapor
accumulators and molten salt tanks. Both systems use sensible heat storage: the
first, in saturated liquid water at high temperature and pressure, and the second
system stores energy by increasing the temperature of molten salts.
As alternative to current technologies, this thesis investigates the use of a) the latent
heat of fusion of phase change materials (PCM) as a thermal storage mechanism
complementary to vapor accumulators, and b) the modification of the
thermophysical properties of the molten salts through the addition of nanoparticles
in order to increase the energy density of the molten salt tanks.
The use of phase change materials as thermal storage requires a screening of
materials based on melting temperature and latent heat values from the literature
in the temperature range of interest (~ 300 °C), validating experimentally these
values using a differential scanning calorimeter. The heat exchange between the
storage material and the heat transfer fluid by using a packed bed required the
design, development and testing of a PCM-capsule system. Borosilicate has been
identified as encapsulating material and it has been used to develop different proof
of concepts.
The PCM-capsules have been manufactured and tested in an experimental facility,
where the main objective was to assess the functionality of the capsules by
performing thermal cycling with a convective flow of air at temperatures between
200 and 400 ° C in order to melt and freeze the material. A finite difference 1-D
numerical model was developed trying to reproduce the phenomena taking place
in the experimental installation. It has been used to understand the influence of
parameters such as the nature of the phase change material (salt - metal), the
capsule material, its thickness, etc… on the start and end times of the phase change
process and on the temperature gradients within the capsule. The model is able to
capture the main physics of the process despite its simplicity and it is used to help
understand the experimental results. The simulations have been compared with
the experiments qualitatively and quantitatively, identifying some sources of
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uncertainty that could explain the mismatch between them.
In conclusion, the use of PCM as thermal storage makes sense from an energy
efficiency point of view. This analysis demonstrates that a single capsule functions
properly, exchanging heat with the surrounding hot air as the PCM melts and
solidifies while maintaining its physical integrity. This demonstrates the volume
expansion of the PCM during the phase transition is managed adequately in the
capsules. The next step would be to test a fixed bed of capsules under conditions
not only of high temperature but also of high pressure. However, the current costs
associated with the fixed-bed container (steam tank) outweigh the benefits of these
PCM capsules, leading to evaluate alternative heat-exchanger designs.
Finally, the addition of nanoparticles has been investigated in order to improve the
storage capacity of the systems based on molten salts. The initial objective was to
reproduce the specific heat increase reported in the literature by several research
groups. Unfortunately, the different concentrations of nanoparticles, sizes, types
and different base fluid compositions tested did not show any substantial or
statistically significant improvement over the base fluid. Slight modifications of the
synthesis process were introduced, finding that the evaporation stage of the
solution appears to separate the salt in different regions with slightly different
compositions, which could explain slight modifications in specific heat and latent
heat.
During this research, some common assumptions regarding the synthesis process
have been questioned. The widely used synthesis protocol based on the dissolution
of the salt in water, sonication and vaporization of the solvent water, does not seem
appropriate since it is possible to find samples with higher or lower Cp even
without adding nanoparticles to the base salt. The ability of the molten salt
nanofluids to keep homogeneously dispersed nanoparticles in suspension has also
been questioned. Scanning electron microscopy images did not show the presence
of "special" or "anomalous" nanostructures in the salt.
The latent heat of the molten salt nanofluids has also been investigated. It has been
observed that latent heat decreases by a greater amount than theoretically
anticipated, unlike the limited existing research with molten salts, but in line with a
large number of studies of phase change materials with lower temperature
nanoparticles. Finally, we have hypothesized the existence of a liquid layer at the
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particle-fluid interface as responsible for this greater reduction in the latent heat.
Although the addition of nanoparticles to heat transfer fluids could be very
interesting to improve their thermophysical properties, the technology is far from
being industrially viable in a solar thermal plant. This detailed analysis has
identified many of the problems to be solved: the difficulty in carrying out
measurements at very high temperatures and with suspended nanoparticles, the
lack of consensus on the possible property improvement, the lack of a synthesis
method that does not influence the results and, above all, the particles’ ability to
remain uniformly dispersed during the work cycles at the required temperatures.
So far, none of the authors who have evaluated these salts with nanoparticles have
reported details of the stability of their dispersions in the liquid state, an essential
prior step to any future work.
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Contents
Acknowledgements ix
Resumen xi
Abstract xv
Contents xix
List of Figures xxiii
List of Tables xxxv
Nomenclature xli
1 Introduction 1 1.1 Concentrating Solar Thermal Power Systems 2 1.2 Thermal Energy Storage (TES) 3 1.3 Mechanisms to store thermal energy 4 1.4 Classification of TES 5
1.4.1 TES in commercial central receiver solar power plants 6 1.4.2 Steam accumulators 8 1.4.3 Two molten salt tanks direct system 9
1.5 Main objectives of the thesis and structure 9
2 Phase Change Materials 13 2.1. Introduction and main objectives 13
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2.1.1. Objectives 16 2.2. PCM screening, characterization and selection 16
2.2.1. Metallic PCM candidates 19 2.2.2. Inorganic salts PCM candidates 27
3 Macro Encapsulation of PCM 53 3.1. Introduction and Main Objectives 53 3.2. Background on High Temperature Encapsulation 56 3.3. Shell Material Selection 66 3.4. Capsule Design 70 3.5. Capsule Manufacturing 71 3.6. Capsule Testing 80
3.6.1. Set-up Design 80 3.6.2. Experimental Procedure 85 3.6.3. Experimental Results and Discussion 88 3.6.4. Melting Results 99 3.6.5. Freezing Results 102
3.7. Conclusions 104
4 Single Capsule Model 107 4.1. PCM-capsule heat transfer model 108 4.2. Grid and time-step convergence 116 4.3. Model Validation 118 4.4. Material properties 121 4.5. Boundary condition 125 4.6. Results and Discussion 127
4.6.1. Effect of the new boundary condition on the phase change times127 4.6.2. Effect of capsule size on phase change 130 4.6.3. Effect of capsule shell thickness on phase change 132 4.6.4. Effect of shell material on phase change times: borosilicate vs. steel 134 4.6.5. Effect of the latent heat on the phase change times 139 4.6.6. Effect of PCM characteristics on phase change times 141 4.6.7. Effect of experimental conditions on the phase change times 144
4.7. Comparison to experimental results 147 4.7.1. Convection correlation for experimental set-up 147
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4.7.2. Model vs. Experiments 150 4.8. Double PCM solution 164 4.9. Discussion and conclusion of encapsulated PCM as TES system 166
5 Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage 169 5.1. Introduction 170
5.1.1. Background on Nano-enhanced HTF-TES materials 170 5.1.2. Impact of a Cp enhancement on a CSTP plant cost 182 5.1.3. Aim and objectives 185
5.2. Materials and Methods 186 5.2.1. Synthesis of nanofluids 186 5.2.2. DSC Measurements 188
5.3. Specific heat of nitrate base nanofluids 194 5.3.1. Results 194 5.3.2. Discussion 207 5.3.3. Results new synthesis method 212 5.3.4. Discussion new synthesis method 215 5.3.5. Conclusions 226
5.4. Latent heat of nitrate base nanofluids 227 5.4.1. Results 232 5.4.2. Discussion 235 5.4.3. Conclusions 240
5.5. Stability of the molten salt nanofluid 241 5.5.1. Motivation 241 5.5.2. Materials and methods 242 5.5.3. Results and discussion 243 5.5.4. Conclusions 250
5.6. Summary and conclusions 251
6 General Conclusions and Future Challenges 253
References 259
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List of Figures
Figure 1-1: Schematic representation of the study presented ....................................... 11
Figure 2-1: Classification of performance enhancement techniques. .......................... 15
Figure 2-2: Schematic representation of the study presented with the contents of
this chapter marked in yellow. ............................................................................................. 16
Figure 2-3: Heat of fusion vs. melting point for metals, fluorides, chlorides, nitrates
and other salts (data extracted from 11). .............................................................................. 18
Figure 2-4: Schematic representation of a typical tube and housing thermal storage
unit .............................................................................................................................................. 20
Figure 2-5: MgZn alloying with Aluminum crucibles. ................................................... 23
Figure 2-6: Lead (Pb) and Tin (Sn) phase change properties: onset melting
temperature (Pb and Sn, left), and lead melting and freezing latent heat (right) ..... 24
Figure 2-7: Specific heat of Pb and Sn ................................................................................. 25
Figure 2-8: KNO3-KCl-KBr phase diagram, composition (1) and (2) .......................... 28
Figure 2-9: NaNO3-NaCl-Na2CO3 phase diagram, composition (3) and (4) .............. 29
Figure 2-10: NaNO3-NaBr-NaCl phase diagram, composition (5) and (6) ................. 29
Figure 2-11: Initial mixture of components (left), manual milling (center) and
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melting in a beaker on a hot plate (right) ........................................................................... 32
Figure 2-12: KNO3 heat flow DSC curves during three subsequent melting/freezing
loops at 10 ºC·min-1 showing the solid-solid transition peaks (a)heating, (d)cooling
and the solid-liquid transition peaks for (b)melting (c)crystallization. Water is
released during the 1st heating ramp, leading to a reduction in the area integrated in
peak (a). ...................................................................................................................................... 35
Figure 2-13: KNO3-KCl-KBr (1) melting curves at 5 ºC·min-1 ........................................ 37
Figure 2-14: KNO3-KCl-KBr (1) freezing curves at 5ºC·min-1......................................... 38
Figure 2-15: Heat flow curves for three subsequent loops of KNO3-KCl-KBr (1);
freezing curves at 1) 20ºC·min-1, 2) 20ºC·min-1, and 3) 2ºC·min-1. Top curves
represent cooling (heat is released during crystallization) and bottom curves
represent heating (heat is absorbed during melting). ..................................................... 39
Figure 2-16: KNO3-KBr-KCl (2) DSC curves heating/cooling at 20ºC·min-1: (a)/(d)
solid-solid transition and (b)/(c) solid-liquid transition. ................................................. 40
Figure 2-17: KNO3 – KCl (6 mol%) heat flow curve, solid-liquid transition. ............. 43
Figure 2-18: Heat flow curve of mixtures (3) and (4) and NaNO3 while heating at 20
ºCmin-1 ........................................................................................................................................ 46
Figure 2-19: Durferrit heat flow curves: phase change evaluation (left) and
variability between samples (right) ..................................................................................... 47
Figure 2-20: Latent heat and melting temperature of several PCM tested. ................ 51
Figure 3-1: Schematic representation of a PCM-capsule ................................................ 54
Figure 3-2: KNO3 encapsulated particles53,54 ...................................................................... 57
Figure 3-3: Encapsulated PCM: Encapsulated NaCl-MgCl2 (left)65; Encapsulated
MgCl2 (center)66; Encapsulated Ternary carbonate eutectic (lithium, sodium and
potassium carbonates) (right)67 ............................................................................................. 60
Figure 3-4: Vshell/Vpcm ratio vs Rpcm/Rcapsule for spherical and cylindrical capsules ...... 61
Figure 3-5: High temperature PCM successfully encapsulated .................................... 64
Figure 3-6: Shell/solid PCM volume ratio vs. capsule external diameter. .................. 65
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Figure 3-7: Shell materials used to encapsulate high temperature PCM. (*Present
work) .......................................................................................................................................... 67
Figure 3-8: SWOT (Strengths-Weaknesses-Opportunities-Threats) analysis
performed to evaluate potential of borosilicate as a PCM shell material. .................. 69
Figure 3-9: Shaping a spherical capsule performed at the University of Zaragoza –
glass blowing service. ............................................................................................................. 71
Figure 3-10: Pre-shaped capsules (left) and local stresses in the sphere (right) ........ 72
Figure 3-11: Annealing treatment of the pre-shaped capsules performed at the
University of Zaragoza glass blowing service. ................................................................. 72
Figure 3-12: Crucibles created for the capsule filling and filled capsule performed at
the University of Zaragoza glass blowing service. .......................................................... 73
Figure 3-13: Capsule closure procedure performed at the University of Zaragoza
glass blowing service. ............................................................................................................. 73
Figure 3-14: Pressure inside the capsules for different PCM and different capsule
filling percentages (in solid state) at room temperature (worst-case scenario, no
vacuum inside). The dots represent the expected pressure inside the capsules
manufactured ........................................................................................................................... 76
Figure 3-15: NaNO3 capsules after the last heat treatment (1-2-3-4) ............................ 78
Figure 3-16: Durferrit capsules after the last heat treatment, capsules (5-6-7) ........... 78
Figure 3-17: Capsules after the last heat treatment: lead capsules (11-12-13) (left
image); tin (14) and KNO3 (10) capsules (right image) ................................................... 79
Figure 3-18: Intermediated relative pressure before the blow heater vs. volumetric
flow rate (liters per minute, LPM) ....................................................................................... 81
Figure 3-19: Experimental set-up schematic. All dimensions are in mm. .................. 81
Figure 3-20: Measured temperature profile in the experimental set-up: average
temperature thermocouple 1 to 3, 4 to 6, and capsule estimated temperature. ........ 82
Figure 3-21: Schematic representation of the experimental set-up installation ......... 83
Figure 3-22: Initial experimental set-up installation performed at the laboratories of
Yflow Sistemas y Desarrollos, S.L. under the collaboration with Prof. I. G.
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Loscertales from the University of Malaga. ....................................................................... 83
Figure 3-23: Schematic of the expected thermal response of the set-up ...................... 84
Figure 3-24: Temperature vs. Voltage applied for different experiments performed
with the set-up at different volumetric flow rates (LPM, liters per minute). The
melting temperatures of two PCM (Durferrit, NaNO3) are shown as reference. ..... 85
Figure 3-25: Capsule temperature measured and approximated equation adjusted
to be used in the capsule model for two experiments ..................................................... 86
Figure 3-26: Total optical transmittance of borosilicate.71 ............................................... 87
Figure 3-27: NaNO3 capsule freezing in free flow: Image A (top left, 400 s) shows a
completely liquid salt-borosilicate capsule, while image B (top right, 410 s) shows
the beginning of the salt crystallization process, as marked by a change in slope in
the IR camera temperature traces (bottom right). The different temperature traces
correspond to the locations numbered 1-5 in the IR image (bottom left). .................. 89
Figure 3-28: Sequential images of a NaNO3 capsule during the solidification process
as recorded with a visual camera (top) and an IR camera (bottom). ........................... 90
Figure 3-29: NaNO3 capsule freezing with test duct: IR camera temperature trace
and video snapshots during the liquid-solid phase change highlighted in the red
area. ............................................................................................................................................. 91
Figure 3-30: NaNO3 capsule freezing with test duct ....................................................... 92
Figure 3-31: Capsule temperature trace and sequential images from IR camera (top
row) and visual camera (bottom row) of a NaNO3 capsule during the melting
process under free flow heating, showing: A melting starts at the upper border, B
melting extends over the complete capsule external surface, C, D, E during the
melting process, F completely liquid capsule after melting ends. ................................ 93
Figure 3-32: Visual and IR images at different stages of the melting process of a
NaNO3 capsule inside a borosilicate duct. ......................................................................... 94
Figure 3-33: NaNO3 capsule melting with test duct recorded with visual and IR
camera. Slope changes in the IR temperature traces correspond to melting. ............ 95
Figure 3-34: Tin (Sn) capsule. Change in reflectivity when melting. Completely
solid Sn (left), during the solid-liquid phase change (middle), and completely liquid
xxvii
Sn (right). ................................................................................................................................... 96
Figure 3-35: Determination of the phase change process for melting and freezing
experiments on the Pb-Capsule number 11 based on the IR camera’s temperature
curves. ........................................................................................................................................ 97
Figure 3-36: Comparison of IR camera temperature traces from Pb and NaNO3
PCM capsules during sequential melting and solidification in free flow experiment.
Shadowed area represents the standard deviation due to capsule surface location.
..................................................................................................................................................... 98
Figure 3-37: Thermocouple temperature (red), IR camera temperature (blue) and
derivative of the IR temperature (green).......................................................................... 101
Figure 3-38: Recorded images of the melting experiment number 6. At t~60 seconds
the melting process seems to begin, confirmed by the IR temperature. ................... 102
Figure 3-39: Example of a Durferrit capsule melting experiment. ............................. 102
Figure 3-40: Example of a Durferrit capsule freezing experiment ............................. 103
Figure 3-41: NaNO3 capsule freezing experiment (Number 15). ............................... 103
Figure 4-1: Schematic of the problem to be solved ........................................................ 111
Figure 4-2: Melting starting time and melting duration time as a function of time
steps. ......................................................................................................................................... 117
Figure 4-3: Start melting and melting duration time as a function of mesh size. ... 118
Figure 4-4: Temperature at a various radial locations as a function of time: Zhao et
al.44 (left) and present work (right). Ni-Zn capsule ........................................................ 119
Figure 4-5: Location of the interface as a function of time: Zhao et al.44 (left) and this
work (right). Ni-Zn capsule ................................................................................................ 120
Figure 4-6: Temperature at a various radial locations as a function of time: Zhao et
al.44 (left) and This work (right). Steel NaCl-MgCl2 capsule. ...................................... 121
Figure 4-7: Location of the interface as a function of time: Zhao et al.44 (left) and this
work (right). Steel-(NaCl-MgCl2) capsule. ....................................................................... 121
Figure 4-8: Borosilicate thermo-physical properties71 (Black dots extrapolated at
higher temperature) .............................................................................................................. 123
xxviii
Figure 4-9: Schematic of air temperature boundary condition: previous model vs.
experiment. SM: start melting, EM: end melting............................................................ 125
Figure 4-10: Example of the new boundary condition introduced in the
mathematical model and temperature evolution for different capsule radial
positions ................................................................................................................................... 127
Figure 4-11: Effect of the new boundary condition applied to the capsule in the
present work compared to Zhao’s boundary condition on the temperature profiles
at the center of the capsule (R=0) and at the shell-PCM interface (R=R2) for a fixed
hconv=150 Wm-2K-1. .................................................................................................................. 129
Figure 4-12: Effect of the new boundary condition applied to the capsule in the
present work compared to Zhao’s boundary condition on the temporal evolution of
the melt fraction for two different convective heat transfer coefficients (50 and 150
Wm-2K-1). .................................................................................................................................. 130
Figure 4-13: Dimensionless temperature at the center of the capsule (R=0) and at the
shell-PCM interface (R=R2) for three different capsule radii. ...................................... 131
Figure 4-14: Location of the solid-liquid interface for three different capsule radii
(left) and melt fraction for three different capsule radii (right) ................................... 132
Figure 4-15: Effect of the capsule thickness on the capsule temperature: at the center
(R=0) and at the shell-PCM interface (R=R2). .................................................................. 133
Figure 4-16: Location of the solid-liquid interface vs. time for different capsule
thickness (left); Melt fraction vs. time for different capsule thickness (right) .......... 133
Figure 4-17: Dimensionless temperature for different radial positions: center (R=0),
shell-PCM interface (R=R2) and capsule surface (R=R1) for NaNO3-capsule with
different shell materials. Convective heat transfer coefficient around the capsule 150
W m-2 K-1. ................................................................................................................................. 135
Figure 4-18: Melt fraction vs time (left) and Location of the solid liquid interface
(right) for different shell materials and different convective heat transfer coefficients
around a NaNO3 capsule ..................................................................................................... 137
Figure 4-19: Dimensionless temperature for different radial positions: center (R=0),
shell-PCM interface (R=R2) and capsule surface (R=R1) for a Sn-capsule with
different shell materials. Convective heat transfer coefficient around the capsule 150
xxix
Wm-2K-1. ................................................................................................................................... 138
Figure 4-20: Location of the solid liquid interface vs time (left) and Melt fraction vs
time (right) in the Sn capsule for different shell materials and different convective
heat transfer coefficients around the capsule. ................................................................. 139
Figure 4-21: Melting time duration vs. latent heat times the latent heat of NaNO3
(left); Effect of the latent heat on melt fraction for three different latent heats tested
(right) ........................................................................................................................................ 140
Figure 4-22: Dimensionless temperature of the capsule at the center (R=0), PCM-
shell interface (R=R2) and the capsule surface (R=R1) for two different PCM
(NaNO3 and Sn) encapsulated with borosilicate. Convective heat transfer
coefficient 50 Wm-2K-1 ........................................................................................................... 142
Figure 4-23: Location of the solid-liquid interface vs. time (left) and melt fraction vs.
time (right) for two different PCM (NaNO3 and Sn) encapsulated with borosilicate.
Convective heat transfer coefficient 50Wm-2K-1. ............................................................. 143
Figure 4-24: Start melting and melting duration time for borosilicate capsules with
Sn and NaNO3 as PCM for different convective heat transfer coefficients. ............. 144
Figure 4-25: Melting start time (left) and Melting duration (right) vs. heat transfer
coefficient for different temperature steps, fixed dimensionless melting
temperature θm= 0.5 and time constant τ=45.7 s. ............................................................ 145
Figure 4-26: Melting start time (left) and Melting duration time (right) for different
convective heat transfer coefficients ‘h’ and dimensionless melting temperature θm
for a fixed T=100ºC .............................................................................................................. 146
Figure 4-27: Convective heat transfer coefficient vs. Reynolds number (at 300ºC) 149
Figure 4-28: Experimental infrared melting and freezing curves for a metallic (Pb,
blue) and Salt (NaNO3, brown) borosilicate capsule ..................................................... 153
Figure 4-29: Melting (left) and freezing (right) comparison: experimental infrared
temperature history curves (blue) for a metallic borosilicate capsule compared to
the model results (brown) ................................................................................................... 154
Figure 4-30: Melting (left) and freezing (right) comparison: experimental infrared
temperature history curves (orange) for a NaNO3 borosilicate capsule compared to
the model results (blue and green) .................................................................................... 154
xxx
Figure 4-31: Experimental freezing start time vs. dimensionless phase change
temperature for freezing experiments .............................................................................. 156
Figure 4-32: Model and experimental results comparison: Freezing start time vs
dimensionless phase change temperature for freezing experiments ........................ 157
Figure 4-33: Experimental melting start time vs. dimensionless melting
temperature. ............................................................................................................................ 159
Figure 4-34: Model and experimental results comparison: Melting start time vs.
dimensionless melting temperature. ................................................................................. 160
Figure 4-35: Experimental phase change start time vs. dimensionless phase change
temperature. ............................................................................................................................ 161
Figure 4-36: Model and experimental results comparison: Melting duration time vs.
temperature step applied for different dimensionless melting temperatures and
convective heat transfer coefficients. ................................................................................. 162
Figure 4-37: Double PCM TES solution ............................................................................ 165
Figure 5-1. Main research groups analyzing molten salt nanofluids (main author
marked in red) ........................................................................................................................ 172
Figure 5-2. Experimental studies (39) on molten salt nanofluids measuring the
specific heat capacity of the liquid salt.............................................................................. 174
Figure 5-3. Specific heat enhancement vs. Nanoparticle concentration. ................... 178
Figure 5-4. a) Fractal-like fluid nanostructures formed by nanoparticles in a
conventional nanofluid. b) Fractal-like fluid nanostructures formed by separated
base molten salts in a molten salt nanofluid.132 ............................................................... 180
Figure 5-5. Schematic representation of two possible predicted thermal behavior of
adsorbed layer in nanofluid: a) extended solid-liquid phase transition (left); b)
nanoporous substrate studies and experimental observation (right).133 ................... 181
Figure 5-6. Break-Down TES cost of a 50MWe -TES 15h central receiver CSP
Plant.135 ..................................................................................................................................... 183
Figure 5-7. Break-Down cost of a 50MWe -TES 15h central receiver CSTP Plant.135
.................................................................................................................................................... 184
xxxi
Figure 5-8. Comparison between a conventional molten salt Tower (50MW, 15h
TES) CSP plant cost vs. the potential use of a nano-HTF TES with an increase of
25% of the base salt specific heat. ....................................................................................... 185
Figure 5-9. Schematic representation of the synthesis method ................................... 187
Figure 5-10. Neat and nanofluid solar salt water solution after sonication, and flasks
during solvent evaporation ................................................................................................. 187
Figure 5-11. Powder-form nanofluid (1% SiO2 10nm) after synthesis, before and
after scratching ready for testing ....................................................................................... 188
Figure 5-12. Schematic showing onset and peak temperatures, width at half peak
height, and latent heat as determined by the DSC tests. .............................................. 189
Figure 5-13. Specific heat of sapphire: theoretical (blue), measured (red) and
corrected (green) for the temperature range 100 - 250 ºC ............................................. 191
Figure 5-14. Position of nitrate salt inside the aluminum crucibles after testing in the
DSC: salts creep up the crucible walls away from the base center. ........................... 192
Figure 5-15 Standard 40μl aluminum crucible after testing KNO3 ............................ 193
Figure 5-16. Specific heat of neat and nanofluid solar salt in solid and liquid phase.
................................................................................................................................................... 194
Figure 5-17. Average specific heat of neat solar salt vs. solar salt nanofluid (10 and
30 nm SiO2 nanoparticles at a concentration of 1 wt %) ............................................... 195
Figure 5-18. Freezing (left) and melting (right) average latent heat of the neat salt
and nanofluids synthesized. Measurements performed at 20 ºC/min and 5 ºC/min
respectively. ............................................................................................................................ 197
Figure 5-19. Effect of 1% of SiO2 (10 nm diameter) nanoparticles on the specific heat
(average 260 – 400ºC for mixtures and 350 – 400ºC for pure components) for
different base fluid mixture compositions. ...................................................................... 198
Figure 5-20. Neat and nanofluid (1% SiO2 10 nm in diameter) specific heat heat
(average over 260 – 400ºC for mixtures and 350 – 400ºC for pure components) vs.
base fluid composition (NaNO3 wt %) in a NaNO3-KNO3 mixture. ......................... 198
Figure 5-21. Specific heat capacity vs. nanoparticle concentration by mass (10 nm
diameter SiO2 nanoparticles) for two different base fluids Na-KNO3 (60-40 wt. %,
xxxii
solar salt) and Na-KNO3 (30-70 wt. %) (at 400ºC) .......................................................... 200
Figure 5-22. Na-KNO3 (60-40 wt. %, solar salt) neat and nanofluid, specific heat for
different SiO2 (20 - 60 nm in diameter) nanoparticles (0 – 0.5 – 1 – 3.21 – 5.35 wt. %)
at 400 ºC. .................................................................................................................................. 201
Figure 5-23. Effect of 1 wt. % of nanoparticles (CuO and Al2O3) on the specficic heat
of the eutectic NaNO3-KNO3 (45-55 wt. %) (average over 260 – 400ºC) .................... 202
Figure 5-24. SEM (left) and SEM-EDS analysis (right) of the neat NaNO3-KNO3 (30-
70 wt. %) ................................................................................................................................... 203
Figure 5-25. SEM (left) and SEM-EDS analysis (right) of the nanofluid 1% SiO2 (10
nm) NaNO3-KNO3 (30-70 wt. %)........................................................................................ 204
Figure 5-26. SEM images of the nanofluid 1% SiO2 (10 nm) NaNO3-KNO3 (30-70 wt.
%) ............................................................................................................................................... 204
Figure 5-27. SEM (left) and SEM-EDS analysis (right) of the nanofluid NaNO3-
KNO3 (30-70 wt. %) with 5% SiO2 (10 nm) ....................................................................... 205
Figure 5-28. SEM images of the nanofluid NaNO3-KNO3 (30-70 wt. %) with 5% SiO2
(10 nm) ..................................................................................................................................... 205
Figure 5-29. SEM (left) and SEM-EDS analysis (right) of the nanofluid eutectic
NaNO3-KNO3 (45-55 wt. %) with 1% CuO ...................................................................... 206
Figure 5-30. Vial (left) vs. larger surface area evaporation receptacles such as glass
petri-dish (center) and steel pan (right, from Schuller et al. (2012)139). Larger surface
area leads to a shorter evaporation times. ........................................................................ 212
Figure 5-31. Different areas tested on a petri-dish. Solar salt nanofluid with 1wt. %
of SiO2 (10 nm) ........................................................................................................................ 213
Figure 5-32. Solar salt nanofluid (1 wt. %, 10 nm SiO2 nanoparticles) specific heat
results vs. Temperature synthesized using petri-dish (PD) evaporation. Neat solar
salt is mixed and evaporated in a vial. .............................................................................. 214
Figure 5-33. Latent heat of Type A and B nanofluid vs. Neat solar salt .................... 217
Figure 5-34. Freezing onset temperature difference between Type A and B
nanofluid. ................................................................................................................................ 219
xxxiii
Figure 5-35. Specific heat and latent heat results of the base salt and nanofluid (1%
of SiO2 10nm) synthesized using petri-dish and mixing completely the entire batch
of salt before testing instead of selectively choosing samples. .................................... 221
Figure 5-36. Specific heat and latent heat results of the neat solar salt (vial) and the
neat solar salt synthesized with petri dish selecting the highest and lowest Cp
value. ........................................................................................................................................ 222
Figure 5-37. Heat flow curves vs. temperature for the sample with the maximum
and minimum Cp synthesized with Petri dish for a representative test from the
data shown in Figure 5-36. .................................................................................................. 223
Figure 5-38. Reported effect of nanoparticles on the latent heat of fusion of organic
PCM .......................................................................................................................................... 228
Figure 5-39. Summary of the effect of nanoparticle on the latent heat of high
temperature inorganic PCM ............................................................................................... 229
Figure 5-40. Phase diagram NaNO3-KNO3 from Factsage43 highlighting the
compositions tested: a hypoeutectic at 34 mol% NaNO3, the eutectic at 49 mol%
NaNO3, a hypereutectic at 64 mol % NaNO3, and the pure components KNO3 and
NaNO3. ..................................................................................................................................... 230
Figure 5-41. Example of DSC cooling and heating curve example, showing
temperature vs. time in blue plotted on the right axis and heat flow vs. time in
green plotted on the left axis. .............................................................................................. 231
Figure 5-42. Heat flow curves vs. temperature for the different compositions tested.
................................................................................................................................................... 232
Figure 5-43. Latent heat of fusion results for different NaNO3-KNO3 mixtures ..... 233
Figure 5-44. Latent heat vs. nanoparticle concentration for different NaNO3-KNO3
mixtures ................................................................................................................................... 234
Figure 5-45. Latent heat vs. NaNO3 content in a Na-KNO3 mixture for different
nanoparticle concentrations ................................................................................................ 234
Figure 5-46. Normalized latent heat vs. nanoparticle concentration for different
NaNO3-KNO3 mixtures. Theory corresponds to a simple mixing rule calculation.
................................................................................................................................................... 235
xxxiv
Figure 5-47. Neat salt and nanofluids (dissolved in water) after sonication. ........... 243
Figure 5-48. The nanofluid powder after solvent water evaporation is ground in a
mortar and placed in smaller vials for the thermal cycling tests ................................ 244
Figure 5-49. Solid-state nanofluid before melting, from left to right: neat salt
NaNO3-KNO3 eutectic, SiO2 nanofluid, Al2O3 nanofluid, CuO nanofluid, and CNT
nanofluid with gum arabic (GA) dispersions. ................................................................ 244
Figure 5-50. Molten state nanofluid during the first melting process on the hot plate
at 350ºC from left to right: neat salt NaNO3-KNO3 eutectic, SiO2 nanofluid, Al2O3
nanofluid, CuO nanofluid, and CNT with gum arabic nanofluid. ............................ 245
Figure 5-51. Difference between SiO2, Al2O3, and CuO nanofluids after 6 melting
and freezing cycles. ............................................................................................................... 245
Figure 5-52. Molten SiO2-nanofluid on a hot plate at 350ºC after 2, 3, 5 and 6 (left to
right) thermal cycles. ............................................................................................................. 246
Figure 5-53. Molten Al2O3-nanofluid (left) and in the hot plate at 350ºC. CuO
nanofluid (right) in molten state and solidified after several freeze/thaw cycles
showing nanoparticle stratification ................................................................................... 247
Figure 5-54. SEM (left) and SEM-EDS analysis (right) of the nanofluid eutectic
NaNO3-KNO3 (45-55 wt. %) with 1% CuO after the stability test .............................. 248
Figure 5-55. SEM analysis of the nanofluid eutectic NaNO3-KNO3 (45-55 wt. %)
with 1% CuO after the stability test ................................................................................... 248
Figure 5-56. SEM (left) and SEM-EDS analysis (right) of the nanofluid eutectic
NaNO3-KNO3 (45-55 wt. %) with 1 wt. % of SiO2 after the stability test ................... 249
Figure 5-57. SEM analysis of the nanofluid eutectic NaNO3-KNO3 (45-55 wt. %)
with 1 wt. % of SiO2 after the stability test ....................................................................... 249
xxxv
List of Tables
Table 1-1. List of commercial CSP central receiver plants with TES operating, under
construction (UC) or under development (UD).13 ............................................................. 6
Table 2-1: Desirable PCM properties.7,17,24 .......................................................................... 17
Table 2-2: Metals price29 ......................................................................................................... 21
Table 2-3: Estimated specific cost of different metal alloys and cost per thermal
storage unit ............................................................................................................................... 22
Table 2-4: Average (cycle 2 to 8) onset melting temperature and melting latent heat
for lead and tin (first melting discarded). .......................................................................... 24
Table 2-5: MgZn measured onset, peak, and endset temperature (melting and
freezing) at 10ºC·min-1 during ten subsequent heating and cooling ramps. .............. 26
Table 2-6: Melting temperature and latent heat of Mg-Zn alloys, literature values
vs. measurements. ................................................................................................................... 26
Table 2-7: Nitrate based compositions to be tested and predicted melting
temperature. *Estimated based on FactSage43 , **literature value 41,44,45 ...................... 27
Table 2-8: Purity grade and supplier of the chemical components used in this study,
and estimated market price ................................................................................................... 31
Table 2-9: KNO3 measured latent heat, onset, peak, and endset temperatures
xxxvi
(melting and freezing) at 10ºC·min-1 during three subsequent heating and cooling
ramps. ......................................................................................................................................... 34
Table 2-10: Heating rate effect on phase transition properties: KNO3 measured
latent heat, onset, peak, and endset temperature (melting and freezing) at 10
ºC·min-1 and 20 ºC·min-1 .......................................................................................................... 36
Table 2-11: KNO3-KCl-KBr(1) measured latent heat, onset, peak and endset
temperature (melting and freezing) at 5 ºCmin-1 (average values of three samples,
5th thermal cycle) ...................................................................................................................... 37
Table 2-12: KNO3-KBr-KCl (2) solid-solid transition measured values ...................... 41
Table 2-13: KNO3-KBr-KCl (2) solid-liquid transition measured values .................... 41
Table 2-14: KNO3 – KCl (6 mol %) solid-liquid transition measured values ............. 42
Table 2-15: Summary of solid-liquid transition properties for KNO3 vs three
different mixtures at a heating rate of 20ºC/ min (except for mixture (1) measured at
10 ºC/min). All compositions are in wt. % ......................................................................... 44
Table 2-16: DSC analysis of NaNO3 (melting and freezing results). Average values
of three different samples. ..................................................................................................... 45
Table 2-17: Melting DSC analysis: comparison between pure NaNO3 vs mixtures
(3), (4), (5), (6) (20 ºC·min-1). Average values of three different samples. .................... 45
Table 2-18: Freezing DSC analysis: comparison between pure NaNO3 vs mixtures
(3), (4), (5), (6) (20 ºC·min-1) .................................................................................................... 46
Table 2-19: Average results for four Durferrit samples .................................................. 48
Table 2-20: Specific cost analysis of the different mixtures calculated with the
experimental latent heat measurement. (*FactSage, **literature) ................................. 49
Table 3-1: High temperature PCM candidates34 ............................................................... 58
Table 3-2: Summary of experimental studies on high temperature encapsulated
PCM. Spherical capsules are marked in blue, cylindrical capsules in black. ............. 61
Table 3-3: Solid and liquid PCM density and calculated amount of PCM added to
each borosilicate capsule type. (*measured) ...................................................................... 74
Table 3-4: List of spherical PCM capsules manufactured included in this study ..... 77
xxxvii
Table 3-5: Variability of the capsule thickness .................................................................. 79
Table 3-6: Melting experiments summary (NaNO3 Tm 302ºC) .................................... 100
Table 3-7: Freezing experiments summary.*SF: start Freezing ................................... 104
Table 4-1: Comparison Zhao et al. vs. present work Zn-Ni capsule (50 mm in
diameter) ................................................................................................................................. 119
Table 4-2: Comparison Zhao et al. vs. present work Steel-NaCl-MgCl2 capsule .... 120
Table 4-3: Thermo-physical properties used in the model. (*Measured) ................. 122
Table 4-4 Properties of the HTF (air) used in the experiments to estimate the
convective heat transfer coefficient used in the model.85.............................................. 124
Table 4-5: Melting start time and melting duration for three different capsule sizes.
................................................................................................................................................... 131
Table 4-6: Shell material properties ................................................................................... 134
Table 4-7: Melting start time and melting time duration for different convective
heat transfer coefficients and different shell materials. NaNO3 as PCM. ................. 136
Table 4-8: Melting start time and melting time duration for different convective
heat transfer coefficients and different shell materials. Tin (Sn) as PCM. ................ 138
Table 4-9: Melting start time and melting time duration for three different latent
heats tested. ............................................................................................................................. 140
Table 4-10: Nusselt number and convective heat transfer coefficient using different
correlations .............................................................................................................................. 149
Table 4-11: Estimated experimental convective heat transfer coefficient that will
make the simulations fit the experimental times. .......................................................... 163
Table 4-12: Material properties ........................................................................................... 165
Table 5-1. Number of publications on the specific heat capacity of high temperature
nanofluids. .............................................................................................................................. 174
Table 5-2. Review on molten salt nanofluids including the base fluid, nanoparticle
concentration and specific heat enhancement. Nitrate, when not specified, refers to
solar salt (NaNO3-KNO3 60-40 wt. %). Studies marked in blue correspond to
references that do not belong to Texas A&M University. ............................................ 175
xxxviii
Table 5-3. Hypothesis test between the base solar salt and solar salt nanofluid (1 wt.
% 30 nm SiO2 nanoparticles) ............................................................................................... 196
Table 5-4. Onset and peak melting and freezing temperatures .................................. 197
Table 5-5 Specific heat of neat salt and nanofluid (1 wt. % SiO2 10 nm
nanoparticles) modifying the base fluid (average over 260 – 400ºC for mixtures and
350 – 400ºC for pure components) ..................................................................................... 199
Table 5-6. Properties of common nanoparticle materials (bulk properties).90.......... 207
Table 5-7. Solar salt specific heat measurement in molten state by different authors.
(two step nanofluid synthesis procedure) ....................................................................... 208
Table 5-8. Chieruzzi et al. (2013)110 results for solar salt modified with different
nanoparticle types and concentrations. ............................................................................ 210
Table 5-9. Differences in the solar salt nanofluid synthesis procedure among
researchers. Enhancement at 1wt. % of nanoparticles concentration if not specified.
.................................................................................................................................................... 211
Table 5-10. New synthesis method nanofluids - Cp results ......................................... 214
Table 5-11. Type A nanofluid SS Vial 1%SiO2 (10 nm) vs neat solar salt .................. 215
Table 5-12. Type A vs Type B nanofluid SS Vial 1%SiO2 (10 nm) .............................. 215
Table 5-13. Possible explanation regarding the specific heat enhancement ............. 217
Table 5-14. New synthesis method nanofluids – Solid-liquid phase change results
.................................................................................................................................................... 218
Table 5-15. Hypothesis testing for the onset freezing temperatures for neat solar salt
vs. type A and type B nanofluid ......................................................................................... 219
Table 5-16. Onset melting and freezing temperatures values corresponding to the
tests shown in Figure 5-35 ................................................................................................... 221
Table 5-17.Maximum and minimum Cp neat solar salt (PD) sample compared to
the average values of the neat SS (vial) ............................................................................. 222
Table 5-18. Specific heat results: present work vs. Andreu-Cabedo et al. (2014). Neat
solar salt vs. solar salt nanofluid (1 wt. % of SiO2 nanoparticles). *Type A
nanofluid. ................................................................................................................................ 224
xxxix
Table 5-19. Solubility of nitrate salts in water.141 ............................................................. 225
Table 5-20. NaNO3-KNO3 compositions tested .............................................................. 230
Table 5-21. Slope of the linear fit of the normalized latent heat vs. the nanoparticle
concentration. ......................................................................................................................... 236
Table 5-22. Estimated thickness (∆) of the hypothetical interfacial liquid layer that
could explain the larger reduction on the latent heat observed in nitrate base
nanofluids. .............................................................................................................................. 239
xl
xli
Nomenclature
Abbreviations
CSP Concentrated solar power
DSC Differential Scanning Calorimeter
DSG Direct Steam Generation
HTF Heat transfer fluid
PCM Phase change materials
SEM Scanning Electron Microscope/Microscopy
TES Thermal energy storage
Symbol
Unit
Meaning
Latin symbols
BR - blockage ratio
Cp J/(kg K) Specific isobaric heat capacity
D m Diameter
e m thickness
h W/(m2 K) Convective heat transfer coefficient
k W/(m K) Thermal conductivity
Kg/s Mass flow
xlii
Nu - Nusselt numbers
P Bar Pressure
Pr - Prandtl
R m Radius
Re - Reynolds
T ºC/K Temperature
V m3 Volume
w - Weight fraction
Greek symbols
α m²/s Thermal diffusivity
η - Thermal efficiency
ρ kg/m³ Density
∆ nm Width of the interfacial liquid layer
∆T ºC Temperature difference
θ - Dimensionless temperature
μ kg/(s·m) Dynamic viscosity
Indices
bf Base fluid
C cold
H hot
i Interfacial liquid layer
nf Nanofluid
nm Nanomaterial
np Nanoparticle
p particle
1
1 INTRODUCTION
he increase in the global energy demand in the last decades, linked to the
rapid growth of the population has resulted in a human activity strongly
dependent on fossil fuels. Nowadays, eighty percent of the present
worldwide energy use is based on fossil fuels according to the World Energy
Council.1
This situation of unprecedented energy consumption has led to concerns about the
security of the energy supply in the midterm. As a response to this energy context,
the big challenge for the society is to achieve sustainable development supported
by the use of clean and renewable energy. The production of clean energy using
renewable energy sources is crucial and it is a necessary component to address the
climate change issue.
From this perspective, solar energy stands as the most abundant permanent energy
resource on earth. The potential of the solar energy becomes even clearer when the
world’s total annual primary energy consumption is compared with total annual
solar radiation falling on the earth, being the latter more than 7500 times higher.1 Its
endless nature makes it a very interesting energy source in the development of
renewable energy.
T
Introduction
2
1.1 Concentrating Solar Thermal Power Systems
Concentrating solar thermal technologies are based on the concept of concentrating
the radiation from the sun using reflective mirrors in order to achieve a high
temperature heat source. This heat source is used to generate electricity through
thermal power cycles.
The different concentration technologies can be classified depending on how the
solar radiation is focused: line-focus (linear Fresnel and parabolic trough) and
point-focus technologies (dish-Stirling and CSP central receiver plant). The main
differences between these two technologies are the sun tracking system and
concentration factor, defined as the ratio between the collector aperture area and
the receiver aperture area. Whereas line-focus systems track the sun along a single
axis achieving a low concentration factor (i. e. approximately 30 for Linear Fresnel),
point focusing systems track the sun along two axis and present considerable
higher concentrating factors. The concentration factor is directly related to higher
working operation temperatures which in turn define the maximum possible
thermodynamic cycle efficiency by Carnot’s theorem:
Equation 1-1
Where TC and TH are the sink and source temperatures respectively. This leads to a
higher efficiency in converting heat into mechanical motion and, hence, to
electricity. This means that point-focus systems can convert into electricity a larger
fraction of the energy that falls on the receiver than linear systems.
According to the International Energy Agency2,3 the central receiver CSP
technology is the one with the highest outlook for improvements linked to the
increase in the maximum operation temperature. Although at the present time line-
focus systems are a more mature technology, tower technology could be the
winning concept in a near future when some challenges such as the receiver
technology and the stability of the fluid flowing through it, called heat transfer
fluid (HTF), are overcome. The new development relies on the use of advanced
power cycles and new heat transfer fluids capable of operating at higher
temperatures.
3
Thermal Energy Storage for High Temperature Applications
There are, however, other more important challenges to be solved before solar
power becomes a feasible alternative capable of, for example, supplying the base
load of a whole country. The main challenge of solar energy nowadays is its
intrinsic intermittency, which makes it difficult to match supply and demand at
any time. The variability of the heat source, unlike traditional systems such as
conventional power plants, comes from the fluctuation of the solar radiation due to
the day and night cycle and intermittencies in cloudy periods. Therefore, a lot of
efforts are being put on trying to decouple power output and solar radiation. There
is only one known way to achieve this: through energy storage.
In this aspect, concentrating solar thermal technology is one step ahead of other
technologies such as photovoltaics: maybe due to the historical predominance of
heat utilization for different purposes, heat storage is a rather well-known issue
and poses nowadays minor technological problems when compared to the still
incipient –but very promising- electricity storage in batteries. Thanks to this
advantage, together with the higher conversion efficiencies compared to
photovoltaics, concentrated solar thermal power is bound to play a major role in
the solar race.
1.2 Thermal Energy Storage (TES)
A thermal energy storage system consists of an insulated container and a storage
medium which absorbs (releases) heat from (to) the heat transfer fluid. Due to the
fact that a CSP plant uses thermal energy as a primary energy form, thermal energy
storage can be direct and more easily integrated than other systems. The energy can
be stored through different mechanisms, described in the following section.
The design and operation of thermal energy storage in a solar thermal power plant
requires specific strategies for the accumulation of energy and production of
electricity depending on the season, location, climatology, number of sun light
hours, and thermal energy storage type and capacity. Nevertheless, it allows a
more manageable electricity production, reduces the transient periods in the plant
operation, and the operation time can be extended beyond the hours of sunlight.
Introduction
4
1.3 Mechanisms to store thermal energy
There are three main types of thermal energy storage (TES) according to the storage
mechanism involved: sensible heat storage, latent heat storage and thermo-
chemical storage.
Sensible heat storage: this type of storage relies on a TES medium’s
temperature change to store energy. The energy is stored/ recovered by
heating up/cooling down the TES medium. The amount of thermal energy
stored in the system depends on the temperature change, the mass, and
specific heat of the energy storage material.
Latent heat storage: this system utilizes the energy change involved in a
phase change transformation to store and release heat. The large enthalpy
change in these transformations provides a high storage density. This type
of TES system usually operates over a much narrower temperature range
than those of the sensible heat storage systems because the phase change
transformation occurs isothermally. Among the different phase change
transformations, latent heat storage by solid–liquid transformations
provides high energy storage density with a relatively low change in
volume.
Thermo-chemical storage: this system utilizes the solar energy to drive
reversible chemical reactions which store energy in chemical bonds. When
a chemical TES system is discharged, the chemical bonds are broken and
the thermal energy can be extracted as needed.
So far, the few thermal storage systems introduced in some solar plants have
followed the path initiated by Solar Two4 in the nineties: they store thermal energy
in form of sensible heat, i.e. as a temperature increase of the storage material.
Lately, the search for improvements in the field of thermal energy storage is
shifting interest towards new solutions, namely to latent heat thermal energy
storage, based on the solidification and melting of the storage material and not on
its temperature change. In comparison with sensible heat technology, latent heat
features a much higher energy density, which in turn means smaller volume of
material and lower heat losses. It also works at a nearly constant temperature,
something that can be especially interesting for the evaporation of steam in a
Rankine cycle. The development of the latent heat technology represents the
5
Thermal Energy Storage for High Temperature Applications
cutting edge in the field of thermal energy storage and has the potential to increase
greatly the competitiveness of solar power. For this reason, there is a growing
amount of studies on the possible materials, how to overcome the challenges that
arise when using phase change materials (PCM) for latent heat thermal storage.5–12
1.4 Classification of TES
According to Gil et al. (2010)8 high thermal storage systems can be classified into
active or passive systems:
The active storage system is characterized by forced convection heat transfer
into the storage material. The storage medium itself circulates through a heat
exchanger. Active systems can be subdivided into direct systems using the heat
transfer fluid as the storage medium, which reduced the number of heat
exchangers, and indirect systems where a second medium is used for storing
the heat.
Passive storage systems are generally dual medium storage systems. The
storage medium itself does not circulate, being the HTF which passes through
the storage only for charging and discharging. The HTF carries energy received
from the energy source to the storage medium, which is mainly solid systems
(concrete, PCM…) during charging, and receives energy from the storage
when discharging. The main disadvantages of passive storage systems are on
one hand, that the HTF decreases in temperature during the discharge as the
storage material cools down. On the other hand, the low heat transfer rates
between the storage material and HTF.
Ideally, one would like to use the same medium as HTF, TES material, and
working fluid in the power block, to avoid any irreversibilities associated with the
exchange of heat and to simplify the installation. However, in practice this can only
be achieved with steam and the quality and duration of the heat storage with steam
to this date is poor.
Introduction
6
1.4.1 TES in commercial central receiver solar power plants
A central receiver solar power plant uses reflective mirrors called heliostats to focus
the radiation from the sun into a small area on top of the tower called the receiver.
A fluid (heat transfer fluid) passes through the receiver absorbing heat. This heat is
used as a heat source in a Rankine Cycle where pressurized water is converted into
superheated steam feeding a steam turbine to generate electricity. This plant
configuration leads to a more compact design where the working fluid piping and
power block are located in a small area.
In the last and present decade some commercial CSP central receiver plants with
thermal storage have started its operation. Data regarding the actual CSP plant
projects that are currently operational or under construction worldwide can be
readily found at the National Renewable Laboratory (NREL) website.13 It is data
compiled by the SolarPACES (Solar Power and Chemical Energy Systems)
organization. It gives a clear picture of the CSP technology maturity, installed
capacity, project location and development trends. Some of the first commercial
plants are presented in Medrano et al.9 describing in more detail the different
technologies and materials used. Table 1-1 summarizes recent commercial solar
power tower plants with TES projects in operation, under construction or under
development.
Table 1-1. List of commercial CSP central receiver plants with TES operating, under
construction (UC) or under development (UD).13
Name Power TES: technology
(hours)
Temperatures Status (Start
Year)
Planta Solar 10
(PS10) (Spain)
11MW Steam
Rankine
(45bar)
50-minute at 50%
load. Steam
accumulator
Water/Steam
(HTF)
250 – 300ºC
(receiver
outlet)
2007
Planta Solar 20
(PS20) (Spain)
20MW Steam
Rankine
(45bar)
50-minute at 50%
load.
Water/Steam
(HTF)
2009
7
Thermal Energy Storage for High Temperature Applications
Gemasolar
Thermosolar
Plant (Spain)
19.9MW Steam
Rankine Molten Salts (MS)
(15h)
290-565ºC
(receiver inlet
outlet)
2011
Khi Solar One
(South Africa)
50MW Steam
Rankine 2h Steam
accumulator
Water/Steam
(HTF)
2014
Crescent Dunes
Solar Energy
Project (USA)
110MW Steam
Rankine
(115bar)
MS (10h) 2015
Rice Solar
Energy Project
(RSEP) (USA)
150MW Steam
Rankine
(115bar)
MS 287-565ºC
(receiver inlet
outlet)
2016 (UD)
NOOR
III(Morocco)
150MW MS (8h)
2017 (UC)
Qinghai
Delingha Solar
Thermal
Generation
Project (China)
270MW Steam
Rankine MS (3.5h)
2017 (UD)
Atacama1
(Chile)
110MW Steam
Rankine
MS (17.5h) 300-550ºC 2018 (UC)
Redstone Solar
Thermal Power
Plant (South
Africa)
100MW Steam
Rankine MS (12h)
288-566ºC
(receiver inlet
outlet)
2018 (UD)
Copiapó (Chile) 260MW Steam
Rankine
MS (14h) 2019 (UD)
Supcon Solar
Project (China)
50MW Steam
Rankine MS (2.5h)
(UC)
As can be seen from Table 1-1, there are two main TES commercial technologies for
this type of plants: saturated steam storage and molten salts (MS) storage. These
two TES technologies would fall within the active storage systems using the same
Introduction
8
fluid as heat transfer fluid and thermal storage material. The main difference
between these two technologies is the number of hours of storage due to the
optimization of each technology leads to different storage capacities.14
1.4.2 Steam accumulators
During the normal operation of the plant, part of the high temperature steam
produced in the solar receiver is derived to the insulated stainless steel tanks, called
steam accumulators, instead of being piped to the steam turbine to generate
electricity. This energy is stored in the form of pressurized saturated liquid water at
high temperature.
In special conditions such as transitory cloudy periods without sunshine in which
the vapor from the receiver is insufficient to maintain the turbine operation, steam
is produced by lowering the pressure of the saturated liquid during discharge.
Steam accumulators operate in many industrial processes and also since the fifties
in some conventional thermal power plants. The energy density of this technology
can reach 20-30 kWh/m3.15 PS10 (Seville, Spain), the first commercial solar tower
power plant in the world with a nominal power of 11 MW, is one of the
commercial solar power plants using this technology. The storage provides 50
minutes of storage capacity at 50% load (50 bars and 285 °C) to handle cloud
transients.
The main advantages of the plants incorporating this technology include: a lower
investment and operation and maintenance costs and greater simplicity of the
overall plant configuration, allowing the installation to operate at higher
temperatures compared to facilities using a different heat transfer fluid than water.
Other advantage is the rapid availability of the storage energy.
Since the heat transfer fluid is the same water used in the power block, the
intermediate heat exchanger between the heat transfer fluid and the steam
generation can be eliminated. This results in a higher steam temperatures and
consequently higher power cycle efficiencies. On the other hand instabilities of the
two phase flow inside the receiver together with the increase of the pipe installation
cost because of the high pressure required are the main drawbacks.8
9
Thermal Energy Storage for High Temperature Applications
1.4.3 Two molten salt tanks direct system
In this thermal storage system molten salts are used as a heat transfer fluid as well
as the storage medium, absorbing the solar thermal energy in the receiver and
storing the fluid in two tanks at different thermal level, one at high temperature
and the other at low temperature, both above the freezing temperature of the salt.
The fluid, known as solar salt, is a well-known off-eutectic mixture of sodium
nitrate (60 wt. %) and potassium nitrate (40 wt. %).
The storage material from the low temperature tank is pumped through the solar
receiver, where it heats up to a higher temperature (565ºC), flowing to the high
temperature tank. The fluid is kept in the hot tank until extra heat is required to
operate the power block. At this point the system is discharged by pumping the
fluid through a heat exchanger where it generates steam for electricity production
while it decreases its temperature. The cycle ends with the fluid in the cold tank as
the original starting point.
The main limitations of this storage system are the high investment costs of the
thermal storage material/HTF, the storage tanks and the heat exchangers.
Furthermore, working with molten salts with relatively high freezing temperature
has the additional risk of solidification of the storage fluid. This increases the
maintenance and operation costs due to the requirement for electric heat tracing on
all salt equipment. On the other hand, the reason why molten salts is the most
widely used thermal storage material in CSP is the combination of heat transfer
and thermo-physical properties together with its very low vapor pressure, which
reduces the thickness of the storage tank and also the piping mechanical stress.
1.5 Main objectives of the thesis and structure
The main objective of the thesis is to improve existing commercial TES solutions for
CSP central receiver plants by increasing their storage density. Two different
research areas can be distinguished: the first part focus on improving the steam
accumulator technology and the second, the two molten salt tanks technology.
The current solution of direct storage in steam accumulators is not cost competitive
for larger storage capacities due to the low volumetric energy density. Research in
Introduction
10
order to increase the storage density of this solution is directed towards latent
storage materials. Phase change materials (PCM) are studied because of its
potential in the system energy density increase for DSG applications. The thermal
storage mechanism based on phase change transformation is used to store the
evaporation enthalpy of water/steam. The main advantage is a temperature profile
in the storage system matches the temperature profile of the water/steam and
therefore higher efficiencies can be obtained in these systems.
Figure 1-1 shows a schematic representation of the approach followed in this study.
Chapter 2 to 4 aim at developing both materials and systems for thermal energy
storage at high temperature based on PCM. The use of PCM consisting of
encapsulating the PCM to exchange heat through capsules in a packed bed system
is investigated. The idea is to provide a suitable design and performance analysis of
the storage units that will form the storage system. A material screening for PCM
candidates is performed in Chapter 2 combining thermo-physical and economic
criteria. Several potential materials are characterized through a differential
scanning calorimeter. Different encapsulation attempts for high temperature PCM
are reviewed in Chapter 3, addressing the most important challenges in the
development and testing of the thermal storage components. For PCM
encapsulation, a new macro-encapsulation procedure is designed, developed, and
tested. In Chapter 4 a single capsule thermal model is used to analyze the impact of
different parameters on the phase change process of the PCM-capsule. This model
provides valuable information to guide the design of PCM capsules. Capsules are
fabricated and a laboratory set-up is used to test single capsules. The encapsulation
procedure, numerical and experimental validation of the proposed solutions are
presented and discussed.
The second part of this thesis is focus on improving the commercial solution based
on two molten salt tanks, specifically on improving the specific heat of the molten
salt by adding nanoparticles (Chapter 5). We have also investigated the effect of
nanoparticles on the phase change characteristics of the salts. The use of
nanoparticles could be used to improve both systems: nanoparticle addition on
nitrate salts to modify the phase change properties for PCM applications and to
enhance the specific heat for sensible heat applications.
11
Thermal Energy Storage for High Temperature Applications
Figure 1-1: Schematic representation of the study presented
TES in Commercial CSP
Tower Plants
Steam Acumulator
PCM
Screen & characterize PCM
(Chapter 2)
Packed bed
Macro - encapsulation (Chapter 3-4)
Screen Shell Materials
Fabricate capsules
Model Single Capsule
Testing
Tube & housing
Double PCM (Chapter 4)
Molten salt tanks
Nano enhanced HTF-TES
(Chapter 5)
Specific heat
Latent heat
Stability
Introduction
12
13
2 PHASE CHANGE MATERIALS
oday’s thermal energy storage commercial solutions in large scale CSP plants
are based on sensible heat materials such as inorganic molten salts and steam
accumulators. The molten salt storage can provide sufficient thermal energy,
not only to avoid the natural intermittencies of the solar resource but also to
produce power throughout the entire night (~ 15 hours), while steam accumulators,
using sensible heat storage in pressurized saturated liquid water are used for peak
power, although this is an inefficient and not economically attractive solution for
large storage capacities and high pressure. On the other hand, higher energy
density systems utilizing latent heat storage are still being developed.
2.1. Introduction and main objectives
Many latent heat TES have been tested at laboratory scale and in demonstrator
prototypes, but they are not currently cost competitive with sensible heat storage.
The most interesting property of these materials is their ability to absorb and
release heat at almost constant temperature.7 This temperature corresponds to the
phase transition (i.e. melting and solidification) of a phase change material (PCM).
T
Phase Change Materials
14
This approach is of particular interest in Direct Steam Generation (DSG) solar
thermal systems, which currently do not have a cost-effective storage solution.
Since the major part of the energy is required in the evaporator ~55%16 this
technology might allow a more efficient heat exchange between the storage system
and the water/vapor by maintaining the temperature difference along the heat
exchanger, thus reducing the heat exchanger pinch point.
Abhat (1983)17 described the three challenging elements involving the design of a
latent heat thermal energy storage system:
1. The phase change material: a heat storage substance that undergoes a
solid-to-liquid phase transition within the desired operating temperature
range and where the bulk of the heat added is stored as the latent heat of
fusion.
2. The containment for the thermal storage substance: this could be
micro/macro-encapsulation of the PCM, for example.
3. A heat exchanging surface for transferring heat from the heat source to the
heat storage substance and from the latter to the heat sink. This heat
exchanger is inherently related to the heat transfer fluid (HTF) of choice
(for DSG this HTF is pressurized steam at high temperature, while for
molten salts tower plants the salts are both the TES material and HTF, and
in parabolic trough plants synthetic oil is typically the HTF).
The preferred latent heat TES candidates for DSG are inorganic salts due to their
relatively low cost, high latent heat, and appropriate melting temperature for this
application (between 200-400ºC). However, the main limitation of these PCM is
their low thermal conductivity (on the order of 1 W·(m K)-1)18, which governs the
heat exchange rate between the storage material and the working fluid. The low
thermal conductivity causes:
Long response times in the thermal storage discharge process
High temperature gradients
Limits the heat flux absorbed/ released from the PCM
This low thermal conductivity has resulted in researchers developing different
15
Thermal Energy Storage for High Temperature Applications
techniques to improve this property. The goal of all these methods is to achieve a
higher thermal conductivity, preferably in the range of 5-20W·m-1·K-1. These
different approaches to improve the thermal performance of PCM TES systems are
presented schematically in Figure 2-1. Fabricating PCM-composite materials with
increased thermal conductivity can be achieved through 1) highly conductive
matrices (carbon foams19, metal foams20, meshes21, wool, sponges) where the
composite is created by infiltrating the molten PCM through the porous matrix;
and 2) dispersing high conductivity fillers (particles, flakes, fibers22 in the PCM,
commonly mixed in solid state at room temperature, although in some cases the
dispersion can be done in molten state or in water suspension/solution.
Alternatively, the thermal conductivity of PCM can be enhanced by adding
traditional extended surfaces to improve the heat transfer. The extended heat
transfer surface techniques include the insertion of fins fabricated with high
thermal conductivity materials (metallic fins on the heat exchanger tubes in contact
with the PCM23 or encapsulating the PCM. Heat transfer effectiveness in this last
solution increases as capsule size decreases, minimizing the influence of a PCM
with low thermal conductivity. PCM encapsulation will be reviewed and analyzed
in detail in Chapter 3. On the other hand, metallic PCM, which present other
drawbacks and generally higher specific cost, have been also studied due to their
higher thermal conductivities.
Figure 2-1: Classification of performance enhancement techniques.
Composite materials with increased
thermal conductivity
Matrix infiltrated with
PCMs
Dispersed filler-PCM
Extended heat transfer surface
Fins
Capsules
Phase Change Materials
16
2.1.1. Objectives
A material screening for potential PCM candidates is performed in this chapter
(Chapter 2) combining thermo-physical and economic criteria. Several potential
materials are characterized through calorimetry comparing with the literature
values. Figure 2-2 shows a schematic representation of the approach followed in
this study.
Figure 2-2: Schematic representation of the study presented with the contents of
this chapter marked in yellow.
2.2. PCM screening, characterization and selection
An extended literature review has been performed for the PCM selection. The main
criteria are materials with a melting temperature in the range 300-400 ºC and a high
latent heat of fusion. However, there is a long list of appropriate physical, thermal,
chemical and economic properties, often competing with each other. Table 2-1
shows the desirable characteristics that the storage materials must possess to enable
a more cost-effective system.
PCM TES heat
exchanger system
Packed bed
Macro-encapsulation
Screen & characterize
PCMs
Screen Shell Materials
Model Single Capsule
Fabricate capsules
Test single
capsule
Compare Experiments
& Model
Evaluate performance & challenges
Tube & housing
Metal PCM
Double PCM
17
Thermal Energy Storage for High Temperature Applications
Table 2-1: Desirable PCM properties.7,17,24
Thermal
properties
Physical
properties
Kinetic
properties
Chemical
properties
Economics
(i) Suitable phase-
transition
temperature.
(ii) High latent
heat of transition.
(iii) High thermal
conductivity.
(iv) Thermal
stability.
(i) High density.
(ii) Low density
variation
during phase
change.
(iii) Low vapor
pressure at the
operating
temperature.
(i) Little or no
super-cooling
during
freezing.
(ii) Sufficient
crystallization
rate.
(i) Long-term
chemical
stability.
(ii) Compatibility
between storage
medium and
containment
material.
(iii) No toxicity.
(iv) No fire
hazard.
(i) Abundant.
(ii) Available.
(iii) Cost
effective.
(iv) Minimal
environmental
and health
impact.
Each of the properties in Table 2-1 comes from the inherent drawbacks that appear
when PCM are used in thermal storage systems. For example, heat transfer
properties are important when the PCM starts solidifying on the heat transfer
surface impeding the heat transfer process; the density change between solid and
liquid PCM is an important issue that must be addressed for encapsulated PCM.
Even if some candidates meet most of the requirements, they can be discarded if
the appropriate balance among the different requirements is not achieved. Finding
an ideal PCM that is able to satisfy all these different requirements is very
challenging.
Storage materials at high temperature are mainly inorganic salts or metals, pure or
often eutectic mixtures, which means that these compositions present minimum
melting temperature melting and freezing congruently like a pure substance.
A first list of possible PCM candidates for a specific TES application can be chosen
based on having a melting point in the rage of interest in order to be able to store
the energy coming from a source or process, and a high latent heat of fusion. Many
extended lists of PCM candidates with their melting temperature and latent heat of
fusion can be found in different literature reviews.5,8,25–27 Zalba et al. (2003) 5
Phase Change Materials
18
presented inorganic substances with potential use as PCM in the temperature
range 100 - 900 ºC. Apart from the melting point and latent heat, Kenisarin (2010)
summarized other thermo-physical properties such as specific heat, conductivity,
safety and compatibility with container materials such as stainless steel. 11
Figure 2-3: Heat of fusion vs. melting point for metals, fluorides, chlorides, nitrates
and other salts (data extracted from 11).
Figure 2-3 summarizes the different PCM candidates (metals, fluorides, chlorides,
hydroxides and nitrates) arranged considering the heat of fusion versus their
melting point. The temperature range used in different CSP plant thermodynamic
cycles has been superposed. For conventional (subcritical) Rankine cycles, the PCM
melting temperature should be in the 250-450ºC range with the highest possible
0
100
200
300
400
500
600
700
800
900
1000
200 300 400 500 600 700 800 900
Heat
of
fusi
on
[k
J·k
g-1
]
Melting Point [ºC]
Alloy Carbonate Chloride Fluoride Hydroxide Nitrate
Rankine
Subcritical
Cycle
Rankine
Supercritical
Cycle
Brayton Cycle
19
Thermal Energy Storage for High Temperature Applications
heat of fusion and best combination of properties as shown in Table 2-1. However,
for CSP plants with more advanced power cycles such as supercritical-H2O or
Brayton-air cycles, PCM with higher melting temperatures should be utilized.
The different PCM candidates for high temperature applications have been divided
into salts and metals, since each group presents their own challenges and
limitations.
2.2.1. Metallic PCM candidates
Pure metals and metal alloys have been proposed as possible candidates for TES
using their phase change enthalpy between solid and liquid state. The main
components of these alloys are zinc, magnesium, aluminum, copper, silicon and
calcium. The melting point (Tm) in the 250-500ºC temperature range and high latent
heat makes these materials competitive with inorganic salts for the same TES
application. The most interesting property of metallic PCM, which gives them an
advantage over salts, is their high thermal conductivity. This implies higher
charging and discharging rates in the thermal energy storage tank heat exchangers.
As an example, the alloy Al-12wt.%Si with a Tm = 576 ºC and heat of fusion = 560
kJ/kg presents a thermal conductivity of 160 W/(m·K), which is 100 times higher
compared to inorganic salt PCM. The effect of this large difference in thermal
conductivity can be understood based on the comparison performed in Hoshi et
al.18 where the use of lead (Pb) versus potassium nitrate (KNO3) as a phase change
material in a shell and tubes TES configuration (similar to Figure 2-4) was
evaluated (dimensional parameters: ri=0.01 m, ro=0.1 m, ∆T = 10 ºC). The charge of
100MJ·m-3 of thermal energy as a latent heat is 8.25 times faster in the case of lead
compared to the inorganic salt. However, the differences become much higher in
the discharge process. The discharging speed for potassium nitrate is considerably
slower than for lead. The salt system is unable to discharge the same amount of
energy as the metal system even in times that are an order of magnitude higher,
since the discharging time is substantially longer for equal volumetric energy
density. These large discrepancies are caused mainly by the differences in the
thermal conductivity (k) of the two media (kPb is 30 to 60 times higher than kKNO3).
Phase Change Materials
20
Figure 2-4: Schematic representation of a typical tube and housing thermal storage
unit
On the other hand, the main drawback of using these metallic materials is their
high cost and limited availability in general. Nevertheless, metallic PCM can
become competitive since a simple design and a reduction in the heat transfer area
(less number of pipes embedded in the bulk material) can be achieved for charging
and discharging due to their high thermal conductivity.
In the temperature range of 300-400 ºC one of the interesting PCM candidates is the
Mg-Zn binary system. Rodríguez-Aseguinolaza et al.28 studied this PCM,
specifically Mg-Zn (49 - 51 wt. %). Its thermal diffusivity, heat capacity, and energy
density were measured and compared to nitrate salts. This alloy shows a thermal
diffusivity two orders of magnitude higher than salts, more than two orders of
magnitude higher thermal conductivity, similar latent heat and volumetric energy
density (155 kJ·kg-1, 442 MJ·m-3) to sodium nitrate’s (172 kJkg-1, 389 MJ·m-3 ). The
price of these metals is around ~2 €/kg29,30 compared to ~0.7 €/kg31 for sodium
nitrate. This means that, for these two PCM’s to be comparable, the difference in
price gives the maximum allowable cost of any heat transfer enhancement
PCM
Heat Transfer
Fluid
x
ro
ri
ξ
HTF
solid
liquid
21
Thermal Energy Storage for High Temperature Applications
technique for sodium nitrate to approximate its heat transfer properties to those of
the alloy.
In Adinberg & Epstein32 Sn-Zn alloys were proposed in order to stabilize the steam
temperature in direct steam solar power plants. The results of this numerical study
showed that the steam sent to a turbine after the thermal buffer may have stable
temperature near 330ºC during the entire charge-discharge cycle. The same idea
was applied in Bellard (2012)33 in concentrated solar power plants using alloy Al-
12wt%Si as PCM in the solar receiver (with a higher melting temperature). This
alloy has been also studied in Yagi et al.34 and Wang et al.35 using a stainless steel
capsule and a clay-ceramic container respectively. Table 2-2 shows the market
prices for some metals in two different dates (at the present time and when this
study was performed) and a thermo-economic evaluation is presented in Table 2-3.
As this data shows, the competitiveness of a metallic PCM is highly dependent on
the fluctuations of the market.
Table 2-2: Metals price29
Price [€/ton]
(accessed 1/11/2012)
Price [€/ton]
(accessed 27/05/2016)
Copper 6017 4208
Zinc 1422 1703
Nickel 12570 7499
Aluminum 1403 1379
Lead 1636 1517
Tin 15761 14551
Phase Change Materials
22
Table 2-3: Estimated specific cost of different metal alloys and cost per thermal
storage unit
Phase Change Materials Melting
Temperature
Heat of
Fusion Price
Price/Stored
Energy
Composition (wt. %) [ºC] [kJ/kg] [€/kg] [€/kWh]
Sn 227 60 15.07 904.2
Pb 327 23 1.50 234.78
Zn(52)–48Mg 340 180 2.10 41.94
Zn(53.7)–46.3Mg 340 185 2.08 40.42
Zn(96)–4Al 381 138 1.54 40.15
Mg(55)–28Ca–17Zn 400 146 1.75 43.07
Zn 419 112 1.76 56.57
Al(59)–35Mg–6Zn 443 310 1.93 22.46
Mg(60)–25Cu–15Zn 452 254 3.38 47.97
Mg(52)–25Cu–23Ca 453 184 2.94 57.48
Al(65.35)–34.65Mg 497 285 1.93 24.36
Al(60.8)–33.2Cu–6.0Mg 506 365 3.12 30.80
Al(64.1)–5.2Si–28Cu–2.2Mg 507 374 2.88 27.70
Al(54)–22Cu–18Mg–6Zn 520 305 2.75 32.45
Al(68.5)–5.0Si–26.5Cu 525 364 2.77 27.44
Al(64.3)–34.0Cu–1.7Sb 545 331 3.32 36.09
Al(66.92)–33.08Cu 548 372 3.05 29.48
Al(83.14)–11.7Si–5.16Mg 555 485 1.65 12.28
Al(87.76)–12.24Si 557 498 1.60 11.55
Al(65)–30Cu–5Si 571 422 2.94 25.05
Al(46.3)–4.6Si–49.1Cu 571 406 3.81 33.83
Al(86.4)–9.4Si–4.2Sb 575 471 2.15 16.40
23
Thermal Energy Storage for High Temperature Applications
In the temperature range of interest three metallic PCM have been selected and
characterized: Mg-Zn alloy due to its thermo-economic properties and two pure
metals: lead, also used as PCM in 34,36, and tin. Most of the other alloys from Table
2-3 have higher melting temperatures which may be more convenient for PCM
cascade configurations in the superheated steam region.
A Differential Scanning Calorimeter (DSC1 from Mettler-Toledo) is used to
thermally characterize the PCM candidates. The calibration procedure is performed
by using the melting temperatures and latent heat of standard certified reference
materials (In, Zn), at a heating rate of 10ºC·min-1, resulting within the limits
specified by the equipment manufacturer. The interactions between the crucible
material and the sample tested must be evaluated before the measurements are
performed. Figure 2-5 shows how the solubility of the aluminum crucibles on the
MgZn sample damages the crucible and can damage the DSC sensor. Sapphire
crucibles have been used instead to avoid any possible alloying between the PCM
and crucible.
Figure 2-5: MgZn alloying with Aluminum crucibles.
The preparation of the Mg-52wt%Zn alloy was slightly more cumbersome than
initially anticipated. It was achieved by melting the magnesium at 665ºC before
adding zinc, both of them in the form of solid bricks. The mixture was then melted
and crystallized twice (30 minutes from 300 to 500ºC and 60 minutes from 300 to
400ºC) in a stainless steel A316 crucible cover by Ar-1%SF6 atmosphere.
The melting temperature, the latent heat and specific heat has been measured
through different freezing /melting cycles. The thermal cycle heats up from 230 ºC
Phase Change Materials
24
to 400 ºC for lead, from 100 ºC to 300 ºC for tin, and 60 ºC to 460 ºC for MgZn alloy,
and cools down to the initial temperature at a heating rate of 10 ºCmin-1. Eight
thermal cycles have been programmed. Nitrogen is used (50 ml·min-1) as over gas.
Figure 2-6: Lead (Pb) and Tin (Sn) phase change properties: onset melting
temperature (Pb and Sn, left), and lead melting and freezing latent heat (right)
Figure 2-6 shows the onset melting temperature (on the left) for tin and lead, and
the melting and freezing latent heat for lead (on the right). The onset melting
temperature is stable over eight thermal cycles. Only the first melting presents
significant differences for both metals (Table 2-4). It might be caused by a larger
energy required to break down the crystalline lattice the first time. This difference
could be also produced by an inadequate contact between the sample and the
crucible before the first melting.
Table 2-4: Average (cycle 2 to 8) onset melting temperature and melting latent heat
for lead and tin (first melting discarded).
Onset Melting T [ºC] Melting Latent Heat [J·g-1]
Average 2-8 melting First melting Average 2-8 melting First melting
Pb 315.5 +/-0.1 317.8 20.9+/-0.1 21.6
Sn 179.2 +/-0.6 182.5 44.4+/-0.7 46.1
150
170
190
210
230
250
270
290
310
330
350
0 1 2 3 4 5 6 7 8 9
On
set
Mel
tin
g T
emp
erat
ure
[ºC
]
Thermal Cycle
Pb Sn
20.5
20.7
20.9
21.1
21.3
21.5
21.7
21.9
0 1 2 3 4 5 6 7 8 9
Lat
ent
Hea
t [J
/g]
Thermal cycle
Pb Melting LH
Pb Freezing LH
25
Thermal Energy Storage for High Temperature Applications
The expected melting temperature and latent heat of these metals is 231.9 ºC and 59
J/g for tin and 327.5 ºC and 23 J/g for lead. Our measurements indicate a low purity
grade of the metals tested leading to a lower melting point and melting enthalpy.
Only the latent heat of lead is near its theoretical value. The differences between the
freezing and melting latent heat (+2% for freezing) can be neglected.
The specific heat of lead and tin has been also measured for different thermal
cycles (Figure 2-7) using ASTM E1269 protocol37. The measured values in molten
state are slightly higher than the theoretical values in solid state at room
temperature 0.13 J·(gK)-1 for lead and 0.21 J·(gK)-1 for tin. Even though the relative
differences (14% and 31%) for tin and lead respectively are not negligible, the
absolute differences are within the accuracy limits of the DSC.
Figure 2-7: Specific heat of Pb and Sn
The characterization of MgZn alloy has been also performed (table 2-5). The
measured latent heat of this alloy is 152.5 +/- 4.6 J·g-1 which is consistent considering
recent literature values (Table 2-6). The onset melting temperature is 342 ºC for the
first melting changing to 336.6 +/- 0.2 ºC for subsequent melting ramps (average
cycles 3 to 10). The onset freezing temperature is stable (331.1+/-0.1 ºC) from the
0.17 0.17
0.24 0.27
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
160 180 200 220 240 260 280 300 320 340 360 380 400
Sp
ecif
ic h
eat
[J/
(g·K
)]
Temperature [ºC]
Pb 2nd Melt
Pb 3rd Melt
Pb 4th melt
Sn 2nd Melt
Sn 3rd Melt
Sn 4th Melt
Phase Change Materials
26
first freezing.
Table 2-5: MgZn measured onset, peak, and endset temperature (melting and
freezing) at 10ºC·min-1 during ten subsequent heating and cooling ramps.
Onset
Temperature [ºC]
Peak
Temperature [ºC]
Endset
Temperature [ºC]
Melting 336.6 +/- 0.2
(342 1st melting)
339.2 +/- 0.1
(344.8 1st melting)
343.2 +/- 1.1
(351.6 1st melting)
Freezing 331.1 +/- 0.1 327.1 +/- 0.3 320.9 +/- 0.6
Table 2-6: Melting temperature and latent heat of Mg-Zn alloys, literature values
vs. measurements.
Melting/solidification
Temperature [ºC]
Melting
Latent
Heat [J·g-1]
Present work (Mg-52wt%Zn) 336.6/331.1 152.5
Blanco et al (2014)28 (Mg-52wt%Zn) 342/337 155
Gasanaliev & Gamataeva (2000)38
(Mg-53.7wt%Zn) 340/- 185
Farkas & Birchenall (1985)39 (Mg-52wt%Zn) 340/- 180
Birchenall & Riechman (1980)40 (Mg/Mg2Zn) 343/- 138
As a summary, among the different metallic PCM considered and thermo-
economically compared, three different metals have been characterized with DSC
measuring the phase change properties and specific heat. The pure metals selected
have shown significan impurities content based on its lower latent heat and
melting temperature. On the other hand, the MgZn alloy has shown agreement
with recent literature values reported for the same alloy in terms of latent heat.
However, the similarities in the melting tempearture can only be observed for the
first melting cycle, reducing the melting temperature by ~6ºC for the third and
subsequent melting cycles. The onset freezing temperture is stable upon cycles.
27
Thermal Energy Storage for High Temperature Applications
2.2.2. Inorganic salts PCM candidates
Different nitrate based mixtures have been pre-selected as possible encapsulated
PCM candidates with a melting temperature in the range 290-340ºC. Other authors
such as Gomez (2011) and Tamme et al. have also analyzed single to quaternary
salt mixtures.41,42 The nitrate based compositions, with an expected melting
temperature in the desired temperature range, evaluated in this study are
described in Table 2-7. Hydroxide and chloride based mixtures were also initially
considered but discarded due to their corrosiveness and their high hygroscopic
nature which makes them extremely complicated to synthesize and to measure
their properties.24
Table 2-7: Nitrate based compositions to be tested and predicted melting
temperature. *Estimated based on FactSage43 , **literature value 41,44,45
Phase Change Materials Composition (wt. %)
Melting
Point
[ºC]
Heat of
Fusion
[kJ·kg-1]
NaNO3 306* 172**
KNO3 335* 95**
Durferrit (Hitec XL) 120** -
NaNO3, NaCl (4.5) 284** 171**
KNO3, KCl (4.5) 320** 74**
(1) KNO3 (91.43)-KCl (7.32)-KBr (1.25) (wt. %) 306.7* -
(2) KNO3 (80.7)-KBr (11.9)-KCl (7.4) (wt. %) 342.0** 140**
(3) NaNO3 (86.67)-NaCl (8.27)-Na2CO3 (5.06) (wt. %) 358.9* -
(4) NaNO3 (93.11)-NaCl (4.67)-Na2CO3 (2.22) (wt. %) 294.6* -
(5) NaNO3 (90.01)-NaCl (2.64)-NaBr (7.35) (wt. %) 290.5* -
(6) NaNO3 (89.21)-NaCl (1.35)-NaBr (9.44) (wt. %) 290.0* -
Sodium and potassium nitrate are the main components in the different mixtures
evaluated. All mixtures are thermally characterized to confirm the reported data.
These pure components (sodium and potassium nitrate) are measured as a
reference in order to analyze whether the combination with other components
Phase Change Materials
28
produce significant differences in their phase change properties. DurferritTM
(another commercial name for HitecXLTM) has been also considered as a PCM. This
mixture is composed by NaNO3, KNO3 and Ca(NO3)2 with a very low melting
point, around ~120 ºC.45 It will be used as a proof of concept for the initial
manufactured PCM-capsule because it makes testing at lower temperatures
possible. The initial tests with this salt mixture are used to demonstrate the ability
of the capsule to withstand initial thermal shock and cycling at lower temperatures
before testing other higher temperature PCM. The mixtures (1, 3-6) have been
selected based on their phase change diagrams43 (Figure 2-8 to Figure 2-10), while
looking for mixtures with eutectic points around 300 ºC. The new compositions
also provide some a priori economic benefits such as lowering the price of the
mixtures using carbonate or chloride. The mixture (2) has been evaluated based on
Gomez (2011)41.
Figure 2-8: KNO3-KCl-KBr phase diagram, composition (1) and (2)
29
Thermal Energy Storage for High Temperature Applications
Figure 2-9: NaNO3-NaCl-Na2CO3 phase diagram, composition (3) and (4)
Figure 2-10: NaNO3-NaBr-NaCl phase diagram, composition (5) and (6)
Phase Change Materials
30
Zhao et al. 44 also consider the melting temperature and latent heat of some of the
nitrate mixtures in Table 2-7. Different trends can be observed in the values
reported: the addition of a small amount of NaCl to NaNO3 seems to lower the
melting point but the latent heat remains unchanged. On the other hand, the
addition of a similar amount of KCl to KNO3 reduces in the same order of
magnitude the melting point (-15 ºC), although a significant change in the latent
heat is observed. In contrast, the addition of a third component (KBr) to the KNO3-
KCl mixture seems to increase significantly both, the melting point (+22 ºC) and the
latent heat (+66 J·g-1) compared to the binary composition, and +7 ºC and +45 J·g-1
compared to the pure KNO3. These observations indicate that further research is
needed to clarify the discrepancies on these novel compositions on the
melting/solidification characteristics.
Thermo-physical properties are only one criterion to take into account when
selecting an appropriate PCM. Economic considerations also play a major role (see
Table 2-1). The purity, supplier and market price of the different components are
summarized in Table 2-8. The market price of different chemical components has
been obtained based on quotes for large purchase volume (hundreds of tons). A
quick assessment of this information gives an idea of the economic impact on the
final price of the mixture.
31
Thermal Energy Storage for High Temperature Applications
Table 2-8: Purity grade and supplier of the chemical components used in this study,
and estimated market price
Component Purity Supplier
Market price
(quotes for tons)
[€/kg] 31
NaNO3 Technical
grade
Thermosolar - Crystals, SQM
Northamerica Corporation,
Atlanta, GA
0.77
KNO3 Technical
Grade
Thermosolar - Crystals, SQM
Northamerica Corporation,
Atlanta, GA
1.08
Na2CO3 99.5% Panreac Química S.L.U.,
Barcelona, Spain 0.23
NaBr 99% Panreac Química S.L.U.,
Barcelona, Spain 2.03
NaCl 99% Panreac Química S.L.U.,
Barcelona, Spain 0.23
KBr 99.5% Panreac Química S.L.U.,
Barcelona, Spain 1.89
KCl 99.5% Panreac Química S.L.U.,
Barcelona, Spain 0.5
2.2.2.1. Synthesis Method
The amount of salt prepared for the thermal analysis for each composition is 3g.
The different compositions have been prepared using the method described in
Gimenez & Fereres (2015)46: firstly, prills from each component are milled and
mixed in a mortar. The resulting powder is melted into a glass beaker on a hot plate
at 400 ºC. Once the content is in liquid state, it is homogenized by stirring
manually. The resulting liquid is poured over the mortar for fast cooling. The
resulting frozen thin layer of salt is easily milled again and the aforementioned
process is repeated three times (milling, mixing, melting) to guarantee
homogenization (Figure 2-10).
Phase Change Materials
32
Figure 2-11: Initial mixture of components (left), manual milling (center) and
melting in a beaker on a hot plate (right)
2.2.2.2. Thermal characterization (Differential Scanning Calorimeter)
A Differential Scanning Calorimeter (DSC1 from Mettler-Toledo) is used to
thermally characterize the PCM candidates by measuring the melting temperature
and latent heat. Aluminum crucibles (40 µl) are filled with salt powder (~15 mg)
and the salt is pre-melted on a hot plate before sealing the crucibles to eliminate the
presence of moisture and to ensure good salt contact with the bottom surface of the
crucible.
The samples are introduced in the DSC equipment, heated to 420 ºC and cooled to
80 ºC at 10 ºC·min-1 five times. Other heating rates 2, 5 and 20 ºC·min-1 have been
also investigated. Nitrogen (50 ml·min-1) is used as inert gas in the thermal
program. Parameters such as the onset and endset temperatures are estimated as
the intersection point between the baseline connecting the points before and after
the transition and the tangent at the point of largest positive and negative slope on
the heat flow DSC curve respectively. The peak temperature is the temperature at
which heat flow during phase changes reaches the maximum. The latent heat of
fusion is determined by numerical integration of the area under the peaks.
33
Thermal Energy Storage for High Temperature Applications
2.2.2.3. Potassium nitrate mixtures
KNO3 is the main component of mixtures (1) and (2). The phase change diagram
from FactSage Thermochemical Database 43 predicts a solid-solid transformation at
122 ºC and a liquid-solid phase change at 335 °C. Both transitions are measured and
presented in Table 2-9 and Figure 2-12, showing the results for three subsequent
melting/freezing cycles performed at a heating/cooling heating rate of 10 ºC·min-1.
Subsequent melting cycles are needed in many cases because salts are highly
hygroscopic and sample moisture can have a significant effect on the measured
latent heat. For instance, water absorption can modify the sample mass, leading to
a reduction in the measured value of the latent heat of the PCM. Additionally, the
latent heat of evaporation of water could be mistaken for a particular transition in
the 120 ºC range in certain salts, as is the case with KNO3.
The results in Table 2-9 show that the main phase transition properties vary mostly
after the first melting process takes place and they remain constant over
subsequent cycles. The energy absorbed by KNO3 in the first solid-solid transition
is different to the same transition after the melting temperature of KNO3 has been
reached, once moisture is assumed to be completely removed. The energy absorbed
changes from approximately 51 J·g-1 to 25 J·g-1, indicating that one should perform a
pre-melt for accurate results (or discard the initial melting DSC curve). Pre-melts
are otherwise typically recommended to improve the contact between the sample
and the crucible base. If water absorption is suspected, the samples should be
measured in a crucible with a small perforation on the cover, allowing water to be
evaporated during the sample heating without creating a pressure buildup. This
has been known to cause the abrupt rupture of crucibles during salt testing with
damaging consequences for the DSC equipment. Another solution could be to pre-
melt the salt inside the crucibles before sealing them hermetically.
Phase Change Materials
34
Table 2-9: KNO3 measured latent heat, onset, peak, and endset temperatures
(melting and freezing) at 10ºC·min-1 during three subsequent heating and cooling
ramps.
KNO3 Enthalpy
[J·g-1]
Onset
[ºC]
Peak
[ºC]
Endset
[ºC]
S-S
transformation
1st melt 51.52 130.24 136.74 143.93
2nd melt 25.09 128.80 131.23 134.7
3rd melt 24.45 129.11 131.25 134.42
1st crystallization 24.8 119.33 117.13 113.73
2nd crystallization 22.18 119.62 116.97 113.72
3rd crystallization 22.06 118.84 117.13 114.02
S-L
transformation
1st melt 96.78 332.74 333.16 341.97
2nd melt 95.93 331.32 332.25 340.25
3rd melt 95.4 331.3 332.10 339.94
1st crystallization 97.81 329.85 329.61 320.26
2nd crystallization 97.05 329.77 329.51 321.34
3rd crystallization 96.37 329.84 329.41 321.88
35
Thermal Energy Storage for High Temperature Applications
Figure 2-12: KNO3 heat flow DSC curves during three subsequent melting/freezing
loops at 10 ºC·min-1 showing the solid-solid transition peaks (a)heating, (d)cooling
and the solid-liquid transition peaks for (b)melting (c)crystallization. Water is
released during the 1st heating ramp, leading to a reduction in the area integrated in
peak (a).
Another parameter to take into account is the velocity of the dynamic segments
(heating/cooling rates). The experiments shown in Figure 2-12 were also performed
at a higher heating rate of 20ºC·min-1. The results are shown in Table 2-10. There is
essentially no difference between the phase transitions at 10 and 20 ºC·min-1 because
there are two clear and narrow peaks for each transition. The melting and freezing
onset temperature and latent heat remain unchanged, while a higher heating rate
extends artificially the phase transition into a wider temperature range.
1st
2nd
3rd
(a) (b)
(c) (d)
HEATING
COOLING
Phase Change Materials
36
Table 2-10: Heating rate effect on phase transition properties: KNO3 measured
latent heat, onset, peak, and endset temperature (melting and freezing) at 10
ºC·min-1 and 20 ºC·min-1
Heating rate
[ºC/min]
Enthalpy
[J·g-1]
Onset
[ºC]
Peak
[ºC]
Endset
[ºC]
S-S transformation
10 25.09 128.80 131.23 134.7
20 24.65 129.68 130.65 136.43
% Difference -2% 1% 0% +1%
Melting
10 96.78 332.74 333.16 341.97
20 96.4 332.89 334.19 346.43
% Difference 0% 0% 0% 1%
Freezing
10 97.81 329.85 329.61 320.26
20 97.47 329.91 329.31 316.70
% Difference 0% 0% 0% -1%
However, the analysis of the mixture (1) KNO3-KCl-KBr (Table 2-11) shows the
presence of a sharp peak together with a smaller one with wider temperature
amplitude. The limits of integration are taken between 312ºC and 347ºC for melting
and 300°C to 340°C for freezing using a straight based line between the flat
segments before and after the phase transition. Figure 2-13 and Figure 2-14 show
the selected integration limits. The shape of the curves suggests that either a) the
mixture is not an eutectic composition and the solid-liquid phase transition extends
over a temperature range or that b) there are two peaks overlapping in the phase
change region that are not entirely visible when sampling at that specific
heating/cooling rate. The temperatures (onset, peak and endset) reported in Table
2-11 correspond to the sharpest peak while the latent heat corresponds to the total
integrated area between the base line and the heat flow curve. The standard
deviation found among three different samples is less than 0.5 J/g for latent heat
measurements and below 0.5ºC in all the transition temperatures.
37
Thermal Energy Storage for High Temperature Applications
Table 2-11: KNO3-KCl-KBr(1) measured latent heat, onset, peak and endset
temperature (melting and freezing) at 5 ºCmin-1 (average values of three samples,
5th thermal cycle)
(1) KNO3-KCl-KBr Melting Freezing
Latent heat [J·g-1] -73.48+/-0.45 72.73+/-0.32
Onset [ºC] 319.45+/-0.09 319.14+/-0.06
Peak [ºC] 321.36+/-0.15 319.04+/-0.15
Endset [ºC] 324.29+/-0.31 314.96+/-0.45
Figure 2-13: KNO3-KCl-KBr (1) melting curves at 5 ºC·min-1
Phase Change Materials
38
Figure 2-14: KNO3-KCl-KBr (1) freezing curves at 5ºC·min-1
Comparing the melting process (only latent heat and onset temperature) between
pure KNO3 (Table 2-9) with the composition tested (1) (Table 2-11) to analyze the
effect of adding a small amount of KCl and KBr, the latent heat for the pure
component is absorbed and released in a Tendset-Tonset ~13.5 ºC temperature range.
The latent heat of the mixture (1) is ~24% lower than the pure component and part
of the energy is absorbed in a narrow temperature range (~5 ºC, peak) while the
rest is absorbed in the range 319-345 ºC. Note that the measured enthalpy of fusion
is 73 J/g, in contrast to 140 J/g reported by Zhao et al.44. This highlights the
importance of testing and understanding the phase transitions of each of these
PCM materials, as the composition in (1) is not a eutectic mixture and,
consequently, does not have a clear, narrow melting/freezing peak.
Testing at lower heating rates can sometimes separate phase transition peaks, but
they are often avoided as they increase testing time. Figure 2-15 shows how the
double hump shown in Figure 2-13 and Figure 2-14 can be separated into two
independent transitions by decreasing the heating rate to 2 ºC/min. These
transitions are very clear in the cooling (upper curve) segment.
39
Thermal Energy Storage for High Temperature Applications
Figure 2-15: Heat flow curves for three subsequent loops of KNO3-KCl-KBr (1);
freezing curves at 1) 20ºC·min-1, 2) 20ºC·min-1, and 3) 2ºC·min-1. Top curves
represent cooling (heat is released during crystallization) and bottom curves
represent heating (heat is absorbed during melting).
Summarizing, the tested mixture (1) shows values an onset temperature and latent
heat very similar to KNO3-KCl mixture reported in Table 2-7. However, based on
the heat flow curves, this composition does not behave as a eutectic. The phase
change expands from 319 ºC to 345 ºC with a latent heat ~73 J·g-1 and with a
pronounced peak around 321 ºC that represents between 34-44% of the latent heat.
Mixture number (2) (KNO3(80.7)-KBr(11.9)-KCl(7.4) (wt%)) has been analyzed.
DSC curves are shown in Figure 2-16. The melting is not congruent, as mixture (1),
and the freezing presents an extended phase change, as it is not a eutectic
composition. There is a solid-solid transition with onset at approximately 128 ºC
and a solid-liquid transition beginning around 320 ºC (solidus) and finishing
Phase Change Materials
40
around 400 ºC (liquidus). The measured values and comparison with recent
literature41 are shown in Table 2-12 and Table 2-13. Because it is not a eutectic
composition, melting occurs over a temperature range starting at the solidus
temperature, extending over the “mushy region” where solid and liquid phases
coexist, until the liquidus temperature, where the mixture is completely liquid. If
the integration intervals are not adequately adjusted, it is possible to obtain
different values for the latent heat of this solid-liquid transition that could explain
differences with the literature. It is unclear why this system has been previously
proposed as a potential PCM since it has a phase change process takes place over
such a large temperature range (~80 ºC).
Figure 2-16: KNO3-KBr-KCl (2) DSC curves heating/cooling at 20ºC·min-1: (a)/(d)
solid-solid transition and (b)/(c) solid-liquid transition.
2nd CYCLE
(a)
(c)
1st CYCLE
HEATING
COOLING
(b)
(d)
41
Thermal Energy Storage for High Temperature Applications
Table 2-12: KNO3-KBr-KCl (2) solid-solid transition measured values
solid-solid
transformation Enthalpy [J·g-1] Onset T [ºC]
Heating
Cycle 1 30.31 128.51
Cycle 2 20.01 127.61
Gomez2011 17.92 129.47
Cooling Cycle 1 18.92 107.1
Cycle 2 19.1 107.1
Table 2-13: KNO3-KBr-KCl (2) solid-liquid transition measured values
solid-liquid transformation
Enthalpy 1
(narrow
peak) [J·g-1]
Enthalpy 2
(extended
phase change)
[J·g-1]
Total
Enthalpy
[J·g-1]
Onset T
[ºC]
Heating /
melting
Cycle 1 12.53 63.28 75.81 322
Cycle 2 23.18 55.41 78.59 322
Gomez 2011 - - 75.89 326.6
Cooling /
solidification
Cycle 1 19.85 63.34 83.19 320.8
Cycle 2 19.45 61.71 81.16 320.7
Gomez 2011 - - 77.3 324.4
The shape of the curves, temperatures and latent heats are similar to mixture (1)
indicating that we are moving in a small area along the ternary phase change
diagram (Figure 2-8). The differences between these two compositions
(KNO3(91.43)-KCl(7.32)-KBr(1.25)(wt%))(1) and KNO3(80.7)-KBr(11.9)-
Phase Change Materials
42
KCl(7.4)(wt%)(2)) is an increase of the KBr content and a consequent reduction on
the KNO3 content, fixing the percentage of KCl. Although mixture (1) contains
higher amount of KNO3, its latent heat (72.7 - 73.5 J·g-1) is lower than mixture
(2)(78.6 - 83.2 J·g-1).
KNO3-6mol%KCl (4.5wt. %) from Table 2-7 was included in the testing plan to
further analyze these differences. This composition has been tested at 2 ºC·min-1
and 20 ºC·min-1 in order confirm whether it behaves as a eutectic mixture and,
consequently be of interest as a PCM or not. Moreover, it is important to clarify if
the presence of KBr is responsible for the extended solid-liquid phase change in
mixtures (1) and (2). Table 2-14 contains the results of the analysis and Figure 2-17
shows the heat flow curves.
Table 2-14: KNO3 – KCl (6 mol %) solid-liquid transition measured values
solid-liquid transformation Enthalpy
[J·g-1]
Onset T
[ºC]
Peak T
[ºC]
Endset T
[ºC]
Heating /
melting
Cycle 2
2 ºC/min 81.1 321.0 322.4 324.5
Cycle 3
20 ºC/min 77.54 321.9 323.2 325.6
Zhao et al. 44 74 320 - -
Cooling /
solidification
Cycle 2
2 ºC/min 80.46 322.9 321.6 319.1
Cycle 3
20 ºC/min 77.96 321.0 321.4 317.2
43
Thermal Energy Storage for High Temperature Applications
Figure 2-17: KNO3 – KCl (6 mol%) heat flow curve, solid-liquid transition.
A comparison of the potassium nitrate mixtures with the pure salt results analyzed
above is shown in Table 2-15 to summarize the findings. Adding a small amount of
KCl to KNO3 reduces the melting point by 11 ºC but also reduces the latent heat
from 96.4 J·g-1 to 77.5 J·g-1. Considering that the theoretical value of the latent heat
of KCl is over three times that of KNO3 47, a reduced latent heat is an undesired and
unexpected result. Adding KBr to the KNO3-KCl mixture does not seem to modify
this outcome and essentially results in the same melting point. Moreover, the
KNO3-KCl mixture behaves as a eutectic even though, based on its binary phase
diagram from Factsage, is an off-eutectic. Since the ternary mixtures (1) and (2)
clearly have extended phase transitions, it seems that the addition of KBr to the
binary mixture KNO3-KCl might be responsible of the extended phase change
observed.
-10
-5
0
5
10
15
Hea
t F
low
(W
/g)
305 310 315 320 325 330 335 340Temperature (°C)
KNO3-KCl_TA_s1_L2-2ºCmin– – – – KNO3-KCl_TA_s1_L3-20ºCmin––––– ·
Exo Up
Phase Change Materials
44
Table 2-15: Summary of solid-liquid transition properties for KNO3 vs three
different mixtures at a heating rate of 20ºC/ min (except for mixture (1) measured at
10 ºC/min). All compositions are in wt. %
Potassium based mixtures (wt. %) Onset T [ºC] Latent heat [J·g-1]
KNO3 332.9 96.4
KNO3-KCl (4.5) 321.9 77.5
(1) KNO3 (91.43) – KCl (7.32) – KBr (1.25) 319.5 73.5
(2) KNO3 (80.7) – KBr(11.9) – KCl (7.4) 322 78.6
2.2.2.4. Sodium nitrate mixtures
Apart from potassium nitrate mixtures, other mixtures such as (3) to (6) have been
evaluated, with NaNO3 as the main component. The reference melting point of
NaNO3 is 306 ºC47. Additionally, this salt is reported to have a solid-phase
transition around 275 ºC48,49.
Table 2-16 shows the melting and freezing results for the pure component. There
are no significant differences between melting and freezing latent heats. The
enthalpy of the second order transition in heating and cooling is also similar. In
freezing experiments 1.5 ºC of supercooling is observed (the freezing onset
temperature is lower than the peak temperature). The width of the phase change
transition for melting and freezing are similar (~7.4 ºC). The second order transition
in cooling is about ~273 ºC, similar to the literature value.
Table 2-17 and Table 2-18 compares the average values of the DSC for 4 different
samples, for each NaNO3 based mixtures, 5 thermal cycles per sample, averaging
cycles 2 to 5, in melting and freezing respectively.
45
Thermal Energy Storage for High Temperature Applications
Table 2-16: DSC analysis of NaNO3 (melting and freezing results). Average values
of three different samples.
NaNO3 2nd Order
transition
Melting Freezing 2nd Order
transition
Latent heat [J·g-1] -17.15+/-0.22 -176.75+/-1.95 175.81+/-2.04 15.02+/-0.4
Onset [ºC] 264.46+/-1.39 302.58+/-0.13 301.33+/-0.47 273.24+/-0.05
Peak [ºC] 273.75+/-0.04 303.93+/-0.3 302.86+/-0.52 271.41+/-0.23
Endset [ºC] 275.20+/-0.11 309.94+/-0.58 293.94+/-3.8 259.78+/-1.02
Table 2-17: Melting DSC analysis: comparison between pure NaNO3 vs mixtures
(3), (4), (5), (6) (20 ºC·min-1). Average values of three different samples.
Melting Latent Heat [J·g-1] Onset [ºC] Peak [ºC] Endset [ºC]
NaNO3 -176.75+/-1.95 302.58+/-0.13 303.93+/-0.30 309.94+/-0.58
NaNO3-NaCl-Na2CO3(3) -171.36+/-0.53 286.73+/-0.13 288.78+/-0.06 293.80+/-0.52
NaNO3-NaCl-Na2CO3(4) -181.78+/-2.46 286.36+/-0.11 288.85+/-0.18 294.11+/-0.62
NaNO3-NaCl-NaBr(5) -178.30 +/-0.80 286.31+/-0.14 289.02+/-0.30 294.78+/-0.50
NaNO3-NaCl-NaBr(6) -173.18 +/-2.08 286.41+/-0.10 289.45+/-0.175 295.26+/-0.50
The standard deviation of the measurements is small. Compared to pure NaNO3
the latent heat of the mixtures does not follow any significant trend, a reduction on
the latent heat is observed for the mixtures (3) and (6), while an increase is observed
for the mixtures (4) and (5). On the other hand, the results for the different
temperatures (onset, peak and endset) show practically the same value regardless
the composition tested. The onset melting temperature shows a reduction of 16.1 ºC
for the different compositions tested compared to the pure salt. Figure 2-18 shows
the heat flow curves of mixture (3) and (4) compared to NaNO3. The samples
behave as NaNO3 lowering their temperatures (onset, peak and enset).
Phase Change Materials
46
Figure 2-18: Heat flow curve of mixtures (3) and (4) and NaNO3 while heating at 20
ºCmin-1
Table 2-18: Freezing DSC analysis: comparison between pure NaNO3 vs mixtures
(3), (4), (5), (6) (20 ºC·min-1)
Freezing Latent Heat [J·g-1] Onset [ºC] Peak [ºC] Endset [ºC]
NaNO3 175.81+/-2.04 301.33+/-0.47 302.86+/-0.52 293.94+/-3.8
NaNO3-NaCl-Na2CO3(3) 168.77+/-0.56 284.56+/-0.58 286.28+/-0.15 279.91+/-0.30
NaNO3-NaCl-Na2CO3(4) 179.39+/-2.17 285.06+/-0.39 287.20+/-0.17 280.53+/-0.68
NaNO3-NaCl-NaBr(5) 176.09+/-0.10 284.05+/-0.40 286.05+/-0.42 279.01+/-0.36
NaNO3-NaCl-NaBr(6) 171.15+/-1.88 285.15+/-0.30 286.42+/-0.20 279.55+/-0.53
Similar observations can be done for the freezing results. Considering the
equipment accuracy in the measurement of the latent heat (+/- 5%), no trends
appears for this parameter. Similar reduction on the freezing onset temperature are
observed (-16 ºC) regardless the composition. The accuracy of the temperature
results is very high and the four mixtures shows the same value for the onset (286
°C) which is very different from the expected value from the phase diagrams.
-250
-200
-150
-100
-50
0
200 250 300 350 400
Heat
Flo
w [
mW
]
Temperature [ºC]
NaNO3
Mix(3)
Mix(4)
47
Thermal Energy Storage for High Temperature Applications
It seems that for the four compositions the peak amplitude (OnsetT–EnsetT) has
been shifted to a lower temperature without any other modification, neither latent
heat, nor shape. For NaNO3 the difference between onset and endeset temperature
(7.5 ºC) is very similar to the difference between the onset and endset temperature
for the four compositions. Therefore the stored energy is about the same as for pure
NaNO3 with an offset of -16 °C.
2.2.2.5. Other compositions
DurferritTM salt has been also characterized through DSC. Figure 2-19 shows the
results for four different samples tested during 5 thermal cycles (25-200ºC). The
integration of the initial heating peak on the first heating of each sample can be
used to determine the amount of water contained, assuming that the absorbed
energy is due to evaporation of water from the sample. Knowing the enthalpy of
evaporation of water, 2257 J·g-1, a water content of about ~0.26 wt. % has been
calculated for each of the sample.
Figure 2-19: Durferrit heat flow curves: phase change evaluation (left) and
variability between samples (right)
The salt freezing occurs in a narrow temperature range, while the heat absorbed in
the melting process is produced on a wider temperature range. The peaks
amplitude are ~11ºC and ~48 ºC for freezing and melting respectively. Freezing
Phase Change Materials
48
process starts at 121 °C with a clear peak. On the other hand, in order to melt the
salt completely it is required to increase the temperature up to 154 °C. The heat of
fusion of the salt is 22 J·g-1 considerably lower than the sodium and potassium
nitrate based compositions analyzed. Table 2-19 shows the average results for the
four samples analyzed. The standard deviation of the measurements is low. This
indicates the homogeneity of the salt.
Table 2-19: Average results for four Durferrit samples
Freezing Melting
Latent heat [J·g-1] 17.75 +/- 0.38 -21.80 +/- 1.00
Onset [ºC] 121.20 +/- 1.43 106.00 +/- 5.24
Peak [ºC] 117.00 +/- 1.60 142.56 +/- 1.12
Endset [ºC] 110.52 +/- 2.03 153.92 +/- 1.69
49
Thermal Energy Storage for High Temperature Applications
2.2.2.6. Nitrate results
Finally, the specific cost of the different compositions has been calculated with the
measurement results (Table 2-20).
Table 2-20: Specific cost analysis of the different mixtures calculated with the
experimental latent heat measurement. (*FactSage, **literature)
Estimated Experimental
PCM
Composition (wt. %)
€/kg €/kWh Melting
T [ºC]
LH
[J·g-1]
Onset
T[ºC]
LH
[J·g-1]
NaNO3 0.77 15.6 306* 172** 302.6 176.8
KNO3 1.08 40.4 335* 95** 332.9 96.4
(1) KNO3(91.44)-KCl
(7.32)-KBr (1.24) 1.05 51.4 306.7* - 319.5 73.5
(2) KNO3(80.7)-
KBr(11.9)-KCl(7.4)
(wt%)
1.13 51.1 342.0* 140** 321.4 79.9
(- ) KNO3(95.5)-KCl(4.5)
(wt%) 1.05 47.9 320** 74** 321.7 79.3
(3) NaNO3 (86.66)-
NaCl(8.27)-
Na2CO3(5.07)
0.69 14.6 358.9* - 286.7 171.4
(4) NaNO3 (93.11)-
NaCl(4.67)-
Na2CO3(2.22)
0.73 14.4 294.6* - 286.4 181.8
(5) NaNO3 (90)-
NaCl(2.64)-
NaBr(7.36)
0.84 17.0 290.5* - 286.3 178.3
(6) NaNO3 (89.20)-
NaCl(1.35)-
NaBr(9.45)
0.88 18.2 290.0* - 286.4 173.2
Phase Change Materials
50
Comparing the literature values with the results of this study, the following
conclusions can be made:
In general, the melting temperatures coincide with previously reported
data, except for mixtures (1), (2), and (3).
Mixtures (1) and (2) have surprisingly similar results (even though the
literature suggested otherwise) and present an off-eutectic behavior,
suggesting that it might be difficult to synthesize and/or measure the
theoretical eutectic.
None of the KNO3-based mixtures improved the latent heat with respect to
the main pure material and are more costly.
The NaNO3-based mixtures essentially have the same latent heat as the
main pure component, they all have lower melting temperatures slightly
lower than 300ºC, and mixtures (3) and (4) are slightly cheaper than the
pure salt.
As confirmed by the experimental measurements, the reduction on the melting
temperature observed in the sodium nitrate mixtures brings them out of the
desired temperature range. In addition to this effect, there is no remarkable latent
heat enhancement observed to justify their use. Moreover, the KNO3 ternary
mixtures show off-eutectic behavior or non-congruent melting which are not
favorable for a TES based on PCM, and all of them with a lower latent heat.
Durferrit has been also selected because of its extremely low melting point,
although it shows a low latent heat. This salt will be encapsulated as an initial test
in the 100-200 ºC temperature range, prior to higher temperature tests. It is
important to highlight the importance of testing the salts experimentally due to the
different discrepancies observed between previously reported temperatures and
those calculated through thermodynamic programs.
These three inorganic salts (NaNO3 (Tm 302 ºC), KNO3 (Tm 332 ºC) and Durferrit
(Tfreeze 121 ºC)) and two pure metals characterized (lead (Tm 315 ºC) and tin(Tm 179
ºC)) have been selected as PCM for the capsule manufacturing. The DSC measured
melting temperature and latent heat is presented in Figure 2-20. Lead, NaNO3, and
KNO3 having the melting temperatures in the range of interest; tin and Durferrit
will also be used because having a significant lower melting temperature with
similar materials can simplify some of the initial experimental measurements.
51
Thermal Energy Storage for High Temperature Applications
Figure 2-20: Latent heat and melting temperature of several PCM tested.
315ºC;
21kJ/kg
179ºC;
44 kJ/kg
332ºC;
96kJ/kg
302ºC;
177kJ/kg
121ºC;
18kJ/kg
0
25
50
75
100
125
150
175
200
100 130 160 190 220 250 280 310 340 370
Lat
ent
Hea
t (k
J/k
g)
Temperature [ºC]
Pb Sn KNO3 NaNO3 Durferrit
Phase Change Materials
52
53
3 MACRO ENCAPSULATION OF
PCM
acro-encapsulated phase change materials (PCM) are potentially an
interesting high energy density solution to store thermal energy near
isothermal conditions. They are usually implemented in a packed bed
latent heat storage system, consisting of a storage medium divided into small
encapsulated particles which increase the specific surface area to exchange heat
with the working fluid (synthetic oil, molten salts or steam). This technology is
expected to yield to a much more compact storage system compared to its sensible
heat storage counterpart.
3.1. Introduction and Main Objectives
The capsule-PCM is formed by a core material, the PCM, in charge of the storage of
energy using its phase change enthalpy, and a shell material surrounding the PCM,
M
Macro Encapsulation of PCM
54
acting as an interface between the heat transfer fluid and the storage material
(Figure 3-1).
Figure 3-1: Schematic representation of a PCM-capsule
An elastic shell or a void space is required to accommodate the volumetric
expansion that most PCM suffer when they melt. The storage of thermal energy
using encapsulated PCM offers potential benefits whenever the capsule
accomplishes the following main requirements50:
i. The capsule has to meet the requirements of strength, corrosion
resistance, and thermal stability, giving mechanical integrity to the
capsule when the PCM is in liquid state.
ii. Act as barrier to protect the PCM from damaging interaction with the
surrounding HTF.
iii. Provide sufficient surface for an efficient heat transfer.
iv. Provide structural stability and easy handling.
The available encapsulating technologies change dramatically depending on the
temperature limits of the applications. There are a large number of micro-
encapsulation methods for low temperature PCM: physical methods (pan coating,
air-suspension, centrifugal extrusion, coacervation, spray drying…) and chemical
methods (interfacial polymerization, in situ polymerization, matrix
polymerization)51,52. Thin, sealed, and high molecular weight polymeric films are
commonly used to encapsulate these PCM maintaining the shape and preventing
PCM from leakage during the phase change process.
Shell
Core
PCM(solid)
Void
Shell
Core
PCM(liquid)
55
Thermal Energy Storage for High Temperature Applications
On the other hand, high temperature (T>300ºC) encapsulation has been less
developed. For example, among the 43 heat transfer studies in capsules of various
geometries and in packed bed storage systems summarized in Regin et al.50 only 5
of them are dealing with high temperature materials. Cost-effectiveness, corrosion,
and thermal stability concerns are also important aspects that differ significantly at
high temperature applications.
The objectives of this chapter are to 1) develop a functional macro-encapsulation
concept for high temperature PCM (inorganic salts or metals) with melting
temperatures in the 300-450ºC range to be used as latent heat storage in direct
steam generation (DSG) solar thermal plants and to 2) develop an experimental rig
to test such concept (Figure 2-2).
Figure 2-2. Schematic representation of the PCM study with the contents of this
chapter marked in yellow.
The target operating conditions of steam in DSG solar thermal plants is T300-
320ºC at 100 bars. Ideally a latent heat storage is designed to be used in an
isothermal system (i.e. saturated steam at 312ºC, 100 bar), but there is interest in
PCM TES heat
exchanger system
Packed bed
Macro-encapsulation
Screen & characterize
PCMs
Screen Shell Materials
Model Single Capsule
Fabricate capsules
Test single
capsule
Compare Experiments
& Model
Evaluate performance & challenges
Tube & housing
Metal PCM
Double PCM
Macro Encapsulation of PCM
56
exploring the use of a single storage system to also cover the superheated regime
(up to 450 ºC) for engineering simplicity. The goal is to design a concept that might
be valid for both saturated and superheated modules with slight modifications for
each case.
3.2. Background on High Temperature Encapsulation
An efficient and cost-effective PCM encapsulation has to be achieved for latent heat
packed beds to be reliable and economically attractive. The challenges in high
temperature encapsulation are mainly related to finding suitable, compatible
materials to exchange heat between heat transfer fluids and PCM working under
high temperature operating conditions.
One of the major barriers of the high temperature encapsulations is preventing the
shell rupture when the core PCM melts and expands in volume due to the
difference in density between the solid and liquid PCM. This phenomenon might
occur when the mechanical integrity is not intrinsic to the capsule, but depends on
the core material, for example, when the capsule is created as deposited layers
around a solid PCM. To avoid the overpressure created by the PCM’s melting (and
volume expansion), an empty space between the PCM and the shell or a flexible
shell is required. Since the 1990s, researchers have tried many different approaches
to encapsulate high temperature PCM. The main challenges and proposed
solutions are reviewed below.
In Mathur (2011)53 and Mathur & Kasetty (2012) (US Patent 20120018116 A1)54 two
different encapsulation methods are developed around different nitrate salts
(Figure 3-2). The first one consists of the creation of a void space between the shell
and the PCM through an organic sacrificial layer. The second approach introduces
agglomerated metal particles between the PCM and the shell. In the first case, the
thermal degradation of the sacrificial layer will leave enough empty space; in the
second case the agglomerated metal particles will act as a flexible layer absorbing
the volumetric expansion. The encapsulation method used in both cases is a
fluidized bed coating. The shell materials are identified as thermally stable after
several thousands of freeze-thaw cycles and compatible with the core PCM.
57
Thermal Energy Storage for High Temperature Applications
Figure 3-2: KNO3 encapsulated particles53,54
Alam et al. 55 developed an innovative technique that does not require a sacrificial
layer to accommodate the volumetric expansion of the PCM upon melting. A non-
reactive polymer is used over the PCM (NaNO3 and NaNO3-KNO3) pellet followed
by deposition of a metal layer (non-vacuum metal deposition technique). The
polymer is not eliminated from the capsule during the manufacturing process but
remains, stable and non-reactive with the PCM at the operation temperature. The
authors validate this new manufacturing process by testing the capsules more than
2200 thermal cycles between 250 - 325 ºC. The integrity of the capsules is surprising
especially considering that the polymer used (Teflon) melts at 327 ºC, very close to
the upper temperature limit.
A thick Cr-Ni bi-layer is used in Zhang et al. 56 to encapsulate copper. A chromium
periodic-barrel electroplating method and nickel barrel-plating method is used.
The same PCM is also encapsulated in Maruoka et al. 57, where an intermediate
layer of carbon or ruthenium is introduced between the copper core and the nickel
shell. A carbon or ruthenium layer, in Mauroka at al.57, and chromium oxide layer,
in Zhang et al.56, are used as inhibition layers to avoid the direct solution between
copper and nickel, because they have complete solubility in the phase diagram at
the operating temperature. In the case of Al-Si alloys, Nomura et al. 58 successfully
encapsulated micro-spheres using stable α-Al2O3 shell with interesting self-
repairing properties oxidizing the core aluminum.
In the temperature range of ~400ºC Zhao et al. (2013)44 considered 2 types of
spherical capsules (20-50 mm in diameter): Zn encapsulated in Ni and eutectic salt
mixtures (NaCl - 43mol% MgCl2) in stainless steel capsules. These results are also
presented in the patent US2011/0259544 A159 which covers encapsulation materials
and PCM at temperatures higher than 400ºC for capsules with a nominal
dimension between 1-100 cm. The compatibility between the materials enables the
use of a 1018 carbon steel capsule for MgCl2-NaCl; a stainless steel 304 capsule for
Macro Encapsulation of PCM
58
NaNO3 and nickel or 316 stainless steel for Zn encapsulation. Spherical and tubular
encapsulation and other PCM including NaNO3, KNO3, NaNO3-KNO3, MgCl2,
MgCl2-NaCl, MgCl2-KCl, NaCl-KCl, inorganic salts, and combinations are
proposed. The experimental study performed by Zhang H.L. et al. 60 would fit into
the patent claims, testing the binary system NaNO3-KNO3, although stainless steel
AISI 321 was used to encapsulate.
In Blaney et al.61 the same group analyses the capsule mechanical resistance due to
stresses from the thermal expansion and volume change of the PCM. The study
considers a three-dimensional finite element model simulating the stress
distribution in a spherical nickel shell of 250 μm thickness formed around a sphere
of Zn by electroless deposition, and a stainless steel cylindrical shell containing Zn.
The nickel shell produced by the electroless deposition process was not
demonstrated to be a feasible way of containing the molten zinc, presenting too
many potential modes of failure and being unlikely to survive multiple cycles of
heating and cooling.
Similar conclusions are found experimentally in Maruoka et al.36 in which lead
pellets are encapsulated in nickel through electroplating. Although the capsule film
shows enough strength when it is thick enough or the PCM diameter is small, the
stress analysis and observations after thermal cycling tests suggest that the coated
film is not uniform and an inactive weak layer exists.
Metallic spherical macro-capsules for high temperature PCM were studied already
in 1995 by Yagi et al. 34 to recover heat of industrial processes using a packed bed.
Six different PCM were considered: two inorganic salts: KNO3-NaNO3 eutectic and
NaCl; and four metallic materials: lead, aluminum and two Al-Si alloys.
Table 3-1: High temperature PCM candidates34
PCM KNO3-NaNO3
eutectic Pb
Al-Si
(87.4-12.6
wt. %)
Al-Si
(74.9-25.1
wt. %)
Al NaCl
Melting point
[ºC] 222 328 577 577 660 800
Latent Heat [J/g] 94 23 516 441*
(estimated) 397 482
59
Thermal Energy Storage for High Temperature Applications
The heating and cooling by convection of a single metallic capsule containing PCM
in a nitrogen gas stream was conducted in Yagi et al.34. The metallic PCM material,
due to its high thermal conductivity, behaved as a thermally uniform material,
while in the melting process in molten salt PCM the thermal gradients inside the
capsule were important, for the capsule size tested (ϕ=4cm, thickness 2 mm). It is
important to mention that the PCM were poured into the hollow capsules in liquid
state. When the PCM freezes and contracts, a void space is created which leaves the
required empty space for subsequent melting and freezing cycles.
Solomon’s and Zhao’s theses62,63 developed metallic cylindrical macro-capsules
used to encapsulate some of the previous high temperature PCM (Table 3-1): salts
(NaNO3 and NaCl) and metals (Al). The thermal properties of the capsule are
tested in a home-made macro-DSC (Differential Scanning Calorimeter), analyzing
the specific heat, the latent heat, the energy stored, and the evolution of the storage
capacity when the capsules are exposed to different thermal cycling conditions.
Although the high thermal conductivity and latent heat of some metallic PCM
makes them an interesting solution for TES, interactions between the metallic PCM
and metallic shell material (e.g. alloying) can result in a reduction of the initial
latent heat storage capacity of the PCM. Aluminum (PCM)-stainless steel capsules
exposed to 720ºC for 500 and 1000 hours showed a 4.1% and 10.4% decrease in
storage capacity respectively.62 Also, in Zhao (2013)63 zinc and aluminum used as
PCM showed a drop in the storage capacity of 12% and 5% respectively after 7
heating/cooling thermal cycles (450ºC) for the zinc, and after 500h at 720ºC for the
aluminum. Therefore, novel encapsulation material such as non-reactive metals,
ceramic capsules or different non-reactive coatings must be developed to solve this
problem. The use of high temperature paints and sodium silicate to protect carbon
steel capsule from highly corrosive chloride mixture in Nath R. 64 is one of the
attempts to solve this problem.
The feasibility of PCM encapsulated in metallic cylinders has been re-evaluated in
recent publications (Figure 3-3). However, publications from the 90s use the same
technology. For this type of capsules, their filling is more easily controlled and,
consequently, the internal stresses caused by the PCM volumetric expansion can be
accommodated. The encapsulation material must be sufficiently thick to maintain
the structural integrity of the EPCM and it has to provide sufficient void space in
the manufacturing process for the PCM’s volumetric expansion.
Macro Encapsulation of PCM
60
Figure 3-3: Encapsulated PCM: Encapsulated NaCl-MgCl2 (left)65; Encapsulated
MgCl2 (center)66; Encapsulated Ternary carbonate eutectic (lithium, sodium and
potassium carbonates) (right)67
The content of this extended review on high temperature encapsulated PCM’s
experimental studies is summarized in Table 3-2. Recently the number of
publications in this subject has risen dramatically, as solar thermal power plant
developments require new, more efficient energy storage solutions. There is only
one study of high temperature macro-encapsulation of PCM from the 1990s and
the rest are mainly investigations carried out in the past five years.
In a PCM-capsule, the amount of material that does not undergo a phase change
should be minimized because the objective is to store thermal energy primarily in
latent heat and not in sensible heat. Thus, capsule filling ratios and shell thickness
must be evaluated in detail. Considering spherical and (long) cylindrical capsules,
Figure 3-4 represents the ratio between the shell volume (Vshell) and capsule core
volume (Vcore) for different dimensionless PCM-radii (Rpcm/Rcapsule) based on
Equation 3-1 (a-b), considering that the PCM completely fills the capsule.
a) For a sphere:
b) For a cylinder:
Equation 3-1
61
Thermal Energy Storage for High Temperature Applications
Figure 3-4: Vshell/Vpcm ratio vs Rpcm/Rcapsule for spherical and cylindrical capsules
As Figure 3-4 shows, a spherical capsule with a lower PCM radius (Rpcm) than
0.87·Rcapsule will contain a shell volume higher than 50% of the PCM, where at least
one third of the total capsule material is not being used for latent heat storage. This
ratio would be Rpcm <0.8·Rcapsule for cylindrical capsules. This an arbitrary limit for
the ratio Rpcm/Rcapsuel where the volume of the shell material represents 50% of the
volume of PCM or lower, but it highlights the importance to maximized the ratio
Rpcm/ Rcapsule to minimize the shell volume for a given capsule diameter and
geometry. At the same time, the encapsulation material has to be sufficiently thick
to prevent the deformation of the capsule caused by stresses on the shell. Therefore,
this is an important consideration in the capsule design and subject to compromise.
Parameters such as the PCM/Capsule diameter ratio, which determines the
shell/core volume ratio, have been also included in Table 3-2.
Table 3-2: Summary of experimental studies on high temperature encapsulated
PCM. Spherical capsules are marked in blue, cylindrical capsules in black.
0
0.5
1
1.5
2
0 0.2 0.4 0.6 0.8 1
Vsh
ell/
Vp
cm
Rpcm/Rcapsule [-]
Sphere
Cylinder
Macro Encapsulation of PCM
62
PC
M T
melt
(ºC
)
LH
(kJ/
kg
)
NaN
O3
306
172
PT
FE
(0.5
-0.7
mm
) –
Nic
kel
(10-8
0μ
m)
PT
FE
(0.5
-0.7
mm
)
27.4
31.4
8-
40.9
%250-3
26 (
2200 c
ycle
s)
250-3
26 (
1000 c
ycle
s)24.4
70.8
92
KN
O3
334
92
PT
FE
(0.5
-0.7
mm
) –
Nic
kel
(50-8
0μ
m)
280-3
50 (
110 c
ycle
s)
NaN
O3-
50w
t.%
KN
O3
222
120
(PT
FE
-FE
P)–
Nic
kel
180-2
42(1
000 c
ycle
s)
NaN
O3-
KN
O3-L
iNO
3
122
140
FE
P–
Nic
kel
100-1
44 (
440 c
ycle
s)
Nom
ura
et
al 5
82015
Al-
40 w
t.%
Si
573
247
(ME
PC
M)
α-A
l2O
3
0.0
407
0.0
022
-40.9
%500-8
00 (
10 c
ycle
s)0.0
363
0.8
92
Zhan
g, G
. et
al 5
62014
Cu
1083
53.2
(ME
PC
M)
Ch
rom
ium
–n
ickel
2.9
06
0.5
53
-320.8
%1050 -
1150 (
1000 c
ycle
s)1.8
0.6
19
Zhan
g, H
.L. e
t al
60
2014
NaN
O3-
40w
t.%
KN
O3
230
-Sta
inle
ss s
teel
AIS
I 321
75
1.5
-72
0.9
60
Zn
420
113
Sta
inle
ss s
teel
304
25.4
1.5
875
~20%
30.6
%
27-4
85 (
7 c
ycle
s,
-12.3
% S
tora
ge
Cap
acit
y)
22.2
30.8
75
Al
660
397.3
Sta
inle
ss s
teel
304
25.4
1.5
875
~20%
30.6
%
30-7
10 (
6 c
ycle
s)
710 (
500h
, -5
%
Sto
rage
Cap
acit
y)
22.2
30.8
75
NaN
O3
308
162.5
Car
bo
n s
teel
1018
50.8
1.5
875
~20%
13.8
%30-3
50 (
3 c
ycle
s)47.6
30.9
38
NaC
l-M
gCl 2
444
292
Sta
inle
ss s
teel
304
Car
bo
n s
teel
1018
25.4
1.5
875
~20%
30.6
%470 (
300h
)22.2
30.8
75
MgC
l 2714
454
Sta
inle
ss s
teel
304
25.4
1.5
875
~20%
30.6
%850 (
1000h
)22.2
30.8
75
NaC
l800
482
Sta
inle
ss s
teel
304
25.4
1.5
875
~20%
30.6
%30-8
50 (
3 c
ycle
s)22.2
30.8
75
Zhao
et
al 6
62013
MgC
l 2714
454
Sta
inle
ss s
teel
304L
(lo
w c
arb
on
sta
inle
ss s
teel
)25.4
1.5
875
20-3
0%
30.6
%33-7
50 (
60 c
ycle
s; 4
80h
) 22.2
30.8
75
NaN
O3
308
177.1
**C
arb
on
ste
el 1
018
50.8
1.5
875
18-2
0%
13.8
%300-4
70 (
50 c
ycle
s; 3
00h
)47.6
25
0.9
38
NaC
l -
43m
ol%
MgC
l 2444
292**
Sta
inle
ss s
teel
304
Car
bo
n s
teel
1018
25.4
50.8
1.5
875
18-2
0%
30.6
%
13.8
%
22.2
25
47.6
25
0.8
75
0.9
38
Ala
m e
t al
55
2015
2013
Zhen
g et
al
65
Au
tho
r(s)
Year
Th
erm
al Sta
bil
ity
T t
est
ed
(ºC
)
PC
M
dia
mete
r
(mm
)
Ex
tern
al
dia
mete
r
(mm
)
Sh
ell
thic
kn
ess
(mm
)
Vo
idSh
ell/
Co
re
Vo
lum
e r
ati
o
φ P
CM
/
φ C
ap
sule
Co
re m
ate
rial
Sh
ell m
ate
rial
Zhao
, W. 6
32013
63
Thermal Energy Storage for High Temperature Applications
PC
M T
melt
(ºC
)
LH
(kJ/
kg
)
Al
660
397
stai
nle
ss s
teel
304L
25.4
1.5
875
20-3
0%
30.6
%720 (
1000h
lea
ds
to
-10%
sto
rage
cap
acit
y)22.2
30.8
75
NaC
l800
430
stai
nle
ss s
teel
304L
25.4
1.5
875
20-3
0%
30.6
%850 (
1000h
)22.2
30.8
75
carb
on
ste
el
50.8
1.5
875
-13.8
%620-6
80 (
10 c
ycle
)47.6
30.9
38
carb
on
ste
el
Co
atin
gs (
Hig
h T
pai
nts
or
sod
ium
silic
ate
)
50.8
1.5
875
-13.8
%680 (
72h
)47.6
30.9
38
mild
ste
el c
oat
ed w
ith
seal
met
50.8
1.5
875
-13.8
%650-6
80 (
20 c
ycle
s)47.6
30.9
38
Pen
dya
la, S.
2012
NaN
O3
306**
*172**
*si
lico
n d
ioxid
e30.1
50.0
75
20-3
5%
1.5
%25-3
50 (
7 c
ycle
s)30
0.9
95
Ter
rafo
re2011
KN
O3
(In
org
anic
sal
ts)
333
92
met
allic-
cera
mic
2.1
69
0.2
92
-156.3
%300-8
00
1.5
85
0.7
31
0.5
50.0
25
33.1
%400
0.5
0.9
09
1.1
0.0
533.1
%1
0.9
09
3.1
0.0
510.3
%3
0.9
68
4.1
0.0
57.7
%4
0.9
76
carb
on
- n
ickel
(0.0
53 -
0.9
40m
m)
4.9
86
0.9
93
-25-1
100 (
50 c
ycle
s)3
0.6
02
Ruth
eniu
m-n
ickel
(1 -
0.9
40)m
m6.8
81.9
4-
30.4
36
KN
O3-N
aNO
3
eute
ctic
222
94
Lea
d328
23
Al-
12.6
Si (w
t.%
)577
516
Al -
25.1
Si (w
t.%
)577
441
Al
660
397
NaC
l800
482
NaN
O3
301.9
174.1
119.7
11.1
326.5
0%
44.1
%250-3
50 (
10 c
ycle
s)17.4
50.8
85
KN
O3
331.6
95.6
19.3
11.1
322.8
0%
45.3
%280-3
80 (
10 c
ycle
s)17.0
50.8
83
Durf
erri
t121.2
19.7
19.8
31.1
324.6
5%
43.8
%90-1
80 (
10 c
ycle
s)17.5
70.8
86
Sn
179.6
44.5
918.6
81.1
317.1
0%
47.2
%140-2
40 (
10 c
ycle
s)16.4
20.8
79
Pb
314
22.8
520.1
81.1
317.9
0%
42.8
%280-3
50 (
10 c
ycle
s)17.9
20.8
88
23**
*n
ickel
2016
Pre
sent
work
Gim
enez
-Gav
arre
ll, P
.
2002
Mar
uoka
et a
l 57
Yag
i &
Akiy
ama
34
1995
2003
Mar
uoka
& A
kiy
ama
36
Cu
1083
209
NaC
l -
56w
t% K
Cl
Nat
h, R
. 64
Solo
mon, L
.D.
62
2013
2012
-~
674
Au
tho
r(s)
Year
Th
erm
al Sta
bil
ity
T t
est
ed
(ºC
)
PC
M
dia
mete
r
(mm
)
Ex
tern
al
dia
mete
r
(mm
)
Sh
ell
thic
kn
ess
(mm
)
Vo
idSh
ell/
Co
re
Vo
lum
e r
ati
o
φ P
CM
/
φ C
ap
sule
Co
re m
ate
rial
Sh
ell m
ate
rial
Pb
Sta
inle
ss s
teel
40
2-
37.2
%
Bo
rosi
lica
te g
lass
36
0.9
00
328
Macro Encapsulation of PCM
64
Spherical capsules (in blue) and cylindrical capsules (in black) studies are included
in Table 3-2. The latent heat of the PCM corresponds to the latent heat of the raw
material; (**) indicates the latent heat of the raw material measured using
MacroDSC; (***) indicates the latent heat of the raw material extracted from the
literature; and (MEPCM) indicates the latent heat of the microencapsulated PCM as
a whole. The melting temperature and latent heat of the different encapsulated
high temperature PCM reviewed are represented in Figure 3-5. For the existing
high temperature encapsulated PCM studies, there seems to be a casual and almost
linear correlation between the chosen PCM latent heat and melting temperature
values. The represented materials are expected to be the consequence to select the
most cost-effective PCM at each temperature.
Figure 3-5: High temperature PCM successfully encapsulated
Most of the capsules analyzed contain a void space left to accommodate the PCM’s
volumetric expansion. In cylindrical capsules this space is left during the filling
process. On the other hand, PCM made out of pellets, the void space is left by
controlling the compactness of the PCM powder. This void space can be included
0
100
200
300
400
500
600
0 200 400 600 800 1000 1200
Late
nt
Heat
[kJ/
kg
]
PCM Melting Temperature [ºC]
Metal
Chloride
Nitrate
Present work Metal
Present work nitrate
65
Thermal Energy Storage for High Temperature Applications
in the Shell/Core volume ratio in order to calculate the effective Shell/solid PCM
volume ratio by:
Shell-solid PCM volume ratio:
Equation 3-2
Figure 3-6 represents the shell-solid PCM volume ratio as a function of the external
diameter for different types of PCM. Metallic PCM are more often used for small
capsules, while nitrate salts are encapsulated using larger capsules. For capsules
diameter lower than 5 mm we can find shell/solid PCM volume ratios varying
from ~10% to ~300%. These large variations indicate the difficulties to create thin,
homogeneous, and resistant barriers around the core PCM. On the other hand, as
the capsule diameter increases the shell-solid PCM ratio seems to progressively
reduce yielding much more efficiently filled PCM capsules. The target dimensions
and filling ratios for the capsules developed in this study (20 mm external
diameter) are also included in Figure 3-5 and Figure 3-6 to show they are in line
with similar studies but present a higher filling ratio than other successful macro-
capsules.
Figure 3-6: Shell/solid PCM volume ratio vs. capsule external diameter.
1%
10%
100%
1000%
0 20 40 60 80
Sh
ell/
so
lid
PC
M v
olu
me
rati
o [
%]
External Diameter [mm]
Metal
Chloride
Nitrate
Present work Metal
Present work nitrate
Large variability
Macro Encapsulation of PCM
66
3.3. Shell Material Selection
A successful encapsulation procedure must meet these three requirements: 1)
material compatibility with both: PCM and heat transfer fluid, 2) thermal
cycling/stability and 3) mechanical stresses under operating conditions, as
commented previously. It must also provide an adequate surface for heat transfer.
The problem is complex due to the combination of high temperature and high
pressure requirements, since the capsule has to exchange heat with water vapor at
T> 300ºC and P100 bar. To simplify the analysis, the thermal and pressure
problem have been decoupled. As a first step, the thermal problem can be
evaluated in a straightforward experiment, concentrating on the high temperature
aspect to characterize the melting/freezing process and analyze the thermal cycling
behavior. Once the thermal resistance is evaluated, the following step would be to
couple high temperature and pressure and analyze the capsule behavior under
such conditions.
Some potential shell materials can be highlighted (Figure 3-7) from the
experimental studies on high temperature PCM-Capsule previously summarized
in Table 3-2. Most of these studies have used stainless steel to encapsulate different
PCM, investigating additional intermediate layers to prevent or reduce possible
interactions between the PCM and the stainless steel shell. Different coatings are
also used as an additional barrier between the capsule and the heat transfer fluid. It
is important to mention the novelty of some polymers to successfully encapsulate
high temperature PCM around 325ºC.55
67
Thermal Energy Storage for High Temperature Applications
Figure 3-7: Shell materials used to encapsulate high temperature PCM. (*Present
work)
Based on some unsuccessful experiences trying to encapsulate salts with metals68
and metals with metals62,63, other potential shell candidates are explored.
Encapsulating metallic PCM with metals is difficult because the core and shell
materials can alloy during the encapsulation procedure or during thermal cycling.
Encapsulating salts with metals can accelerate container corrosion issues due to the
high working temperatures, requiring specific coatings for both the HTF-shell and
the shell-PCM interfaces. Some ceramic shells also require coatings due to their
porous nature which can lead to leakages. Although polymers could be ideal
candidate to encapsulate due to their flexibility, low cost, and variety of
encapsulation methods, when this analysis was performed there was no
commercial polymer meeting the requirements of thermal stability above 400 ºC.
In the search for a compatible, impermeable medium to both the HTF (steam) and
potentially corrosive core materials (inorganic salts), borosilicate was proposed and
discussed in collaboration with Prof. I. G- Loscertales from the University of
Málaga as shell material. Several authors have used borosilicate tubes for hydrogen
Nitrates
•Polymer (PTFE)
•Coating: Ni
•Polymer (PTFE-FEP)
•Coating: Ni
•Polymmer (FEP)
•Coating: Ni
•Stainless steel AISI 321
•Carbon steel 1018 with Intermediate Layer of SiO2
•Metal+Clay
•Organic binder
•Borosilicate glass*
Chlorides
•Stainless steel 304L
•Carbon steel 1018 with Intermediate Layer :
•High T paints
•Na2SiO3
Metals
•α-Al2O3
•Stainless steel 304L
•Ni
•Ni with Intermediate Layer of:
•Cr
•Carbon
•Ruthenium
•Borosilicate glass*
Macro Encapsulation of PCM
68
encapsulation, which can withstand 400bar.69,70 It means that with an appropriate
geometry it is able to resist the required pressure. On the other hand, it is able to
withstand high temperatures (500ºC for short-term usage (<10h); 450ºC for long-
term usage (>10h)). Its high thermal shock resistance is well known (borosilicate,
commonly known as PyrexTM, is a common cookware glass) making it an
interesting shell candidate together with its thermal stability and non-reactivity.71
Considering that the PCM in question are salts (typically highly corrosive) and the
heat transfer fluid is high pressure steam, finding an impermeable,
thermally/chemically stable, and non-reactive shell compatible with both external
and internal materials is very challenging. A SWOT analysis (Strengths-
Weaknesses-Opportunities-Threats) is shown for the borosilicate capsule in Figure
3-8.
One of the added benefits of using a glass shell is its transparency, allowing the
visualization of the phase change process within the capsule. Up to this date, the
visual observation of the melting process has been only carried out with organic
low temperature PCM (mainly paraffin wax n-octadecane), such as Moore and
Bayazitoglu (1982), Revankar et al. (2007), Tan et al. (2008 and 2009)72–75.
Therefore, a transparent high thermal resistance glass sphere could allow a similar
analysis for high temperature PCM to complement the recently increasing number
of numerical studies. Thus, this study presents a unique contribution by
developing a new encapsulation method for high temperature PCM based on
borosilicate capsules. We take advantage of the non-reactivity of this material, its
high thermal resistance, and the optical properties of the shell and the PCM,
developing an experimental set up able to reach temperatures up to 400 ºC, high
enough to melt some inorganic salt mixtures and metals while varying the external
flow conditions.
69
Thermal Energy Storage for High Temperature Applications
Figure 3-8: SWOT (Strengths-Weaknesses-Opportunities-Threats) analysis
performed to evaluate potential of borosilicate as a PCM shell material.
Strengths
- Material compatibility/ non-reactivity
- Thermal stability
-Inexpensive
-Available raw materials
-Easy to coat
Weaknesses
- Risk of mechanical failure (fragile)
- Management of PCM volume expansion
-Low thermal conductivity
Opportunities
- Transparent allows testing visualization
- Can encapsulate highly reactive materials
Threats
- Unknown mass production process or cost
-Patentability
Macro Encapsulation of PCM
70
3.4. Capsule Design
The capsule dimensions are a compromise between achieving a maximum
allowable size and system design requirements (as smaller capsules have better
heat transfer behavior but produce a larger pressure drop along the packed bed),
while also taking into consideration the availability of raw materials and ease of
manufacturing. This size (diameter of 20 mm diameter with 1 mm thickness) is in
the same range as contemporary theoretical modeling studies by Ramos-Archibold
et al.76,77 and other salt capsules as shown previously in Figure 3-6. Additional
considerations should be taken into account once the encapsulating concept is
proven such as optimizing the packing degree by using multiple size capsules, for
example.
Two different capsule shapes are initially considered for this study: spherical and
cylindrical. Spherical capsules present some advantages in terms of minimizing the
surface and, consequently, the shell material for a given PCM volume. Cylinders
seem a priori easier to fabricate and mass produce, but natural convection effects
might be important if the capsule aspect ratio is large and capsule orientation will
also have an influence on system behavior.
The main heat transfer mechanisms controlling the melting process are heat
conduction and natural convection. Regardless of the heat source and boundary
conditions (constant heat flux at the wall or constant wall temperature) during the
initial stages of the melting process heat conduction plays a dominant role, until a
significant volume of material is melted and buoyancy-driven flow becomes
important. The thermophysical properties of the PCM, the heat supplied, and the
capsule characteristics (shape, orientation, and size) will have an important effect in
the dynamic behavior of the convective dominated melting process.78 To minimize
orientation and geometry effects, this study will focus on spherical capsules.
One of the main challenges regarding PCM encapsulation is the management of
the PCM volume expansion during the melting process without breaking the
capsule due to internal stress. As mentioned above, different solutions have been
proposed to overcome this difficulty, such as a polymer sacrificial layer that is
burned off as part of the manufacturing procedure. In this study, the use of a void
space inside the capsule will be tested as a method to manage the PCM volume
changes during the solidification/melting process. The minimum void space
71
Thermal Energy Storage for High Temperature Applications
required for each PCM capsule depends on the difference in density between liquid
and solid PCM. The metallic and salt PCM tested in this study have a volume
expansion below 10% and 20% respectively (specific values are shown in Table
3-3). For 20 mm diameter capsules, an empty space of 4 mm is left on top of the
PCM. This void space will cover the volume expansion of the different PCM tested.
3.5. Capsule Manufacturing
The PCM candidates for the capsule manufacturing are lead (Pb), tin (Sn), NaNO3,
KNO3 and Durferrit. As mentioned above, lead, NaNO3, and KNO3 have melting
temperatures in the range of interest; tin and Durferrit will also be used because
having a significant lower melting temperature with similar materials can simplify
some of the initial experimental measurements.
The capsule manufacturing procedure that will be described is used as a proof of
concept to develop the conceptual design. For the capsule mass production a
different manufacturing technique would be required.
The process uses hollowed borosilicate cylinders as raw material. The cylinders are
exposed initially to an annealing treatment (550ºC 1h and slow cooling inside the
furnace). One end of the tube is sealed. A spherical shape is achieved by keeping
the sealed end at higher temperature than the borosilicate softening temperature
(>550ºC) and blowing through the open side (Figure 3-9). The diameter is
controlled manually during the process.
Figure 3-9: Shaping a spherical capsule performed at the University of Zaragoza –
glass blowing service.
Once the initial shape is achieved and cooled down, the capsule is inspected to
analyze the residual thermal stresses in the glass. Polarized light is used for this
Macro Encapsulation of PCM
72
purpose. The transitional areas between the spherical shape and the cylinder
contain residual stresses which represent weak points for crack initiation (Figure 3-
10).
Figure 3-10: Pre-shaped capsules (left) and local stresses in the sphere (right)
A new annealing process at 550ºC is required to effectively remove these residual
thermal stresses (Figure 3-11). The idea is to prevent the breakage of the capsule by
thermal shock when high temperature liquid PCM is poured inside. A second
visual inspection after the annealing and before filling the capsules is performed
with polarized light to ensure that there are not residual thermal stresses on the
capsule.
Figure 3-11: Annealing treatment of the pre-shaped capsules performed at the
University of Zaragoza glass blowing service.
73
Thermal Energy Storage for High Temperature Applications
The weight of the initial pre-shaped capsule is recorded. The capsules are
preheated before filling. It avoids the salt freezing when it is poured in liquid state
and it also minimizes the thermal shock. The salt is melted and poured into the
capsule controlling its level visually and its weight (Figure 3-12). The final weight
of the PCM is adjusted.
Figure 3-12: Crucibles created for the capsule filling and filled capsule performed at
the University of Zaragoza glass blowing service.
The closure of the capsule is performed, with the PCM at room temperature, by
connecting a vacuum pump to the open side of the tube while melting the
intermediate narrow section created for this purpose as shown in Figure 3-13.
Figure 3-13: Capsule closure procedure performed at the University of Zaragoza
glass blowing service.
The negative pressure created pulls in the soft and viscous glass, sealing the
remaining aperture when it solidifies. The final shape achieved is a uniform sphere
except in the closure section where a small elongation can be observed. Finally, a
Macro Encapsulation of PCM
74
new annealing process inside a muffle furnace at 550ºC is required to eliminate the
closure residual thermal stresses. This means that the different encapsulated salts
have to withstand the temperature and duration of this treatment without
degradation. Thermo-gravimetric tests (1h 550ºC) were performed using
hermetically sealed and pin holed aluminum crucibles on the inorganic salts. No
weight loss was observed in the pin holed crucibles tests. The hermetically sealed
crucibles filled with salt tested did not open during the experiment. These results
guarantee the thermal stability of the salts in the last annealing process on the filled
and closed capsules.
The total volume for a 20 mm spherical capsule is ~4,189 mm3. For a shell thickness
of 1 mm, the shell volume represents 27.1% of the total capsule volume. Each
capsule is filled up to 87.4% of its maximum capacity, which is ~3,054 mm3 (with
the PCM in liquid state) considering a perfect spherical capsule, leaving
approximately 4 mm in the radial direction on top of the capsules. The ratio
between the PCM volume and the total capsule volume is 87.4% *(100-27.1) = 63.7%
with the PCM in liquid state. Because each material has different volumetric
expansion coefficients, a constant PCM liquid volume corresponds to different
filling ratios in the solid state for each PCM. The calculated amount of PCM for
each capsule type and the percentage of capsule filling in solid state are shown in
Table 3-3.
Table 3-3: Solid and liquid PCM density and calculated amount of PCM added to
each borosilicate capsule type. (*measured)
ρliquid
[g/cm3]
ρsolid
[g/cm3]
Volumetric
expansion
[%]
Capsule filling
in solid state
[Volume %]
Weight of
PCM
in the
capsule [g]
Latent
Heat*
[J·g-1]
Storage
Density
[J/capsule]
NaNO3 1.9 2.26 18.9% 73.5% 5.07 176.8 896.4
KNO3 1.865 2.11 13.1% 77.2% 4.98 96.4 480.1
Sn 6.99 7.365 5.4% 82.9% 18.65 44.4 828.1
Pb 10.66 11.34 6.4% 82.1% 28.44 20.9 594.4
Furthermore, the capsule has to withstand the pressure from the internal air
expansion. The inside initial pressure at room temperature, after the capsule
75
Thermal Energy Storage for High Temperature Applications
closure, is under atmospheric pressure because of the vacuum pump connected.
However the degree of vacuum is not specifically measured or controlled, but
could be in future mass production processes. Hence, it is assumed that there is
some air remaining inside the capsule during the fabrication process. The internal
pressure increases because the volume occupied by the internal air is reduced
when the PCM expands during melting. Besides this effect, the change in
temperature from 25ºC to 550ºC also increases the gas internal pressure. The
pressures inside the capsule in the last annealing treatment (most unfavorable case)
can be estimated using the combined gas law, assuming that the spherical capsule
does not expand at all compared to the PCM that expands much more, mainly
because of the phase change volumetric expansion.
The fraction of the initial volume that is occupied by the PCM at room temperature
is denoted by %PCMvolume. The densities of each PCM in solid and liquid state are
shown in Table 3-3. The thermal expansion in liquid state has not been considered.
The initial (Tinitial) and final (Tfinal) temperatures are 293 and 823 K. In the worst-case
scenario, where the created vacuum is neglected, the initial gas pressure (Pinitial air)
in the capsule is 1 atm. As the PCM expands, the effective volume of the gas left in
the capsule is reduced resulting in an increased air pressures. Using ideal gas law
for a volume domain described in Blaney et al.61 the final air pressure is determined
using Equation 3-3:
Equation 3-3
Figure 3-14 shows the pressure inside the spherical capsule filled with different
amount of PCM in solid state and exposed to the annealing temperature (550ºC).
The theoretical pressure inside the capsules manufactured during the annealing
process has been represented also in the graph. The design criterion for the capsule
filling percentage (“capsule filling in solid state volume” from Table 3-3) was
chosen to ensure that in the worst case scenario (capsule completely full of air, no
vacuum) the internal pressure does not dramatically increase as shown by the
vertical slopes in the curves in Figure 3-14 for each PCM.
Macro Encapsulation of PCM
76
Figure 3-14: Pressure inside the capsules for different PCM and different capsule
filling percentages (in solid state) at room temperature (worst-case scenario, no
vacuum inside). The dots represent the expected pressure inside the capsules
manufactured
It is worth mentioning that because the capsule is not a perfect sphere there is an
extra volume that will reduce the real pressure inside the capsule. This means that
the represented dots in Figure 3-14 overestimate the real pressure as long as salt
does not degrade 1) because there is some degree of vacuum inside and 2) because
the capsules are not perfectly spherical. Table 3-4 shows the list of capsules
manufactured without defects which can be seen in Figure 3-15 to Figure 3-17.
There is certain variability among capsules (diameter, thickness, filling, closure
gap) inherent to the manual blowing manufacturing process, which can be
minimized when the fabrication is later standardized.
0
2
4
6
8
10
12
14
16
0% 20% 40% 60% 80% 100%
Pre
ssu
re i
nsi
de
the
cap
sule
[b
ar]
Capsule filling (% of PCM)
NaNO3
KNO3
Sn
Pb
77
Thermal Energy Storage for High Temperature Applications
Table 3-4: List of spherical PCM capsules manufactured included in this study
Number Material Diameter1
[mm]
Diameter2
[mm]
Diameter3
[mm]
Avg φ
[mm]
SD φ
[mm]
PCM
[g]
Borosilicate
[g]
Total
Weight
[g]
1 NaNO3 20.55 20.63 20.56 20.58 0.04 5.07 3.354 8.424
2 NaNO3 19.55 19.53 19.62 19.57 0.05 5.07 3.014 8.084
3 NaNO3 19 19.03 18.97 19.00 0.03 5.07 2.831 7.901
4 NaNO3 19.66 19.71 19.73 19.70 0.04 5.07 3.058 8.128
5 Durferrit 19.3 19.28 19.26 19.28 0.02 5.00 2.921 7.921
6 Durferrit 21.04 20.99 21.03 21.02 0.03 5.00 3.508 8.508
7 Durferrit 19.24 19.17 19.2 19.20 0.04 5.00 2.896 7.896
8 KNO3 19.32 19.35 19.4 19.36 0.04 4.98 2.946 7.926
9 KNO3 19.66 19.75 19.68 19.70 0.05 4.98 3.056 8.036
10 KNO3 18.73 19.02 18.91 18.89 0.15 4.98 2.796 7.776
11 Pb 19.88 19.93 19.88 19.90 0.03 28.44 3.123 31.563
12 Pb 19.88 19.83 19.77 19.83 0.06 28.44 3.099 31.539
13 Pb 20.79 20.8 20.82 20.80 0.02 28.44 3.432 31.872
14 Sn 18.66 18.68 18.7 18.68 0.02 18.65 2.731 21.381
Macro Encapsulation of PCM
78
Figure 3-15: NaNO3 capsules after the last heat treatment (1-2-3-4)
Figure 3-16: Durferrit capsules after the last heat treatment, capsules (5-6-7)
(1)
(2)
(3)
(4)
NaNO3
(5) (6) (7)
Durferrit
79
Thermal Energy Storage for High Temperature Applications
Figure 3-17: Capsules after the last heat treatment: lead capsules (11-12-13) (left
image); tin (14) and KNO3 (10) capsules (right image)
Finally, two capsules have been broken to analyze the variability of the capsule
thickness through nine measurements along the capsule. Table 3-5 shows the
results. The deviations from the average value are lower than 3.5%.
Table 3-5: Variability of the capsule thickness
Thickness
Capsule A
[mm]
Thickness
Capsule B
[mm]
1.13 1.11
1.07 1.12
1.12 1.13
1.18 1.05
1.08 1.17
1.17 1.13
1.07 1.17
1.11 1.20
1.22 1.21
Average [mm] 1.13 1.14
Standard Deviation (SD) [mm] 0.05 0.05
Standard Error of the Mean (SEM) [mm] 0.02 0.02
Confidence Intervals (95%) [mm] 1.09 - 1.17 1.10 - 1.18
Sn (14) KNO3 (10)
Macro Encapsulation of PCM
80
3.6. Capsule Testing
3.6.1. Set-up Design
The experimental set-up to test a single PCM capsule was conceived in
collaboration with Prof. I. G. Loscertales from the University of Malaga, with initial
experiments performed at the installations of Yflow Sistemas y Desarrollos, S.L.
Further modifications to the set-up and the main tests were carried out at the
laboratories of Abengoa Research.
The experimental set-up consists of a glass cylinder 2 mm thick, 550 mm long, with
a 36 mm outer diameter, insulated with 15 mm of mineral wool and an air blow
heater connected to one end of the cylinder which is fed from a pressurized air line
at 5 bars. A volumetric flow meter and a pressure meter are connected between the
blow heater and the pressurized line to control the volumetric flow rate and air
density and hence the air mass flow is determined. The blow heater (BH) causes a
pressure drop, reflected in an increase in the upstream pressure. This increase of
pressure has to be considered for the mass flow calculation due to the
measured/controlled variable is the volumetric flow rate. Figure 3-18 shows the
correlation between the intermediate pressure and the volumetric flow rate.
The set up developed allows the test of borosilicate-PCM capsules analyzing the
melting and solidification process. The PCM-capsule is placed inside the tube using
a home-made sample holder consisting of stainless steel mesh (5x5 mm squared)
used to fix the position of the capsule in the tube. The capsule is exposed to the air
flow, melting or freezing the PCM depending on the stream temperature. Figure
3-19 shows a schematic representation of the set-up (dimensions in mm).
81
Thermal Energy Storage for High Temperature Applications
Figure 3-18: Intermediated relative pressure before the blow heater vs. volumetric
flow rate (liters per minute, LPM)
Figure 3-19: Experimental set-up schematic. All dimensions are in mm.
Six thermocouples monitor the system to estimate the real gas temperature applied
to the capsule. Thermocouples T1 to T3 are placed 200 mm downstream from the
blow heater. The thermocouples T4 to T6 are located at 150 mm from the end of the
borosilicate tube. The capsule is placed 1cm downstream from thermocouples T1 to
T3. The temperature upstream the capsule can be estimated assuming linear
thermal profile by:
Equation 3-4
y = 0.00122x1.541969 R² = 0.99015
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 20 40 60 80 100
Inte
rmed
iate
Pre
ssu
re[b
ar]
LPM
36
2
550
20200 150
TC1,TC2,TC3 TC4,TC5,TC6
air
g
Macro Encapsulation of PCM
82
Figure 3-20 shows an example of the estimated temperature around the capsule in
a freezing experiment using Equation 3-4, where T1-3 and T4-6 are the average
temperatures of thermocouples 1-3 and 4-6. The temperature of the air around the
capsule is estimated assuming linear thermal profile between the two groups of
thermocouples.
Figure 3-20: Measured temperature profile in the experimental set-up: average
temperature thermocouple 1 to 3, 4 to 6, and capsule estimated temperature.
The air crossing the blow heater is heated up by an electrical resistance. The power
supplied to the resistance is controlled modifying the input voltage through a
voltage transformer. The set-up schematic and images of the testing devices can be
seen in Figure 3-21 and Figure 3-22 respectively.
170
190
210
230
250
270
290
310
0 10 20 30 40 50 60 70 80 90 100 110 120 130
Tem
pera
ture
[ºC
]
Time [s]
Avg T1-3
Avg T4-6
Tcapsule
83
Thermal Energy Storage for High Temperature Applications
Figure 3-21: Schematic representation of the experimental set-up installation
Figure 3-22: Initial experimental set-up installation performed at the laboratories of
Yflow Sistemas y Desarrollos, S.L. under the collaboration with Prof. I. G.
Loscertales from the University of Malaga.
The blow heater outlet temperature depends on the power supplied and the mass
flow. A detail characterization of the blow heater determining parameters such as
the temperature and mass flow limits of the installation and the response against a
step-wise change in voltage are required for the definition of the experimental
procedure. Figure 3-23 shows a schematic representation of the expected response
BH
air
g
IR
Camera
Video
Camera
Data
logger
TCs
V
IR
Camera
Video
Camera
Macro Encapsulation of PCM
84
of the set-up. The step in voltage applied is shown in blue, the blow heater outlet
temperature in red and the estimated air temperature around the capsule using
Equation 3-4 in green.
Figure 3-23: Schematic of the expected thermal response of the set-up
There is a transient for the outlet temperature of the blow heater to reach a specific
outlet temperature and also a delay caused by the thermal inertia of the glass tube.
Therefore, the air around the capsule increases gradually until it reaches the blow
heater outlet temperature.
Figure 3-24 represents the experimental characterization of the flow parameters.
For each volumetric flow rate and voltage there is a stable temperature around the
capsule. It determines the initial and final voltage in order to apply a desired initial
and final temperature around the PCM-capsule for a specific volumetric flow rate.
Two different volumetric flow rates will have different heat transfer convection
coefficient around the capsule. In order to test with the same initial and final
temperatures for two different volumetric flow rates, different initial and final
voltages must be applied.
Capsule External
Temperature
(Experiment)
∆V set up
Tem
pera
ture
Time
Blow Heater Outlet temperature
85
Thermal Energy Storage for High Temperature Applications
Figure 3-24: Temperature vs. Voltage applied for different experiments performed
with the set-up at different volumetric flow rates (LPM, liters per minute). The
melting temperatures of two PCM (Durferrit, NaNO3) are shown as reference.
3.6.2. Experimental Procedure
The experimental procedure to melt/freeze a single capsule is the following:
1) place a PCM-capsule in the metallic sample holder
2) set the volumetric air flow rate (40-50-60 LPM) and the initial voltage,
heating the capsule from room temperature to the initial temperature
3) hold the initial temperature for 25 min as a homogenization
temperature step
4) heat up the air to the desired final temperature by changing the
applied voltage
5) hold at this temperature while the PCM melts
6) maintain this temperature for 25 min apply the initial voltage, cooling
down the capsule to its initial temperature, freezing the PCM
The procedure is repeated up to 10 times to ensure the capsule integrity to several
thermal cycles. The PCM melting/freezing process is observed through a
transparent window in the set-up and recorded by a digital camera during the
0
50
100
150
200
250
300
350
400
450
100 150 200 250 300
Tem
pera
ture
[ºC
]
Voltage [V]
40 LPM
50 LPM
60 LPM
80 LPM
NaNO3
Durferrit
Macro Encapsulation of PCM
86
thermal cycle in order to determine:
the melting start and end time, and consequently the melting duration;
the freezing starting time
to examine any crack formation
In both types of experiments (melting and freezing) the thermocouples measured
temperature has been used to estimate the temperature around the capsule. This
applied temperature is adjusted to an exponential curve to simplify the boundary
condition form using Equation 3-5. This boundary condition will be used in the
capsule model.
Equation 3-5
Where, ∆T is the temperature step applied, and τ is the time where the measured
temperature has reached 63.2% of the temperature step. These parameters have
been calculated for each experiment, shown in Table 3-6 and Table 3-7. As an
example, Figure 3-25 shows the estimated temperature based on Equation 3-5
compared to the measured temperature in the experimental set-up for a freezing
and for a melting experiment.
Figure 3-25: Capsule temperature measured and approximated equation adjusted
to be used in the capsule model for two experiments
180
200
220
240
260
280
300
320
340
0 50 100 150 200 250 300 350
Tem
per
atu
re [º
C]
Time [s]
T capsule [ºC]
T model [ºC]
EXP 14 280
290
300
310
320
330
0 50 100 150 200 250
Tem
per
atu
re [º
C]
Time [s]
T capsule [ºC]
T model [ºC]
EXP 7
87
Thermal Energy Storage for High Temperature Applications
The experiments are recorded simultaneously with both a visual and an infrared
(IR) camera (FLIR model SC7200 F/3 MW InSb) during the thermal cycling. The
infrared radiation comprises wavelength from 0.7 μm to 1000 μm. However, the
InSb detector can capture the spectral range of from 1.5 - 5.1 µm 79 covering the
Short-wavelength IR and Mid-wavelength IR.
The test duct and capsule shell are made out of borosilicate. The total optical
transmittance of borosilicate is higher than 90% for wavelengths 0.3 μm to 2.2 μm
for thicknesses between 0.7 to 5 mm. The transmittance of 1 mm borosilicate
thickness is higher than 40% for wavelengths between 0.3 μm to 3.5 μm (Figure
3-26). The temperature range between 25ºC and 300ºC does not change significantly
the transmittance, modifying slightly only the lower wavelength limit which is in
the UV range. The borosilicate is opaque for wavelength higher than 5 μm which is
out of the spectral range of the IR detector. Therefore, temperature differences
between the capsule and the test duct should be visible with this equipment.
Figure 3-26: Total optical transmittance of borosilicate.71
Macro Encapsulation of PCM
88
The other important property to consider is the emissivity, which is ~0.9 for
borosilicate (Pyrex glass) for a temperature range from 25 ºC to 300 ºC.80 When the
capsule surface is at higher temperature than the tube surface, its radiation is
expected to be distinguished from the test duct’s emitted radiation. The emissivity
of lead and tin from 25ºC to 250°C80 is Pb = 0.05-0.1 (polished), 0.3 (oxidized), and
0.65-0.4 (rough lead) and Sn = 0.05-0.1 (for unoxidized tin). Due to the low
emissivity of these materials compared to the emissivity of borosilicate the
radiation detected by the IR camera is expected to be due to the borosilicate
capsule.
3.6.3. Experimental Results and Discussion
Several tests were performed without test duct to guaranty that the information
about the phase change process from the IR camera and visual camera match and
to assist evaluating the images using a borosilicate tube, where the radiation might
be attenuated. Figure 3-27 shows one of the freezing tests on a NaNO3 capsule
performed without a confined flow. There, a previously melted capsule is cooled
by blowing hot air at 100ºC over its top surface. Due to subsequent melting and
freezing cycles and the effect of gravity, the salt moves to the bottom of the capsule.
The freezing start time is determined by a visible change in salt color (Figure 3-27,
top) and by an abrupt change in slope of the IR temperature traces (Figure 3-27,
bottom). The solidification of the first initial layer of salt is a very rapid
phenomenon lasting less than 10 seconds, as shown in the graph in Figure 3-27 and
qualitatively in Figure 3-28. The formation of this first external solid layer is quite
uniform and almost concentrically, if it were not for the uneven distribution of salt
and existence of the void inside the capsule.
89
Thermal Energy Storage for High Temperature Applications
Figure 3-27: NaNO3 capsule freezing in free flow: Image A (top left, 400 s) shows a
completely liquid salt-borosilicate capsule, while image B (top right, 410 s) shows
the beginning of the salt crystallization process, as marked by a change in slope in
the IR camera temperature traces (bottom right). The different temperature traces
correspond to the locations numbered 1-5 in the IR image (bottom left).
A BA B
A B
Time
Tem
per
ature
Macro Encapsulation of PCM
90
Figure 3-28: Sequential images of a NaNO3 capsule during the solidification process
as recorded with a visual camera (top) and an IR camera (bottom).
The addition of the borosilicate test duct attenuates the radiation from the capsule
and the acuteness that the phase change process causes on the radiation emitted by
the capsule surface and detected by the IR camera. However, it reduces the heat
losses and provides a more uniform and controlled heating environment. Figure
3-29 and Figure 3-30 show the freezing curves of two NaNO3 experiments, now
with the borosilicate test tube and a metallic sample holder. When the external
layer of the PCM freezes, the thermal curve (temperature vs. time) recorded by the
IR camera changes abruptly in slope (dT/dt). This change is clearer without a test
duct (Figure 3-27) but it can be also observed using the test duct (Figure 3-29 and
Figure 3-30). The initiation of the crystallization process in the salt can be observed
also visually by a change in color (opaqueness) and the formation of dendritic
shaped crystals (Figure 3-29 at 314 and 316 seconds) but unfortunately the end of
the process cannot be observed neither by the IR nor the visual camera.
91
Thermal Energy Storage for High Temperature Applications
Figure 3-29: NaNO3 capsule freezing with test duct: IR camera temperature trace
and video snapshots during the liquid-solid phase change highlighted in the red
area.
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
160
180
200
220
240
260
280
300
320
290 300 310 320 330 340 350 360
dT
/d
t [º
C/
s]
Tem
pera
ture
[ºC
]
Time [s]
T1
dT1/dt
Vid
eo C
amer
a im
ages
310 s 312 s 314 s 316 s 318 s 320 s
Macro Encapsulation of PCM
92
Figure 3-30: NaNO3 capsule freezing with test duct
The identification of the melting process initiation and duration is slightly different
from the crystallization. Again, the process is more visible without the duct and
sample holder, in the free flow heating experiment (Figure 3-31) than with the
borosilicate test tube and metallic sample holder (Figure 3-32). The melting start
time corresponds to the first abrupt change in the temperature profile that appears
at ~115-120 seconds. Due to the low thermal diffusivity of NaNO3 allowing large
thermal gradients within the PCM, the end of the melting process cannot be
observed with the IR camera. However, the visual camera is used to estimate the
end of the melting process in NaNO3 capsules, because the liquid salt is translucent
and visibly different from the solid opaque salt.
The melting process inside the capsules shows clearly an unconstrained melting
behavior 74: the process is initially dominated by heat conduction across the shell
wall but as the PCM melts, it sinks to the bottom of the capsule (Figure 3-31 D,
Figure 3-32 C). Natural convection can be observed inside the capsule: the hotter
liquid PCM rises to the top as the cooler liquid and solid pieces remain at the
bottom.
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
160
200
240
280
320
360
280 290 300 310 320 330 340 350 360
dT
/d
t [º
C/
s]
Tem
pera
ture
[ºC
]
Time [s]
T1
dT1/dt
A
1
B B
C C
A
A B C
93
Thermal Energy Storage for High Temperature Applications
Figure 3-31: Capsule temperature trace and sequential images from IR camera (top
row) and visual camera (bottom row) of a NaNO3 capsule during the melting
process under free flow heating, showing: A melting starts at the upper border, B
melting extends over the complete capsule external surface, C, D, E during the
melting process, F completely liquid capsule after melting ends.
Macro Encapsulation of PCM
94
Figure 3-32: Visual and IR images at different stages of the melting process of a
NaNO3 capsule inside a borosilicate duct.
The IR camera temperature traces should be analyzed with care, because not all the
changes in slope indicate the beginning or end of the phase change process. For
example, abrupt capsule movements within the sample holder due to large pieces
suddenly melting (Figure 3-31 C, D), large bubbles or even the thermocouples
Melting
Melting
Melting
Time
Tem
per
ature
Time
Time
Time
Tem
per
ature
Tem
per
ature
Tem
per
ature
A
B B
C C
A
D D
B
C
A
D
95
Thermal Energy Storage for High Temperature Applications
moving can appear to have a similar change in capsule temperature. The only way
to definitively confirm that a modification in slope corresponds to a phase change
in these experiments is by comparing the IR data with the visual camera. Figure
3-31- Figure 3-33 show visual images during the complete melting process and
their corresponding temperature trace from the IR camera. At certain points there
are changes in slope during the melting process and it is sometimes difficult to
assess from the IR data only if it is indicating melting start times.
Figure 3-33: NaNO3 capsule melting with test duct recorded with visual and IR
camera. Slope changes in the IR temperature traces correspond to melting.
Vid
eo C
amer
a im
ages
Time
Tem
per
atu
re
120 s 130 s 140 s 150 s
Macro Encapsulation of PCM
96
In the case of metallic PCM, the reflectivity of the surface changes when the PCM
melts (Figure 3-34). Small bubbles are formed every time the PCM freezes because
of the PCM contraction and they disappear when melting. This is the only
information available for metallic PCM with the video camera and unfortunately
this information is too ambiguous to be used to determine the phase change
process. However, the infrared images allow a clear determination of the phase
change process for both melting and freezing experiments as shown in Figure 3-35
for a Pb-capsule.
Figure 3-34: Tin (Sn) capsule. Change in reflectivity when melting. Completely
solid Sn (left), during the solid-liquid phase change (middle), and completely liquid
Sn (right).
97
Thermal Energy Storage for High Temperature Applications
Figure 3-35: Determination of the phase change process for melting and freezing
experiments on the Pb-Capsule number 11 based on the IR camera’s temperature
curves.
Time
Tem
per
ature
FreezingMelting
A B C D E
A B C D E
A B C D E
Temperature
Temperature
Temperature
Heating Cooling
Macro Encapsulation of PCM
98
Figure 3-36: Comparison of IR camera temperature traces from Pb and NaNO3
PCM capsules during sequential melting and solidification in free flow experiment.
Shadowed area represents the standard deviation due to capsule surface location.
Overall, for salt PCM capsules, the melting front is hard to trace with the IR
camera, firstly because of the camera’s resolution and secondly because what we
measure is a combination between the radiation emitted from the capsule surface
and the radiation emitted by the PCM. In contrast, the IR camera provides much
more useful information for metallic PCM capsules.
Due to the large difference in thermal conductivity of some of the PCM tested (salt
vs metal) the IR temperature trace behavior is dramatically different in each case
(Figure 3-36). For low conductivity PCM (salts), the capsule shell will see only a
change in slope when the phase change begins, as higher thermal gradients appear
inside the salt core which is slowly melting. For high conductivity PCM (metals),
the core melts quickly throughout the capsule radius keeping the overall capsule
isothermal during the process and, consequently, showing a flatter temperature
profile in the IR camera temperature traces. This happens during both the melting
and the freezing process. This explanation will later be confirmed by the model
analysis in Chapter 5, showing the different type of behavior depending on the
120
140
160
180
200
220
240
260
280
300
320
340
360
380
0 100 200 300 400 500
Tem
pera
ture
[ºC
]
Time [s]
Pb
NaNO3
PbNaNO3
99
Thermal Energy Storage for High Temperature Applications
type of PCM but it can also be seen qualitatively in the comparison shown in
Figure 3-36. Here, a lead (Pb) and a salt (NaNO3) capsule are compared under the
same external heating (Thot=430ºC) and cooling (Tcold=100ºC) temperatures in the
free flow set-up, although the lead capsule is only heated for 280 s whereas the salt
capsule is heated for 380 s.
3.6.4. Melting Results
Table 3-6 shows the results for different melting experiments on NaNO3 capsules in
the test duct. The volumetric air flow in the set-up is adjusted to 40-50-60 LPM in
order to evaluate the effect of different convective heat transfer coefficients around
the capsule. Different initial and final voltages have been adjusted for each
experiment resulting in different initial and final temperatures and different
dimensionless melting temperature θ
. For each experiment the
starting time of the melting process and the end time have been estimated
analyzing the recorded images, calculating the melting duration time.
The analysis of the melting experiment number 6 (Table 3-6) on a NaNO3 capsule is
described in detail as an example. The initial capsule temperature measured by the
thermocouple is 234ºC while the initial temperature measured by the IR camera is
203ºC. This difference might be caused by the radiation absorbed by the
borosilicate tube. Analyzing the temperature vs. time curve of the IR images, a
small variation is observed in t~60 seconds since step in voltage (Figure 3-37). The
derivative of the temperature recorded by the IR camera is represented in green.
The small temperature slope variation (blue) is clearly observed with its derivative
(green) indicating that the melting process of the first layer of PCM has begun.
Macro Encapsulation of PCM
100
Table 3-6: Melting experiments summary (NaNO3 Tm 302ºC)
Num Material
PCM
Q
[LPM]
T
Initial
[ºC]
T
Final
[ºC]
∆T
[ºC]
τ
[s]
Start
Melting
[s]
End
Melting
[s]
Melting
Duration
[s]
θm
1 NaNO3 40 204.9 368.9 164.0 51.4 170 830 660 59.2%
2 NaNO3 50 256.6 415.0 158.4 47.8 50 380 330 28.7%
3 NaNO3 50 266.6 387.1 120.5 51.8 50 450 400 29.4%
4 NaNO3 50 270.0 381.0 111.0 45.5 60 570 510 28.8%
5 NaNO3 40 270.6 404.9 134.3 43.6 70 425 355 23.4%
6 NaNO3 50 233.9 387.9 154.0 64.8 60 410 350 44.2%
7 NaNO3 60 282.1 329.0 46.9 46.6 40 601 561 42.4%
8 NaNO3 60 278.5 348.0 69.6 49.6 30 460 430 33.9%
101
Thermal Energy Storage for High Temperature Applications
Figure 3-37: Thermocouple temperature (red), IR camera temperature (blue) and
derivative of the IR temperature (green).
The recorded images of the capsule melting confirm that the melting process
begins around 60 seconds after the step in voltage was applied (Figure 3-38). The
end melting time is estimated using the recoded video, when the capsule is
completely transparent.
-0.05
0
0.05
0.1
0.15
0.2
200
225
250
275
300
325
350
375
400
-20 0 20 40 60 80 100 120 140 160
dT
emp
erat
ure
/dt
[ºC
/s]
Tem
per
atu
re [
ºC]
Time [s]
T capsule (Thermocouple) T (IR) dT/dt (IR) Melting
Experiment 6
Macro Encapsulation of PCM
102
Figure 3-38: Recorded images of the melting experiment number 6. At t~60 seconds
the melting process seems to begin, confirmed by the IR temperature.
Durferrit capsules have been also thermally cycled to confirm the capsule integrity
(Figure 3-39). The phase change of this salt extends to a wide temperature range, as
observed in the DSC measurements, consequently the beginning and end of the
phase change process is harder to estimate.
Figure 3-39: Example of a Durferrit capsule melting experiment.
3.6.5. Freezing Results
Similarly to the melting experiments, freezing experiments have been also
performed. Table 3-7 summarizes the results. The start of the freezing process can
be estimated visually from the video recording. Unfortunately, in this case the end
time of the freezing process cannot be estimated because when the first layer of salt
solidifies, it loses its transparency. Figure 3-40 and Figure 3-41 are examples of
freezing experiment on a Durferrit capsule and NaNO3 capsule respectively.
40 s 60 s Melting 120 s 240 s 400 s280 s
250s 270s 290s 310s230s
103
Thermal Energy Storage for High Temperature Applications
Figure 3-40: Example of a Durferrit capsule freezing experiment
Figure 3-41: NaNO3 capsule freezing experiment (Number 15).
Time: 15s Time: 17s Time: 20s
Macro Encapsulation of PCM
104
Table 3-7: Freezing experiments summary.*SF: start Freezing
Num Material
PCM
Q
[LPM]
T
Initial
[ºC]
T
Final
[ºC]
∆T
[ºC] τ [s]
Start
Freezing
[s]
θm
9 NaNO3 40 372.0 228.6 -143.4 37.8 80 48.8%
10 NaNO3 40 369.8 244.9 -124.9 37.1 75 54.3%
11 NaNO3 50 349.6 163.9 -185.7 32.5 50 25.6%
12 NaNO3 50 357.9 223.3 -134.6 41.2 70 41.5%
13 NaNO3 50 359.4 256.0 -103.5 46.5 90 55.5%
14 NaNO3 50 330.7 205.0 -125.7 49.5 45 22.8%
15 NaNO3 50 314.3 185.4 -128.9 45.7 20 9.5%
16 NaNO3 50 379.8 282.1 -97.7 38.0 110 79.6%
17 NaNO3 50 380.5 273.3 -107.2 39.7 105 73.2%
18 NaNO3 50 404.2 280.1 -124.1 38.9 130 82.4%
19 NaNO3 50 345.0 238.0 -107.0 46.5 80 40.2%
20 NaNO3 60 347.9 237.6 -110.3 39.8 80 41.6%
3.7. Conclusions
An extended literature review on high temperature encapsulation has been
performed in this chapter. Borosilicate as an encapsulating material is proposed
based on the different problems identified for high temperature PCM. It is
compatible with the PCM (salt and metal) evaluated and with the HTF (high
pressure steam). The capsules are designed, manufactured, and tested in an
experimental rig to qualitatively and quantitatively analyze the PCM melting and
solidification process. The phase change process is identified using a combination
of visual and infrared images. Changes in the radiation from the capsule, detected
by the IR camera and transformed to temperature curves, are clearly correlated
with the phase change process.
105
Thermal Energy Storage for High Temperature Applications
For inorganic salt PCM, changes in the slope of the temperature curves from the IR
images facilitate estimating the melting start time and freezing start time,
complementing the information observed visually recorded with the video camera.
The melting start time is hard to appreciate with the video camera, but is more
easily estimated based on the IR temperature profiles. The melting end time is
clearly observed with the video camera as well as the freezing start time.
For metallic PCM, the visual images do not provide any useful information.
However, based on the IR information, the beginning and the end of the phase
change process (in both melting and freezing) can be estimated.
Comparing the different PCM cores, the IR temperature history traces also show a
qualitative difference: metallic PC due to their higher thermal conductivity allow
smaller gradients within the capsule; when the outer surface melts, the inner core
quickly reaches the melting temperature, resulting in a flat (almost isothermal)
profile during the melting process. Salt PCM, with lower thermal conductivity, take
a longer time to melt and present larger gradients within the capsules themselves.
Qualitatively, the melting process shows a typical unconstrained melting behavior
with natural convection inside the capsule and solid portions sinking to the bottom
during the phase change, as confirmed by the visual camera images. The freezing
of the most outer layer shows in contrast a more uniform, almost concentric
behavior, if it were not for the existing asymmetries in the capsule due to the void
and salt location.
The analysis of the melting start time and melting duration as well as the freezing
start time for NaNO3-capsules is performed in Chapter 5 comparing with
numerical simulations to further explain the different results and trends.
Important questions regarding the borosilicate shell capsules appeared during their
manufacturing and testing such as the possibility of mass production to reduce
fabrication costs and their fragility. However, the capsules tested show mechanical
integrity and thermal stability over 10-15 freeze-thaw cycles. Similar processes in
glass laboratory test tube manufacturing also suggest approaches to automate and
simplify the fabrication process.
Macro Encapsulation of PCM
106
107
4 SINGLE CAPSULE MODEL
n this section the heat transfer of a single capsule is analyzed numerically. A
qualitative and quantitative study is provided to help understand the phase
change process and the PCM-capsule behavior. The objective is twofold: 1) aid
in the design of the PCM capsules and 2) compare to the single capsule
experimental results to further validate the proof-of-concept (Figure 2-2).
I
Single Capsule Model
108
Figure 2-2 Schematic representation of the PCM study with the contents of this
chapter marked in yellow.
4.1. PCM-capsule heat transfer model
The phase change inside a capsule is a highly complicated process; the
understanding of the phenomena and their simulation present several challenges50:
The motion of the solid-liquid interface (moving boundary problem)
makes it non-linear.
The heat transfer process at the interface is difficult to predict due to
buoyancy-driven natural convection in the fluid.
The thermal resistance between shell and solid PCM remains uncertain
and it is hard to quantify.
Some other phenomena may also appear during phase change which can
have a determinant influence in the heat transfer process such as the
volumetric expansion resulting in over-pressurization at melting, or the
creation of voids during solidification.
PCM TES heat
exchanger system
Packed bed
Macro-encapsulation
Screen & characterize
PCMs
Screen Shell Materials
Model Single Capsule
Fabricate capsules
Test single
capsule
Compare Experiments
& Model
Evaluate performance & challenges
Tube & housing
Metal PCM
Double PCM
109
Thermal Energy Storage for High Temperature Applications
The enthalpy formulation of the energy equation, Equation 4-1, simplifies some of
the difficulties linked to the moving boundary problem:
Equation 4-1
The solution of Equation 4-1 requires knowledge of the enthalpy–temperature
functional dependency. This is the preferred method amongst authors, its major
advantage being that an explicit treatment of the moving boundary is not required,
which simplifies the phase change problem. Other advantages are:
The governing equation is similar to the single phase equation.
There is no condition to be satisfied at the solid–liquid interface as it
automatically obeys the interface condition.
Enthalpy formulation allows a mushy zone between the two phases.
Modeling the melting process of a material in a spherical enclosure has received
much attention by the research community in the past decades. Assis et al.81
explored numerically and experimentally the process of melting of paraffin wax
(PCM) in a spherical geometry, including a parametric study about the influence of
capsule radius and HTF temperature on melting process. The simulations provide
detailed phase and flow fields inside the system and incorporate such phenomena
as convection in the liquid phase, volumetric expansion due to melting, sinking of
the solid in the liquid, and close contact melting. The sinking solid phase and the
thin liquid layer between it and the shell are shown. Moore et al. in 1982 studied
the same phenomenon numerically and experimentally, investigating the melting
of n-octadecane in a glass spherical enclosure.72
Archibold et al.77 also analyzed numerically the heat transfer during melting of a
PCM inside a closed and uniformly heated spherical shell. The results were
compared with the experiments performed by Tan et al.75. In a similar study, the
same authors further developed the model to consider partially filled spherical
shells.76
Despite the high complexity of the melting process and, consequently, the
complexity of the different models reviewed which could be implemented, a
compromise between complexity and computational time is finally met by
implementing a finite differences scheme to solve a one-dimensional heat transfer
Single Capsule Model
110
model for a PCM-capsule based on Zhao et al.44. The boundary fixing method is
used to solve the motion of the solid-liquid interface. The heat transfer within the
molten material is assumed to be purely a conduction-dominated problem. The
solid portion of the PCM is considered to remain at the center of the sphere,
neglecting the solid core motion away from the center due to the density
differences between the solid and liquid phases. Due to the symmetry it can be
reduced to a 1D problem, as it is assumed that the convective heat transfer
coefficient is uniform around the capsule (diffusion occurs only in radial direction).
Several modifications are introduced in the approach including a gradual
temperature boundary condition (see 4.5), adjustments to account for void space
inside the capsules, and the general resolution of the equations at the interface.
The variability in the capsule manufacturing and the uncertainty regarding the
convective heat transfer coefficient in the experimental set up make meaningless
trying to solve a more complex model for this first approach. On the other hand,
the model implemented has sufficient complexity to estimate the main capsule
design parameters, demonstrate their feasibility, and validate the proof-of-concept
experiments. It also allows the study of different factors such as the influence of the
particular PCM properties on the phase change times, the impact of different shell
characteristics, capsule size, and the effect of the convective heat transfer coefficient
on the charging/discharging process.
The problem to be solved is the heating/cooling of a spherical capsule of outer
diameter R1=20 mm, with a shell of thickness e= R1-R2= 1.1 mm. The material
representing the PCM core (r < R2) undergoes melting/solidification during this
process, which is tracked with the melting front situated at r=s(t) (Figure 4-1). The
position of the melting/solidification front is forced to move two nodes each time
step calculating the time step required.
111
Thermal Energy Storage for High Temperature Applications
Figure 4-1: Schematic of the problem to be solved
Solid Liquid
S(t)
R2
R1
PCM
Shell
Single Capsule Model
112
Governing Equations Dimensionless Discretization
Diffusion equation for a sphere, being j=1,
2, 3 the shell, the liquid PCM and the solid
PCM respectively.
Boundary conditions Dimensionless Discretization
is the capsule radius divided by the PCM
radius
is the thermal diffusivity, ‘j’ is 1-2-3 for shell,
liquid and solid respectively,
; is the thermal
conductivity [W (m K)-1]; is the density [kg·m-3], and
113
Thermal Energy Storage for High Temperature Applications
is the specific heat [J (g K)-1]
L: latent heat [J kg-1]
Dimensionless temperature, and
dimensionless melting temperature. Tm is the melting
temperature; Tf is heat transfer fluid temperature and To
is the initial temperature.
Dimensionless radial position, and
dimensionless melting front position.
dimensionless time
dimensnsionless heat transfer coefficient
For charging process, initially:
Single Capsule Model
114
The linear system that is solved is:
Where P, T and MB are:
·
+
·
P·θk + T = MB· θk+1
θk+1= MB-1·[P·θk + T]
θk+1= MB-1· P·[MB-1·(P·θk-1 + T) ] + MB-1 T
θk+1= MB-1 P·MB-1· P·θk-1 + MB-1·P· MB-1 T + MB-1 T
θk+1= (MB-1 P)2 ·θk-1 + (MB-1·P + I)· MB-1 T
θk+1= (MB-1 P)2 · (MB-1·[ P·θk-2 + T]) + (MB-1·P + I)· MB-1 T
115
Thermal Energy Storage for High Temperature Applications
θk+1= (MB-1 P)3 ·θk-2 + (MB-1 P)2MB-1 T + (MB-1·P + I)· MB-1 T
θk+1= (MB-1 P)3 ·θk-2 + ((MB-1 P)2+MB-1·P + I)· MB-1 T
…
θk+1= (MB-1 P)k+1 ·θ0 + MB-1 T]
Where Ωi, Ai, Bi, Ci and Di are:
Ω Ω Ω
Ω
The sub-index ‘i’ indicates the shell (1), liquid (2) and solid (3) region for Ω, D and
B. On the other hand, for each i-row, A and C are calculated with their radial
position (Ri is the radial position of each node).
The matrices built when there is no phase change involved are unchanged. The
boundary conditions between heat transfer fluid-shell and the adiabatic conditions
at the center of the capsule are also unchanged during the whole problem.
However, the boundary condition between the storage material and the shell
depends on the state of the PCM (liquid or solid). K1 or K2 is used to build this
condition depending on having liquid PCM-shell or solid PCM-shell respectively.
Finally, in order to solve the phase change problem, the moving boundary
condition is introduced between the solid-liquid interface. The model uses an
iterative process to calculate the time step and the temperature profile at the
interface when it moves two nodes inwards. The temperature in the following
node is forced to be the melting temperature (melting front position) and the matrix
MB is divided in two square sub-matrixes solving the problem by blocks, obtaining
the temperature from the node 1 to the melting front ‘node-1’, and from the melting
front ‘node+1’ to the external node. The new matrices in order to solve the phase
change problem are:
Single Capsule Model
116
·
+
·
4.2. Grid and time-step convergence
The convergence tests are conducted by using various numbers of nodal points in
radial direction and various time steps. For a 50 mm diameter NaCl-MgCl2 eutectic
PCM capsule the melting start time does not vary significantly with the time step
for ∆τ<0.002. Similarly, the melting duration time does not vary significantly for a
longer time step, ∆τ<0.05. Consequently, the most restrictive time step ∆τ=0.002 has
been selected for the simulations. Figure 4-2 shows the result of the time step
convergence test.
117
Thermal Energy Storage for High Temperature Applications
Figure 4-2: Melting starting time and melting duration time as a function of time
steps.
The result for the spatial convergence test is displayed in Figure 4-3. For a number
of nodes N>455 the changes in melting start time and melting duration become
small (∆τ=0.002).
Compared to Zhao et al.44 (∆τ=0.01 and N=55), the time step and number of nodal
points selected in this study (∆τ=0.002 and N=455) are significantly shorter and
larger respectively. This is due to the fact that in this analysis the comparison has
been performed comparing times in seconds instead of minutes with only one
significant decimal value. This analysis compares also the start melting time, which
is an order of magnitude less than the melting duration.
1820
1822
1824
1826
1828
1830
1832
1834
1836
1838
1840
140
142
144
146
148
150
152
154
156
158
160
0.001 0.01 0.1
Melt
ing
tim
e D
ura
tio
n [
s]
Melt
ing
Sta
rt t
ime [
s]
∆τ
Start Melting
Melting Duration
Single Capsule Model
118
Figure 4-3: Start melting and melting duration time as a function of mesh size.
4.3. Model Validation
The model implemented has been validated by obtaining the results presented in
Zhao et al. 201344 with the same material properties (Zn-Ni capsule) and
dimensions (50 mm diameter). In Zhao’s simulation the time for the PCM to reach
the melting temperature is 204 seconds, with a convective heat transfer coefficient
of h=95.77 Wm-2K-1. Table 4-1 compares the results.
1750
1760
1770
1780
1790
1800
1810
1820
1830
1840
1850
130
132
134
136
138
140
142
144
146
148
150
0 100 200 300 400 500
Melt
ing
tim
e D
ura
tio
n [
s]
Melt
ing
Sta
rt t
ime [
s]
Nodes
Start Melting
Melting Duration
119
Thermal Energy Storage for High Temperature Applications
Table 4-1: Comparison Zhao et al. vs. present work Zn-Ni capsule (50 mm in
diameter)
Reference Point Zhao et al.44 Present work Difference (%)
Start phase change 204 s 202 s -1%
Melting front at R=0.5 650 s 649 s -0.1%
End phase change 744 s 750 s +0.8%
Melting process 540 s 548 s +1.5%
Phase change from solid to liquid starts at the zinc/nickel interface when the
temperature reaches the melting point in about 3.4 min (204 s) compared to 202 s in
the implemented model (error: -1%) (Figure 4-4). The interface will pass through
R=0.5 after approximately 10.8 min (650 s), and will reach to the center after about
12.4 min (744 s), compared to 649 s and 750 s in the implemented model (error: -
0.1% and +0.8%). The melting process takes 540 s compared to 548 s in the
implemented model (error +1.5%) (Figure 4-5 to Figure 4-7).
Figure 4-4: Temperature at a various radial locations as a function of time: Zhao et
al.44 (left) and present work (right). Ni-Zn capsule
0 200 400 600 800 1000 1200 1400 1600 1800320
340
360
380
400
420
440
460
480
500
520
T in
sid
e c
ap
su
le [
ºC]
Time [s]
Temperature for different radial positions
T Center
T Shell int
T Shell ext
T experiment
Single Capsule Model
120
Figure 4-5: Location of the interface as a function of time: Zhao et al.44 (left) and this
work (right). Ni-Zn capsule
The results for the steel & NaCl-MgCl2 (shell-core) 50 mm capsule diameter have
been also compared in Table 4-2. The deviations from the reported results are lower
than 1.5%.
Table 4-2: Comparison Zhao et al. vs. present work Steel-NaCl-MgCl2 capsule
Reference Point Zhao et al. Present work Difference (%)
Start phase change 144 s 146 1.4%
End phase change 1992 s 1978 -0.7%
Melting process 1848 s 1832 -0.8%
0 100 200 300 400 500 6000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Lo
cati
on
of
inte
rface (
Scalin
g)
Time [s]
121
Thermal Energy Storage for High Temperature Applications
Figure 4-6: Temperature at a various radial locations as a function of time: Zhao et
al.44 (left) and This work (right). Steel NaCl-MgCl2 capsule.
Figure 4-7: Location of the interface as a function of time: Zhao et al.44 (left) and this
work (right). Steel-(NaCl-MgCl2) capsule.
4.4. Material properties
The physical properties for this analysis are presented in Table 4-3. The borosilicate
properties have been extracted from Schott Catalog71, at 300ºC when measured or
extrapolated as in Figure 4-8. When possible, measured values of the PCM
properties are used. The latent heat, the specific heat, and the melting temperatures
of the PCM (NaNO3, Sn, and Pb) have been measured while the thermal
0 500 1000 1500 2000 2500340
360
380
400
420
440
460
480
500
520
540
T in
sid
e c
ap
su
le [
ºC]
Time [s]
Temperature for different radial positions
T Center
T Shell int
T Shell ext
T experiment
0 200 400 600 800 1000 1200 1400 1600 1800 20000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1L
ocati
on
of
inte
rface (
Scalin
g)
Time [s]
Single Capsule Model
122
conductivity and density are have been extracted from Zhao et al. (2012).82 Table
4-4 shows the properties of the HTF (air) used in the experiments to estimate the
convective heat transfer coefficient used in the model.
Table 4-3: Thermo-physical properties used in the model. (*Measured)
k
[W m-1 K-1]
(liquid/solid)
ρ
[kg m-3]
(liquid/solid)
Cp
[kJ kg-1 K-1]
(liquid/solid)
LH
[kJ kg-1]
Tm
[ºC]
NaNO3 0.5/0.5 1900/2260 1.650/1.4* 176.8* 302*
Sn 26/57 83 6990/7365 0.24/0.24* 44.4* 179*
Pb 15/29 83 10660 /11340 0.17/0.17* 20.9* 315.4*
Borosilicate71 -/1.55 -/ 2200 -/ 1.23 N/A N/A
Stainless Steel84 -/ 14.9 -/ 7900 -/ 0.477 N/A N/A
In order to approximate the model to the experiments a modification is introduced
to consider the void space in the capsule. The NaNO3 storage material densities
have been multiplied by (1-Voidsolid) = 73.5 %, for solid density, and by (1-Voidliquid)
= 87.4%, for the liquid density. As the model is one-dimensional, the void space in a
specific location of the capsule cannot be considered. However, by changing the
density we can at least consider a capsule which is able to store the same energy as
the real capsule. The Sn-capsule density has been reduced similarly in order to
approximate the latent heat storage capacity of this capsule to the latent heat of
NaNO3-capsule by multiplying the solid density by 88.4 % and the liquid density
by 93.1 %, consequently both capsules are able to store the same amount of energy
as latent heat and phase change times can be compared. The density of solid and
liquid lead has been multiply by 82.2 % and 87.4 % respectively in order to consider
the void space present in this capsule.
123
Thermal Energy Storage for High Temperature Applications
Figure 4-8: Borosilicate thermo-physical properties71 (Black dots extrapolated at
higher temperature)
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
0 50 100 150 200 250 300 350 400
Sp
ecif
ic h
eat
[J/
(g·K
)]
Th
erm
al
Co
nd
ucti
vit
y [
W/
(mK
)]
Temperature [ºC]
Single Capsule Model
124
Table 4-4 Properties of the HTF (air) used in the experiments to estimate the
convective heat transfer coefficient used in the model.85
Property Value
(200ºC)
Value
(300ºC)
Value
(400ºC)
Unit
Medium : Air Air Air
Pressure : 1 1 1 [ bar ]
Temperature : 200 300 400 [ Celsius ]
Density : 0.7356 0.6072 0.517 [ kg / m3 ]
Specific Enthalpy : 476 579.6 685.3 [ kJ / kg ]
Specific Entropy : 7.334 7.532 7.702 [ kJ / kg K ]
Specific isobar
heat capacity Cp
1.026 1.046 1.069 [ kJ / kg K ]
Isobar coefficient of
thermal expansion :
2.115 1.745 1.486 [ 10-3 (1 / K) ]
Heat conductance 37.95 44.09 49.96 [ 10-3 (W / m·K)]
Dynamic viscosity : 26.09 29.86 33.35 [ 10-6 (Pa s) ]
Kinematic viscosity : 35.468 49.177 64.507 [ 10-6 m2 / s]
Thermal diffusivity : 503 694.3 903.8 [ 10-7 m2 / s]
Prandtl-Number : 0.7051 0.7083 0.7137
125
Thermal Energy Storage for High Temperature Applications
4.5. Boundary condition
In the previous simulations, and those performed in Zhao’s investigation44,63 the
capsule starts at constant temperature and at t=0 a temperature step from 250 ºC to
500 ºC is applied, since the surrounding heat transfer fluid is “hot” at 500 ºC during
charging. Similarly, a step from 500 ºC to 250 ºC is used for discharging where the
surrounding heat transfer fluid is “cold” at 250 ºC. This boundary condition has
been used to validate the implemented model. However, this is not the boundary
condition applied in the experimental set-up. In the experiments the thermal inertia
of the system makes the temperature around the sphere change gradually and not
stepwise. Moreover, considering any industrial systems, which are expected to last
a long operation time, thermal shocks like the one simulated (∆T=200 ºC) are
usually avoided.
The numerical model has been adapted to the experimental boundary conditions.
The air temperature boundary condition has been modified to approximate to the
experimental conditions. Figure 4-9 shows the Start Melting (SM) time and End
Melting (EM) time for the implemented model compared to the expected times
based on the temperature boundary condition applied to the capsule in the
experiment.
Figure 4-9: Schematic of air temperature boundary condition: previous model vs.
experiment. SM: start melting, EM: end melting.
Single Capsule Model
126
The dimensionless temperature is normalized by the heat transfer fluid
temperature. However, the HTF temperature boundary condition applied to the
capsule is time-dependent in the experiments.
Boundary condition
Dimensionless boundary condition
In the experiments the temperature of the final air temperature around the capsule
has been used to define the dimensionless temperature, while the time-dependent
air temperature has been introduced as a new term as follows:
Equation 4-2
with
Equation 4-3
The analytical approximation of the experimental air temperature around the
capsule has been used in order to facilitate its implementation in the model. The
discretization of the new boundary condition will be as follows:
127
Thermal Energy Storage for High Temperature Applications
4.6. Results and Discussion
4.6.1. Effect of the new boundary condition on the phase change times
As explained above, in Zhao’s simulation44,63 the step in temperature is applied in
t=0 s. However, in the experimental set-up a gradually increasing temperature is
applied. This led to a modification in the boundary condition to adapt the
simulations to the experimental set-up. An example of the time evolution of the
capsule temperature and heat transfer fluid temperature for the modified
boundary condition can be seen in Figure 4-10. As Figure 4-10 shows, the PCM
starts melting when R=R2 reaches the melting temperature (300 ºC approximately).
The melting process duration is described by the constant temperature trace inside
the capsule (R=0) finishing around 465 seconds.
Figure 4-10: Example of the new boundary condition introduced in the
mathematical model and temperature evolution for different capsule radial
positions
250
260
270
280
290
300
310
320
330
340
350
0 100 200 300 400 500 600
Tem
pera
ture
fo
r d
iffe
ren
t ra
dia
l p
osi
tio
ns
[ºC
]
Time [s]
HTF
T Shell ext (R=R1)
T Shell int (R=R2)
T Center (R=0)
Start
Melting
Single Capsule Model
128
The change in the boundary condition has an important impact on the melting start
time and in the melting end time as shown in Figure 4-11, where both boundary
conditions are compared. As expected, the melting process is shifted, starting and
finishing earlier when a discrete temperature time step (i.e. Zhao´s boundary
condition) is applied. However, the phase change process almost remains
unchanged (Figure 4-12). It means that the new boundary condition only delays the
phase change process without any other modification. When the melting process
starts, the dimensionless HTF temperature is approximate ~0.9 for the new
boundary condition, not too different than Zhao’s boundary condition where the
dimensionless temperature is 1. Then, as the temperature applied to the system
when the melting process starts is similar, the melting process is almost identical
for both types of boundary conditions. This suggests, as we will see later on, that
the melting process is governed mainly by the PCM thermal properties and the
external flow conditions, which are kept constant for both simulations shown in
Figure 4-11. The simulation conditions are: NaNO3 –Borosilicate capsule with
r1=0.01 m, a capsule thickness of 0.0011 m, the number of nodes N = 451, time step
∆τ = 0.002, temperature step applied ∆T = 100 ºC and dimensionless melting
temperature θm = 0.5. Two different set-up time constants have been chosen: τexp =
0.0001 s for Zhao’s Boundary condition and τexp = 50 s used in the present work;
and two convective heat transfer coefficients, 50 and 150 W m-2 K-1. The simulation
is extended up to θcenter = θ (R=0) = 0.9.
129
Thermal Energy Storage for High Temperature Applications
Figure 4-11: Effect of the new boundary condition applied to the capsule in the
present work compared to Zhao’s boundary condition on the temperature profiles
at the center of the capsule (R=0) and at the shell-PCM interface (R=R2) for a fixed
hconv=150 Wm-2K-1.
The melt fraction is defined as the amount of molten PCM divided by the initial
PCM amount (Equation 4-4). The density change has been neglected.
Equation 4-4
Changing the boundary condition barely affects the melt fraction. As shows Figure
4-12, this small change is only noticeable when the convective heat transfer
coefficient is high.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 100 200 300 400 500 600
Dim
en
sio
nle
ss T
em
pera
ture
θ
[-]
Time [s]
HTF BC Zhao
T (R=0) BC Zhao
T (R=R2) BC Zhao
HTF BC Gimenez
T (R=0) BC Gimenez
T (R=R2) BC Gimenez
Single Capsule Model
130
Figure 4-12: Effect of the new boundary condition applied to the capsule in the
present work compared to Zhao’s boundary condition on the temporal evolution of
the melt fraction for two different convective heat transfer coefficients (50 and 150
Wm-2K-1).
4.6.2. Effect of capsule size on phase change
For this analysis a NaNO3-Borosilicate capsule has been considered. The convective
heat transfer coefficient is fixed to 50 Wm-2K-1. Three different capsule radii (r1)
have been simulated: 0.01, 0.02 and 0.03 meters (r1, 2·r1, 3·r1). The ratio R2/R1 is
fixed for all cases to 0.89 (Table 4-5).
Figure 4-13 shows the temperature profiles, Figure 4-14 (left) the location of the
solid-liquid interface, and Figure 4-14 (right) the melt fraction vs. time for the three
radii simulated. The melting start time and the melting time duration increases as
the radius of the capsule increases. When the radius is increased (r1, 2·r1, 3·r1) the
surface area increases as well, but squared (1A, 4A, 9A). Since the convective heat
transfer coefficient is fixed, the input power increases in the same proportion as the
surface area. However, the amount of PCM has increased to the third power
(1·Volume, 8·Volume, 27·Volume) consequently the melting process takes
significantly longer time.
0%
20%
40%
60%
80%
100%
0 100 200 300 400 500 600 700
Melt
Fra
cti
on
Time [s]
BC Zhao h=50 BC Gimenez h=50 BC Zhao h=150 BC Gimenez h=150
131
Thermal Energy Storage for High Temperature Applications
Table 4-5: Melting start time and melting duration for three different capsule sizes.
Melting Start time [min] Melting Duration [min]
Radius x1 (0.01m) 2.7 10.0
Radius x2 (0.02m) 4.3 28.6
Radius x3 (0.03m) 5.6 55.1
Figure 4-13: Dimensionless temperature at the center of the capsule (R=0) and at the
shell-PCM interface (R=R2) for three different capsule radii.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 500 1000 1500 2000 2500 3000 3500 4000 4500
Dim
en
sio
nle
ss T
em
pera
ture
θ
[-]
Time [s]
HTF T (R=0) Radius x1 T (R=R2) Radius x1 T (R=0) Radius x2 T (R=R2) Radius x2 T (R=0) Radius x3 T (R=R2) Radius x3
Capsule Size ↑
Single Capsule Model
132
Figure 4-14: Location of the solid-liquid interface for three different capsule radii
(left) and melt fraction for three different capsule radii (right)
4.6.3. Effect of capsule shell thickness on phase change
For this analysis a NaNO3-Borosilicate capsule has been considered. The convective
heat transfer coefficient is fixed to 50 Wm-2K-1. Three different capsule thickness ‘e’
have been simulated: 0.0011, 0.0022 and 0.0033 meters (e1, 2·e1, 3·e1). It changes the
ratio R2/R1. The capsule external radius is fixed to 0.01 m. As the capsule thickness
increases maintaining the capsule radius, the amount of PCM is reduced.
Consequently the melting time is reduced. The melting start time is also reduced
slightly as the capsule thickness is increased because more material has to be
heated up before the PCM reaches its melting temperature, taking longer time
when the PCM is further away from the capsule surface (Figure 4-15 and 4-16).
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 1000 2000 3000
Lo
cati
on
of
the i
nte
rfase
S
(t)
[-]
Time [s]
Radius x1
Radius x2
Radius x3
0%
20%
40%
60%
80%
100%
0 1000 2000 3000
Melt
Fra
cti
on
[%
] Time [s]
Radius x1
Radius x2
Radius x3
Capsule Size ↑
Capsule Size ↑
133
Thermal Energy Storage for High Temperature Applications
Figure 4-15: Effect of the capsule thickness on the capsule temperature: at the center
(R=0) and at the shell-PCM interface (R=R2).
Figure 4-16: Location of the solid-liquid interface vs. time for different capsule
thickness (left); Melt fraction vs. time for different capsule thickness (right)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 200 400 600 800 1000
Dim
en
sio
nle
ss T
em
pera
ture
θ
[-]
Time [s]
HTF T (R=0) Thickness x1 T (R=R2) Thickness x1 T (R=0) Thickness x2 T (R=R2) Thickness x2 T (R=0) Thickness x3
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 100 200 300 400 500 600 700
Lo
cati
on
of
the i
nte
rfase
S(t
) [-
]
Time [s]
Thickness x1
Thickness x2
Thickness x3
0%
20%
40%
60%
80%
100%
0 100 200 300 400 500 600 700
Melt
Fra
cti
on
Time [s]
Thickness x1
Thickness x2
Thickness x3
Thickness↑
Thickness↑
Capsule Thickness ↑
Single Capsule Model
134
4.6.4. Effect of shell material on phase change times: borosilicate vs.
steel
The effect of the shell material on the melting process of a NaNO3-capsule is
evaluated by comparing two materials with very different thermal conductivity: a
metal shell (stainless steel) and a glass (borosilicate). The materials´ thermal
properties are shown in Table 4-6. Stainless steel has a significantly higher thermal
conductivity (approximately 10 times higher) and higher thermal diffusivity than
borosilicate. For this analysis, the capsule geometry (volume, shell thickness), type
of PCM material, and external conditions are kept constant. The simulation
conditions are: r1 = 0.01 m, capsule thickness 0.0011 m, number of nodes N = 451,
∆τ = 0.002, ∆T = 100 ºC, dimensionless melting temperature θm = 0.5, set-up time
constant τexp = 50 s, simulation until θcenter = 0.9.
Table 4-6: Shell material properties
k
[W/(mK)]
ρ
[kg/m3]
Cp
[kJ/(kgK)]
Cp
[kJ/(m3K)]
=k/Cp
[m2/s]
Borosilicate71 1.55 2200 1.23 2706 5.73·10-7
Stainless Steel84 14.9 7900 0.477 3768.3 3.95·10-6
The temperature history at various locations (center, shell-PCM interface and
capsule surface) of a NaNO3 capsule, comparing different shell materials
(borosilicate vs. steel) for fixed geometry and external flow conditions can be seen
in Figure 4-17. For a constant shell thickness, the stainless steel shell takes slightly
longer time (approximately +3%) to reach the melting temperature at the shell-
PCM interface R=R2. In other words, the melting start time is longer. This can be
explained because the volumetric energy density “ρ·Cp” [Jm-3K-1]) of steel is ~40%
higher than that of borosilicate. This means that to reach the same temperature for a
constant volume we need to apply more heat to the steel capsule. However, once
the capsule starts melting, the process is faster with the higher conductivity metallic
shell (about 2% faster), since the center R=0 finishes the isothermal segment (=m)
earlier. The borosilicate and stainless steel present ~3 and ~20 times higher thermal
135
Thermal Energy Storage for High Temperature Applications
diffusivity than NaNO3. Therefore the salt PCM, with lowest thermal diffusivity, is
the limiting factor for the melting duration. Consequently, changes on the shell
material with higher thermal diffusivities within the range evaluated, do not affect
the melting process significantly.
Figure 4-17: Dimensionless temperature for different radial positions: center (R=0),
shell-PCM interface (R=R2) and capsule surface (R=R1) for NaNO3-capsule with
different shell materials. Convective heat transfer coefficient around the capsule 150
W m-2 K-1.
The melting start time and the melting duration of a NaNO3 capsule, comparing
different shell materials (borosilicate vs. steel) and different convective heat transfer
coefficients are shown in Table 4-7. The stainless steel capsule delays the beginning
of the phase change process (longer melting start time) compared to a borosilicate
capsule due to its higher volumetric energy density, as explained before. For low
convective heat transfer coefficients this effect in the melting start time is slightly
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 100 200 300 400 500 600
Dim
en
sio
nle
ss T
em
pera
ture
θ
[-]
Time [s]
HTF
T (R=0) NaNO3-Borosilicate
T (R=R2) NaNO3-Borosilicate
T (R=R1) NaNO3-Borosilicate
T (R=0) NaNO3-STEEL
T (R=R2) NaNO3-STEEL
T (R=R1) NaNO3-STEEL
Single Capsule Model
136
more noticeable.
On the other hand, the borosilicate capsule delays the melting process (Figure 4-18).
The differences on the melting duration seem more noticeable for high convective
heat transfer coefficient, although these differences are very small.
Table 4-7: Melting start time and melting time duration for different convective
heat transfer coefficients and different shell materials. NaNO3 as PCM.
Melting Start
time [s]
Melting time
Duration [s]
h=50
[W m-2 K-1]
h=150
[W m-2 K-1]
h=50
[W m-2 K-1]
h=150
[W m-2 K-1]
Borosilicate and NaNO3 163.6 85.6 599.5 379.6
Stainless steel and NaNO3 175.3 87.8 596.9 371.2
Figure 4-18 (left) represents the melt fraction versus time for the four cases
simulated (different shell material and convective heat transfer coefficient). As
explained, for a fixed geometry, changes in shell material are negligible within the
range of convective heat transfer coefficients simulated. The location of the melting
front s(t) is also compared in Figure 4-18 (right). The effect of changing the shell to a
higher conductivity material to reduce the overall melting process time is slightly
more noticeable at higher heat transfer coefficients as mentioned above.
137
Thermal Energy Storage for High Temperature Applications
Figure 4-18: Melt fraction vs time (left) and Location of the solid liquid interface
(right) for different shell materials and different convective heat transfer coefficients
around a NaNO3 capsule
The same analysis on the effect of the shell material has been performed using a
metallic PCM core instead. The melting start time and the melting duration of
encapsulated tin (Sn), comparing different shell materials (borosilicate vs steel) and
convective heat transfer coefficients are shown in Table 4-8. Similarly to the NaNO3
case, the melting start time and the melting duration for the Sn-capsule are not
affected significantly by the shell material type when the convective heat transfer
coefficient is fixed (Figure 4-19). As in the NaNO3 case, the melting starts slightly
earlier for a borosilicate capsule compared to the steel capsule as before, because of
its lower ‘ρCp’ and the melting duration is shorter for the stainless steel capsule
compared to the borosilicate capsule.
The differences on the melting start time are reduced as the convective heat transfer
coefficient is increased (Figure 4-20). On the other hand, the borosilicate shell slows
down the melting process by approximately 10 seconds for the convective heat
transfer simulated. It might be caused by the difference in thermal diffusivity. The
difference in melting duration increases slightly as the convective heat transfer
increases.
0%
20%
40%
60%
80%
100%
0 100 200 300 400 500 600
Melt
Fra
cti
on
[%
]
Time [s]
NaNO3-Borosilicate h=50
NaNO3-Borosilicate h=150
NaNO3-STEEL h=50
NaNO3-STEEL h=150
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 100 200 300 400 500 600
Lo
cati
on
of
the i
nte
rfase
S(t
) [-
]
Time [s]
NaNO3-Borosilicate h=50
NaNO3-Borosilicate h=150
NaNO3-STEEL h=50
NaNO3-STEEL h=150
hconv↑ hconv↑
Single Capsule Model
138
Table 4-8: Melting start time and melting time duration for different convective
heat transfer coefficients and different shell materials. Tin (Sn) as PCM.
Melting Start time [s] Melting Duration [s]
h=50
[Wm-2K-1]
h=150
[Wm-2K-1]
h=50
[Wm-2K-1]
h=150
[Wm-2K-1]
Borosilicate and Sn(PCM) 144.91 80.64 298.94 128.65
Stainless steel and Sn(PCM) 156.95 83.49 289.07 118.06
Figure 4-19: Dimensionless temperature for different radial positions: center (R=0),
shell-PCM interface (R=R2) and capsule surface (R=R1) for a Sn-capsule with
different shell materials. Convective heat transfer coefficient around the capsule 150
Wm-2K-1.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 50 100 150 200 250
Dim
en
sio
nle
ss T
em
pera
ture
θ
[-]
Time [s]
HTF T (R=0) Sn-Borosilicate T (R=R2) Sn-Borosilicate T (R=R1) Sn-Borosilicate T (R=0) Sn-STEEL T (R=R2) Sn-STEEL T (R=R1) Sn-STEEL
139
Thermal Energy Storage for High Temperature Applications
Figure 4-20: Location of the solid liquid interface vs time (left) and Melt fraction vs
time (right) in the Sn capsule for different shell materials and different convective
heat transfer coefficients around the capsule.
4.6.5. Effect of the latent heat on the phase change times
The effect of the latent heat on the phase change times is evaluated. The simulation
conditions are: NaNO3 – Borosilicate capsule with r1 = 0.01 m, a capsule thickness
of 0.0011 m, the number of nodes N = 451, time step ∆τ = 0.002, temperature step
applied ∆T = 100 º C and dimensionless melting temperature θm = 0.5. The new
boundary condition is used with τexp = 50 s. The convective heat transfer coefficient
is set to 150 Wm-2K-1. The simulation is extended till θcenter = 0.9. The latent heat of
NaNO3 has been used together with an ideal PCM with a latent heat half of this
value and another with two times this latent heat. The results are shown in Table
4-9 and Figure 4-21.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 100 200 300 400 Lo
cati
on
of
the i
nte
rfase
S(t
) [-
]
Time [s]
Sn-Borosilicate h=50
Sn-Borosilicate h=150
Sn-STEEL h=50
Sn-STEEL h=150
0%
20%
40%
60%
80%
100%
0 100 200 300 400
Melt
Fra
cti
on
Time [s]
Sn-Borosilicate h=50
Sn-Borosilicate h=150
Sn-STEEL h=50
Sn-STEEL h=150
hconv↑
hconv↑
Single Capsule Model
140
Table 4-9: Melting start time and melting time duration for three different latent
heats tested.
Melting Start
time [min]
Melting time
Duration [min]
0.5 ·Latent Heat 1.4 4.0
1.0 ·Latent Heat 1.4 6.3
2.0 ·Latent Heat 1.4 10.7
The melting start time is unchanged for the three cases because the latent heat does
not affect the sensible heat of the PCM-capsule. On the other hand, the latent heat
affects linearly the melting time duration as shown in Figure 4-21.
Figure 4-21: Melting time duration vs. latent heat times the latent heat of NaNO3
(left); Effect of the latent heat on melt fraction for three different latent heats tested
(right)
0
2
4
6
8
10
12
0.0 1.0 2.0
Melt
ing
tim
e d
ura
tio
n [
min
]
Multiples of the latent heat of NaNO3
0%
20%
40%
60%
80%
100%
0 200 400 600 800
Melt
Fra
cti
on
Time [s]
0.5·Latent Heat
1.0·Latent Heat
2.0·Latent Heat
141
Thermal Energy Storage for High Temperature Applications
4.6.6. Effect of PCM characteristics on phase change times
In this section the effect of the PCM material (Salt NaNO3 vs Metal Sn) on the
melting start time, melting duration, melt fraction, and location of the solid-liquid
interfaces as a function of time, as well as the temperature profiles in the PCM-
capsule are compared. It is interesting because the choice of material implies a
combination of properties frequently with opposing effects and not merely an
increase in thermal conductivity or an increase in latent heat. Figure 4-22 shows the
temperature evolution at 3 different radial positions for a Sn-Borosilicate capsule
and NaNO3-borosilicate capsule. As can be observed the metallic PCM does not
allow large thermal gradients within the capsule because of its high thermal
conductivity and diffusivity. The capsule surface closely follows the temperature
profile inside the capsule; the surface does not increase its temperature until the
phase change process has finished. On the other hand, the salt PCM shows large
thermal gradients within the capsule. The surface temperature increases
progressively until the PCM outer surface (R=R1) reaches the phase change
temperature. At this point, the shell temperature profile changes its slope, but the
surface and the PCM-shell interface keeps heating up, whereas any location inside
the PCM (0<R<R1) will show an isothermal segment corresponding to the phase
change, confirming the appearance of large gradients inside the PCM.
In the case of metallic PCM the phase change duration (melting start and end
times) could be estimated by measuring the capsule surface temperature. However,
only the melting start time could be estimated based on the NaNO3-capsule surface
temperature. This observation can be perfectly correlated with the infrared images
recorded in previous section, where the temperature traces of the capsule surface
show an isothermal melting segment in the case of a metallic PCM but not in the
case of a salt PCM capsule.
Single Capsule Model
142
Figure 4-22: Dimensionless temperature of the capsule at the center (R=0), PCM-
shell interface (R=R2) and the capsule surface (R=R1) for two different PCM
(NaNO3 and Sn) encapsulated with borosilicate. Convective heat transfer
coefficient 50 Wm-2K-1
Analyzing the differences in the melting process of Sn and NaNO3 borosilicate
capsule, comparing capsules with the same latent heat capacity, Figure 4-23 shows
how tin (PCM)-capsule, with a similar melting start time and a thermal diffusivity
more than 2 orders of magnitude higher than NaNO3, shows around half melting
time. The heat goes through the molten PCM until the phase change front that is
why the thermal diffusivity of the PCM is important and a low value slows down
the melting process.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 200 400 600 800 1000
Dim
en
sio
nle
ss T
em
pera
ture
θ
[-]
Time [s]
HTF
T (R=0) Sn-Borosilicate
T (R=R2) Sn-Borosilicate
T (R=R1) Sn-Borosilicate
T (R=0) NaNO3-Borosilicate
T (R=R2) NaNO3-Borosilicate
T (R=R1) NaNO3-Borosilicate
Sn Melting
NaNO3 Melting
143
Thermal Energy Storage for High Temperature Applications
Figure 4-23: Location of the solid-liquid interface vs. time (left) and melt fraction vs.
time (right) for two different PCM (NaNO3 and Sn) encapsulated with borosilicate.
Convective heat transfer coefficient 50Wm-2K-1.
Finally, the study has been extended to convective heat transfer coefficients far
from the experimental conditions, comparing the melting start time and melting
duration for borosilicate capsules using tin and sodium nitrate as a PCM. The
results are shown in Figure 4-24. As we increase the heat transfer coefficient the
two times (phase change start and duration) for the two types of capsules tend to
stabilize becoming independent of the convective heat transfer coefficient and
being only dependent on the diffusion of heat through the capsule. The melting
start time is almost identical for convective heat transfer coefficients higher than
100 Wm-2K-1. As the convective heat transfer increases, the melting duration of both
capsules tend to a constant value significantly lower for a tin-capsule compared to
a NaNO3-capsule.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 100 200 300 400 500 600 700 Lo
cati
on
of
the i
nte
rfase
S(t
) [-
]
Time [s]
NaNO3-Borosilicate
Sn-Borosilicate
0%
20%
40%
60%
80%
100%
0 100 200 300 400 500 600 700
Melt
Fra
cti
on
Time [s]
NaNO3-Borosilicate
Sn-Borosilicate
Single Capsule Model
144
Figure 4-24: Start melting and melting duration time for borosilicate capsules with
Sn and NaNO3 as PCM for different convective heat transfer coefficients.
4.6.7. Effect of experimental conditions on the phase change times
A parametric study has been performed on borosilicate capsules (20 mm in
diameter, 1.1mm thickness) with NaNO3 as PCM to evaluate the effect of changes
in the experimental conditions (temperature step applied ‘∆T’, convective heat
transfer coefficient ‘h’ and dimensionless melting temperature ‘θm’) on the phase
change times. For different values of the convective heat transfer coefficient, h (50-
75-100-200-300W/m2K), temperature step, ∆T (60-100-140ºC), (with 0.5 as
dimensionless melting temperature, i.e. the melting temperature falls in the center
of the temperature range), and different dimensionless melting temperatures
θm(0.25-0.5-0.75), (fixing the temperature step ∆T=100ºC), the start and end melting
times and melting duration are analyzed. The average time constant (τ=45.7s)
calculated from the experiments has been used in the simulation to represent the
real air temperature profile applied to the capsule.
0
2
4
6
8
10
12
0 250 500 750 1000
Tim
e [
min
]
Convective heat transfer coefficient h [Wm-2K-1]
Melting Duration NaNO3-Borosilicate
Start Melting NaNO3-Borosilicate
Melting Duration Sn-Borosilicate
Start Melting Sn-Borosilicate
145
Thermal Energy Storage for High Temperature Applications
The first parametric study modifies the convective heat transfer coefficient and the
temperature step applied, fixing the dimensionless melting temperature (θm) to 0.5.
The “melting start time” and the “melting duration” results are represented in
Figure 4-25.
Figure 4-25: Melting start time (left) and Melting duration (right) vs. heat transfer
coefficient for different temperature steps, fixed dimensionless melting
temperature θm= 0.5 and time constant τ=45.7 s.
The starting time for the melting process seems dependent only on the convective
heat transfer coefficient, and independent of the externally applied temperature
step (∆T). The time to reach the PCM the dimensionless melting temperature of 0.5
does not depend on the temperature step applied. On the other hand, the melting
duration depends on both: the heat transfer fluid and the temperature step. The
relative effect of the heat transfer coefficient on the melting duration is similar for
each temperature step. The melting duration is reduced almost by half when the
heat transfer coefficient is increased from 50 to 300Wm-2K-1. A similar reduction is
observed when the temperature step is increased from 60ºC to 140ºC.
50
70
90
110
130
150
170
190
50 100 150 200 250 300
Melt
ing
Sta
rt t
ime [
s]
Heat transfer coefficient [Wm-2K-1]
∆T 60ºC
∆T 100ºC
∆T 140ºC
50
150
250
350
450
550
650
750
850
950
50 100 150 200 250 300
Melt
ing
Du
rati
on
tim
e [
s]
Heat transfer coefficient [Wm-2K-1]
∆T 60ºC
∆T 100ºC
∆T 140ºC
∆T 220ºC
Single Capsule Model
146
The second parametric study modifies the convective heat transfer coefficient and
the non-dimensional melting temperature (θm = 0.25-0.50-0.75), fixing the
temperature step ∆T=100ºC, because the experiments start at different temperature
with different θm and it is expected that changes in the boundary conditions and
the θm affect significantly the melting times. The “melting start time” and the
“melting duration” results are represented in Figure 4-26.
Figure 4-26: Melting start time (left) and Melting duration time (right) for different
convective heat transfer coefficients ‘h’ and dimensionless melting temperature θm
for a fixed T=100ºC
In this case the melting start time depends on both the convective heat transfer
coefficient and the dimensionless melting temperature. There is a pronounced
reduction in the melting start time at low convective heat transfer coefficients (50-
100Wm-2K-1) compared to (h>100Wm-2K-1) when the dimensionless melting
temperature is reduced. The dimensionless melting temperature has an important
impact on the melting start time at 50Wm-2K-1 reducing its effect when the heat
transfer coefficient is increased. The relative effect of the heat transfer coefficient on
the melting start time is similar for each dimensionless melting temperature. An
0
50
100
150
200
250
300
350
50 100 150 200 250 300
Mel
tin
g S
tart
tim
e [s
]
Heat transfer coefficient
[Wm-2K-1]
θm=0.25
θm=0.50
θm=0.75
0
200
400
600
800
1000
1200
50 100 150 200 250 300
Mel
tin
g D
ura
tio
n t
ime
[s]
Heat transfer coefficient
[Wm-2K-1]
θm=0.25
θm=0.50
θm=0.75
147
Thermal Energy Storage for High Temperature Applications
average reduction of 62% in the melting start time is observed when the heat
transfer coefficient is increases for 50 to 300Wm-2K-1. Increasing the dimensionless
melting temperature (starting the experiments at higher initial temperature) by
+0.25 the average increase on the melting start time is +83.6%.
Similar effect on the melting duration compared to the melting start time is
observed. On average a 46% reduction on the melting duration time is observed
when the heat transfer coefficient is increased from 50 to 300Wm-2K-1. This
reduction is similar for the three dimensionless melting temperatures. For low θm
the effect on the melting time of the convective heat transfer coefficient is lower. For
high θm and for the convective heat transfer coefficient around ~50 Wm-2K-1,
expected in the experimental set-up, changes in the θm and the convective heat
transfer coefficient has an important effect on the melting time duration.
4.7. Comparison to experimental results
4.7.1. Convection correlation for experimental set-up
The heat transfer coefficient between the surface of the capsule and the air stream is
an important parameter for estimating the heat exchange rate. Although the
distribution of the local Nusselt number is affected by the air pattern around the
sphere, Whitaker’s Nusselt number correlation86 is often employed in practice for
estimating the average heat transfer coefficient between a spherical surface and a
free stream (Equation 4-5). This correlation is valid for 3.5 < Rep < 7.6E4 and 0.7 < Pr
< 380 with the fluid properties evaluated at the free stream temperature T∞, except
for μs which is evaluated at surface temperature.
Equation 4-5
However, the experimental set-up is far from an air free stream around a sphere.
The air is expected to be affected by the blockage ratio, defined as the ratio between
diameters (Dsphere/Dtube). The effect of blockage ratio on the heat transfer for a
centrally located sphere in a pipe water flow (Prandlt=5.15) was investigated in
Krishnan et al.87 using CFD simulations. To validate the model the authors
Single Capsule Model
148
compared the CFD predicted transient Nusselt numbers at a low blockage ratio
(BR=0.02) with the steady classical solution of Kramers (1946) Equation 4-6
(1<Rep<2000).
Equation 4-6
The proposed correlation presented in Equation 4-7 considers the effect of the
blockage ratio on the Nusselt number. The equation fitted the data for
and where refers to Kramers solution (Equation 4-6).
Equation 4-7
In order to evaluate the different correlations the mass and volumetric flow rate
around the sphere in the experimental set-up is required. The mass flow rate is
calculated using the volumetric flow rate and the intermediate pressure, which
depends on the volumetric flow rate. The mass flow rate, at atmospheric pressure
and at different temperatures, determines the air velocity in the tube. The blockage
ratio in the set-up is BR = Dsphere /Dtube =0.625 (Dsphere =0.02 m and Dtube =0.032 m).
Table 4-10 and Figure 4-27 show the Nusselt number, the Reynolds number, and
the convective heat transfer coefficient using the proposed correlations. For
different volumetric flow rate and temperature, the standard deviation among the
three convective coefficients calculated with different correlation is lower than
2.7%.
149
Thermal Energy Storage for High Temperature Applications
Figure 4-27: Convective heat transfer coefficient vs. Reynolds number (at 300ºC)
Table 4-10: Nusselt number and convective heat transfer coefficient using different
correlations
Q
[LPM]
Tsphere
[ºC]
h Whitaker
[W/(m2K)]
h Kramer
[W/(m2K)]
h Krishnan
[W/(m2K)]
Nup
Whitake
Nup
Kramer
Nup
Krishnan
Rep vtube
[m·s-1]
40 250 41.01 40.22 40.4 19.82 19.44 19.53 947.1 1.99
40 300 42.71 41.92 42.12 19.23 18.87 18.97 888.6 2.18
40 350 44.35 43.55 43.77 18.72 18.39 18.48 838.9 2.371
50 250 48.23 47.03 47.21 23.31 22.73 22.81 1313 2.758
50 300 50.2 49.01 49.2 22.6 22.07 22.15 1232 3.022
50 350 52.09 50.91 51.12 22 21.5 21.58 1163 3.286
60 250 55.74 53.99 54.16 26.94 26.09 26.17 1748 3.672
30
35
40
45
50
55
60
65
70
75
80
500 1000 1500 2000 2500 3000
Co
nv
ecti
ve
hea
t tr
ansf
er
coef
fici
ent
[W·(
m2·K
)-1]
Re
h Whitaker
h Kramer
h Krishnan
Single Capsule Model
150
60 300 57.98 56.25 56.44 26.11 25.33 25.41 1640 4.024
60 350 60.15 58.43 58.62 25.4 24.67 24.75 1548 4.375
70 250 63.58 61.14 61.3 30.73 29.55 29.63 2259 4.747
70 300 66.12 63.7 63.87 29.77 28.68 28.76 2120 5.201
70 350 68.56 66.15 66.34 28.95 27.93 28.01 2001 5.655
80 250 71.77 68.5 68.65 34.69 33.11 33.18 2854 5.997
80 300 74.61 71.36 71.52 33.6 32.13 32.2 2678 6.571
80 350 77.35 74.1 74.28 32.66 31.29 31.36 2528 7.144
It is important to note the limits of the Krishnan correlation (Rep≤500 and
0.02≤BR≤0.5), and Kramers correlation (1<Rep<2000) and take into account that the
experimental set-up conditions are slightly above the limits of the proposed
correlations Rep ≈ (1000-1500), BR=0.625. In Whitaker’s correlation, for a
temperature difference of 100ºC between the HTF and the capsule surface, the term
(μ∞/μs)1/4 ≈ 1.03 in the worst-case scenario, which means that this 3% correction can
be neglected. Another difference is the use of water (Pr=5.12) in Krishnan´s
simulation instead of air (Pr=0.7) in the experimental set-up. This means that the
thermal diffusivity is low compared with the momentum diffusivity for water,
compared to the case of air, with a higher thermal diffusivity than momentum
diffusivity, but this should not affect the validity of the correlation.
4.7.2. Model vs. Experiments
There are certain limitations that have to be considered when comparing the
experimental results to the simulations. Some of them are described below:
The implemented model is one-dimensional which means that it might be
more appropriate to describe the freezing process of a uniformly heated and
completely filled capsule, because it is expected to show a radial evolution of
the freezing front. Whereas the fabricated capsules have a small void (to
manage volume expansion during melting) and are not perfectly spherical,
151
Thermal Energy Storage for High Temperature Applications
introducing spatial asymmetries in the melting process.
Only on a two-dimensional model the specific location of the capsule void
space can be considered. The simplest way to introduce a first approximation
correction for this empty space is by calculating an effective density of the
PCM-void system. It this sense, the equivalent completely filled PCM capsule
with the new density will store the same energy as the real capsule.
The set-up air conditions are slightly out of the validity limits of the heat
transfer convection coefficient correlations. Even so, they have been used
because these correlations are the closest to the experiments that have been
found in the literature.
The effect of the capsule holder is not considered, and it is expected to increase
slightly the turbulence of the air and the convective heat transfer coefficient.
Wall effects have also not been considered.
The thermocouples are not in contact with the capsule; therefore their
temperature might be slightly overestimated leading to higher temperatures in
the model than in the experimental set-up.
The melting process can be considered a two-dimensional problem as can be
seen from the video recording where the density differences are not negligible.
The solid fraction of the PCM sinks to the bottom of the capsule since the
density of solid PCM is higher than that of liquid PCM. These differences in
density might accelerate the real melting process, and they are not considered
in the implemented one-dimensional model.
There is another expected deviation based on the capsule position in the set-up.
The heat transfer fluid flows around the capsule from the top to bottom in a
channel. The highest rate of heat transfer should occur at the top. However,
over subsequent melting/freezing cycles, due to liquid/solid PCM density
differences and convective buoyant flows inside the capsule, the void space
moves to the top. The presence of a void located at the top of the capsule acts
as an insulator and is expected to slow down the heat flux into the PCM.
The melting process is expected to be affected by the difference in density between
the solid and liquid PCM, as commented before. On the other hand, the freezing
process is expected to behave more like a radial evolution, more similar to the
implemented one dimensional model. However, in the case of NaNO3 capsules,
Single Capsule Model
152
the phase change duration can be estimated only for melting experiments because
in the freezing experiments the opacity of the external layer of frozen salt, the lack
of thermocouple inside the capsule, and the challenges in seeing a clear
temperature trace with the IR camera do not allow to estimate the end time for
freezing experiments.
In contrast, the experimental melting and freezing start times can be clearly
measured and compared with their corresponding times in the one-dimensional
model. In this case we can expect the opposite behavior: the phase change start
time in the melting experiments are expected to behave closer to the model than the
phase change start time in the freezing experiments. The reason is the following: in
the melting experiments, the PCM starts in solid state and because of the low
thermal diffusivity of the NaNO3, the simulations indicate that finding thermal
differences of ~30 ºC between the shell-PCM interface (at R=R2) and center of the
capsule (at R=0) is not unexpected.
On the other hand, in freezing experiments the PCM starts in molten state. Based
on literature values, we have used the same thermal conductivity for solid and
liquid NaNO3. However, the convection in the liquid PCM inside the capsule (not
considered in the model), might reduce the thermal gradients compare to the solid
PCM case. Then, we will need to cool down the capsule to a lower temperature to
start the freezing process in our experiments and this process is expected to take
longer.
In order to have an order of magnitude of this effect we have simulated two
NaNO3-capsules with two different values of liquid thermal conductivity: 0.5 and 5
Wm-1K-1. For h=50 and 150 Wm-2K-1 the melting start time are 154 and 77 seconds
respectively. The increase of an order of magnitude in the thermal conductivity of
the liquid phase, trying to simulate the expected convection inside the capsule,
increases the melting start time by only ~7 seconds. This means that the effect
described can be neglected and it is not expected to change significantly the results.
4.7.2.1. Reproducing different PCM qualitative behavior
Figure 4-28 shows the experimental infrared temperature curves for a Pb-
Borosilicate capsule (blue) and a NaNO3-Borosilicate capsule (brown). In the
metallic capsule the complete phase change process can be distinguished by the
153
Thermal Energy Storage for High Temperature Applications
roughly isothermal segments. On the other hand, in a NaNO3-capsule only the
beginning of the phase change can be seen with the IR camera, identified by a
change in slope temperature history data. These qualitative differences are clearly
reproduced in the model by looking at the outer shell surface temperature (R=R1)
(Figure 4-22, Figure 4-29, Figure 4-30) and are a consequence of the thermal
resistance of the different PCM.
Figure 4-28: Experimental infrared melting and freezing curves for a metallic (Pb,
blue) and Salt (NaNO3, brown) borosilicate capsule
120
140
160
180
200
220
240
260
280
300
320
340
360
380
0 100 200 300 400 500
Tem
pera
ture
[ºC
]
Time [s]
Pb
NaNO3
PbNaNO3
Single Capsule Model
154
Figure 4-29: Melting (left) and freezing (right) comparison: experimental infrared
temperature history curves (blue) for a metallic borosilicate capsule compared to
the model results (brown)
Figure 4-30: Melting (left) and freezing (right) comparison: experimental infrared
temperature history curves (orange) for a NaNO3 borosilicate capsule compared to
the model results (blue and green)
140
160
180
200
220
240
260
280
300
320
340
360
0 50 100 150 200 250 300
Tem
per
atu
re [
ºC]
Time [s]
Pb experiment
Pb model
Melting (Model)
Melting(Experiment)
140
160
180
200
220
240
260
280
300
320
340
360
0 20 40 60 80 100
Tem
pera
ture
[ºC
]
Time [s]
Pb experiment
Pb modelFreezing(Model)
Freezing(Experiment)
50
70
90
110
130
150
170
190
210
230
250
270
290
310
330
350
0 100 200 300 400 500
Tem
per
atu
re [
ºC]
Time [s]
NaNO3 Experiment
T_HTF Model
T(R=R1) NaNO3 Model
T (R=0) NaNO3 Model
Start melting (Exp)
Start melting (Model)
50
70
90
110
130
150
170
190
210
230
250
270
290
310
330
350
250 350 450 550
Tem
pera
ture
[ºC
]
Time [s]
NaNO3 Experiment
T_HTF Model
T (R=R1) NaNO3 Model
T (R=0) NaNO3 Model
Start freezing (Exp)
Start freezing (Model)
155
Thermal Energy Storage for High Temperature Applications
Different qualitative and quantitative observations can be made:
Different types of PCM based on their thermal conductivity and diffusivity
present different behavior: metallic PCM barely have any temperature
gradients inside the capsule and the shell closely follows the core temperature;
but salt PCM exhibit large temperature gradients between the outer shell and
inner PCM core.
As perfectly reflected in the model, only the phase change process can be
distinguished in metallic PCM based on the capsule surface temperature
history data (Figure 4-29). In salt PCM the surface temperature can be only
used to estimate the melting and freezing start time, not the end time due to
the existence of large temperature gradients inside the PCM (Figure 4-30). This
explains why the IR camera temperature traces (corresponding to irradiation of
the surface temperature of the capsules) only show the complete melting
process in metallic PCM and not in salt PCM capsules.
The melting starting time is observed at higher temperature than the melting
temperature of each material. The IR camera data is the capsule surface
temperature. Because of the low thermal conductivity of the capsule, when the
PCM in contact with the capsule reaches the melting temperature, the capsule
surface temperature has increased (~+10ºC for our experimental conditions). In
spite of the model´s simplicity, it is able to reproduce this observation.
The same phenomenon is produced in the freezing experiments but in the
opposite direction (freezing will occur at lower temperatures than the PCM
melting temperature); consequently the temperature of the start melting and
start freezing observed with the infrared camera should differ by the sum of
these two ∆T.
4.7.2.2. Freezing Experiments
In the freezing experiments, only the freezing start time can be used to compare to
the model. This time is calculated in the model as the time when the shell-PCM
interface reaches the phase change temperature. On the other hand, this time has
been estimated in the video recording of the freezing experiments as the time when
a NaNO3 solid (opaque) layer is completely created, which can be also identified
Single Capsule Model
156
based on the infrared images. It means that this time is not affected by the phase
change process because it has not started yet.
As the parametric study concluded, the freezing start time depends mainly on the
dimensionless melting temperature (
) and the convective heat transfer
coefficient (h), being independent of the temperature step (∆T = Tf - To) applied.
However, analyzing the different volumetric flow rates used in the experiments
and their calculated convective heat transfer coefficient it seems that the convective
heat transfer coefficient may only vary between 40 and 60 Wm-2K-1. The vast
majority of the experiments have been performed with ~50 Wm-2K-1 (50LPM).
Figure 4-31 represents the estimated freezing start time vs. the dimensionless
melting temperature for the freezing experiments. Freezing start time increases
with the dimensionless phase change temperature as expected. Unfortunately, with
the experiments performed do not have a large difference in flow rates to extract
experimental trends with the heat transfer coefficient.
Figure 4-31: Experimental freezing start time vs. dimensionless phase
change temperature for freezing experiments
In order to compare with the experimental results, we can simulate the freezing
process for the NaNO3-Borosilicate capsule, using average values for the set-up
0
20
40
60
80
100
120
140
0.0 0.2 0.4 0.6 0.8
Fre
ezin
g S
tart
tim
e [s
]
Dimensionless melting temperature θm [-]
Exp h=40 W/(m2 K)
Exp h=50 W/(m2 K)
Exp h=60 W/(m2 K)
157
Thermal Energy Storage for High Temperature Applications
condition (∆T and τexp), simulating different dimensionless melting temperatures
and different convective heat transfer coefficients, as shown in Figure 4-32.
Figure 4-32: Model and experimental results comparison: Freezing start time vs
dimensionless phase change temperature for freezing experiments
As the model suggests, freezing start time increases with increasing dimensionless
phase change temperature and with higher heat transfer coefficients. As the
convective heat transfer coefficient increases, the differences in freezing start time
for different connective heat transfer coefficients are reduced. The experimental
results at 50 Wm-2K-1 show the same qualitative behavior as the model, fitting
reasonably well to a linear curve. However, the experimental times behave closer to
curves with a convective heat transfer coefficient h between 100 and 150 Wm-2K-1,
even though the estimated experimental convective heat transfer coefficient is
significantly lower (50 Wm-2K-1). These findings indicate that the calculated
convective heat transfer correlation might underestimate this coefficient. This
0
50
100
150
200
250
300
350
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Fre
ezin
g S
tart
tim
e [
s]
Dimensionless melting temperature θm [-]
Exp h=40 W/(m2 K) 40 LPM
Exp h=50 W/(m2 K) 50 LPM
Exp h=60 W/(m2 K) 60 LPM
Model h=50 W/(m2 K)
Model h=100 W/(m2 K)
Model h=150 W/(m2 K)
Single Capsule Model
158
deviation could be due to:
Inaccuracies measuring the volumetric flow rate
Higher heat transfer coefficient in the experiments due to turbulent flow, not
fully developed temperature profiles, higher heat losses than expected, the
presence of the capsule holder introducing any one of these effects, etc.
Underestimated experimental heat transfer coefficient due to simplifications
and the use of correlation out of its validity limits
Considering the number of experiments under the same conditions, the
inaccuracies/difficulties in measuring phase change times visually from a video
recording, and the simplicity of the model it is quite remarkable that the trend with
the dimensionless temperature can be experimentally reproduced even though the
heat transfer coefficient is underestimated.
4.7.2.3. Melting experiments
Similar to the freezing experiments, the melting start time has been represented vs.
the dimensionless melting temperature (Figure 4-33). The experiments show the
expected behavior of the effect of the convective heat transfer coefficient: lower heat
transfer coefficients tend to increase the melting start time, while higher convective
heat transfer coefficients will tend to start the melting process sooner. The trend
with the dimensionless melting temperature is also as expected: higher
lead to longer melting start times.
159
Thermal Energy Storage for High Temperature Applications
Figure 4-33: Experimental melting start time vs. dimensionless melting
temperature.
The simulations for different heat transfer coefficients have been superposed to the
experimental results (Figure 4-34). The general trends are reproduced: melting start
times increase with higher and lower h and there is a greater slope for lower
heat transfer coefficients. However, as before with the freezing experiments, the
calculated experimental hconv is underestimated especially for h=50 Wm-2K-1. We can
observe that the experiments with an estimated convective heat transfer coefficient
of ~40 Wm-2K-1 fit the simulated curve for h=50 Wm-2K-1. However, as in the
freezing experiments, the experiments with an estimated convective heat transfer
coefficient of ~50 Wm-2K-1 fit simulated curves with ‘h’ between 100 and 150 Wm-2K-
1. The experiments with an estimated h~60 Wm-2K-1 fit simulated curves with
higher ‘h>150 Wm-2K-1 ’.
0
20
40
60
80
100
120
140
160
180
200
0.0 0.2 0.4 0.6 0.8
Mel
tin
g S
tart
tim
e [s
]
Dimensionless melting temperature θm [-]
Exp h=40 W/(m2 K) 40 LPM
Exp h=50 W/(m2 K) 50 LPM
Exp h=60 W/(m2 K) 60 LPM
Single Capsule Model
160
Figure 4-34: Model and experimental results comparison: Melting start time vs.
dimensionless melting temperature.
Finally, the phase change start time for melting and freezing experiments can be
combined in a single graph (Figure 4-35). We observe a clear tendency and
alignment for the experiments performed with an estimated convective heat
transfer coefficient of ~50 Wm-2K-1 for both, melting and freezing experiments. This
shows the symmetry of the problem when comparing heating and cooling
experiments. Results for different heat transfer coefficients have not been
represented.
0
20
40
60
80
100
120
140
160
180
200
0.0 0.2 0.4 0.6 0.8
Melt
ing
Sta
rt t
ime [
s]
Dimensionless melting temperature θm [-]
Model h=50 W/(m2 K)
Model h=100 W/(m2 K)
Model h=150 W/(m2 K)
Exp h=40 W/(m2 K)
Exp h=50 W/(m2 K)
Exp h=60 W/(m2 K)
161
Thermal Energy Storage for High Temperature Applications
Figure 4-35: Experimental phase change start time vs. dimensionless phase
change temperature.
For melting experiments we can estimate the melting duration calculating the
difference between the melting start time (based on the infrared images and video
recording) and the end melting time (based on the video recording). Figure 4-36
represent the experimental melting duration vs. the temperature step applied in
each melting experiments for different dimensionless melting temperatures.
Different simulated curves have been included for the same dimensionless melting
temperature using 50 Wm-2K-1 as heat transfer fluid. Other convective heat transfer
coefficients have been also included as reference. As shown, the numerically
predicted melting time duration shows reasonably good agreement when
compared to the results obtained in the experiments for three of them (θm=0.23 &
h=40 and 2 out of 3 experiments of θm=0.29 & h=50) fitting perfectly to the
simulated curve. On the other hand, some experiments deviate from their
simulated curves. One of the experiments ‘θm=0.29 & h=50’ shows melting time
significantly longer than the simulation (+19%) and one of the experiments ‘θm=0.43
& h=50’ shows melting time significantly shorter than the simulation (-17.5%).
However, considering all the limitations exposed previously these deviations could
be considered reasonable.
The experiment ‘θm=0.59 & h=40’ would fit a simulated curve with a lower
convective heat transfer coefficient (h=30). It means that it shows longer time than
0
20
40
60
80
100
120
140
160
180
200
0.0 0.2 0.4 0.6 0.8 1.0
Ph
ase
ch
an
ge S
tart
tim
e [
s]
Dimensionless phase change temperature θm [-]
Freeze h=50 W/(m2 K)
Melt h=50 W/(m2 K)
Single Capsule Model
162
simulated with h=40. And the experiments with ‘h=60’ fit a simulated curve with a
higher convective heat transfer coefficient. This might indicate, as in the phase
change starting time analysis was pointed out, that the convective heat transfer
coefficient estimated for the experimental set-up underestimate the real convective
heat transfer coefficient.
Figure 4-36: Model and experimental results comparison: Melting duration time vs.
temperature step applied for different dimensionless melting temperatures and
convective heat transfer coefficients.
In Table 4-11 the experimental convective heat transfer coefficient that will make
the simulated curves fit the experimental melting start time and melting duration
has been estimated. The melting start times for all the melting experiments fit
curves with higher convective heat transfer coefficient. It means that the melting
start process seems to be faster than expected numerically and the experimental
convective heat transfer coefficient was underestimated. Experiments 2, 3 and 5
show faster melting start time than expected but they fit very well the melting
200
300
400
500
600
700
800
900
1000
40 60 80 100 120 140 160 180
Melt
ing
Du
rati
on
tim
e [
s]
∆T [ºC]
Model θm=0.60 & h=30
Exp θm=0.59 & h=40
Model θm=0.23 & h=40
Exp θm=0.23 & & h=40
Model θm=0.29 & h=50
Exp θm=0.29 & h=50
Model θm=0.43 & h=50
Exp θm=0.43 & h=50
Model θm=0.37 & h=140
Exp θm=0.34 & h=60
Exp θm=0.43 & h=60
163
Thermal Energy Storage for High Temperature Applications
duration time. Experiment 1 and 4 follow the same trend: they fit simulated curve
with a higher convective heat transfer coefficient than the experimentally estimated
value for the melting start time but the opposite for the melting duration.
The melting start time and melting duration for the experiment 6 is shorter than
simulated. Consequently the experiment fits simulated curve with higher
convective heat transfer coefficient. Finally, experiments 7 and 8 follow the same
trend, the PCM reaches its melting temperature in a significantly shorter time, and
the melting duration takes a moderate shorter time to finish. This means that
simulations with higher ‘h’ than the experimentally estimated would fit the
experimental results.
Table 4-11: Estimated experimental convective heat transfer coefficient that will
make the simulations fit the experimental times.
Melting Experiment Number
(estimated experimental ‘h’)
Melting start time (‘h’ that makes simulation
fit the experiments)
Melting duration (‘h’ that makes simulation
fit the experiments)
1 (h=40 Wm-2K-1) h↑ (~50 Wm-2K-1) h↓ (~30 Wm-2K-1)
2 (h=50 Wm-2K-1) h↑ (~100 Wm-2K-1) Ok
3 (h=50 Wm-2K-1) h↑ (~100 Wm-2K-1) Ok
4 (h=50 Wm-2K-1) h↑ (~100 Wm-2K-1) h↓
5 (h=40 Wm-2K-1) h↑ (~50 Wm-2K-1) Ok
6 (h=50 Wm-2K-1) h↑ (~100 Wm-2K-1) h↑
7 (h=60 Wm-2K-1) h↑ (~350 Wm-2K-1) h↑ (~140 Wm-2K-1)
8 (h=60 Wm-2K-1) h↑ (~350 Wm-2K-1) h↑ (~140 Wm-2K-1)
Single Capsule Model
164
4.8. Double PCM solution
A packed bed solution with steam as the heat transfer fluid requires a costly high
pressure vessel to contain the heat exchanger, increasing the system cost
dramatically. Alternative HTF-storage heat exchanger solutions were also
examined as part of this study. A more complex yet economical solution consisting
of a “double-PCM” system is proposed in order to avoid direct heat exchange of
the capsules with high steam pressure. Changing the heat exchanger surface to
tubes instead of a packed bed vessel can enable further cost reductions.
This solution, represented schematically in Figure 4-37, has been covered by the
Patent PCT/ES2015/070452-WO/2015/18945088. The invention relates to a thermal
storage system which includes a container (1) in which several components shown
in Figure 4-37 are arranged: a) a set of capsules (3) which form a porous bed and
contain a phase-change material having high energy density and consisting of
inorganic salts; and b) a matrix (2) which consists of a metal phase-change material
having high thermal conductivity and located in the interstices of the capsules (3).
The combination of two phase-change materials provides enhanced effective
conductivity and high energy density in the energy storage system. The invention
also relates to the method for charging and discharging said system by using a
heat-transfer fluid which flows through heat-exchange tubes (4) that pass through
the container (1).
The proposed innovative thermal energy storage system is composed by two
specific materials with a matching temperature in the range of the high pressure
Rankine cycles (150bar). These PCM are: a metallic MgZn alloy and chloride
quaternary salt mixture (LiCl-KCl-LiCO3-LiF) encapsulated and submerged on the
metallic alloy (properties can be seen in Table 4-12).
165
Thermal Energy Storage for High Temperature Applications
Figure 4-37: Double PCM TES solution
Table 4-12: Material properties
Material ρ [kg m-3] ∆Hf [J kg-1] Tm [ºC]
Mg(49)-Zn(51) 2850 155 342
LiCl-KCl-LiCO3-LiF 3584 375 340
The usage of two PCM materials with different nature (metallic and inorganic salts)
allows a combination of their properties and makes possible to increase the thermal
conductivity of the embodiment without losing energy density. The study of the
optimal fraction of each material in terms of conductivity, energy density and cost
as well as other design consideration as the capsule fabrication and materials, and
the experimental characterization of both mixtures is out of the scope of this thesis.
Single Capsule Model
166
4.9. Discussion and conclusion of encapsulated PCM as TES system
A new encapsulation method for high temperature Phase Change Materials (PCM)
is developed for Direct Steam Generation (DSG) applications in Solar Thermal
Power Plants. A solution consisting of borosilicate and NaNO3 as a shell and PCM
core respectively has been studied focusing on the thermal behavior of the PCM-
capsule system. The phase change properties of some inorganic salts and metal
alloys have been thermally characterized using conventional DSC. A novel
encapsulation procedure has been developed together with an experimental setup
aimed at analyzing through visual observation and with an infrared camera the
melting and freezing behavior of high temperature PCM. The infrared images have
been demonstrated as a feasible method to determine the complete phase change
process in metallic PCM –borosilicate shell capsules.
A one-dimensional finite difference heat transfer model simulates the phase
transition within a single capsule. The model has the objective of a) aiding and
guiding further PCM capsule designs and b) is used to validate the concept of
borosilicate shell capsules in the 300-400ºC range. Experimental conditions and
empirical correlations have been used to determine the average convective
coefficient around the sphere. This parameter and the measured capsule
surrounding air temperature have been used as a boundary condition in the model.
The melting start time, melting duration and freezing start time have been taken to
compare both experiments and simulations.
The comparison of the model to the experimental data can be performed thanks to
the transparency of the shell and the salt in liquid state, a property of the system,
which allows the study of the behavior of the capsules through visual observation,
being able to estimate the start and finish of the melting process in heating
experiments and the start of the freezing process in cooling experiments.
In spite of the many simplifications implemented, considering the complexity of
the moving boundary phase change problem plus the asymmetries of a partially
filled, almost spherical borosilicate capsule, the model accomplishes the proposed
goals reasonably well. It helps understand the effects of different parameters on the
phase change process of PCM-capsules such as the nature of the PCM (salt or
metal), the shell material nature (borosilicate or steel), the thickness and size of the
167
Thermal Energy Storage for High Temperature Applications
capsule; as well as the effect of experimental conditions such as the convective heat
transfer coefficient, the temperature step applied or the dimensionless melting
temperature.
As a summary, the key points are:
Using borosilicate as a shell material instead of encapsulating with metals will
not significantly change the melting times, even though the thermal resistance
of the shell wall does increase. For thin shells and within the range of the
parameters evaluated in this study, melting times are much more sensitive to
the capsule heat transfer with its surroundings, i.e. the convective heat transfer
coefficient values.
The PCM´s thermo-physical properties will have a large influence on the
temperature profiles inside the capsules and, consequently, on the information
which can be obtained by non-invasive measuring techniques such as real-time
imaging (visible and IR). Highly conductive metallic PCM clearly show the
phase change process on the surface, as the temperature is almost uniform
inside the capsules. Inorganic salts, on the other hand, do not show the
isothermal phase change process on the external layers as large temperature
gradients and high thermal inertia exist inside the capsules. The model is used
to corroborate these experimental findings.
The dimensionless melting temperature θm = (Tm-To)/(Tf-To) has a great effect
on melting times and durations, comparable to the effect of high heat transfer
coefficients.
Finally, the numerically predicted phase change start times and phase change
duration show reasonably good agreement when compared to the results
obtained in the experiments. Also, the qualitatively comparison between
different types PCM (metallic vs. salt) is perfectly aligned with the numerical
results. The convective heat transfer coefficients in the experiments are
underestimated, but this result is not surprising considering the limitations of
the correlations and the real flow and temperature distribution complexity in
the experiments.
Once the thermal problem of a single capsule was solved, an innovative solution
was proposed to avoid the existence of a large storage tank under pressure as well
as to address the mechanical integrity of the capsule exposed to high pressure. This
solution, successfully patented, consists of the use of a molten metal surrounding
Single Capsule Model
168
the borosilicate capsules. In the storage process the metal and capsules act as PCM,
storing energy as latent heat and the metal matrix increases the effective thermal
conductivity of the composite.
In conclusion, the main goal was to develop an encapsulation or storage system
with the following properties:
Enough strength and mechanical integrity to hold the PCM inside during
melting and solidification, accommodating the volume expansion during the
phase change process: several melting and freezing cycles have been
performed successfully on different partially filled PCM-capsules.
A non-porous shell to prevent any molten PCM leakage: no leakage has been
observed
Stable at high temperatures and continuous freeze/thaw thermal cycling: tested
successfully
A good thermal conductor to effectively transfer heat from the heat transfer
fluid (HTF) to the PCM: the study performed with the numerical model
indicates small differences between a metallic and a borosilicate shell.
A non-reactive shell material to the molten PCM
A non-reactive shell to the HTF (high temperature, high pressure steam)
Low cost material.
The remaining challenges are mainly economical: the capsule cost (material
and fabrication) and optimal heat exchanger configuration (to avoid costly
pressurized steam tanks).
169
5 NANO-ENHANCED HEAT
TRANSFER FLUID FOR THERMAL
ENERGY STORAGE
olten salts are commonly used as heat transfer fluids and thermal energy
storage media in concentrated solar power central receiver plants.
Systems of heliostats concentrate solar energy in the solar receiver. The
molten salt circulates through the receiver absorbing the energy as sensible heat,
increasing its temperature from 290 ºC to 565 ºC. The hot salt is stored in tanks to be
used later to heat water and produce steam in the steam generator, which is used to
produce electricity in a turbine. The amount of energy one is able to store in the
molten salt ‘Q’ is determined by:
Equation 5-1
Where ‘m’ is the amount of salt, ‘∆T’ is difference between the temperature of the
salt in the hot tank and the temperature of the salt in the cold tank; and ‘Cp’ is the
specific heat of the salt. Consequently, the specific heat is directly proportional to
the storage capacity. This means that any improvement in this property increases
the storage density, and it translates in an increase in the storage capacity. Other
M
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
170
approaches to increase the storage capacity involve increasing the maximum
operation temperature, determined by the thermal decomposition of the salt and/or
reducing the minimum working temperature, limited by the salt freezing
temperature.
The aim of this chapter is to increase the storage capacity of the current TES
systems based on two molten salt tanks. To do so, the addition of nanoparticles to
the base fluid is investigated. Choi in 199589 was the first to use the term
“nanofluid” to refer to this colloidal suspension of nanometer-scale particles in a
fluid. Since then, nanofluids have been extensively studied due to reports of
enhanced thermal and physical properties, such as thermal conductivity and heat
capacity. Small amount of nanoparticles dispersed in the storage media resulted in
important improvements for the thermal energy storage systems, showing a huge
potential to reduce costs in current and future solar thermal power plants.
Moreover, the ability to develop the technical know-how and capability to produce
nanofluids with customizable thermal properties would have a wide variety of
other engineering applications. The effect of nanoparticles in other thermal
properties of the inorganic salt such as the latent heat is also investigated.
5.1. Introduction
5.1.1. Background on Nano-enhanced HTF-TES materials
Romanin & Fereres90 compiled a large number of published experimental heat
capacity data for nanofluids. The authors tried to clarify the magnitude, nature, and
cases where an enhancement of specific heat capacity can be expected and potential
theories to explain the nano-modified specific heat capacity. The different trends
found in this meta-analysis (29 references) are summarized as follows:
1. Overall, water and organic based nanofluids do not show an increase of
the specific heat capacity with respect to the base fluid value; in fact,
adding any type of nanoparticle to these fluids decreases the specific heat
capacity. This effect is more noticeable as particle loading is increased.
2. There is substantial evidence that significant (>20%) heat capacity
enhancement is possible in the case of molten salts with nanoparticles, in
171
Thermal Energy Storage for High Temperature Applications
contrast to water based and organic based nanofluids.
3. The published data also shows that the majority of the reported
enhancement occurs at nanoparticle concentrations around 1% by mass. At
higher concentrations, there might be higher propensity for agglomeration
of the nanoparticles leading to the degradation of the thermal properties
and thermal performance.
Despite the large number of experimental data, only a few research groups have
been working on molten salt nanofluids to this date. The main groups currently
investigating the specific heat of nanofluids based on molten salts are presented in
Figure 5-1.
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
172
Figure 5-1. Main research groups analyzing molten salt nanofluids (main author
marked in red)
D BanerjeeSince 2009(All fluids)
(Texas A&M University)
D Shin (University of
Texas at Arlington) H Tiznobaik
B Jo N SinghS Jung H Kwak
R DevaradjaneJ SeoB Dudda
Ian C. Nelson & Ponnappan, Rengasamy
University of
Perugia (Italy)
2013 (Solar Salt)Chieruzzi, ManilaCerritelli, Gian FMiliozzi, AdioKenny, José M
2015 (Potassium Nitrate)Chieruzzi, ManilaMiliozzi, AdioCrescenzi, TommasoTorre, LuigiKenny, José M
UniversitatJaume I,
Castellon (Spain)
2014 (Solar Salt)Patricia Andreu-Cabedo, Rosa Mondragon,Leonor Hernandez,Raul Martinez-Cuenca,Luis CabedoJ Enrique Julia
National TsingHua
University (Taiwan)
National ChiaoTung
University (Taiwan)
2013 (Solar Salt )Lu MC, Huang CH
Univ. of Leeds & Univ. of Birmingham (UK)
2015 (Solar Salt)Mathew Lasfargue, Qiao Geng, Hui Cao, Yulong Ding
Taiwan
Texas A&M University
(USA)2012 (Solar Salt&Carbonate)Michael Schuller, Frank Little, Darren Malik, Matt Betts, Qian Shao, Jun Luo, Wan Zhong, Sandhya Shankar, AshwinPadmanaban
2015 (Solar Salt)Michael Schuller, QianShao, Thomas Lalk,
MScThesis:D. MalikM. Betts
PhD. Thesis:M. Lasfargue
ChinaXi’an Jiaotong
University
2015 (Carbonate)Y.B. TaoC.H. LinY.L. He
2014 (Hitec Salt)Ho MX, Pan C
PhD ThesisD. ShinH TiznobaikMSc Thesis:B. Dudda
National Renewable
Energy Laboratory Colorado
(USA)
2011 (Nitrate, PAO)Anne K. Starace, Judith C. Gomez, Jun Wang, Sulolit Pradhan, and Greg C. Glatzmaier
173
Thermal Energy Storage for High Temperature Applications
The Texas A&M University research group led by Professor D. Banerjee was the
first and more active group publishing specific heat enhancement of molten salt
based nanofluids. D. Banerjee participated in the first report of specific heat
capacity enhancement using Polyalphaolefin (PAO) as a based fluid in 2009
modified with exfoliated graphite (EG) nanoparticles.91 The specific heat capacity of
the nanofluid was found to be enhanced by 50% compared with PAO at 0.6 wt. %
nanoparticle concentration.
After this initial publication this group moved to a synthetic oil used in CSP plants
known as Therminol VP-192,93 and molten salt such as: chlorides (KCl-CaCl₂-LiCl)94
and BaCl2-NaCl-CaCl2-LiCl95, binary carbonates (Li2CO3-K2CO3 eutectic)96–103 , and
nitrates (NaNO3-KNO3 solar salt)104–106.
After Banerjees’ interesting initial results, Starace et al.107 tried to reproduce
Nelson’s original results with PAO modified with expanded graphite
nanoparticles. The authors re-tested this fluid as well as different combinations of
base fluids (ethylene glycol, water/eth. Glycol, Ca(NO3)2*4H2O, mineral oil) and
other nanoparticle types (fumed silica 20 and 40 nm, 50 nm SiO2, 100 nm Al2O3, 15
and 20 nm Fe@Fe3O4, 40 nm Bi, 100 nm aluminum nitride). However, they found
“no increase in heat capacity upon the addition of the particles larger than the
experimental error”. It is worth mentioning that even though 5-10% of the
measurements performed showed slightly higher Cp than 6% of the base fluid Cp,
the vast majority of their measurement (>90%) fall within +/-6% difference
compared to the base fluid, which the authors considered experimental error.
Despite this first unsuccessful attempt to reproduce Banerjee’s results the number
of publications of molten salt based nanofluids has increased year after year. Table
5-1 shows the number of publications on the specific heat of molten salt nanofluids
published since 2009 classified by base fluid regardless of the Cp result
(enhancement or not). The initial PAO and Therminol VP-1, a conventional heat
transfer fluid used in parabolic trough CSP plants, are also included. The
specific references are presented on Table 5-2.
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
174
Table 5-1. Number of publications on the specific heat capacity of high temperature
nanofluids.
Year 2009 2010 2011 2012 2013 2014 2015 2016
PAO 1
1
Therminol VP-1
2
Carbonates
4 3 2 3 2 5
Nitrates
1 2 3 4 3 1
Chlorides
1 1
Adding up these numbers, Figure 5-2 shows on which materials these different
research groups have been working on. Among the different high temperature
base fluids modified with nanoparticles, it is interesting to note that roughly 50% of
the references investigate carbonate salts (specifically the binary system Li2CO3-
K2CO3)
Figure 5-2. Experimental studies (39) on molten salt nanofluids measuring the
specific heat capacity of the liquid salt
22
19
14
2
PAO
Therminol VP-1
Carbonate
Nitrate
Chloride
175
Thermal Energy Storage for High Temperature Applications
A summary of these publications, with the nanoparticle concentration and specific
heat enhancement, is presented on Table 5-2.
Table 5-2. Review on molten salt nanofluids including the base fluid, nanoparticle
concentration and specific heat enhancement. Nitrate, when not specified, refers to
solar salt (NaNO3-KNO3 60-40 wt. %). Studies marked in blue correspond to
references that do not belong to Texas A&M University.
Year Base Fluid NP
[wt.%]
Enhancement
[%] Reference
2009 PAO 0.6 +34% Nelson et al. (2009)91
2011 PAO 1 +0% Starace et al.(2011)107
2010 Therminol VP-1 1 +5.4%
Shin et al. (2010)92
2010 Therminol VP-1 1 +5.41% Kwak et al. (2010)93
2010 Chlorides 1 +5% Shin & Banerjee (2010)94
2011 Chlorides 1 +14.5% Shin & Banerjee (2011b)95
2011 Nitrates 1 +20% Betts (2011)108
2012 Nitrates
Hitec XL 1 +34.6%
Devaradjane & Shin
(2012)109
2012 Nitrates 1 +25% Dudda & Shin (2012)104
2013 Nitrates 1 +28% Dudda & Shin (2013)105
2013 Nitrates 1 +22.4% Chieruzzi et al. (2013)110
2013 Nitrates 2 -3% Lu & Huang (2013)111
2014 Nitrates
(Li-Na-K)-NO3 1 +13% Seo & Shin (2014)112
2014 Nitrates 1 +25% Andreu-Cabedo et al.
(2014)113
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
176
2014 Nitrates
Hitec 0.063 +19.9% Ho &Pan (2014)114
2014 Nitrates 1 +10% Xiao et al. (2014)115
2015 Nitrates
KNO3 1 +6.1% Chieruzzi et al. (2015)116
2015 Nitrates 0.78 +30.6% Schuller et al. (2015)117
2015 Nitrates 0.1 +10.5% Lasfargues et al. (2015)118
2016 Nitrates
Hitec XL 1 +19%
Devaradjane & Shin
(2016)119
2010 Carbonate 1.5 +75% & +100% Shin & Banerjee (2010)96
2010 Carbonate 1 +22.4% & +17% Shin et al. (2010)92
2010 Carbonate 2.5 +14.6% Kwak et al.(2010)93
2010 Carbonate 1 +15.7% Jo & Banerjee (2010)120
2011 Carbonate 1 +24% Shin & Banerjee (2011a)97
2011 Carbonate 1 23% Shin & Banerjee (2011c)121
2011 Carbonate 1 +21% Jo & Banerjee (2011)122
2012 Carbonate 1 +28.4% Tiznobaik & Shin (2012a)123
2012 Carbonate 1 +83% & +20% Tiznobaik & Shin (2012b)124
2013 Carbonate 1 +26% Tiznobaik & Shin (2013b)98
2013 Carbonate 1.5 118-124% (zoneA)
0% (zoneB) Shin & Banerjee (2013)99
2013 Carbonate 1 +26% Tiznobaik & Shin (2013a)100
2014 Carbonate 1 +32% Shin & Banerjee (2014)101
2014 Carbonate 0.1 +57% Jo & Banerjee (2014)125
177
Thermal Energy Storage for High Temperature Applications
2015 Carbonate 1 22% Tiznobaik et al. (2015)103
2015 Carbonate 1 +15% (Solid) Shin & Banerjee (2015)102
2015 Carbonate 1 +22% Jo & Banerjee (2015a)126
2015 Carbonate 1 +29.3% Jo & Banerjee (2015b)127
2015 Carbonate 1.5 +18.6% Tao et al. (2015)128
The types of nanoparticles have not been included because nearly all enhancements
occur with a wide variety of nanoparticles. The different types of nanoparticles
used include different metal oxides: Al2O3, TiO2, CuO and SiO2 (in various forms);
and carbon based nanoparticles including carbon black (amorphous carbon), multi-
wall (MW) and single-wall (SW) carbon nanotubes (CNT), and graphite platelets or
exfoliated graphite. No significant trend was found when the enhancement was
represented vs. the normalized nanoparticle diameter or the particle specific
surface area.90
If we represent the different enhancements grouped by type of base fluid (Figure
5-3) we can observe that there are only two PAO references with contradictory
results. On the other hand, Therminol VP-1 shows a marginal +5.4% of
enhancement, considered within the experimental error in Starace et al.107.
Chloride salts are well known due to its hygroscopicity and corrosiveness, for this
reason they were discarded.
Finally, carbonate and nitrate base nanofluids seem to show higher potential Cp
enhancement compared to the other base fluids. Although carbonate salts have
shown the highest Cp enhancement among the different molten salts, its
applicability as HTF and TES in solar power plans seems complicated due to its
high melting temperature (near 500ºC).
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
178
Figure 5-3. Specific heat enhancement vs. Nanoparticle concentration.
Another observation is that most of the references on carbonate nanofluid (95%;
18/19) come from the same research group. On the contrary, the number of groups
investigating nitrate base mixtures modified with nanoparticles is larger. This
could be because of the immediate applicability that this technology would have in
today’s thermal storage in conventional solar thermal power plants which widely
use nitrate salt mixtures. Consequently, the nitrate mixture known as “solar salt”
has been the selected material of study.
5.1.1.1. Theories behind Cp enhancement
Shin and Banerjee95 introduced three independent mechanisms to explain the
observed enhancement of the specific heat capacity values, which are itemized as
follows:
-20%
0%
20%
40%
60%
80%
100%
120%
140%
0 0.5 1 1.5 2 2.5 3
Sp
ecif
ic H
eat
En
han
cem
en
t [%
]
Nanoparticle Concentration [wt. %]
PAO
Therminol VP-1
Chlorides
Nitrates
Carbonate
179
Thermal Energy Storage for High Temperature Applications
1. Enhanced specific heat of nanoparticle due to its reduced size (higher
specific surface area per unit mass)
This proposed mechanisms is based on some reports in the literature of
nanoparticle powders, such as Wang et al.129. In these studies nanoparticles show
larger heat capacities than their bulk counterparts. If nanoparticles have the same
crystal structure as their bulk counterparts, the nanoparticles have higher heat
capacities because of their much larger proportion of surface atoms. However, this
first mechanism was discarded 90,107 because an increase of the reported 25%
between the nanoparticle Cp compared to the heat capacity of the bulk material
would have a negligible effect on the nanofluid Cp for the small mass fractions
used in the literature. Then, the result in nanofluidCp/basefluidCp ratios should be
close to one, as they were measured in Starace et al.107 for mass fractions of around
1 wt. % and lower.
2. Enhanced thermal properties of a dense semi-solid liquid layer of
eutectic molecules formed on the nanoparticle surface.
This second proposed mechanism was again discarded in Starace et al.107. Although
it has been demonstrated with molecular dynamic simulations and experiments
that liquid metals Al–Al2O3 form atomic layers at the interface with a flat solid130,
the layering dissipates within 1 or 2 nm at most. This means that at small
nanoparticle loadings, the liquid fraction that forms this layer would be also small.
As a result, in order to induce a large change in the overall heat capacity of the
system: a) the layered structure would need to have a heat capacity several times
higher that of the bulk base fluid which is improbable because this layer is made
out of the base salt and nanopaticles; or b) the layered structure must be wider than
2 nm which contradicts the thickness shown experimental and numerically.
3. Additional heat storage in the interfacial interactions (interfacial thermal
resistance) between the condensed phase and the nanoparticle
lattice.95,121
Apart from these three initially postulated mechanisms to justify the specific heat
enhancement, in 2012 Banerjee’s group proposed several new mechanisms to
explain the same phenomenon.
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
180
4. Heat storage mechanisms due to chemical interactions between the
nanoparticle and the solvent phase.131
5. The formation of a percolation network composed of semi-solid layering
of salt eutectic induced by the presence of the nanoparticles in a
significant part of the bulk solvent phase. The percolation network
would form a “web” nanostructure that also “traps” the rest of the
nanoparticles in the percolation network.124,131
In 2013 the same group moved to needle like nano-structures instead of percolation
networks.99,100 They claim that the nanoparticles induce the formation of needle-like
structures in molten salts (Figure 5-4).
In 2014 Shin and Banerjee101 accepted that there is no direct contribution of
nanoparticles on enhanced specific heat capacity (Mechanism 1), therefore the
chain-like nanostructures formed is mainly responsible of this enhancement. The
same year Shin et al. (2014)132 distinguish between fractal-like fluid nanostructures
formed by nanoparticles or formed by the molten salt:
a)
b)
Figure 5-4. a) Fractal-like fluid nanostructures formed by nanoparticles in a
conventional nanofluid. b) Fractal-like fluid nanostructures formed by separated
base molten salts in a molten salt nanofluid.132
Nanoparticle
Fluid molecules
Aggregated nanoparticles nanoparticle
Ionic compound
Aggregated ionic compounds
181
Thermal Energy Storage for High Temperature Applications
Finally, Tiznobaik et al. in 2015103 proposed the existence of “secondary long range”
nanostructures which primarily dominate the level of enhancement of the specific
heat capacity values, which is less sensitive to the material composition of the
nanoparticle (for a similar size, shape and mass concentration of the nanoparticles).
Apart from all these mechanisms, Thoms (2012)133 postulated the possibility of a
reversible adsorption type interaction on the nanoparticle surface exceeding the
melting point of the base fluid. Two different desorption behaviors are
hypothesized which could be accompanied by a reduction on the latent heat
(Figure 5-5 I&II)
I II
Figure 5-5. Schematic representation of two possible predicted thermal behavior of
adsorbed layer in nanofluid: a) extended solid-liquid phase transition (left); b)
nanoporous substrate studies and experimental observation (right).133
Summarizing, since 2011 the TexasA&M group has postulated various different
mechanisms to justify the specific heat enhancement of nanofluids. It is worth to
mention that all the mechanisms proposed by the Texas A&M’s group are
supported mainly by SEM images, which are taken at room temperature (e.g.
solidified nanofluids). Consequently, there is no physical evidence that these
different “structures” exist at high temperature with the salts in molten state.
Finally, Thoms proposed the existence of a reversible adsorption layer around the
nanoparticle that could correlate specific heat with latent heat. Unfortunately, the
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
182
latent heat is not reported in any of the Texas A&M’s papers, even though the
phase change of the nanofluid is produced prior to in any Cp measurement in
molten state. This means that we cannot validate Thoms’ hypothesis using Texas
A&M’s results.
5.1.2. Impact of a Cp enhancement on a CSTP plant cost
The molten salt used is a mixture of salts composed by sodium nitrate (NaNO3)
and potassium nitrate (KNO3) in the proportion 60-40 wt. %. In a CSTP plant the
molten salt system consists essentially of:
Molten salt storage tanks
Solar receiver
Interconnecting pipes
Heat exchanger in the steam generator system
Cold and hot pumps for salt
Two types of salt inventory must be considered: active and inactive. The active salt
inventory is the salt in charge of guaranteeing that the turbine can supply the
nominal power during the nominal storage hours. For a 15h molten salt TES and
50MWe solar thermal power plant, the active salt inventory would be 134:
Equation 5-2
Where t is the thermal efficiency converting thermal energy in the molten salt to
electricity ( t ~42%); ~ 1.513 KJ/(Kg K) is the average specific heat capacity of the
salt in the temperature range (290ºC to 565ºC). The active storage would be
approximately ~15450 tons of salt.
The inactive volume consists of the salt volume required for filling all the
equipment: the vapor generation system (~0.5% of the active salt), pipes (~1.5% of
the active salt and the solar receiver (~1% of the active salt). There is also an inactive
volume in the storage tanks required due to the minimum submergence pump
height and the ~1 m operating minimum level of the tanks. Summarizing, the
inactive volume might represent ~10.5% of the active salt.
183
Thermal Energy Storage for High Temperature Applications
According to Konstantin et al.135 the TES system can represent ~9-10% of the total
cost of a CSP plant (including salt inventory, hot & cold tanks, foundations, pumps,
piping, NOx abatement system, melting system, and heat exchangers)
Quotes from different suppliers indicate the price of the raw KNO3 and NaNO3 to
be 860 $/ton and 444 $/ton. An additional 200 $/ton must be added as a transport
cost. This leads to ~810 $/ton of Solar Salt. Other reference values of the solar salt
suggest a price close to ~1 $/kg.12,136 Using the latest reference multiplied by the
total amount of salt required (active and inactive storage) results in a salt inventory
cost of ~17 M$.
As the salt inventory is about ~50% of TES cost (Figure 5-6) this would lead to a
TES cost of ~34 M$ and ~366 M$ of total CSTP plant cost, considering the TES
represents 9.3% of the total cost (Figure 5-7).
Figure 5-6. Break-Down TES cost of a 50MWe -TES 15h central receiver CSP
Plant.135
49%
17%2%
6%
13%
4% 9%Salt
Storage tanks
Insulation materials
Foundation
Heat Exchangers
Pumps
Balance of system
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
184
Figure 5-7. Break-Down cost of a 50MWe -TES 15h central receiver CSTP Plant.135
If we could develop a nano-HTF TES material with a specific heat enhancement of
25% respect to the molten salts, the active salt inventory would be reduced by 20%.
According to the literature review it seems reasonable to assume that this
enhancement could be achieved by dispersing 1wt. % of nanoparticles within the
salt. With these assumptions, the breakeven price that makes both salt inventories
cost the same is 23 $/kg of nanoparticles (assuming that this cost includes the
nanofluid synthesis process cost). This means that a first cost reduction in the
reference salt inventory could be achieved by finding nanoparticle suppliers with a
cheaper price than the breakeven price.
However, the salt inventory cost would not be the only saving, since the TES size
would also be reduced. Figure 5-8 compares the conventional CSTP plant (50MW,
15h TES) with the same system using a hypothetical nano-HTF TES. As Figure 5-8
shows, even without considering any cost reduction in the salt inventory, the
reduction in the TES size represents a TES cost reduction of 9% and 0.9% of the
total CSP cost.
3.8%
31.3%
16.2%
1.7%
9.3%
12.4%
5.7%
6.4%
8.0%
5.2% Site Preparation
Heliostat Field
Reveiver System
Tower
TES
Power Block
Balance of Plant
EPC Contractors Engineering
Contingencies
Owners Costs
185
Thermal Energy Storage for High Temperature Applications
Figure 5-8. Comparison between a conventional molten salt Tower (50MW, 15h
TES) CSP plant cost vs. the potential use of a nano-HTF TES with an increase of
25% of the base salt specific heat.
Both potential cost reductions (cost of the salt inventory and reduction in the TES
size) in a CSTP plant motivate the investigation of this technology and its potential
use.
5.1.3. Aim and objectives
The aim of this work is to explore and understand the mechanisms behind the
effect of the addition of nano-particles on the thermo-physical properties of nitrate
based salt mixtures. The specific objectives are:
a) Reproduce the different specific heat enhancements produced by
nanoparticles dispersed in solar salt (NaNO3-KNO3 60-40 wt. %)
b) Explore different synthesis procedures
c) Evaluate the effect of nanoparticles on other properties such as the phase
change characteristics
d) Evaluate the stability of nanofluid in molten state
CSPcost
TES
Saltcost Active
TEScost
TES 10% CSP cost
Inactive
Active
Salt Inventory
Othercosts
Salt 50% TES cost
InactiveNanoSaltcost
Othercosts
TEScost
TES
Other
Conventional CSP Nano-HTF TES CSP
Other
-9%
TES cost reduction of 9%
CSP cost reduction of 0.9%
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
186
5.2. Materials and Methods
The materials used to produce the nanofluids are: NaNO3 (Spectrum Chemical,
min 99% Crystal, Reagent, A.C.S); KNO3 (Spectrum Chemical P1345 125gr min 99%
Crystal, Reagent, A.C.S); and SiO2 (10 and 30 nm) from Meliorum Technologies
Nanomaterials, 620 Park Ave, Rochester, NY 14607 10gr).
5.2.1. Synthesis of nanofluids
The initial synthesis procedure to prepare the salt mixture and the nanofluid is the
two-step method described in Dudda & Shin (2013)105 and schematized in Figure
5-9. The salt components and the nanoparticles in the form of powder are pre-dried
for 1h at 200ºC on a hot plate to ensure the absence of moisture. The three dry
components (nanoparticles, NaNO3, and KNO3) are then weighed and mixed in a
flask in the appropriate proportion. Milli-Q water is added as a solvent. The
dilution used for the salt is 10 mL of water per 100 mg of salt mixture. The amount
of salt synthesized per batch is 200 mg. The flasks (vials) are then sonicated for 200
minutes (Branson 3510, Branson Ultrasonics Co.) and the well-dispersed mixture is
evaporated on a hot plate at 200ºC inside a laboratory hood for 5-6 hours (Figure 5-
10), until the solvent water evaporates completely, leaving a powder-form
nanofluid (Figure 5-11).
The evaporated salt from the vial is scratched and mixed. The different vials with
salt are closed, hermetically sealed with paraffin and kept in a desiccator. Before
opening the vials for testing they are placed on a hot plate at 200 ºC for 30 minutes.
Then, the vial is opened and dried for another 30 min before testing.
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Thermal Energy Storage for High Temperature Applications
Figure 5-9. Schematic representation of the synthesis method
Figure 5-10. Neat and nanofluid solar salt water solution after sonication, and flasks
during solvent evaporation
Weigh & mix
components:
nanoparticles
salt
Add distilled
water
Sonicate
~200 min
Evaporate water in vial 5-6 h @ 200ºC Preheat 1 h @
200ºC
Prepare sample in crucible
Measure in Differential
Scanning Calorimeter
(DSC)
NaNO3
KNO3
SiO2
+ Water
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
188
Figure 5-11. Powder-form nanofluid (1% SiO2 10nm) after synthesis, before and
after scratching ready for testing
5.2.2. DSC Measurements
The DSC Q20 and Q2000 from TA instruments have been used for the differential
scanning calorimeter analysis. The calibration procedure is performed by using the
melting temperatures and latent heat of standard certified reference materials (In,
Zn), at a heating rate of 10 ºC·min-1 (for the cell constant calibration).
In order to test the salt samples 11-18 mg are introduced in Tzero standard
aluminum crucibles. Modulated DSC is used for the specific heat capacity
measurement. The following thermal cycle is applied to the samples: the initial
temperature is 80 ºC; the samples are heated up from 80 ºC to 420 ºC at 20 ºC/min to
pre-melt the sample as recommended in Boettinger et al.137 for powder samples.
Then the sample is cooled down to 150 ºC at the same heating rate. At this point,
the modulation starts with the temperature amplitude of 0.5 ºC every minute at a
heating rate of 5 ºC/min up to 420 ºC, recommended by the equipment
manufacturer (TA instruments). Two minutes isothermal segments are added
before each dynamic segment. Nitrogen is used as inert gas during the thermal
program (50 ml/min).
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Thermal Energy Storage for High Temperature Applications
Figure 5-12. Schematic showing onset and peak temperatures, width at half peak
height, and latent heat as determined by the DSC tests.
The melting and freezing onset temperatures are estimated as the intersection point
between the baseline connecting the points before and after the transition and the
tangent at the point of largest slope on the heat flow DSC curve (Figure 5-12). The
latent heats of phase change are determined by numerical integration of the area
under the peaks. The temperature at which heat flow during phase changes
reaches the maximum (peak temperature) is also reported as representatives of the
phase change heat flow curve.
5.2.2.1. Measurement errors and statistical analysis
The different specific heat, latent heat, and onset temperatures are presented within
a confidence interval at 90% (α=0.1). Minimum and maximum values of each
measurement are calculated according to Equation 5-3 (exemplified for Cp)
Equation 5-3
Peak T heating
Hea
t in
(h
eati
ng
)H
eat o
ut
(co
oli
ng
)
Cooling
Heating
Peak T cooling
Width at½ height
Width at½ height
Onset T heating
Onset T cooling
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
190
Being tα/2,n-1 the value of the t-Student for the sample and selected confidence
interval, s the standard deviation of the data and n the number of repetitions of the
measurement.
In order to verify that the base fluid and nanofluids have different average Cp
values (latent heat, onset temperatures, etc.) a hypothesis test is performed:
Equation 5-4
The sub index 1 and 2 refer to the nanofluid and base fluid respectively, s the
standard deviation of the data and n the number of samples measured. This
statistic is compared with the value of the t-Student (tα/2, n1+n2-2) for selected α (0.1)
and the degrees of freedom equal to the total number of measurements minus 2.
The rejection of the null hypothesis (equal mean values) is done when |ttest| > tα/2,
n1+n2-2. It assumes that the samples are independent, from normal populations and
with equal variance.
5.2.2.2. Sapphire correction
The modulation cell constant (K, typically close to 1), which is the ratio between the
true-certified specific heat value of the reference material (sapphire disc) and its
measured value is used to correct the specific heat capacity measurement of the salt
sample:
Equation 5-5
Equation 5-6
The TA software allows the introduction of one single cell constant value in order
to correct subsequent measurements. For measurements performed on a
temperature spam of 50ºC this correction has been found to be accurate enough.
However, the use of a single value correction might be not appropriate for
measurement performed on a temperature range higher than 100º C. Figure 5-13
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Thermal Energy Storage for High Temperature Applications
shows an example of how correcting by a fixed cell constant over a temperature
range might affect the results.
Figure 5-13. Specific heat of sapphire: theoretical (blue), measured (red) and
corrected (green) for the temperature range 100 - 250 ºC
In Figure 5-13 the blue line is the theoretical Cp value of sapphire and the red line is
the measured raw Cp value for the sapphire reference disc. To obtain the reversing
Cp constant in the temperature range 100 - 250 ºC, the ratio between the theoretical
Cp and the measured one is calculated and averaged in this temperature range.
This single value, required by the TA software, is used to correct subsequent
measurements. If we correct the sapphire measured using this value, the green line
is obtained, which might differ in the limits of the temperature range from the real
sapphire value even though it has been corrected. In this specific example, the
maximum deviations are -3.5% (at 100 ºC) and +3.2% (at 260 ºC). On the other hand,
if the raw data is corrected by a temperature dependent MDSC constant we do not
introduce any additional uncertainty on the measured data correction.
0.6
0.7
0.8
0.9
1
1.1
1.2
100 150 200 250 300 350 400
Sp
ecif
ic H
eat
[J/(
g K
)]
Temperature [ºC]
Theoretical
RAW Measure
Corrected Measure
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
192
5.2.2.3. Other measurement considerations
Along this investigation different uncertainty sources in the DSC measurements
have been identified. Based on the lessons learned measuring the specific heat of
nitrate salts some difficulties and recommendations are highlighted regarding both
equipment and sample (nature and preparation) in order to obtain reliable
measurements.
Inherent to the material (molten salts):
Salt behavior: salts creep in the crucibles. Figure 5-14 shows nitrate salt
inside standard aluminum DSC crucibles after testing. It is observed that
the salt tends to move towards the crucible borders when it is melted. This
behavior is an important source of uncertainty and can lead to DSC sensor
contamination.
Figure 5-14. Position of nitrate salt inside the aluminum crucibles after testing in the
DSC: salts creep up the crucible walls away from the base center.
Moisture control: salts are tremendously hygroscopic, thus the ambient
moisture can affect the results. Preheating samples, working in a dry
environment or even readjusting sample mass to account for excess
moisture is recommended.
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Thermal Energy Storage for High Temperature Applications
The flatness of the reference and sample crucibles is extremely important
for Cp measurements. For example, Figure 5-15 shows, for some type of
crucibles, how the solidification of KNO3 curves the bottom of the
crucibles.
Figure 5-15 Standard 40μl aluminum crucible after testing KNO3
Inherent to the equipment:
Crucible selection: certain equipment requires testing with a pin hole in the
lid of the crucible. However, hermetically sealed crucibles are preferred to
minimize ambient moisture and salt creeping effects. Absorbed moisture
during sample preparation can increase the pressure inside the crucibles
and in some cases (depending on crucible type) can lead to sudden
crucible bulging or even rupture during testing. On the other hand, the salt
creeping behavior might result in salt leaving the crucible when pin-holed
crucibles are used. Both phenomena might result on salt spilling over the
sensor affecting both the equipment and measurement results.
For Cp measurements the flatness of the reference and sapphire crucibles
is very important. The use of pin-holed crucible is recommended to
guarantee the shape of the crucible.
Crucible geometry: shape and size is limited by the equipment
manufacturer, but certain geometries seem more prone to salt creeping
and spilling.
If the Cp is to be measured over a temperature range, the sapphire
correction, as mentioned above, must be performed using a temperature
dependent reversing modulated DSC constant by selecting the default cell
constant to 1 and manually correcting the measured data for each
temperature.
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
194
Sapphire uncertainty: it is recommended to run reference materials, not
only sapphire but base fluid samples to guarantee that any effect is due to
the sample and not the equipment.
5.3. Specific heat of nitrate base nanofluids
5.3.1. Results
The initial nanofluid synthesized is solar salt (SS, NaNO3-KNO3 60-40 wt. %) with
1% of SiO2 (10 nm diameter) nanoparticles. Two batches of neat solar salt (without
nanoparticles) and two batches of nanofluid are synthesized following the
procedure described above; testing 3-4 crucibles from each batch (~8 samples in
total). By following the same procedure with the neat salt we try to eliminate any
effect the synthesis process might have on the salt.
Figure 5-16. Specific heat of neat and nanofluid solar salt in solid and liquid phase.
0
2
4
6
8
10
12
190 210 230 250 270 290 310 330 350 370 390
Re
v C
p [
J/(g
K)]
Temperature [ºC]
neat_SS
nf_ss_SiO2(10nm)
1
1.2
1.4
1.6
1.8
2
190 230 270 310 350 390
Re
v C
p [
J/(g
K)]
Temperature [ºC]
195
Thermal Energy Storage for High Temperature Applications
A typical Cp curve can be seen in Figure 5-16 for two samples tested. The peak
indicates the solid-liquid transition. The specific heat capacity of the salts in both
solid (to the left of the peak) and liquid phase (to the right of the peak) is roughly
constant or slightly increasing with temperature. The average specific heat capacity
results of the solar salt base nanofluids and the neat solar salt for specific
temperatures are shown in Figure 5-17.
Figure 5-17. Average specific heat of neat solar salt vs. solar salt nanofluid (10 and
30 nm SiO2 nanoparticles at a concentration of 1 wt %)
Nanofluids with different particle size were tested. The nanofluid with 10 nm SiO2
particles does not show enhancement in the temperature range (260 – 400 ºC) of
study (Figure 5-17). The average Cp values match the average base solar salt Cp,
although according to Dudda’s results105 a +13% specific heat enhancement on
average could be expected for this particular salt+nanoparticle combination. The
authors also reported that a higher enhancement can be expected (+21% on
average) using 1% of 30 nm SiO2 nanoparticles, which were tested in the following
batches.
The initial results for this nanofluid (1% of SiO2 30 nm) were promising. They
showed little enhancement. However, as the number of samples increased (28
1.00
1.10
1.20
1.30
1.40
1.50
1.60
1.70
1.80
1.90
2.00
250 275 300 325 350 375 400
Sp
ecif
ic H
eat
[J/(
g·K
)]
Temperature [ºC]
Neat SS (Vial)
NF SS 1% SiO2(10nm) (Vial)
NF SS 1% SiO2(30nm) (Vial)
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
196
crucible tested from 6 different batches) the average Cp also fell into the confidence
interval for the base salt as can be seen in Figure 5-17. Based on the hypothesis test
(“the average Cp value is different”), we cannot reject the null hypothesis of the
equality of the two means as shown by the results presented in Table 5-3.
Table 5-3. Hypothesis test between the base solar salt and solar salt nanofluid (1 wt.
% 30 nm SiO2 nanoparticles)
Temperature (260ºC) Neat SS (Vial) NF SS 1% SiO2 (30nm) (Vial)
Avg Cp [J/ (g K)] 1.44 1.50
Std Cp [J/ (g K)] 0.11 0.21
Number of samples 8 28
Confidence Interval (90%) 1.37 – 1.51 1.43 – 1.57
This indicates that the maximum difference of +4.1% on average between neat salt
and the nanofluid (30 nm SiO2) observed at 260 ºC does not represent a statistically
significant difference between the mean value of the base fluid and the nanofluid.
Moreover, this enhancement seems to disappear at higher temperatures.
The latent heat of the different fluids is shown in Figure 5-18. The latent heat of
nanofluids is expected to decrease linearly on a per-mass basis as particles not
contributing to phase change are added to the base fluid. Since both nanofluids
contain the same mass fraction of nanoparticles (1 wt. % regardless of the
nanoparticle size) one would expect the same reduction of the latent heat for both.
An average reduction on the latent heat of nanofluid is observed for both types of
nanoparticles with no significant difference among them. There are no differences
between the melting and freezing latent heat. The confidence interval is smaller for
the 30 nm nanofluid because a larger number of samples tested.
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Thermal Energy Storage for High Temperature Applications
Figure 5-18. Freezing (left) and melting (right) average latent heat of the neat salt
and nanofluids synthesized. Measurements performed at 20 ºC/min and 5 ºC/min
respectively.
The melting and freezing onset and peak temperatures are shown in Table 5-4. No
significant difference is observed between the temperatures of solar salt and
nanofluids. The temperatures include the confidence interval (90%).
Table 5-4. Onset and peak melting and freezing temperatures
Temperatures [ºC] Neat SS (Vial) NF SS 1%SiO2
(10nm) (Vial)
NF SS 1%SiO2
(30nm) (Vial)
Onset (melting) 216.7 +/- 1.2 216.3 +/- 0.8 216.3 +/- 0.4
Peak (melting) 224.8 +/- 0.3 224.2 +/- 0.4 224.3 +/- 0.2
Onset (freezing) 232.9 +/- 1.3 232.8 +/- 1.8 233.9 +/- 1.2
Peak (freezing) 218.5 +/- 0.6 217.6 +/- 0.7 218.1 +/- 0.3
At this point we decided to explore other nanoparticle concentrations and other
base fluid compositions within the same phase diagram Figure 5-19 and Figure
5-20 show the effect of 1% of SiO2 (10 nm) nanoparticles on different base fluid
compositions. There is not significant effect of the nanoparticles regardless the
composition. The confidence interval is slightly higher for the nanofluids compared
to the neat salts. This is a consequence of a larger variability of the measurements of
the nanofluid and higher number of samples tested for the base salt.
100
105
110
115
120
125
130
Lat
ent
Hea
t (f
reez
ing
) [J
/g] Neat SS (Vial)
NF SS 1% SiO2(10nm) (Vial)
NF SS 1% SiO2(30nm) (Vial)
100
105
110
115
120
125
130
Lat
ent
Hea
t (m
elti
ng
) [J
/g] Neat SS (Vial)
NF SS 1% SiO2(10nm) (Vial)
NF SS 1% SiO2(30nm) (Vial)
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
198
Figure 5-19. Effect of 1% of SiO2 (10 nm diameter) nanoparticles on the specific heat
(average 260 – 400ºC for mixtures and 350 – 400ºC for pure components) for
different base fluid mixture compositions.
Figure 5-20. Neat and nanofluid (1% SiO2 10 nm in diameter) specific heat heat
(average over 260 – 400ºC for mixtures and 350 – 400ºC for pure components) vs.
base fluid composition (NaNO3 wt %) in a NaNO3-KNO3 mixture.
1.20
1.30
1.40
1.50
1.60
1.70
1.80
0% 1%
Sp
ecif
ic H
eat
[J/(
g·K
)]
Concentration of SiO2 (10 nm) [wt. %]
KNO3 Na-KNO3(45-55wt%) Na-KNO3(60-40wt%) NaNO3
1.20
1.30
1.40
1.50
1.60
1.70
1.80
0% 20% 40% 60% 80% 100%
Sp
ecif
ic H
eat
[J/(
g·K
)]
NaNO3 (wt %) on a NaNO3-KNO3 mixture
Neat Nanofluid
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Thermal Energy Storage for High Temperature Applications
The specific heat of the different base salt mixtures depends on the composition. A
higher Cp for sodium rich mixtures and lower Cp for potassium rich mixtures
could be expected. The different composition seems to follow the weight average
mixing rule between the pure components. Some of the composition deviates
slightly from the expected Cp based on its composition, although inside the
experimental error. The specific heat of the different compositions tested does not
show any statistically significant differences between the neat salt and nanofluid
for any of the compositions tested. The specific heat values are shown in Table 5-5
with their confidence interval.
Table 5-5 Specific heat of neat salt and nanofluid (1 wt. % SiO2 10 nm
nanoparticles) modifying the base fluid (average over 260 – 400ºC for mixtures and
350 – 400ºC for pure components)
Specific heat [J/(g K)]
SiO2 (10 nm) concentration 0% 1%
KNO3 1.32 +/- 0.03 1.34 +/- 0.07
Na-KNO3 (30-70 wt. %) 1.42 +/- 0.03 1.43 +/- 0.05
Na-KNO3 (45-55 wt. %) 1.41 +/- 0.10 1.40 +/- 0.11
Na-KNO3 (60-40 wt. %) 1.47 +/- 0.07 1.47 +/- 0.12
NaNO3 1.60 +/- 0.05 1.58 +/- 0.11
Different SiO2 (10 nm) nanoparticle concentrations (0 - 1 - 3 - 5 -10 wt. %) have been
tested using Na-KNO3 60-40 wt. % (solar salt) and 30-70 wt. %. The results are
shown in Figure 5-21. A reduction on the Cp is observed with increasing
nanoparticle concentration. A larger dispersion in results is also seen with
increasing nanoparticle loading. None of the fluids tested show anomalous Cp
enhancement for the different nanoparticle concentrations tested.
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
200
Figure 5-21. Specific heat capacity vs. nanoparticle concentration by mass (10 nm
diameter SiO2 nanoparticles) for two different base fluids Na-KNO3 (60-40 wt. %,
solar salt) and Na-KNO3 (30-70 wt. %) (at 400ºC)
Figure 5-22 shows a similar study using 20-60 nm SiO2 nanoparticles. The lower
number of samples in this study is translated into larger confidence intervals.
Again no Cp enhancement is observed for the different nanoparticle concentrations
tested.
1.20
1.30
1.40
1.50
1.60
1.70
1.80
0% 1% 2% 3% 4% 5% 6% 7% 8% 9% 10%
Sp
ecif
ic H
eat
[J/(
g·K
)]
Concentration of SiO2 (10 nm)
Na-KNO3 (60-40 wt.)
Na-KNO3 (30-70 wt.)
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Thermal Energy Storage for High Temperature Applications
Figure 5-22. Na-KNO3 (60-40 wt. %, solar salt) neat and nanofluid, specific heat for
different SiO2 (20 - 60 nm in diameter) nanoparticles (0 – 0.5 – 1 – 3.21 – 5.35 wt. %)
at 400 ºC.
Note that the confidence intervals of the different nanofluis are larger because a
low number of samples (3-4) for each nanofluid have been tested.
Finally, the eutectic Na-KNO3 composition (45-55 wt. %) has been modified with
different types of nanoparticles: CuO (Io-li-tec nanomaterials, 40-80 nm, 99.9%) and
Al2O3 (Aldrich Chemistry, 13 nm, 99.8%). The specific heat results are shown in
Figure 5-23. The specific heat of CuO and Al2O3 nanofluids are 1.50 +/- 0.09 J/ (g K)
and 1.45 +/- 0.05 J/ (g K) compared to 1.41 +/- 0.10 J/ (g K) of the base fluid.
1.20
1.30
1.40
1.50
1.60
1.70
1.80
0% 1% 2% 3% 4% 5% 6%
Sp
ecif
ic h
eat
[J/(
g K
)]
Concentration of SiO2 (20-60 nm)
Solar Salt
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
202
Figure 5-23. Effect of 1 wt. % of nanoparticles (CuO and Al2O3) on the specficic heat
of the eutectic NaNO3-KNO3 (45-55 wt. %) (average over 260 – 400ºC)
A not-statistically significant enhancement of +5.9 % and +2.6% are observed for
CuO and Al2O3 nanofluid respectively.
5.3.1.1. Scanning Electron Microscopy Characterization
The characterization of several samples has been performed using Scanning
Electron Microscopy (SEM) in order to analyzed structural changes in the
nanofluids produced by the addition of nanoparticles to the base salt. In SEM
measurements, a focused electron beam is scanned over the surface of the sample.
When the electrons strike it, different interactions can occur: emission of
backscattered electrons (BE), auger electrons (electron bombarded 2-50 keV),
secondary electrons (SE) and X-rays. All these signals can be detected revealing
morphology, chemical composition or microstructure information of the sample.
1.20
1.30
1.40
1.50
1.60
1.70
1.80
0% 1% CuO
(40-80 nm)
1% Al2O3
(13 nm)
Sp
ecif
ic H
eat
[J/(
g K
)]
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Thermal Energy Storage for High Temperature Applications
A Hitachi S5200 SEM equipped with a field emission gun (FEG), located at
“Instituto de Ciencia de Materiales de Sevilla” (ICMS) has been employed in this
study to analyze the microstructure of base salt and nanofluid samples (2 kV and 7
mm of working distance). The microscope is dotted with an energy dispersive X-
ray (EDX) spectroscopy Bruker X Flash Detector 4010 which allows quantifying the
composition. The EDX analysis has been performed at 10kV.
The microstructure of the neat salt NaNO3-KNO3 (30-70 wt. %) and nanofluids with
1 wt. % and 5 wt. % of SiO2 nanoparticles (10 nm in diameter) are shown in Figure
5-24 to Figure 5-28. As Kramer et al. indicated, a tendency towards clustering of
similar ions in the solid solutions can be observed. The flat solidus boundaries
between solid and liquid solutions (a horizontal solidus, observed in the phase
change diagram Figure 5-40) may indicate a eutectic with limited solid solution.138
Figure 5-24. SEM (left) and SEM-EDS analysis (right) of the neat NaNO3-KNO3 (30-
70 wt. %)
This composition shows well defined sodium and potassium crystals. The structure
of the nanofluid (1 wt. % of SiO2, Figure 5-25) is similar to the base salt, with
identified well-dispersed silica nanoparticles.
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
204
Figure 5-25. SEM (left) and SEM-EDS analysis (right) of the nanofluid 1% SiO2 (10
nm) NaNO3-KNO3 (30-70 wt. %)
Figure 5-26. SEM images of the nanofluid 1% SiO2 (10 nm) NaNO3-KNO3 (30-70 wt.
%)
The addition of nanoparticles to the base salt does not modify its appearance. The
silica nanoparticles seem to be well dispersed and do not show clusters (Figure
5-26). However, Figure 5-27 and Figure 5-28 show that significant clusters are
formed for the same base salt composition with 5 wt. % of SiO2 nanoparticles.
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Thermal Energy Storage for High Temperature Applications
Figure 5-27. SEM (left) and SEM-EDS analysis (right) of the nanofluid NaNO3-
KNO3 (30-70 wt. %) with 5% SiO2 (10 nm)
Figure 5-28. SEM images of the nanofluid NaNO3-KNO3 (30-70 wt. %) with 5% SiO2
(10 nm)
From the SEM analysis, it is observed that while the NaNO3-KNO3 (30-70 wt. %)
nanomaterial with 5 wt. % of SiO2 contains significant amounts of agglomerated
nanoparticles (Figure 5-28), the nanofluid with 1 wt. % of SiO2 shows
homogeneously dispersed nanoparticles.
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
206
The eutectic NaNO3-KNO3 (45-55 wt. %) nanofluid with 1wt. % of CuO
nanoparticles is shown in Figure 5-29. The sample, after being tested in the DSC
shows homogeneous nanoparticles dispersion, similar to the silica nanofluid in
Figure 5-25. Higher proportion of sodium crystals is also observed due to its higher
content in this specific base salt.
Figure 5-29. SEM (left) and SEM-EDS analysis (right) of the nanofluid eutectic
NaNO3-KNO3 (45-55 wt. %) with 1% CuO
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Thermal Energy Storage for High Temperature Applications
5.3.2. Discussion
In the classical limit, where size effects and surface effects are neglected and the
nanoparticles and the surrounding fluid are in thermodynamic equilibrium, the
specific heat of a mixture would be a weighted average of the two materials. The
Cp of a nanofluid on a mass basis, in the classical limit, is thus given by Equation
5-7.
Equation 5-7
The specific heat of different common nanoparticles and the specific heat values of
the base salt used in this study are shown in Table 5-6 and Table 5-7 respectively.
Table 5-6. Properties of common nanoparticle materials (bulk properties).90
Material Specific Heat Capacity
[J/(g K)]
Density
[kg/m3]
Al2O3 0.88 3950
SiO2 0.68 - 0.73 2650
CuO 0.53 6310
If the bulk material properties are used, adding any of the materials listed in Table
5-6 (Cp < 1J/gK) to molten salts as a base fluid ( Cpliquid 1.5 J/gK) will decrease the
Cp of the resulting nanofluid according to Equation 5-7. If small particle loadings
are considered, the effective heat capacity should not change and remain close to its
base fluid value.
However, the specific heat of the nanoparticles themselves can change with respect
to their bulk material values. Several authors have attempted to measure the
nanoscale effect on solid Cp. An increase of about 25% with respect to its bulk
value seems like an upper limit of this potential enhancement.90 Small particles
have a large surface area, providing a higher number of surface atoms and
vibrational modes available. If we consider the specific heat of the nanoparticle
with the lowest specific heat and a possible increase due to nanoscale effects of up
to 25% (CuO Cp = 0.53 J/(g K)*1.25) and the specific heat of solar salt (1.47 J/(g K)),
the expected nanofluid Cp using Equation 5-7 would be 1.462, 1.454 and 1.446 J/(g
K) for 1, 2 and 3 wt. % of nanoparticles. The calculated nanofluid Cp compared to
the confidence intervals for the mean value of the base salt suggest that we would
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
208
not be able to measure significant differences between the base salt and nanofluids
because the predicted change in Cp is lower than the uncertainty in the
measurements.
Six similar studies also used nitrates based nanofluid in their research, specifically
solar salt, and the two step nanofluid synthesis procedure. Their measured specific
heat of solar salt is shown in Table 5-7.
Table 5-7. Solar salt specific heat measurement in molten state by different authors.
(two step nanofluid synthesis procedure)
Solar Salt
Cp [J/(g K)] Enhancement [%]
Latent
Heat [J/g]
Dudda & Shin (2012)104 1.38 +/- 0.01 +19% (5 nm SiO2)
+25% (30 nm SiO2) -
Dudda & Shin (2013)105 1.47 +/- 0.02 +13% (10 nm SiO2)
+21% (30 nm SiO2) -
Chieruzzi et al. (2013)110 1.65 +0.8% (7 nm SiO2)
+22.4% (SiO2-Al2O3) 110
Lu & Huang (2013)111 1.59 +/- 0.03 -3% (13 nm Al2O3
at 2 wt.%) -
Andreu-Cabedo et al. (2014)113 1.48 +/- 0.09 +25% (12 nm SiO2) -
Schuller et al. (2015)117
1.47 +/- 0.04
(1.59 two
months later)
+30.6% (40 nm Al2O3
at 0.78 wt.%) -
Present work 1.47 +/- 0.07 0% (10 nm SiO2)
+4.5% (30 nm SiO2) 112 +/- 3
The discrepancies in the base salt Cp (Table 5-7) is a well known problem. This
problem has led to specific activities of the SolarPACES Thermal Energy Storage
Group (an international network of researchers and industry experts for the
development of concentrating solar thermal power systems and solar chemistry
technologies). The proposed activity was a Round Robin Test on the measurement
of specific heat capacity of solar salt in which we have participated. The results
have been published in the SolarPACES conference 2016, showing relative error
values among partners between 5 and 10% for Cp between 200 and 400ºC.
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Thermal Energy Storage for High Temperature Applications
The base salt measurement of the present work matches 3 of the 5 references. The
other two references show ~10% higher Cp of the base fluid. It is worth mentioning
that the solar salt measurement reported in Schuller et al. (2015)117 shows an
inexplicable enhancement of 8% after repeating the measurements 2 months later,
with the samples kept at room temperature under inert atmosphere. These
deviations shown between authors and even within the same investigation indicate
how difficult is to get reliable specific heat capacity measurements when working
with nitrates molten salts.
The other important observation is that only one investigation reports values of the
phase change properties of the salts. In every Cp measurements the salt is melted
and the data is usually recorded by the DSC. Therefore, the phase change
measurement could help in identifying outliers in the data or to develop a global
explanation of any anomalous specific heat measurement.
Our specific heat results using vial evaporated nanofluids (Figure 5-21) are aligned
with Lu & Huang (2013)111 results, where no enhancement was observed but they
differ from Dudda & Shin’s results (Table 5-7).
Chieruzzi’s results110 with for SiO2 (7 nm) nanofluid did not show enhancement
either. In fact, although the authors highlight in the abstract and conclusions the
huge Cp enhancement (+22.5%) observed in one type of nanofluid and one
nanoparticle concentration, their results show more Cp reductions than
enhancements (Table 5-8). In fact, the important reduction on Cp observed at 0.5
wt. % for different nanoparticle types (-19%, -7.6%, -15.7%) seems more remarkable
than the enhancements. Due to the absence of error bars in this study the precision
of their measurements cannot be evaluated in other to clarify if the differences in
Cp are statistically significant.
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
210
Table 5-8. Chieruzzi et al. (2013)110 results for solar salt modified with different
nanoparticle types and concentrations.
Cp Enhancement [%]
Nanoparticle
concentration [wt. %]
SiO2
(7nm)
Al2O3
(13nm)
TiO2
(20nm) SiO2+Al2O3
0.50% -19.36% -7.65% -15.66% -7.46%
1.00% 0.79% 5.89% -6.31% 22.45%
1.50% -1.46% -3.52% -11.77% 1.52%
In the case of Lu & Huang (2013)111 their results might not be directly comparable
with this study because the nanoparticle concentration and nanoparticle type were
different (2 wt. % of alumina instead of silica). However, their results show a
monotonous decreasing tendency on the nanofluid specific heat with the
nanoparticle concentration (with lower Cp than the base salt) increases from 2 wt.
% to higher concentrations, which is similar to our results with silica nanoparticles
(Figure 5-21 and Figure 5-22).
Our latent heat results can be also compared with Chieruzzi (2013)110. Their results
shows an anomalous latent heat enhancement of +14.9% for solar salt nanofluid
with 1 wt. % of SiO2 (7nm) nanoparticles. However, a small reduction on the latent
heat has been measured in this research: -1.5 % and -0.4 % on average for 10 nm
and 30 nm silica nanofluid respectively at 1 wt. %, which is inside the experimental
error.
Summarizing, the synthesized nanofluids in this study do not show anomalous
behavior neither on the specific heat nor the latent heat. The specific heat results are
aligned with Chieruzzi et al. (2013)110 and against Dudda & Shin’s results104,105
regarding solar salt nanofluid modified with silica nanoparticles. The effect of
several parameters were evaluated by changing: a) the base salt composition, b) the
nanoparticle loading (weight fraction), c) the nanoparticle type (SiO2, Al2O3, CuO),
and d) the nanoparticle size (10 and 30 nm). In all these cases, except for large
particle loadings above 5% by wt. the specific heat capacity was modified within
the confidence intervals, showing no statistical difference in average with respect to
the neat salt.
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Thermal Energy Storage for High Temperature Applications
If we analyze carefully the synthesis procedure to prepare the nanofluids followed
in the different studies (Table 5-9), even though all of them use a two step
procedure, some differences can be identified among authors. These differences are
mainly related to the sonication time and the evaporation temperature.
Table 5-9. Differences in the solar salt nanofluid synthesis procedure among
researchers. Enhancement at 1wt. % of nanoparticles concentration if not specified.
Synthesis Procedure Enhancement
[%] Sonication
time
Evaporation
temperature
Evaporation
time
Dudda and Shin
(2012)104 200 min 200ºC 7 h
+19% (5 nm SiO2)
+25% (30 nm SiO2)
Dudda and Shin
(2013)105 200 min 200ºC 7 h
+13% (10 nm SiO2)
+21% (30 nm SiO2)
Chieruzzi et al.
(2013)110 100 min 200ºC 2 h
+0.8% (7 nm SiO2)
+22.4% (SiO2-Al2O3)
Lu and Huang
(2013)105 100 min 90ºC 12 h
-3% (13 nm Al2O3
at 2 wt.%)
Andreu-Cabedo et al.
(2014)113 5 min 100ºC 1 h +25% (12 nm SiO2)
Schuller et al.
(2015)117 120 min 90ºC 45 min
+30.6% (40 nm
Al2O3
at 0.78 wt.%)
Present work 200 min 200ºC 7 h 0%
If we compare the evaporation temperature and time, we identify two main
groups. On the one hand the groups with long evaporation time (most of them
using an evaporation temperature of 200ºC). On the other hand the groups with
shorts evaporation times and 100ºC of evaporation temperature. One of the key
factors to explain how reducing the hot plate temperature can lead to a shorter
evaporation time is the exposed surface area (Figure 5-30).
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
212
Figure 5-30. Vial (left) vs. larger surface area evaporation receptacles such as glass
petri-dish (center) and steel pan (right, from Schuller et al. (2012)139). Larger surface
area leads to a shorter evaporation times.
The other two references (Andreu-Cabedo et al. (2014)113 and Schuller et al. (2015) 117) evaporate in a shorter time, even though the hot plate temperature was set to
100ºC instead of 200ºC. This might be possible by using a larger surface area
evaporation method. This evaporation method will be used in the following section
(5.3.3 Results new synthesis method).
5.3.3. Results new synthesis method
The synthesis procedure is modified after observing that, unlike Dudda’s
results104,105, the nanofluids synthesized using vial did not show Cp enhancement.
Shin et al. (2014)101 reported higher Cp enhancement when evaporating the
nanofluid-water solution using a glass petri-dish on a hot plate (100ºC) instead of a
vial, although they only tested the binary carbonate Li2CO3-K2CO3. The authors
suggest that the larger surface area of the petri-dish leads to a faster evaporation.
The nanoparticles are thought to be less agglomerated, which is hypothesized to
yield a higher Cp enhancement. The change in the synthesis procedure (lower
evaporation temperatures) is also motivated by the data in Table 5-9 from Andreu-
Cabedo et al. (2014)113 and Schuller et al. (2015)117.
In Jo & Banerjee (2015b) and Shin & Banerjee (2013)99,127 the authors differentiated
between 2 areas on the petri-dish after the evaporation: fine powder (type-A,
believed to have a well-dispered nanofluid) and coarse powder (type-B, suggested
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Thermal Energy Storage for High Temperature Applications
to have agglomerated nanoparticles) and selectively tested each area. We proceed
in the same way (Figure 5-31).
Figure 5-31. Different areas tested on a petri-dish. Solar salt nanofluid with 1wt. %
of SiO2 (10 nm)
The results of the different nanofluid types tested are shown in Figure 5-32. The
solar salt nanofluid synthesized with the new method shows different degrees of
enhancement depending on the selected nanofluid type. Type A nanofluid shows
an average +8.4% specific heat enhancement, while Type B nanofluids shows an
average -4.2% decrease.
Type A
Type B
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
214
Figure 5-32. Solar salt nanofluid (1 wt. %, 10 nm SiO2 nanoparticles) specific heat
results vs. Temperature synthesized using petri-dish (PD) evaporation. Neat solar
salt is mixed and evaporated in a vial.
Table 5-10. New synthesis method nanofluids - Cp results
Specific heat Cp [J/gK]
Temperature
[ºC]
Neat SS
(Vial)
Type A NF SS
1% SiO2 (10nm) (PD)
Type B NF SS
1% SiO2 (10nm) (PD)
260 1.44 +/- 0.07 1.54 +/- 0.11 1.41 +/- 0.07
300 1.46 +/- 0.07 1.57 +/- 0.11 1.41 +/- 0.06
350 1.48 +/- 0.07 1.61 +/- 0.12 1.41 +/- 0.06
400 1.50 +/- 0.07 1.64 +/- 0.13 1.41 +/- 0.07
The same statistical significance test is performed to the difference of mean Cp
values for the neat fluid and the new synthesized nanofluids. The results indicate
that the +8.4% average enhancement (+9.6% at 400ºC) on the specific heat of the
0.50
0.75
1.00
1.25
1.50
1.75
2.00
250 275 300 325 350 375 400
Sp
ecif
ic H
eat
[J/(
g·K
)]
Temperature [ºC]
Neat SS (Vial)
Type A NF SS 1% SiO2 (10nm) (PD)
Type B NF SS 1% SiO2 (10nm) (PD)
215
Thermal Energy Storage for High Temperature Applications
Type A nanofluid is not statistically significant ( ),
with n1 and n2 8 and 12 samples respectively (Table 5-11). This means that we
cannot reject the null hypothesis of equality of means between Type A nanofluid
and the base salt. In the case of Type B nanofluid the differences on the Cp mean
value are not statistically significant either.
Table 5-11. Type A nanofluid SS Vial 1%SiO2 (10 nm) vs neat solar salt
Temperature (400ºC) Neat SS Vial Type A NF SS Vial
1%SiO2 (10 nm)
Avg Cp [J/ (g K)] 1.50 1.64
Std Cp [J/ (g K)] 0.10 0.25
Number of samples 8.0 12.0
Confidence Interval (90%) 1.43 – 1.57 1.51 – 1.77
However, the specific heat difference between Type A and B is significant (
) (Table 5-12).
Table 5-12. Type A vs Type B nanofluid SS Vial 1%SiO2 (10 nm)
Temperature (400ºC) Type A NF SS Vial
1%SiO2 (10 nm)
Type B NF SS Vial
1%SiO2 (10 nm)
Avg Cp [J/ (g K)] 1.64 1.41
Std Cp [J/ (g K)] 0.25 0.11
Number of samples 12.0 9.0
Confidence Interval (90%) 1.51 – 1.77 1.33 – 1.48
5.3.4. Discussion new synthesis method
Based on the results shown in Figure 5-32 two different hypotheses are proposed
regarding the specific heat enhancement:
a) A composition shift during the synthesis process: the water solution and
evaporation step is modifying the composition of the base fluid by zones
(type A and B) which can affect the Cp based on Figure 5-20.
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
216
b) Adsorption at the nanoparticle interface: a reversible adsorption type
interaction on the nanoparticle surface, acting as an extended solid-liquid
phase transition, exceeding the melting point of the base fluid.
As the samples are selectively tested, if the process is varying the base fluid
composition (hypothesis (a)), different areas with higher sodium and higher
potassium concentration might appear, with higher and lower Cp respectively
based on Figure 5-20. An artificial specific heat enhancement and reduction could
be produced in each nanofluid type. A change in composition should also affect the
measured latent heat.
We have discarded all the mechanisms involving “needle-like structures”, “nano-
structures” and “fractal-like nano-structures” proposed by Banerjee and Shin
(section 5.1.1.1Theories behind Cp enhancement) based on our SEM images, where
none of these structures have been observed. Most of these structures are observed
in carbonate salts instead of nitrate salts. Besides this, the SEM images shown in
Dudda & Shin (2013)105 relative to solar salt show some agglomerates, very similar
to Figure 5-29, and none of the other authors reporting specific heat enhancement,
such as Andre-Cabedo et al. and Chieruzzi et al. (2013 and 2015)110,113,116 show any
type of nano-structures.
On the other hand, mechanism (b), postulated by Thoms133, has not yet been
confirmed experimentally. It is hypothesized that in both desorption behaviors
(Figure 5-5 I&II) a reduction on the latent heat can be expected.
These two possible explanations should be combined with the expected effect of
nanoparticles added to the molten salts (Table 5-13). As mentioned above, adding
nanoparticles with a lower specific heat capacity than the base fluid should in
theory reduce the nanofluid heat capacity. A reduction on the latent heat should
also be expected because particles do not undergo any phase change. Regarding
the phase change temperatures, a reduction on the onset temperatures could be
anticipated because nanoparticles might act as impurities slightly reducing the
melting onset temperatures as well as they might act as a nucleation sites
increasing the freezing onset temperature. The specific heat and latent heat
reduction should fall within the experimental error because of the low nanoparticle
concentration tested (1 wt. %).
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Thermal Energy Storage for High Temperature Applications
Table 5-13. Possible explanation regarding the specific heat enhancement
Adding
nanoparticles
with lower
Cp
Composition
Shift (a) Adsorbed
layer (b) Na-rich NF |K-rich NF
Cp ↓ (less than 1%) ↑ ↓ ↑ or ↓
LH ↓ (~1%) ↑ ↓ ↓ and ↓
Onset T Melting ↓ ~↑ ~↓ ~
Onset T Freezing ↑ ↑ ↓ ~
If we analyze the latent heat of this nanofluid (Figure 5-33, Table 5-14) we observe a
little increase in Type A (with higher average Cp) nanofluid while a reduction on
Type B nanofluid (with lower average Cp).
Figure 5-33. Latent heat of Type A and B nanofluid vs. Neat solar salt
100
105
110
115
120
125
130
Lat
ent
Hea
t (m
elti
ng
) [J
/g]
Neat SS (Vial)
Type A NF SS 1% SiO2(10nm) (PD)
Type B NF SS 1% SiO2(10nm) (PD)
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
218
Table 5-14. New synthesis method nanofluids – Solid-liquid phase change results
Neat SS
(Vial)
Type A NF SS
1% SiO2 (10nm) (PD)
Type B NF SS
1% SiO2 (10nm) (PD)
Mel
tin
g
Integral
(J/g) 111.9 +/- 3.5 112.6 +/- 2.0 108.5 +/- 1.2
Tonset
(ºC) 216.7 +/- 1.2 215.7 +/- 1.1 215.4 +/- 0.8
Tpeak
(ºC) 224.8 +/- 0.3 223.9 +/- 0.5 224.0 +/- 0.5
Fre
ezin
g
Integral
(J/g) 112.7 +/- 2.6 113.3 +/- 2.2 109.5 +/- 0.7
Tonset
(ºC) 232.9 +/- 1.3 235.1 +/- 1.1 232.5 +/- 1.3
Tpeak
(ºC) 218.5 +/- 0.6 220.9 +/- 3.5 222.8 +/-6.5
During the phase change analysis between neat solar salt and nanofluids Type A
and B a single property showed a statistically significant difference: Type A
nanofluid shows a slightly higher onset freezing temperature Table 5-15 than the
neat solar salt and Type B nanofluid (Figure 5-34). On the other hand, Type B
nanofluid shows no statistically significant lower freezing onset temperature with
respect to the base salt.
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Thermal Energy Storage for High Temperature Applications
Figure 5-34. Freezing onset temperature difference between Type A and B
nanofluid.
Table 5-15. Hypothesis testing for the onset freezing temperatures for neat solar
salt vs. type A and type B nanofluid
Hypothesis testing
Type A vs. Neat SS
(statistically significant difference)
The combination of these three results (slightly higher latent heat and higher
freezing onset temperature and slightly higher specific heat) suggests that Type A
nanofluid could contain a larger amount of NaNO3 than the base fluid. These
results suggest that the synthesis method is producing different composition salts
and that these differences are more obvious when the solvent evaporation takes
place quicker in a large surface area container. Based on these results, one can
hypothesize that the different components of the mixture are not precipitating
homogenously.
Finally, it is important to mention that based on these results it seems as though
selectively choosing samples from a Petri-dish can lead to erroneous and random
0
0.5
1
1.5
2
2.5
3
180 200 220 240 260 280 300 320
Hea
t F
low
[W
/g]
Temperature [ºC]
Type A
Type B
freezing
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
220
results. Also, since the evaporation method affects the results, both base fluids and
nanofluids should be synthesized following the same procedure for a systematic
comparison. The same thought was shared by Lasfargues (2014)140 where the
author “removed the salt crystals from the petri-dish and both coarse and fine
grains (nanofluid types) were mixed together and tested”.
In contrast, selectively choosing samples (Banerjee and Shin’s synthesis and testing
procedure) seems to be accepted by the scientific community. In their publications
slight modifications on the synthesis protocol are usually introduced to synthesize
the nanofluid. However, the synthesis protocol of the base fluid used for
comparison never changes. For instance, Jo & Banerjee (2010)120 evaluate the effect
of the hot plate temperature on the evaporation time and the effect on the Cp on
the nanofluids; only one base fluid is taken as a reference synthesized with a single
hot plate temperature. Another example appears in Shin & Banerjee (2010)96 where
the effect of using a large Petri dish (high surface are to evaporate) in the synthesis
protocol is analyzed. They discovered phase segregation claiming that it has been
promoted by the dispersed nanoparticles. However, the binary carbonate salt used
as a reference is synthesized using a vial (longer evaporation time). In both
examples the authors are comparing fluids synthesized with a different protocol,
which in our opinion leads to erroneous results.
In an effort to improve the methods for a fairer comparison, a petri-dish
evaporation was used to synthesize the base salt and a new nanofluid (1% SiO2
10nm). The salt crystals from the complete petri-dish are removed and mixed
together before testing, as in Lasfargues (2014)140 to avoid bias from selectively
choosing the samples by evaporation areas. The results are shown in Figure 5-35.
On average the specific heat of the solar salt nanofluid with 1 wt. % of SiO2
nanoparticles is between 1.9 and 4.4% higher than the neat solar salt, both
synthesized with petri-dish and mixing completely before testing, although this
enhancement is within the confidence intervals.
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Thermal Energy Storage for High Temperature Applications
Figure 5-35. Specific heat and latent heat results of the base salt and nanofluid (1%
of SiO2 10nm) synthesized using petri-dish and mixing completely the entire batch
of salt before testing instead of selectively choosing samples.
Table 5-16. Onset melting and freezing temperatures values corresponding to the
tests shown in Figure 5-35
Temperatures [ºC] Neat SS (Vial) Neat SS
(PD All mixed)
NF SS 1%SiO2 (10nm)
(PD All mixed)
Onset (melting) 216.7 +/- 1.2 216.4 +/- 1.1 215.2 +/- 0.9
Onset (freezing) 232.9 +/- 1.3 233.9 +/- 1.6 233.5 +/- 1.0
The results are compared with the initial solar salt. The neat salt (with no particles)
using a different evaporation process in a larger surface area container (petri-dish
instead of vial) shows on average a higher specific heat capacity than the same salt
evaporated in a small vial. However, no statistically significant difference between
these two neat fluids and the nanofluid is observed. An expected slight reduction
on the latent heat is observed for the nanofluid (Table 5-17).
1.00
1.25
1.50
1.75
2.00
250 275 300 325 350 375 400
Sp
ecif
ic H
eat
[J/(
g·K
)]
Temperature [ºC]
Neat SS (Vial)
Neat SS (Petri Dish AllMixed)
NF SS 1% SiO2 (10 nm) (PD AllMixed)
100
105
110
115
120
125
130
Lat
ent
Hea
t (m
elti
ng
) [J
/g]
Neat SS (Vial)
Neat SS (Petri Dish AllMixed)
NF SS 1% SiO2 (10 nm) (PD AllMixed)
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
222
Table 5-17.Maximum and minimum Cp neat solar salt (PD) sample compared to
the average values of the neat SS (vial)
Neat SS (Vial) Neat SS PD
Max Cp sample
Neat SS PD
Min Cp sample
Specific Heat (260ºC) 1.44 +/- 0.07 1.591 1.324
Onset (melting) 216.7 +/- 1.2 215.0 216.9
Latent Heat(melting) 111.9 +/-3.5 116.6 108.8
Onset (freezing) 232.9 +/- 1.3 236.2 232.7
Latent Heat(freezing) 112.7 +/- 2.6 115 112
Figure 5-36. Specific heat and latent heat results of the neat solar salt (vial) and the
neat solar salt synthesized with petri dish selecting the highest and lowest Cp
value.
Two samples with the highest and lowest Cp from a petri dish selectively chosen
are compared with the average values of the neat SS synthesized with petri and
mixed completely before testing (Table 5-17, Figure 5-36). We can observed the
large variation of the specific heat of the selectively chosen samples, which
coincides with a higher and lower latent heat and freezing onset temperature for
the higher and lower specific heat sample respectively. These results suggest that
0.50
0.75
1.00
1.25
1.50
1.75
2.00
250 275 300 325 350 375 400
Sp
ecif
ic H
eat
[J/(
g·K
)]
Temperature [ºC]
Neat SS (Vial)
Max Cp neat SS (PD)
Min Cp neat SS (PD)
100
105
110
115
120
125
130
Lat
ent
Hea
t (m
elti
ng
) [J
/g]
Neat SS (Vial)
Max Cp neat SS (PD)
Min Cp neat SS (PD)
223
Thermal Energy Storage for High Temperature Applications
composition shifts can be produced along the different petri dish areas which affect
the specific heat as well as the phase change properties of each sample. The heat
flow curves of these two samples are shown in Figure 5-37.
Figure 5-37. Heat flow curves vs. temperature for the sample with the maximum
and minimum Cp synthesized with Petri dish for a representative test from the
data shown in Figure 5-36.
Summarizing, we have proceeded according to Banerjee & Shin’s procedure99,127 in
both the synthesis procedure and the testing procedure by evaporating the
dissolved nanofluids in a larger surface area container (petri-dish) and selectively
choosing different regions to test. The solar salt nanofluid has shown an
enhancement of +8.4% on average, which is not statistically significant when
compared to the neat salt synthesized and evaporated in a vial. This fluid shows
also a slightly higher latent heat and higher freezing onset temperature. These three
-1
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
190 210 230 250 270 290 310
Hea
t F
low
[W
/g]
Temperature [ºC]
Min Cp neat SS (PD)
Max Cp neat SS (PD)
freezing
melting
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
224
observations, combined with the NaNO3-KNO3 phase change diagram (Figure
5-40), suggest that Type A nanofluid might contain a higher NaNO3 proportion. On
the other hand, Type B nanofluid shows a small reduction on the specific heat and
latent heat that could suggest a higher KNO3 content.
We have also observed that the neat salt samples without nanoparticles selected
from different areas in the Petri dish can show higher specific heat depending on its
composition; these samples show slightly higher latent as well. When the neat salt
is synthesized with petri dish and mixed together before testing, a not-statiscally-
significant small enhancement is also observed.
We can compare our average Cp enhancement (Type A nanofluid, best-case
scenario) with Andreu-Cabedo et al. (2014)113 results. The same nanoparticle type
(SiO2) is used in both studies with a very similar nanoparticle size (12 nm vs. 10
nm). The nanopartcile concentration (1 wt. %) is also the same. Table 5-18 compares
the results.
Table 5-18. Specific heat results: present work vs. Andreu-Cabedo et al. (2014). Neat
solar salt vs. solar salt nanofluid (1 wt. % of SiO2 nanoparticles). *Type A
nanofluid.
Average Cp value (250-420ºC) Base salt
[J/ (g K)]
Nanofluid
[J/ (g K)]
Cp
Enhancement
[%]
Andreu-Cabedo et al. (2014)113 1.48 +/- 0.09 1.85 +/- 0.06 +25
Present work 1.47 +/- 0.07 1.59 +/- 0.12* +8.4
As in the previous section, our results do not show a statistically significant
enhancement on the specific heat for solar salt nanofluid with 1 wt. % of SiO2
nanoparticles, which disagrees with Andreu-Cabedo et al. (2014)113 showing an
average +25% enhancement on the same nanofluid.
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Thermal Energy Storage for High Temperature Applications
5.3.4.1. Solubility as a key factor
Our experimental results suggest that the synthesis process seems to separate the
salt components. This separation is more evident when using larger surface area
evaporation receptacles. This separation might be related to the solubility of each
salt component in water. This parameter has never been mentioned in any of the 33
reference using these synthesis methods (solution-sonication-evaporation).
The solubility is the quantity of solute (nitrate salts in this case) that dissolves in a
given amount of solvent (water in this case). It depends on the nature of the solute
and solvent, the amount of solute, the temperature and pressure of the solvent and
it is often expressed as the quantity of solute per 100 g of solvent at a specific
temperature.
Table 5-19. Solubility of nitrate salts in water.141 Salt Solubility in water [g/100g of water] 20g of water (100ºC)
NaNO3 149 (80°C) 176 (100°C) 35.2 g
KNO3 168.8 (80°) 243.6 (100°) 48.7 g
As can be seen in Table 5-19, the 20 ml of water used in the solution step is large
enough to dissolve 200 mg of salt. However, the amount of water is eliminated
progressively during the evaporation process. As the water is continuously
evaporated there is a point where there is not enough water to keep the salt
dissolved starting its precipitation on the receptacle. The precipitation of the salt is
not expected to be uniform, but it will start with the less soluble component, in this
case NaNO3.
This effect could explain why the different areas in Figure 5-31 show slightly
different properties. The fine powder areas might precipitate first, with higher
content of the less soluble component at evaporation temperature (80 ºC – 100 ºC)
which is NaNO3. At the end of the evaporation process the remaining salt (rich in
KNO3) precipitates and forms the so called “Type B” nanofluid which might
contain a larger proportion of KNO3, as the slightly lower specific heat, latent heat
and freezing onset temperature suggest.
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
226
This is an important observation for nitrate base nanofluid, but it could be even
more important for carbonate base nanofluid because, while the solubility ratio
between NaNO3 and KNO3 is 1.38, in the case of carbonates the solubility ratio
between K2CO3 and Li2CO3 is 216.4. This means that lithium carbonate is 2 orders
of magnitude less soluble than K2CO3. Consequently, lithium salt will start
precipitating sooner forming reach lithium carbonate areas with very different
properties than the remaining salt, which might lead to larger Cp enhancements as
observed in Figure 5-3 for Banerjee and Shin Group’s results 99,121,127
Even though the effect described previously regarding composition shift during the
synthesis might explain the results observed during this investigation, it cannot
justify +20% Cp enhancements on average shown in the literature (which were
however not achieved in this research) or values above the Cp of pure NaNO3 (the
pure component with the highest Cp)
5.3.5. Conclusions
In this study the effect of SiO2 nanoparticles on the specific heat of solar salt has
been investigated. Through different experiments we have tried to replicate
different published studies with unsuccessful results:
Varying nanoparticle size on solar salt
Varying nanoparticle concentration on solar salt
Varying base fluid composition with silica nanoparticles
Varying nanoparticle type with the eutectic composition as base fluid
We have found no statistically significant evidence that the specific heat of molten
salt can increase by the addition of nanoparticles. The variation of the synthesis
process has lead to larger Cp enhancements (no statistically significant either).
However, the results suggest that the composition of the salt is not homogeneous
when the samples are selected based on different visual aspect of the salt in the
petri dish. The two-step water solution and evaporation synthesis is affecting the
salt composition of the nanofluids, explaining to a certain extent positive Cp
enhancement reports. This composition shift can be explained through the
difference in water solubility of each salt component which leads to a segregation
during the evaporation step.
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Thermal Energy Storage for High Temperature Applications
DSC measurements with molten salts should be performed with care, as
corrections such as those with reference materials are temperature specific and
should not be extended over large temperature ranges. Also, molten salts have
been shown to creep and escape certain (not well-sealed or pin-holed) crucibles,
contaminating the equipment sensors throughout testing. More frequent testing to
account for drifts in measurements and baseline references should be performed
when measuring molten salts.
5.4. Latent heat of nitrate base nanofluids
Based on the specific heat results we decided to explore the phase change
properties of the salt. In recent years there has been a dramatic increase in research
analyzing the effect of nanoparticle addition to heat transfer fluids and storage
materials. Nonetheless, there are not that many experimental studies focused on
the phase-change properties of nanofluids as a latent heat storage material.
Therefore, we decide to extend the investigation analyzing the effect of adding
different SiO2 nanoparticle concentrations on the phase change characteristics of
different NaNO3-KNO3 mixtures along its phase change diagram with the aim of
understanding and quantifying these effects.
The latent heat of fusion of nanofluids is expected to decrease linearly as particles
not contributing to phase change are added to the base fluid.142,143 Some of the
research on nanoparticle enhanced phase change materials (PCM) uses materials
that melt at relatively low temperatures (e.g. paraffin with a melting point around
60ºC) and therefore are not suited for CSP applications. Figure 5-38 summarizes the
effect on the latent heat of the addition of nanoparticles to low melting temperature
organic PCM.
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
228
Figure 5-38. Reported effect of nanoparticles on the latent heat of fusion of organic
PCM
Although there is a spread of different results, the general trend indicates that
adding nanoparticles reduces the latent heat of the composite material. However,
the experimental reduction in latent heat generally exceeds the theoretical
predictions. For instance, the magnitude of the effect of Al2O3 on the phase change
properties of paraffin is not clear. Ho et al. (2009)144 shows a reduction on the latent
heat following the predicted curve while Teng et al. (2012)145 shows a larger
reduction on the same material and nanoparticle type.
On the other hand, high temperature molten salt composites, more appropriate for
CSP applications, have been less studied. Figure 5-39 summarizes the effect on the
latent heat of the addition of nanoparticles to high temperature inorganic PCM.
50%
60%
70%
80%
90%
100%
110%
120%
0% 2% 4% 6% 8% 10%
No
rmal
ized
Lat
ent
Hea
t [%
]
Nanoparticle concentration [wt.%]
Theory
Ho et al. Paraffin-Al2O3
Wu et al. Parraffin-CuO
Teng et al. Paraffin-Al2O3 or TiO2
Teng et al. Paraffin-SiO2 or ZnO
Yang et al. Paraffin-Si3N4
Harikrishnan et al. Oleic Acid-CuO
Yavari et al. 1-Octadecanol-Graphene
Parameshwaran et al. Organic ester-Ag
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Thermal Energy Storage for High Temperature Applications
Figure 5-39. Summary of the effect of nanoparticle on the latent heat of high
temperature inorganic PCM
As observed in Figure 5-39, both contradictory 15% enhancements and 10%
reductions are reported at a nanoparticle concentration of 1% wt. These anomalous
and opposite trends on the latent heat in molten salt nanofluid research also
motivates this investigation.
The solid-liquid transitions are dramatically different in the case of eutectic and off-
eutectic mixtures. As the NaNO3-KNO3 binary system phase diagram in Figure
5-40 shows, a eutectic mixture melts at a specific temperature (eutectic
temperature) whereas off-eutectic compositions will melt over a temperature
range. It is unknown if the effect of nanoparticles on the latent heat is the same on
each type of salt mixture. To answer this question, different compositions are tested
in this study, as specified in Table 5-20. The compositions prepared are the pure
components, the eutectic mixture (49 mol% NaNO3), and two off-eutectic mixtures:
a potassium rich mixture with 34 mol% NaNO3 and a sodium rich mixture with 64
mol% NaNO3 (this particular composition is the one previously investigated,
80%
85%
90%
95%
100%
105%
110%
115%
120%
0% 1% 2% 3%
No
rmal
ized
Lat
ent
Hea
t [%
]
Nanoparticle concentration [wt.%]
Theory
Tao et al. Carbonate+CarbonBased
Lasfargues et al. Nitrate+CuO
Lasfargues et al. Nitrate+TiO2
Chierruzzi et al. (2013) Nitrate+SiO2
Chierruzzi et al. (2013) Nitrate+Al2O3
Chierruzzi et al. (2013) Nitrate+TiO2
Chierruzzi et al. (2015) KNO3+SiO2
Chierruzzi et al. (2015) KNO3+Al2O3
Chierruzzi et al. (2015) KNO3+(SiO2+Al2O3)
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
230
known as “solar salt”). The off-eutectic compositions are located symmetrically
from the eutectic point in the phase diagram. For the two off-eutectic compositions
studied, this melting range should theoretically start near or slightly above the
eutectic temperature (223ºC) and span about 25ºC.
Table 5-20. NaNO3-KNO3 compositions tested
NaNO3 KNO3
KNO3 0 mol% (0 wt.% ) 100 mol% (100 wt. %)
K-rich off-eutectic 34 mol% (30 wt.% ) 66 mol% (70 wt. %)
Eutectic 49 mol% (45 wt. %) 51 mol% (55 wt. %)
Na-rich off-eutectic 64 mol % (60 wt. %) 36 mol% (40 wt. %)
NaNO3 100 mol% (100 wt.% ) 0 mol% (0 wt. %)
Figure 5-40. Phase diagram NaNO3-KNO3 from Factsage43 highlighting the
compositions tested: a hypoeutectic at 34 mol% NaNO3, the eutectic at 49 mol%
NaNO3, a hypereutectic at 64 mol % NaNO3, and the pure components KNO3 and
NaNO3.
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Thermal Energy Storage for High Temperature Applications
The synthesis method described previously in section “5.2.1 Synthesis of
nanofluids”. It has not been modified in order to compare it with Chieruzzi et al.
(2013, 2015)110,116, which showed anomalous latent heat results using the same
synthesis procedure. The DSC method described in “5.2.2 DSC Measurement” has
been slightly modified by changing all the different heating rates to 5 ºC/min and
eliminating the modulation for the specific heat measurement (Figure 5-41). The
isothermal segments and inert gas have been maintained.
Figure 5-41. Example of DSC cooling and heating curve example, showing
temperature vs. time in blue plotted on the right axis and heat flow vs. time in
green plotted on the left axis.
For PCM applications, the mentioned eutectic is preferable than off-eutectic
compositions (such as solar salt) to limit the temperature range at which the
mixture solidifies. For HTF application it is interesting to have materials that melt
at lower temperatures and with low latent heat.
The latent heat of fusion and the onset temperature are compared between the five
NaNO3-KNO3 compositions and the effect of nanoparticle addition is evaluated in
each case.
0
50
100
150
200
250
300
350
400
450
-1.2
-0.8
-0.4
0
0.4
0.8
1.2
0 20 40 60 80 100 120 140 160 180
Tem
per
atu
re [
ºC]
Hea
t F
low
[W
/g]
Time [min]
Heat Flow Temperature
freezing melting melting
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
232
5.4.1. Results
The heat flow curves obtained for each composition is shown in Figure 5-42. The
bottom curves (endothermic process, since H<0) correspond to the melting
process and top curves (exothermic, H>0 ) correspond to crystallization. The pure
components as well as the eutectic composition behave very similarly, melting in a
narrow temperature range, since they melt at a specific temperature, while the
phase change of off-eutectic compositions expands over a wider temperature range
corresponding with the “mushy” region (between the solidus and liquidus lines in
the phase diagram).
Figure 5-42. Heat flow curves vs. temperature for the different compositions tested.
The latent heat of fusion results of the five compositions tested agree very well with
other previous studies such as Rogers & Janz (1982)146 evaluating different the
phase change properties of NaNO3-KNO3 mixtures as can be observed in Figure
5-43. These results indicate that the latent heat of a mixture cannot be simply
-3.5
-2.5
-1.5
-0.5
0.5
1.5
2.5
3.5
180 200 220 240 260 280 300 320 340 360
Hea
t F
low
[W
/g]
Temperature [ºC]
KNO3
Na-KNO3 (30-70 wt.%)
Eutectic
Na-KNO3(60-40 wt.%)
NaNO3
233
Thermal Energy Storage for High Temperature Applications
calculated as a linear weighted average of the pure component latent heats and
must be experimentally determined.
Figure 5-43. Latent heat of fusion results for different NaNO3-KNO3 mixtures
A minimum latent heat is observed around 25 - 34 mol% of NaNO3 and the latent
heat of the eutectic composition and the pure KNO3 are very close in both studies.
Once the latent heat of the different mixtures has been verified, the effect of
different SiO2 (10 nm) nanoparticle concentration on the latent heat is evaluated.
The results are presented in Figure 5-44, Figure 5-45, and Figure 5-46. Adding
nanoparticles to any KNO3-NaNO3 mixture decreases the latent heat. This
reduction in latent heat increases with particle loading. Normalizing the results by
the latent heat of the neat salt mixture (Figure 5-46) can indicate whether there are
differences in the behavior between compositions with a single melting
temperature (pure components or eutectic) and mixture compositions melting over
a temperature range.
80
100
120
140
160
180
200
0 20 40 60 80 100
Lat
ent
hea
t o
f m
elt
ing
[J/
g]
NaNO3 (mol %) in a NaNO3-KNO3 mixture
This study
Rogers & Janz (1982)
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
234
Figure 5-44. Latent heat vs. nanoparticle concentration for different NaNO3-KNO3
mixtures
Figure 5-45. Latent heat vs. NaNO3 content in a Na-KNO3 mixture for different
nanoparticle concentrations
60
80
100
120
140
160
180
200
0% 1% 2% 3% 4% 5%
Lat
ent
Hea
t [
J/g
]
Nanoparticle concentration [%]
0% mol NaNO3
34%mol NaNO3
49%mol NaNO3
64%mol NaNO3
100%mol NaNO3
60
80
100
120
140
160
180
200
0 20 40 60 80 100
Lat
ent
Hea
t o
f M
elti
ng
[J/
g]
NaNO3 (%mol) in a NaNO3-KNO3 mixture
0 wt% SiO2
1 wt% SiO2
5 wt% SiO2
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Thermal Energy Storage for High Temperature Applications
Figure 5-46. Normalized latent heat vs. nanoparticle concentration for different
NaNO3-KNO3 mixtures. Theory corresponds to a simple mixing rule calculation.
5.4.2. Discussion
Theoretically the latent heat of fusion of composite materials is expected to linearly
decrease with particle addition due to the nanoparticles which do not undergo a
phase change. A simple mixing rule for the latent heat can be used as a first
approximation to compare the expected latent heat of the nanofluids with their
measured values (Figure 5-46). The nanoparticle mass fraction is calculated as
follows:
Equation 5-8
The nanoparticle mass fraction (w) and the latent heat (LH) of the base fluid is used
to predict the latent heat of the nanofluid:
Equation 5-9
where m stands for mass and subscripts nm, np, and bf denote nanomaterial,
nanoparticle, and base fluid, respectively.
80%
85%
90%
95%
100%
105%
0% 1% 2% 3% 4% 5%
No
rma
lize
d L
aten
t H
eat
[%]
Nanoparticle concentration [%]
Theory
0% mol NaNO3
34%mol NaNO3
49%mol NaNO3
64%mol NaNO3
100%mol NaNO3
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
236
The results shown in Figure 5-46 indicate that adding SiO2 nanoparticles to the base
salt results in a reduction of the latent heat, as expected but larger than predicted by
mixing rule theory, and independently of the salt composition evaluated. The
latent heat of fusion of the nanofluid decreases as the nanoparticle concentration
increases. This is also observed for both: pure components and mixtures.
In Giménez & Fereres (2015)147 we considered the possibility that the nanoparticle
effect on latent heat could be a function of the base salt composition, since latent
heat essentially follows mixing theory predictions for the eutectic mixture but it
decreases slightly more than the predicted values when nanoparticles are added to
an off-eutectic composition. This difference is more evident as nanoparticle
concentrations are increased. The larger number of samples tested complementing
this study on the same salt compositions suggest a similar tendency, although the
confidence intervals are overlapped.
If we analyze the rate of nanofluid latent heat reduction with respect to the particle
concentration, we observe that it is maintained quite constant for the range of
nanoparticle concentration tested (0 wt % to 5 wt. %). The rate of nanofluid latent
heat reduction is in all cases higher than the expected theoretical.
The slope of the linear fit of the latent heat measurements for each composition is
shown in Table 5-21. It is interesting how KNO3 and the eutectic composition,
which showed a very similar latent heat, show also similar latent heat reduction on
average while pure NaNO3 and both off-eutectic composition behave similarly.
Table 5-21. Slope of the linear fit of the normalized latent heat vs. the nanoparticle
concentration.
LH [J/g] Slope
[% latent heat / % of nanoparticle]
KNO3 99.8 +/- 4.4 -1.705
K-rich off-eutectic 91.1 +/- 2.4 -2.344
Eutectic 98.6 +/- 3.2 -1.857
Na-rich off-eutectic 110.1 +/- 2.7 -2.659
NaNO3 184.0 +/- 6.0 -2.515
Theory - -1.000
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Thermal Energy Storage for High Temperature Applications
The results presented here disagree with previous studies on the same salts and
nanoparticle type and concentration. Chieruzzi et al (2013)110 reported an
anomalous latent heat enhancement of +14.8% for 64%mol of NaNO3 composition
at 1% for SiO2 nanoparticles. Similar enhancement was reported for the same
nanoparticle loading but different nanoparticle types such as Al2O3 (+15.54%). On
the other hand TiO2 nanoparticles showed lower enhancement (+4.77%). The same
authors in Chieruzzi et al. (2015)116 tested the same nanoparticle (SiO2) type and
concentration on pure KNO3 reporting +11.84% enhancement on the latent heat of
fusion. However, none of the results reported by these authors have been
replicated in this study. On the contrary, the opposite trend has been measured
showing reductions in latent heat instead of enhancements.
Lasfargues et al. (2015)118 also tested the binary mixture (64%mol of NaNO3)
reporting different behavior depending on the nanoparticle type. While TiO2
nanofluid showed a slight latent heat enhancement of +3.81% and +0.53% (for
0.1wt.% and 1wt.% respectively), different tendency was reported for CuO
nanofluid with +2.46% enhancement on latent heat for 0.1wt.%, but -4% reduction
for 1wt.% of nanoparticle loading. Unlike other authors, Lasfargues’ synthesis
method does not involve the use of water to dissolve the salt and disperse the
nanoparticle through sonication before re-crystallization, but using purely physical
mixing through the use of a ball-mill.
The latent heat enhancement in Lasfargues et al. (2015) 118 was justified with
trapped nanostructures inside tiny agglomerates raising the enthalpy of melting as
more energy would be required to melt the solid trapped within these structures. A
similar explanation has been used to also explain a reduction in latent heat, i.e.
nanoparticle clustering effect in Zabalegui et al. (2014)143, where the base fluid
inside the aggregated nanoparticle clusters increase the strained interface volume
of an ordered structure with weaker molecular bonds.
In the case of Chieruzzi et al. (2013 & 2015)107,113 different hypothesis were used to
explain the enhancement mechanism for the latent heat of nanofluids: one of the
hypothesis was the high specific surface energies associated with the high surface
area of the nanoparticles per unit volume. Other proposed hypothesis was the
possible existence of a layer of small agglomerates in the nanofluids in which the
nanoparticles would be trapped. The melting of the solid entrapped in these
agglomerates would require more energy to occur causing an increase of the latent
heat.
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
238
Unlike Chieruzzi’s and Lasfergues’ research, the measurements performed in this
study do not show any anomalous (positive) enhancement of the latent heat. The
reduction on latent heat observed in this study is larger on average than expected
by the mixing rule theory. This trend was also reported with paraffin and other
organic based PCM where an interfacial liquid layering mechanism was suggested
as a possible explanation.143
5.4.2.1. Interfacial liquid layering
Zabalegui et al. (2014)143 also found a reduction in latent heat in excess of the
mixing rule predictions and attributed it to three potential explanations: 1)
interfacial liquid layering, 2) Brownian motion effects and 3) particle clustering.
Among the different possible mechanisms proposed in Zabalegui et al.143 to explain
this further reduction in latent heat through the weakening of molecular bond
structures the interfacial liquid layering is evaluated. Brownian motion is discarded
as a possible explanation because the Brownian diffusion time scales are two orders
of magnitude larger than the momentum relaxation time (i.e. it is too slow).
The larger reduction of the latent heat suggests that part of the base fluid does not
contribute to the phase change. A new term is introduced on the nanomaterial
latent heat to account for this additional reduction:
Equation 5-10
Where wnp is the nanoparticle mass fraction and wi is the mass fraction of the
interfacial layer. Based on the experimental results the ratio goes from 0.7 to
1.66 based on Table 5-21.
Equation 5-11
It has been established from both experimental studies and molecular dynamics
simulations that the width (∆) of the interfacial or densely packed layer (DPL) is no
more than 1–2 nm.148,149 Since attractive forces dissipate normal to the particle
surface, base molecules further away from the interface migrate shorter distances.
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Thermal Energy Storage for High Temperature Applications
For spherical nanoparticles and concentric semi-solid liquid layering:
Equation 5-12
For 10 nm SiO2 nanoparticles and ∆=1 nm the ratio = 0.728. The silica
nanoparticle density is ρnp=2.65 g/cm3.90 We can consider the interfacial liquid
layering as a semi-solid layer with an average density between the solid and liquid
density of the base fluid density, which depends on the composition.
Using the average density of this layer for each composition, calculated with the
volumetric additivity rule150, as well as the experimental ratio, we can
estimate the thickness (∆) of this interfacial layer for each composition. The solid
and liquid densities are 2.26 and 1.9 g/cm3 for NaNO3, respectively; and 2.11 and
1.865 g/cm3 for KNO3.
Table 5-22. Estimated thickness (∆) of the hypothetical interfacial liquid layer that
could explain the larger reduction on the latent heat observed in nitrate base
nanofluids.
∆i [nm]
KNO3 0.705 0.93 1.23
K-rich off-eutectic 1.344 1.77 2.02
Eutectic 0.857 1.12 1.42
Na-rich off-eutectic 1.659 2.15 2.33
NaNO3 1.515 1.93 2.15
The calculated thickness for each composition (Table 5-22) seems reasonable, not
exciding the suggested 1-2 nm. However, this calculation assumes that the
interfacial liquid layer does not contribute to latent heat. This means that the
calculated thickness is the minimum estimate.
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
240
5.4.2.2. Relationship between an interfacial liquid layer with the Cp
The interfacial compressed liquid layer has been used to justify the anomalous
specific heat enhancement in molten salt nanofluids (mechanism 2 in section
5.1.1.1Theories behind Cp enhancement). The latent heat measurements performed
in this section show that this interfacial liquid layer could exist and could be the
cause of a larger reduction on the latent heat of nanofluids than expected based on
the mixing rule. If a 1-2 nm thick compressed liquid layer exists and one estimates
its contribution to the total nanofluid heat capacity as an additional term in
Equation 5-7, it would need to have an extraordinary heat capacity value given its
small mass. Considering that the solid salts have a lower Cp than the liquid salts,
one might anticipate that the compressed liquid layer (ordered liquid molecules)
will have intermediate properties between the solid and the liquid states. Hence, it
is difficult to understand how this small liquid layer, which should theoretically
have lower Cp than the liquid, can increase the overall Cp.
Moreover, in this investigation a negligible specific heat enhancement has been
measured which means that on the one hand: if this layer exists, it is not causing an
anomalous specific heat enhancement. On the other hand, if we have not measured
specific heat enhancement of nanofluids because the interfacial liquid layer has not
been formed, another hypothesis is needed justifying the larger reduction on the
latent heat measured.
5.4.3. Conclusions
The effect of nanoparticles on the latent heat of different NaNO3-KNO3 mixtures
has been analyzed. Understanding the solid-liquid phase transition process in
molten salt mixtures with and without nanoparticles can help explain the nanofluid
trends in the specific heat capacity in the liquid phase and is also very useful to
characterize the melting/crystallization process in nanoparticle-enhanced phase
change materials.
A reduction in the latent heat of fusion can be explained by the addition of
nanoparticles which do not physically contribute to the phase change, as they are
solid during the melting/solidification of the base fluid. The experimental results
suggest this hypothesis does not fully explain the measured reduction in latent
heat, as there is a larger than expected reduction for all the salt compositions and all
241
Thermal Energy Storage for High Temperature Applications
the nanoparticle concentrations tested. The decrease in latent heat increases as
particle loading increases.
If an adsorption mechanism at the nanoparticle surface is taking place (i.e. a
compressed ordered liquid layer at the fluid/nanoparticle interface), the latent heat
should be further reduced. Using the experimental deviation from the simple
mixing rule accounting for an additional mass (particles) not changing phase, we
have estimated the thickness of an interface to be between 1-2 nm. This is in
agreement with reported measured values and could explain different (higher or
lower) heat capacity values from the base salt. This interfacial semi-solid or
compressed liquid layer would act similar to an extended phase transition during
the temperature range of test.
Nevertheless, if the nanofluid synthesis procedure is affecting the salt composition,
as demonstrated above, the latent heat should be normalized by a neat salt that has
undergone the same synthesis procedure to ensure comparable neat/nanofluid
mixtures.
5.5. Stability of the molten salt nanofluid
5.5.1. Motivation
The applicability of nanofluids at a commercial scale depends on the stability of
nanoparticles in the molten salts. Up to this date and to the author´s best
knowledge, this parameter has only been verified mainly by two different
observations:
1) Good dispersion of nanoparticles in the initial water solution during the
nanofluid synthesis process
2) Thermal cycling of the samples in the DSC
Most authors employing a two-step synthesis method with a solvent (i.e. water)
have shown the nanofluid (salt and nanoparticle mixture) is uniformly dispersed in
the solution during the initial production process.93,126 However, given the high
melting temperature range of these fluids (above 200ºC) and the unavailability of
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
242
standard measurement equipment to test at temperatures above 90-120ºC,
researchers have not evaluated the dispersion stability of these molten salt
nanofluids in the liquid state. SEM images are typically shown, e.g. Shin & Banerjee
(2011b)95, of the solid state mixture, but it seems unlikely that the crystallized
mixture may describe the molten nanofluid.
Checking the repeatability of DSC tests during thermal cycling has also been used
to assess the stability of molten salt nanofluids. In Jo & Banerjee (2014)125 the
stability was tested by 5 times DSC measurements on the same crucible. As the heat
capacity of the nanomaterials were almost uniform after 5 times of melting in the
DSC the authors concluded that it was stable. Similarly Dudda & Shin (2012)104
performed several (4 - 6 times) thermo-cycling (from 140 ºC to 500 ºC) to ensure
repeatability of the measurements and stability. In Schuller et al.117 the stability was
verified by re-testing the DSC samples after 1-2 months.
In Shin & Banerjee (2011c)121 thermo-cycling experiments were conducted for
repeated measurements of the specific heat capacity by using multiple freeze-thaw
cycles of the nanofluids/ nano-composites, respectively. Transmission electron
microscopy of the nanomaterial samples were performed to confirm the stability of
the nanofluids by analyzing the state of aggregation of the nanoparticles before and
after the thermocycling experiments in the DSC. In Shin and Banerjee (2011b)95
Scanning Electron Microscopy images of SiO2 nanofluid were used before
melting/solidification in the DSC to verify the stability.
Through different experiments under high temperature operating conditions, we
demonstrate in this section that the stability of this nanofluid is not guaranteed by
these observations. Testing at high temperatures, as they should operate in an
industrial application, is necessary. Our hypothesis is that the appearance of a well-
dispersed nanofluid in the molten state should be similar to the nanofluid
dissolved in water after sonication, i.e. a homogeneous color mixture.
5.5.2. Materials and methods
Near spherical metal oxide nanoparticles are used to produce the nanoparticle
dispersions. The nanoparticle concentration is 1 wt %. The nanoparticle supplier
information is the following: SiO2 (Meliorum Technologies Nanomaterials, NY, 10
nm), CuO (Io-li-tec nanomaterials, 40-80 nm, 99.9%) and Al2O3 (Aldrich Chemistry,
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Thermal Energy Storage for High Temperature Applications
13 nm, 99.8%). Carbon nanotubes (CNT), produced by Iolitec nanomaterials (CNT
95+%. OD 10-20 nm, L: 5-15μm), have been also investigated using gum arabic as a
surfactant (0.25 wt. % with respect to salt).
The synthesis method is the one described previously in section “5.2.1 Synthesis of
nanofluids”. The eutectic NaNO3-KNO3 (49-51 mol %) has been selected for this
section because of its lower melting temperature. The amount of salt synthesized is
2000 mg per batch (200 ml of water for the solution). This amount of salt is a large
enough sample to be able to observe any stratification in the vials after several
melting and freezing cycles.
5.5.3. Results and discussion
Figure 5-47 shows the nanoparticle dispersions during the water—solution step
after sonicating the dissolved nanofluids. The main difference between the four
nanoparticles dispersions synthesized while they are dissolved in water is the color
of the sonicated solution. While SiO2 and Al2O3 make the salt-water solution
slightly whiter and less transparent, CuO presents brownish translucent solution.
On the other hand CNT shows an opaque black solution. After sonication the
nanofluids are stable and homogeneously dispersed.
Neat salt SiO2 Al2O3 CuO CNT
Figure 5-47. Neat salt and nanofluids (dissolved in water) after sonication.
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
244
The nanofluid -water solution is then evaporated on a hot plate at 200ºC. Once it
has been completely dried, the salt crystals is scratched from the beaker and milled
in a mortar. The resulting nanoparticle-salt powder can be seen in Figure 5-48.
SiO2-nanofluid CuO-nanofluid
Figure 5-48. The nanofluid powder after solvent water evaporation is ground in a
mortar and placed in smaller vials for the thermal cycling tests
Figure 5-49. Solid-state nanofluid before melting, from left to right: neat salt
NaNO3-KNO3 eutectic, SiO2 nanofluid, Al2O3 nanofluid, CuO nanofluid, and CNT
nanofluid with gum arabic (GA) dispersions.
As mentioned above, the homogeneity and dispersion stability of these
nanoparticle suspensions has been typically assessed in previous studies by
ensuring the salt - nanoparticle - water solution is well dispersed during the first
step of the synthesis procedure and subsequent solid phase SEM images to assess
the nanofluid particle dispersion quality.
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Thermal Energy Storage for High Temperature Applications
The first melt of the same vials from Figure 5-49 can be seen in Figure 5-50.
Qualitative differences can be easily seen between each nanoparticle type colloid:
the SiO2 nanofluid seems to have the most homogeneous appearance, followed by
the Al2O3 nanofluid. On the contrary, the CuO mixtures seem clearly stratified even
during this initial melt. The CNT+salt solution presented an interesting
phenomenon, growing in volume as the salt is heated up and melted.
Figure 5-50. Molten state nanofluid during the first melting process on the hot
plate at 350ºC from left to right: neat salt NaNO3-KNO3 eutectic, SiO2 nanofluid,
Al2O3 nanofluid, CuO nanofluid, and CNT with gum arabic nanofluid.
Each of the nanofluid samples is then subjected to the same thermal cycle on hot
plate: each vial is heated until it is fully melted and then cooled down to solidify.
This process is repeated six times.
Figure 5-51. Difference between SiO2, Al2O3, and CuO nanofluids after 6
melting and freezing cycles.
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
246
The different types of nanofluid behave very differently during this
melting/freezing thermal cycling process. Figure 5-51 shows the SiO2, Al2O3 and
CuO nanofluids fully melted where significant qualitative differences can be
observed after the six melting/freezing cycles.
Ideally one would expect to see a homogeneous single color fluid, similar to the
initial water solution after sonication during the synthesis process. If these
nanoparticles are well-dispersed, due to their small size, they might change the
color of the fluid but agglomeration should not occur. However, the observation of
the nanofluid in molten state (Figure 5-50) indicates that having a good dispersion
during the water solution step of the synthesis procedure is a necessary but not a
sufficient condition to ensure a well-dispersed colloid. Moreover, a clear convective
movement is observed in every single nanofluid in the molten state. Additionally,
in some cases, the nanoparticles appear to concentrate at the bottom of the vials
(Al2O3 and CuO nanofluids in Figure 5-51).
The SiO2 nanofluid is the only nanofluid tested able to maintain the nanoparticles
dispersed in the liquid state salt without showing any precipitate (it is not possible
to determine in the solid state since both SiO2 particles and solid salt are white).
However, the SiO2 nanoparticles appear to be fully aggregated forming 0.2-0.5 mm
visible clusters. The convective movement and the SiO2 aggregates can be seen in
Figure 5-52. Note that the average aggregate size does not vary between thermal
cycles in the SiO2 nanofluid case.
Figure 5-52. Molten SiO2-nanofluid on a hot plate at 350ºC after 2, 3, 5 and 6 (left to
right) thermal cycles.
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Thermal Energy Storage for High Temperature Applications
The Al2O3 nanofluid shows a precipitate at the bottom of the vial indicating the
impossibility of maintaining the nanoparticles in suspension.
Figure 5-53. Molten Al2O3-nanofluid (left) and in the hot plate at 350ºC. CuO
nanofluid (right) in molten state and solidified after several freeze/thaw cycles
showing nanoparticle stratification
Similarly to the previous oxide nanoparticles, CuO cannot be maintained
suspended the molten salt. Figure 5-53 (right) shows black CuO nanoparticles
precipitated at the bottom of the vial. Even after slightly stirring the molten
nanofluid with a needle some nanoparticles are still found at the bottom. Some
other micron-sized particle clusters are observed suspended within the salt.
The images presented indicate the inability of different types of nanoparticles at 1
wt % to be kept homogeneously dispersed in the molten eutectic sodium-
potassium nitrate mixture.
5.5.3.1. Scanning Electron Microscopy Characterization
The characterization of several samples has been performed using Scanning
Electron Microscopy (SEM). The samples extracted after the stability test show
significantly bigger clusters which correlate to the macroscopic observations.
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
248
Figure 5-54. SEM (left) and SEM-EDS analysis (right) of the nanofluid eutectic
NaNO3-KNO3 (45-55 wt. %) with 1% CuO after the stability test
Figure 5-55. SEM analysis of the nanofluid eutectic NaNO3-KNO3 (45-55 wt. %)
with 1% CuO after the stability test
Similarly, the eutectic NaNO3-KNO3 (45-55 wt. %) with 1 wt. % of SiO2 shows also
aglomeration of nanoparticles when the samples are observed after the stability
test. Their appearance resembles the previous nanofluid (Figure 5-28) with 5 wt. %
of SiO2 although with smaller silica clusters.
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Thermal Energy Storage for High Temperature Applications
Figure 5-56. SEM (left) and SEM-EDS analysis (right) of the nanofluid eutectic
NaNO3-KNO3 (45-55 wt. %) with 1 wt. % of SiO2 after the stability test
Figure 5-57. SEM analysis of the nanofluid eutectic NaNO3-KNO3 (45-55 wt. %)
with 1 wt. % of SiO2 after the stability test
The SEM analysis of the eutectic nanofluid NaNO3-KNO3 (45-55 wt. %) 1 wt. % of
SiO2 (Figures 5-56 and 5-57) and CuO (Figures 5-54 and 5-55) nanoparticles shows
the presence of micro-size clusters, which is aligned with the macroscopic
observation (sedimentation and visually noticeable aggregates), but it is in
opposition to the well dispersed nanoparticles analyzed once synthesized after
testing in the DSC. These observations suggest that the nanoparticle distribution
could be homogenous after the synthesis process for low nanoparticle
concentrations, and it might remain when testing in the DSC. However, if we melt
a larger batch of nanofluid (grams), the nanoparticles tend to agglomerate forming
micro-size clusters.
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
250
5.5.4. Conclusions
A series of experiments analyzing the macroscopic behavior of the molten eutectic
NaNO3-KNO3 mixture adding 1wt. % of different nanoparticle types (CNT, SiO2,
CuO and Al2O3) have been performed. The objective was to investigate whether
having a homogenous dispersion in the water solution step of the synthesis process
is a sufficient condition to guarantee the dispersion stability once the solvent has
been removed. Experiments we performed to check if different types of
nanoparticles can be kept homogeneously dispersed in nitrate base molten salts
after several melting and freezing cycles. The enhancement of any thermophysical
property suggested in the nanofluid literature such as enhanced thermal
conductivity or enhanced specific heat requires a homogeneous dispersion of the
nanoparticles for any engineering application.
It appears that researchers assume that synthesizing well-dispersed nanofluid in
water will ensure a good dispersion of the nanoparticles in the salt once the water
solvent is evaporated. The experimental results indicate that this assumption is a
necessary but not sufficient condition to ensure a well-dispersed nanofluid in the
molten state.
The visual observation of the salt in liquid state showed that none of the nanofluids
tested are able to maintain the stability of the nanoparticle suspension. Once the
salt is melted for the first time the color of the molten salt changes. Depending on
the nanoparticle type, different degrees of stratification is observed: CNT at the top,
CuO and Al2O3 (bottom). In the case of silica, a whitish cloud is clearly observed
suspended within the salt. Even though the silica nanoparticles do not precipitate
they are far from being well-dispersed.
The qualitative results presented in this section call into question most of the recent
literature on high temperature molten salt nanofluids. The different appearance
than a homogeneous mixture might indicate that regardless the large number of
publications in the field its usefulness is far from being a reality.
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Thermal Energy Storage for High Temperature Applications
5.6. Summary and conclusions
This chapter analyzes the effect of nanoparticles on the specific heat capacity and
latent heat of nitrate molten salts with the goal of increasing the storage capacity of
solar thermal plants. The extensive literature from the past years is to this date still
unclear about the possibility and magnitude of potential specific heat capacity
enhancement mechanisms.
However, the measurements performed in this chapter do not support the
existence of an anomalous specific heat enhancement caused by the addition of
nanoparticles to nitrate base molten salts. No statistical evidence has been found
relative to the moderate enhancement measured in this research. This
enhancement, combined with other phase change parameters suggest that
composition shifts can be produced during the synthesis process. Moreover, the
large uncertainty in the molten salt specific heat capacity measurements (without
nanoparticles) questions the significance of some of the reported data.
The main contributions of this work are the following:
The two-step water solution synthesis process, extensively used in the
literature for these mixtures, leads to component segregation during the
solvent evaporation step. This composition shift effect is more noticeable when
the evaporation is faster or using containers with a larger surface area.
The solubility of the different salt components has been demonstrated to be
key property that might explain the Cp enhancement shown in the literature:
NaNO3 (component with a larger Cp) is less soluble in water and, therefore,
will start precipitating from the mixture first as the solvent water is evaporated.
Initial areas of salt+nanoparticle precipitation form more “homeogenously”
looking crystals, which have been described as less-agglomerated nanofluids
before. Thus, these areas contain a higher content of NaNO3 and will have
consequently a higher Cp, regardless of the nanoparticles added (for low
enough concentrations). This is demonstrated with measurements of neat salts
(without nanoparticles) showing “enhancement” by following different
evaporation processes without adding any particles.
Sapphire corrections should be performed carefully for specific heat capacity
measurements over a wide temperature range, as they can easily increase the
measurement error. DSC equipment calibration must be performed regularly
Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage
252
and more frequently with molten salts, as these materials creep, escape the
crucibles and contaminate the DSC sensors during testing.
The latent heat of nanofluid has been observed to decrease in a larger amount
than the theoretically predicted, against limited existing investigations with
molten salts but aligned with extensive results from lower temperature phase
change materials with nanoparticles for increased thermal conductivity.
The existence of an interfacial liquid layer has been hypothesized as
responsible for this larger reduction in latent heat calculating the layer
thickness for each base fluid. The calculated layer thickness seems to be
reasonable, not discarding this hypothesis. However the phase transition of
this hypothesized compressed liquid layer has not been observed
experimentally.
Finally, and most importantly from an industrial impact point of view, this is
the first time the stability of molten salt nanofluids have been evaluated in the
liquid state at high temperatures (above 300ºC). The results show the inability
of the molten salt to keep different nanoparticle types dispersed, even though
the different steps during the synthesis procedure show homogeneity.
Some thermo-physical properties might be improved trough the addition of
nanoparticles to the base salt, some expected such as thermal conductivity when
high thermal conductivity particles are added and other unexpected enhancement
such as specific heat, which has not been reproduced along this investigation, both
desirable for thermal storage materials, the first one directly related to the heat
transfer rate, and the second one related to the amount of energy that can be stored
in solar thermal plants. However, the nanoparticle agglomeration and stratification
presented in this study is clearly indicating that more effort is needed to design a
stable, functional and realistic heat transfer nanofluid for any engineering
application.
253
6 GENERAL CONCLUSIONS AND
FUTURE CHALLENGES
he aim of this thesis was to explore different techniques in order to increase
the storage capacity of the current TES systems in commercial concentrated
solar tower power plants. The two commercial TES technologies are: steam
accumulators and the two molten salt tanks. Both systems use sensible heat
storage: the first one in pressurized saturated liquid water and the second
increasing the temperature of molten salt. As alternatives to the current TES
technologies this thesis investigates the use of a) latent heat TES by encapsulating
phase change materials, and b) the modification of the thermo-physical properties
of molten salts through nanoparticle addition.
The use of phase change materials (PCM) has been explored in Chapters 2, 3 and 4
to increase the storage capacity of the steam accumulators. The goals accomplished
by this investigation are summarized as follow:
A wide material analysis has been performed starting from literature
values.
In the temperature range of interest the selected materials have been
T
General Conclusions and Future Challenges
254
characterized experimentally using differential scanning calorimetry. The
importance of testing is highlighted due to discrepancies observed
between previously reported values and those calculated through
thermodynamic programs. Some compositions proposed as PCM
candidates have shown off-eutectic behavior without melting in a narrow
temperature range, making them undesirable for PCM applications. For
the thermo-economic comparison of materials, the measured latent heat
has been used. Since the different compositions evaluated have not shown
substantial benefits in terms of latent heat compared to their pure main
constituent and for the sake of simplicity during the encapsulation proof-
of-concept, pure salts (NaNO3 and KNO3) and certain metals (lead and tin)
were selected as PCM for subsequent sections.
The use of PCM requires the development of a functional container where the
phase change takes place and a heat exchanger design incorporating such
container to transfer heat back and forth to the heat transfer fluid (e.g. steam).
Here the use of a packed bed storage system is proposed, which requires the
design, development, and testing of capsules which has been performed in
Chapter 3.
An extended literature review of experimental macro-encapsulated PCM
studies has been performed identifying borosilicate as an interesting and
novel shell candidate because of its thermal properties, corrosion
resistance, and chemical compatibility with both the heat transfer fluid
(high pressure steam) and a wide variety of high temperature PCM (salts
and metals).
Different borosilicate capsules have been manufactured and filled with
different PCM materials. The designed capsules are in line with
contemporary studies in terms of size, temperature range, and shell/PCM
volume ratio. The appropriate degree of filling to minimize internal
pressure has been determined to calculate the maximum allowed amount
of PCM, improving the energy density of previously reported capsules.
An experimental set up has been developed to validate the capsule
concept by melting and freezing different capsules combining the
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Thermal Energy Storage for High Temperature Applications
information provided by a video camera and infrared camera to analyze
the phase change process. It is the first time the solid-liquid transition of
capsules of such sort has been visually analyzed at such temperatures and
through thermal imaging, describing the phase transition behavior
corresponding to the characteristics of each type of PCM.
A numerical model has been implemented (Chapter 4) to help understand the
influence of each capsule design parameter in the phase change time. The model is
capable of capturing the main physics taking place and is used to help understand
the experimental results. Simulations and experiments have been compared
qualitatively and quantitatively identifying some sources of uncertainty that could
explain the mismatch between them.
Finally, the addition of nanoparticles to improve the storage capacity of TES
systems based on molten salts has been investigated in Chapter 5. The initial goal
was to reproduce the specific heat enhancement reported in the literature by
several research groups. Unfortunately, the different nanoparticle concentrations,
sizes, types, and different base fluid compositions tested did not show any
remarkable and statistically significant enhancement of the specific heat of nitrate
salts. Modifications in the synthesis process have been made, discovering that the
water-solution evaporation step seems to separate salt in different regions with
slightly different compositions, which could explain slight modifications in the
specific heat and latent heat.
During this investigation some assumptions made in the synthesis process by
different research groups have been questioned. The water dissolution-sonication-
evaporation synthesis protocol, the most widely used, does not seem to be
appropriate because specific samples with higher (or lower) Cp than the average
could be selected even without nanoparticles. The ability of molten salt nanofluids
to keep the nanoparticles in suspension and well dispersed has been also
questioned. Scanning electron microscopy images did not show the presence of
“special” or “anomalous” nanostructures in the salt.
The latent heat of molten salt nanofluids has been also investigated. The latent heat
has been observed to decrease in a larger amount than the theoretically predicted,
against limited existing investigations with molten salts but aligned with extensive
General Conclusions and Future Challenges
256
results from lower temperature phase change materials with nanoparticles for
increased thermal conductivity. Finally, we have hypothesized the existence of an
interfacial liquid layer as responsible for this larger reduction in latent heat
calculating the layer thickness for each base fluid.
Future challenges involve several aspects: on one hand, finding a scalable
encapsulation fabrication process for high temperature PCM using borosilicate. The
packed bed solution is, however, dependent on the development of cost-efficient
steam tanks to contain the capsules. On the other hand, a more efficient system to
exchange heat with high pressure steam is a casing and tube heat exchanger, which
minimizes the surface exposed to high pressure when the vapor circulates through
the tubes. Following this idea, the solution based on double PCM should be further
analyzed numerically and experimentally.
In the field of high temperature nanofluids a large amount of work is required in
order to apply this technology at a commercial scale such as:
a) Develop a novel, scalable synthesis process that does not require the
dissolution of the salt mixtures in water.
b) Guarantee the stability of the molten salt nanofluid at high temperatures
(e.g. in the liquid state) exploring different additives, since common
surfactants will thermally degrade at these temperatures.
One of the main challenges in this field is to end the controversy on the specific
heat enhancement of nanofluids. A Round Robin test could be proposed where the
same nanofluid is tested by different research groups.
Unfortunately, the corrosive nature of molten salts, their tendency to absorb any
ambient moisture, and the high testing temperatures of interest (above 300ºC)
makes experimentation with molten salt nanofluids extremely difficult. The lack of
standard laboratory equipment to test such high temperature colloids increases the
effort to measure and quantify the effect of such nanoparticles. Further advancing
in the understanding of ionic liquid based nanofluids, liquid at room temperature,
may help evaluating the possibilities of extraordinary enhancements not deducible
a priori from general principles.
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Thermal Energy Storage for High Temperature Applications
The optical properties of the nanoparticles with the transparency of the molten salts
could be used in other applications leading to several research projects. For
example, the evaluation of ceramic/quartz transparent solar receivers using nano-
modified heat transfer fluids could have potential advantages.
General Conclusions and Future Challenges
258
259
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