Selected Solar-to-heat/Solar-to-electricity · PDF fileD4.8 Selected...

SOL2HY2 - Solar To Hydrogen Hybrid Cycles

Contract n. 325320

D4.8 Selected Solar-to-heat/Solar-to-electricity concept

SOL2HY2_ D4.8.doc Page. 1

SEVEN FRAMEWORK PROGRAMME

FCH-JU-2012-1

SP1-JTI-FCH.2012.2.5 :

Thermo-electrical-chemical processes with solar heat sources

Contract for:

Collaborative project


Project acronym: SOL2HY2

Project full title: Solar To Hydrogen Hybrid Cycles

Contract no.: 325320

Date of issue: 11

th July 2014

Revision: 1

Category: PU

Start date of contract: June 1st, 2013

Duration of the project: 36 months

Project coordinator name: Stefano Odorizzi

Project coordinator organisation name: ENGINSOFT S.p.A.


Contract n. 325320



D 4.8 Selected Solar-to-heat/Solar-to-electricity concept

Table of Contents:

1. Introduction Page 3

2. Deliverable main contents Page 5

3 Conclusions

4 Acronyms

5 References

Page 49

Page 50

Page 51


Contract n. 325320



1. Introduction

1.1. Generalities

The present report deals with the theoretical work carried out in the field of thermal storage of the

CSP and its coupling with the plant to produce hydrogen, according to SOL2HY2 project.

ENEA, DLR, AALTO, Erbicol and EnginSoft have collaborated to realize this report.

In particular DLR focused its attention on the High Temperature heat production and storage

providing its experience on Solar Tower applications.

ENEA applied its experience on the molten salts (NaNO3-KNO3 mixtures) utilization as thermal

storage mean and thermal fluid to find the best solutions to couple the Hydrogen production plant with

the Medium Temperature Solar source and analysing different solutions.

AALTO focused its attention on the Phase Change Materials (PCM) as alternative solution for the

heat storage.

ERBICOL supplied some solutions concerning materials and structures for HT heat storage.

ENGINSOFT introduced the MCDM options to be considered to suggest the most promising

integration schemes for coupling the solar plants (including thermal storage) with the Hydrogen

production plant.

1.2. Objectives

The WP4 is focused on the design of the interface between the power source (CSP plants) and the

thermo-chemical plant. This involves the identification/selection/design/scale up of the most suitable

components for the high- (~800-1000°C) and medium- (300-550°C) temperature applications,

including: solar collectors, solar receiver, heat transfer fluid, heat storage system, heat exchangers and

interfaces with the chemical sections developed in other WPs and possible solar/thermo-electrical

systems.

In particular the task 4.1 is focused on the optimization of the coupling of the thermo-electrochemical

plant with the power source, i.e. the Concentrated Solar Power (CSP) plant. The results of the first

analysis incorporate the specifications of the potential selected layouts for the solar/thermo-

electrochemical plant matching.

In fact, the results of this task will contribute to the realization of the virtual plant and to realize the

necessary hardware to provide a suitable coupling between the CSP and the hydrogen production

plant.

Concerning the realization of the real plant, only some critical equipments will be realized at

demonstrative scale and in this case the thermal storage and the heat transfer equipments are already

available but need integration with direct current (DC) and, heat balance and new material solutions,

mainly for the HT aspects.

Nevertheless for the virtual plant, it is necessary to integrate all the involved parts: H2SO4

concentration, H2SO4 vaporization, SDE, H2SO4 cracking and so on. Some of these must work in

continuous and other not, but need high temperature or electricity.


Contract n. 325320



In this field the correct integration between HT and MT plant, the optimized use of energy and storage

is essential and this deliverable contributes to give a panorama of possible solutions, analyses some

critical aspects and contributes to give a first solution to the realization of the whole virtual plant and

to design and run some field tests of key blocks, especially for the production of thermal and electrical

energy.

The results and the solutions individuated in this work package will be also important to the Techno-

economic analysis of the WP6 (T6.3).

With this aim the possibility to assess the mirror surface or thermal storage for different sites or the

implications to vary the percentage of energy provided by MT solar plant on the total is analysed.

Concerning the MT CSP plant, the solution using parabolic solar trough with molten salts as thermal

vector and thermal storage is mostly analysed, because at present guarantees a maximum operating

temperature of 550 °C, modularity, high efficiency of heat to electricity transformation, no toxicity, no

environmental problems, availability of the technology and good integration in the SOL2HY2 system;

nevertheless the use of alternative solutions including PCM systems, mixtures with inclusions,

particular kinds of storage systems are analysed.

The present report initially presents a State of Art on CSPs, in particular Solar Tower and Solar

Trough and makes available the DLR experience on the Julich facilities and the ENEA one for the

Solar Trough.

Successively it describes the principal aspects which affect the solar collection efficiency, in order to

optimize the system, taking into consideration the irradiance available in the analysed sites.

It describes the heat storage concepts concerning the utilization of molten salts in two tanks (the hot

and the cold one), in one tank (exploiting the difference in density with temperature) and in form of

PCM (Phase Change Material with the addition of inclusions as TIO2) and the heat storage concepts

suitable for high temperature storage and suitable for the Solar Tower technology.

Some descriptions on the cycle and the equipments to be used for the electrical energy production and

the related efficiency are also provided.

Finally some important concepts on the material and structures for the heat temperature storage and

the options to be considered to suggest the most promising integration schemes for coupling the solar

with the hydrogen production plant are provided.


Contract n. 325320



2. Deliverable main contents

2.1 CSP introduction

Concentrated solar power has been under investigation for several decades. In the CSP, direct solar

radiation is concentrated to heat a so-called heat transfer fluid, which runs an engine connected to a

generator, producing thus electricity. This heat can also be transferred to a receiver-reactor and be

used for thermo-chemical reactions.

CSP is a variable energy source, like solar photovoltaics and wind, but contrary to these last ones, it

can easily be coupled with thermal energy storage (TES) making it dispatchable. Recently, a lot of

improvements and efforts have been done in order to make CSP more cost effective. Ongoing research

works are in the areas of reflector and collector design and materials, heat absorption as well as

thermal storage [R 1].

CSP plants use mirrors to concentrate the sun rays and produce heat and steam for electricity

generation via a conventional thermodynamic cycle.

Unlike solar photo‐voltaic, CSP uses only the direct component of sunlight, namely the direct normal

irradiance (DNI) and provides carbon‐free, cost‐effective heat and power only in regions and countries

with sufficient level of DNI (DNI>2000 kWh/m2‐yr). DNI represents up to 90% of the total sunlight

during sunny days but is negligible on cloudy days

Countries with high DNI level typically include sunny and dry regions between 15°and 40° latitude

North or South of the Equator. Equatorial regions are not particularly suited to CSP because of

frequent clouds and rain. In contrast, high DNI can also be available at high altitude above the sea

level where the light scattering is lower. In most favorable regions the DNI can be as higher as 2800

kWh/m2‐y.

In these regions, the CSP electricity generation potential can reach 100‐130 GWhe/km2/y. This is the

same electricity amount generated annually by a 20 MW coal‐fired power plant, with a 75% capacity

factor.

Best world's regions for CSP installation include the Middle East and North Africa (MENA), South

Africa, Southwestern United States, Mexico, Chile, Peru, Australia, India, Western China, Southern

Europe and Turkey.

However, latitude and altitude alone are not sufficient to characterize a site for CSP installation. Other

key factors include general and local meteorological conditions (e.g. raining, fog, velocity of

dominant winds, general meteorological stability), and site flatness to facilitate the installation of CSP

components, which require significant land use in comparison with conventional power technologies.

CSP plants can be equipped with a heat storage system to generate electricity even when the sky is

cloudy or after sunset. During sunny hours, solar heat can be stored in high thermal‐capacity media

(e.g. fluids). It is then released upon demand (e.g. after sunset) to produce electricity. Thermal storage

can significantly improve the economic competitiveness and the grid integration of CSP plants

because it increases the plant capacity factor, reduces the electricity cost, and improve the

dispatchability of the CSP electricity, namely the ability of the plant to provide electricity on the

operator’s demand.


Contract n. 325320



Plant Capacity Factor is usually given as a percentage (%) of operating hours in a year. Conventional

base‐load power plants, in fact, are usually operated at full (nominal) power. Therefore, the capacity

factor is usually defined as the number of hours per year the plant produces electricity at full

(nominal) power. In the solar plants, the power level and the electricity production depend on solar

irradiance. Therefore, the capacity factor is often referred to as the ratio of the actual electricity

generation in a year to the electricity that the plant could in theory produce annually by operating

continuously at full (nominal) power.

To provide the required heat storage capacity, the solar field (i.e. solar heat collectors) of the CSP

plant must be appropriately oversized with respect to the nominal electric capacity (MW) of the power

plant. For this purpose, solar multiple (SM) is the ratio of the actual size of the solar field to the solar

field size needed to feed the turbine at nominal design capacity with maximum solar irradiance. To

cope with thermal losses, plants with no storage have a solar multiple between 1.1‐1.5 and up to 2.0

for the Linear Fresnel Reflectors (LFR) while plants with thermal storage may have solar multiples up

to 3‐4.

There is a trade‐off between the incremental cost associated with the thermal storage and the

economic benefit from increased electricity production. Significant research efforts focus on thermal

storage for CSP plants because this is one of most cost‐effective options for large‐scale energy (and

electricity) storage.

While CSP plants produce primarily electricity, they can also produce high‐temperature heat which

can be used for industrial processes, space heating (and cooling), and other processes such as water

desalination.

The first commercial CSP plants (with no thermal storage) were built in California between 1984 and

1991, with a total capacity of 354 MW distributed in nine units (i.e. SEGS project). After a period of

stagnation due to the low price of fossil fuels, the interest in CSP resumed in the 2000s, mainly in the

United States and Spain, as a consequence of energy policies to mitigate CO2 emissions and diversify

the energy supply.

At present, Spain and the United States are leading countries for CSP development and deployment,

while Germany and Italy contribute significantly into the CSP development. Other countries (e.g.

Saudi Arabia) have announced ambitious CSP deployment plans.

CSP plants are now in operation, under construction or planned in a number of countries. In 2012 the

global installed CSP capacity amounted to about 2 GW up from 1.3 GW in 2010, and an additional

15‐20 GW are currently under construction or planned all over the world.

The available operational experience suggests that a CSP plant can be built in 1‐3 years (depending on

its size), can be operated for more than 30 years and returns the energy used for its construction in

about six months of full‐power operation. The gross CSP land‐use is estimated at approximately 2 ha

per MWe.

Although CSP is not yet economically competitive with conventional coal and gas‐based power, the

CSP industry has been growing rapidly over the past decade. In comparison with other renewable

power sources (e.g. PV and wind power) the competitiveness of CSP plants should be assessed taking

into account two key elements: the important role of the energy storage and the significant potential


Contract n. 325320



for reduction of the CSP investment cost due to technology and industrial learning. CSP technology

includes four variants: Solar Tower (ST), Parabolic Trough (PT), Fresnel Reflector (FR), and Solar

Dish (SD).

2.1.1 State of the art of solar tower technology

The solar power tower converts sunshine into electricity for the world’s electricity grids.

The technology utilizes many large, computer controlled, sun-tracking mirrors, so called heliostats to

focus sunlight on a receiver at the top of a tower (Figure 1).

Figure 1: Central tower concept [R 2]

A heliostat field can consist of multiple up to several thousands of heliostats, each individually two-

axis tracked. The heliostats reflect the irradiation onto a receiver which is typically mounted above the

heliostat field on a tower. This receiver transforms the solar radiation into heat. Indeed a heat transfer

fluid heated in the receiver absorbs the highly concentrated radiation reflected by the heliostats and

converts it into thermal energy. This heat is coupled through a heat exchanger to a conventional steam

cycle to produce electricity.

Early power towers (such as the Solar One plant) utilized steam as heat transfer fluid; current US

designs utilize molten nitrate salt because of its superior heat transfer and energy storage capabilities.

Current European designs use air as heat transfer medium because of its high temperature and its good

handleability.

Solar towers typically stand about 75-150 m height [R 3]. These plants are best suited for utility-scale

applications in the 10 to 200 MWe range [R 4].

2.1.2 Molten Salt Solar Power Tower

The viability of solar towers was demonstrated by Solar One, a 10 MW plant, located near Barstow

CA in 1981. Then, Solar Two (Figure 2) was built in 1995, as retrofit of Solar One, to demonstrate the

advantages of molten salt for heat transfer and solar storage. At one point it delivered power to the

grid for seven days, 24 hours a day during cloudy weather. Molten salt solar towers are well suited to


Contract n. 325320



peaking power applications, being able to generate power when most needed, day or night, cloudy or

sunny. In a molten-salt power tower, the molten nitrate salt, which is a clear liquid with properties like

water at temperatures above its 240°C melting point, is pumped from a large storage tank to the

receiver, where it is heated in tubes to temperatures of 565°C [R 5]. The salt is then returned to a

second large storage tank, where it remains until needed by the utility for power generation. At that

time, the salt is pumped through a steam generator to produce the steam to power a conventional,

high-efficiency steam turbine to produce electricity. The salt at 285°C then returns to the first storage

tank to be used in the cycle again.

Figure 3 is a schematic diagram of the primary flow paths in a molten-salt power plant.

Figure 2: Solar Two, Barstow, CA

Figure 3: Molten salt power tower system schematic

The 17 MWel Gemasolar plant (see Figure 4) developed by Torresol Energy and connected to the grid

in 2011 uses also molten salt as heat transfer fluid and for thermal storage. This plant is designed to

operate round-the-clock in summertime [R 6].

Figure 4: Gemasolar plant of torresol Energy in Andalucia, Spain

http://thefraserdomain.typepad.com/photos/uncategorized/solar_two.jpg


Contract n. 325320



2.1.3 Air solar power tower

The 1.5 MWe solar thermal experimental and demonstration power plant in Jülich, Germany (Figure

5) was declared operational in December 2008. It is the world’s first solar thermal power plant erected

which uses air as the medium for heat transport. Air was chosen as the heat transfer medium, since it

is freely available, non-toxic, and does not require freeze protection during times of non-operation.

Over 2,000 movable mirrors concentrate the solar radiation onto the receiver at the top of the 60-metre

tower. The receiver is a volumetric receiver consisting in porous ceramic structure on which the solar

radiation is focused and through which incoming ambient air flows. This receiver was developed and

patented by DLR. The volumetric effect increases the efficiency. The air cools the outer parts of the

receiver and is heated up gradually to the design temperature level at the inner surface. In passing

through the receiver, the air is heated to around 700°C and this heat is delivered to the water-steam

cycle in a heat recovery boiler. Steam parameters may be up to 100 bar and 500°C, which is in the

typical range for medium-sized conventional power plants. The steam generated there drives a

turbine/generator and returns in form of condensate to the steam generator (Figure 5). The receiver

can be a tube receiver or a volumetric receiver, consisting of porous ceramic structures, e.g.

honeycombs or foams, which are heated by the concentrated solar radiation. In such a case air is

sucked through the ceramic structures and hence heated.

Figure 5: Jülich Solar tower plant (left) and plant layout (right) [R 7]

2.2. Solar Trough Technology and ENEA experience

As already mentioned, CSP technology includes four variants: Parabolic Trough (PT), Fresnel

Reflector (FR), Solar Tower (ST) and Solar Dish (SD). In PT and FR plants, mirrors concentrate the

sun’s rays on a focal line, with concentration factors of 60‐80 and maximum operating temperatures of

about 550°C. In ST and SD plants, mirrors concentrate the sunlight on a single focal point, with

higher operating temperatures and concentration factors (up to 10‐fold higher).


Contract n. 325320



At present, Parabolic Trough is the most mature and cost effective CSP technology accounting for

more than 90% of the current installed CSP capacity.

It is based on linear parabolic mirrors that concentrate the sun’s rays on heat receivers (i.e. steel tubes)

placed on their focal line.

Receivers have a special coating to maximize the energy absorption and minimize the infrared

re‐irradiation. The receiver tubes are placed inside an evacuated glass envelope to avoid convection

heat losses.

Solar heat is removed by a heat transfer fluid (e.g. synthetic oil, molten salt) flowing in the receiver

tube. It is then transferred to a steam generator to produce super‐heated steam and run the turbine.

Mirrors and receivers (i.e. the solar collectors) track the sun along a single axis (usually, East to

West).

Most PT plants that are currently in operation have capacities between 15 and 100 MWe, efficiencies

of around 14‐16% (i.e. the ratio of net electric output to solar energy input) and a maximum operating

temperature of 390°C, which is limited by the degradation of the synthetic oil that is used as the heat

transfer fluid. Some of these plants have a thermal storage system based on the use of molten salt as a

storage fluid, which storage temperature is also limited by the oil maximum temperature.

Apart from the SEGS project, recent major PT projects in operation include two 70‐MW units in the

United States (i.e. Nevada Solar One and MNGSEC‐Florida), a number of 50‐MW units in Spain and

smaller units in a number of countries. As mentioned, some 50‐MW power plants in Spain use

synthetic oil as the heat transfer fluid and molten salt as the thermal storage fluid, with a thermal

storage capacity of around 7.5 hours and a capacity factor of up to 40%.

As of January 2013, large PT plants under construction include the Mojave project (250 MW,

California, 2013), the Solana project (280 MW, Arizona, 2013), the Shams 1 project (100 MW,

United Arab Emirates, 2012), the Godawari project (50 MW, India, 2013) and further 50‐MW plants

in Spain.

CSP plants are designed for electricity generation, but they can also produce high‐temperature heat

that can be used for industrial heating, water desalination, production of synthetic fuels (e.g syngas),

oil refinery, and enhanced oil recovery (EOR) in mature oil fields.

The use of CSP plants for production of electricity and heat, and water desalination is of particular

interest in arid regions where CSP can provide either electricity for desalination by reverse‐osmosis or

heat for desalination by thermal distillation. CSP can also be integrated into hybrid CSP/fossil fuel

plants in order to produce fully dispatchable electricity.

In this case, the solar field provides steam to the thermodynamic cycle of the conventional power

plant.

Projects based on this concept are in operation in Algeria, Australia, Egypt, Italy and the United

States.

In the period 2001‐2010 ENEA has developed a novel concept of CSP PT plant using

high‐temperature (550°C) molten salt either for heat transfer and heat storage purposes. The use of

high‐temperature molten salt not only increases the efficiency of the plant but also increases

significantly the capacity of the thermal storage system and/or reduces its costs.


Contract n. 325320



However, the use of high‐temperature molten salt requires new technologies for key components of

the CSP plant such as solar collectors, heat receivers, and the heat storage system. These new

components and systems have been developed and patented by ENEA, and tested at the full‐scale CSP

test facility (Figure 6) that was built since 2004 at the ENEA Casaccia Labs in Rome and has been

working now for more than 20,000 hours. Of particular importance are the high‐temperature

heat‐receiver tubes working under vacuum at 550°C, with a special coating to maximise heat

absorption and minimise losses. Italian companies have been involved in the R&D programme and

currently produce the receiver tubes and other components under ENEA license.

A 5‐MWe demonstration plant (Archimede Solar Plant) with about 6.5 h thermal storage was built by

ENEL, the largest Italian utility, in Sicily (Italy) based on ENEA patents. The plant started the

operation in July 2010 and was the first one in the world using molten salt as either heat transfer and

storage fluid. The novel technology is expected to improve significantly the storage performance and

the capacity factor of CSP plants up to more than 60%.

The Archimede power plant is intended to test components and systems of the new technology under

real operating conditions. The solar plant is part of a 760 MW gas‐fired combined cycle power station

and uses the same steam turbine of the conventional power block. This arrangement allows the

demonstration to focus only on innovative components, i.e. solar field, solar collectors, heat receivers,

storage system, molten salt operation and piping.

The hot tank provides heat to the steam generator to produce superheated steam at 535°C. ENEL is in

charge of the plant demonstration programme.

The use of high‐capacity storage systems improves the flexibility of CSP plants, which can provide

either dispatchable electricity and high‐temperature heat for industrial or residential use. Moreover,

molten salt (a 60‐40% mix of KNO3‐NaNO3) is safer (non‐flammable), cheaper and more

environmentally friendly in comparison with synthetic oil.

ENEA is currently involved in further optimization of the CSP technology, including alternative heat

transfer/storage fluids, streamlined heat storage systems, improved heat receivers, as well as CSP

applications based on the use of high‐temperature solar heat (e.g. production of synthetic fuels via

reforming and thermo‐chemical processes, water desalination via thermal process or reverse osmosis).


Contract n. 325320



Figure 6: Solar collector s' tests at the ENEA Casaccia Labs (Rome)

2.3 Efficiency evaluation

2.3.1 Efficiency evaluation of solar tower technology

Optical efficiency of heliostat field

The solar collector field of central receiver systems is of particular importance as it accounts for the

largest cost fraction of a solar power plant. Several loss mechanisms affect the optical efficiency of

the heliostat field. Most importantly is the cosine effect annually varying between 0.9 and 0.7 [R 4]

which arises from the angle between incoming radiation from the sun and reflected radiation to the

solar receiver. Other losses are attributed to shading (mirror surface shadowed by other heliostats),

reflectivity of the mirror, blocking (reflected radiation hitting other heliostats), atmospheric

attenuation (radiation absorbed by the atmosphere between heliostat and receiver), spillage (reflected

radiation missing the receiver) and, if applicable, transmission losses of secondary optics.

Depending on the geographic latitude and the use of secondary concentrators a surround field or a

north/south field can be favourable. Generally, surround field have advantages close to the equator,

while north (on the northern hemisphere) or south (on the southern hemisphere) fields become


Contract n. 325320



beneficial at higher latitudes. A study of Schmitz et al. [R 8] found that surrounding fields can also

become favourable for large fields if secondary optics are required.

Secondary concentrators are necessary for the clustering of cavity receivers, especially, if windows

are required which is the case for the receiver-reactor for sulphuric acid cracking developed in the

SOL2HY2 project. Particularly for large power plants high towers are required which, however,

increase optical losses due to atmospheric attenuation and spillage. Schmitz et al. [R 8] find that

systems with a single aperture using a north field have optimal tower heights which are about 30 %

higher than for multi aperture receivers using a surround field.

Typical optical efficiencies are reported by Kolb et al. [R 9]: a particle receiver for sulphuric acid

cracking in a hybrid sulphur cycle hydrogen plant is found to have an annual optical efficiency of

57 % resulting from a cosine efficiency of 85 %, a shading and blocking efficiency of 96 %, an

atmospheric attenuation efficiency of 88 % and an intercept efficiency of 90 %.

Thermal efficiency of receivers

The thermal efficiency of solar receivers is mainly determined by the fraction of radiative losses

which can never be fully avoided due to the required opening towards the solar collector field.

Generally, receivers for solar towers can be categorized into those with a cavity design and others

with an external design. In the first concept the area of the aperture is smaller than the absorbing

surface reducing radiative losses while in the latter case aperture and absorber have the same surface

limiting the achievable thermal efficiency. Solar receivers can either be directly irradiated using

volumetric absorbers being porous or consisting of particles with the heat transfer fluid flowing

through the open volume; or the receiver is indirectly irradiated so that the heat has to be conducted

through the wall of a tubular absorber before transferring it to the working fluid. Generally, higher

temperatures and thermal efficiencies are achievable with volumetric designs; however, pressuring

these systems requires the use of windows limiting the operating pressure.

Typical temperature and solar flux ranges of receivers for solar towers are given in Table 1.

Fluid Water steam Liquid sodium Molten salt

(nitrates)

Air

(volumetric)

Solar flux (MW/m2)

Average 0.1-0.3 0.4-0.5 0.4-0.5 0.5-0.6

Peak 0.4-0.6 1.4-2.5 0.7-0.8 0.8-1.0

Fluid outlet

temperature (°C) 490-525 540 540-565 700 to >1000

Table 1: Temperature and solar flux ranges of solar receivers for solar towers [R 4]

Thermal efficiencies of more than 80 % are possible with solar tower receivers. In a screening

analysis of various concepts for thermochemical hydrogen production Kolb et al. [R 9] report an


Contract n. 325320



annual thermal efficiency of 76 % for a solid particle receiver to decompose sulphuric acid in a hybrid

sulphur cycle hydrogen plant.

2.3.2 Solar Trough efficiency

Concerning solar trough efficiency due to plant geometry and design, it includes the optical efficiency

of the overall collector structure and the thermal efficiency of the receiver tube. The optical efficiency

of the solar collectors depends on the mechanical design (collector's geometry, interception factor),

quality of materials (reflectance of the mirrors,) and the accuracy of alignment. Thermal efficiency of

the receiver tube instead depends on the absorbance of the selective coating (emissivity as a function

of temperature), transmittance of tube glass, and intensity of the solar radiation.

However, an important factor is mirror and glass tube fouling during operation. Reflectivity declines

by some 14 percentage points in a month, corresponding to a 0.5% per day reduction.

Degradation rate depends on the site. In any case, periodical washing is needed to restore the design

reflectivity. A trade‐off exist between washing costs (frequency) and efficiency reduction. Weekly

cleaning determines a reflectivity mean value of about 90%. In the absence of fouling site‐relevant

data, an average reduction of the absorbed energy of about 5.5% could be considered, based on 26

cleaning cycle per year, with a water consumption of 0.7 L/m2.

Thermal energy collected by the solar field and efficiency of the heat capture is the key to assess the

annual electricity production and the overall efficiency of the CSP plant. This efficiency, also referred

to as optical efficiency, depends on the optical performance of solar collectors and the thermal

performance of the receiver tubes. Key parameters for solar collectors and receiver tubes, which

determine the optical efficiency are given in Table 2 where the fouling factor is derived from

available operating experience with CSP plants. Based on Table 2, the optical efficiency is 0.756

including the fouling effect. This means that 75.6% of the solar thermal power available at collector's

surface is absorbed by the receiver tube. The actual thermal power that is absorbed by the heat transfer

fluid depends on the thermal efficiency of the receiver tube, which in turn depends on the selective

coating of the tube, its emittance, the operating temperature, and the intensity of the solar irradiance.

As solar collectors are grouped into strings including 6-8 SCA (Solar Collector Assembly) with

in‐series connection, thermal efficiency and optical efficiency refer to the string and are also a

function of the intensity of the solar irradiance.

If the solar irradiance is suitable, thermal fluid in the tubes can flow at a speed such that the turbulence

to have a good thermal exchange, assures at the same time the requested salt warming (from 290 to

550 °C) and an high solar energy absorption by salts. In this case the efficiency will be the highest

possible.

On the contrary, if the solar irradiance decreases, in order to assure the same salt warming gradient (it

is a constraints for the users), it is necessary reduce the salt flow, in fact the absorbed energy depends

on flow, heat capacity and temperature gradient (Q ~ W*cp*DT ). Nevertheless, in this last case, the

turbulence and consequently the heat exchange lower.


Contract n. 325320



Obviously, it is not possible to reduce the salt flow endlessly, because flow would become laminar

and heat exchange too low, with salt solidification risks too. The red line in Figure 7 represents the

trend when the flow must be kept to the minimum flow constantly, but in this case it is not possible to

assure the nominal outlet temperature (550 °C) and under a certain exit temperature the heat collected

will be used to compensate the heat losses.

Therefore, the total collector efficiency ranges from 0.4 and 0.69 (see Figure 7). It should be noted that

this analysis includes a molten salt flow rate in the string ranging between 2.5 kg/s and 7.9 kg/s for

solar irradiance between 380 W/m2 and 1000 W/m

2, respectively, in order to obtain a constant outlet

temperature of 550°C. It should also be noted that in the case of no solar irradiance, the minimum

mass flow rate is setup at 2.5 kg/s . In some cases, it is possible to arrive at 2 kg/s in order to collect

solar irradiance up to 320 kW/m2 (Figure 8).

Parametrs Value

Interception Factor 0.986

Sun tracking Factor 0.99

Mirror Reflectance Factor 0.96

Tube Glass Fouling Factor 0.97

Bellows Shading effect on tube receiver 0.96

Mirror Fouling Factor 0.95

Glass Tube Transmittance Factor 0.96

Receiver Tube Absorbance Factor 0.95

Table 2 : Key Parameters for Solar Collector Performance

‐

Figure 7 : Total Collectors' Efficiency of the String example

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1

Tota

l Eff

icie

ncy

Irradiance [kW/m2]

M = 2.5 kg/s M > 2.5 kg/s

Poli. (M > 2.5 kg/s)


Contract n. 325320



The overall energy balance of the CSP plant includes the solar field, the molten salt distribution

network, the heat storage system, the steam generator, and the thermodynamic cycle.

The energy absorbed by the heat transfer fluid is calculated based on the hourly profile of the effective

solar irradiance (Aperture Normal Irradiance - ANI), the total mirrors surface in the solar field, and

the average efficiency of solar collector strings. The energy absorbed by the fluid minus the network

loss is the energy available for storage.

Figure 8: molten salts flow into the loop.

The energy balance of the storage system takes into account the maximum storage capacity, the

profile of the useful energy to supply to the steam generator in order to produce electricity. If sunlight

is not available (overnight or under cloudy sky), molten salt needs to be re‐circulated through the solar

field and the storage cold tank to keep the required molten salt temperature with a certain safety

margin. This involves (re)circulation loss.

The basic assumption to assess the plant performance is that the power plant works if thermal energy

is available in the storage system, otherwise the steam generator is in the stand‐by condition, with no

electricity production. Actual annual operation of the power plant includes periods with different

power level depending on solar irradiance availability. Based on a nominal thermal power of the

steam generator, the operating window is between 36% (typically, December and January) and 110%

(May to August) of the nominal thermal power. As the efficiency of the power block with low load

level is about 90% of its nominal value, the thermal power that is needed to produce electricity is

about 40% of the nominal thermal power.

The annual energy balance also accounts for the average thermal losses of the solar field including

connection piping (i.e. about 100 W/m) and the thermal losses of main headers and manifolds of the

molten salt distribution network (i.e. about 150 W/m); the weighted average of the total thermal losses

is estimated at about 104 W/m (solar field and network) or 18 W/m2 of reflecting surface.

Given the maximum capacity of the storage system, a part of storable energy can be discharged when

the hot storage tank is full (discharged energy). In this case, some strings of solar collectors can be


Contract n. 325320



misaligned from the focal line in order not to capture further thermal power. In contrast, if the hot tank

is empty and sunlight is not available, an energy addition from an external source may be needed to

ensure molten salt circulation (e.g. overnight circulation).

2.4 Thermal storage

A big advantage of the CSP technology is the capability to provide dispatchable power – by storing

solar energy in thermal reservoirs and releasing it as and when it's needed i.e. during periods of peak

power demand, during cloudy weather or even at night. So storage can eliminate intermittency as well

as extend energy production past sun-set.

Solar heat collected during the day can be stored in liquid or solid media such as molten salts,

ceramics, concrete, phase-changing salt mixtures or in saturated water. At night, the heat can be

extracted from the storage medium to keep the turbine running.

2.4.1 High temperature storage

High temperature heat storage with molten salts

Molten salt is in current applications. This heat storage medium retains thermal energy effectively

over time and matches well with the most efficient steam turbines. Moreover molten salt is a non-

toxic, readily-available material. Molten salt can act as both Heat Transfer Fluid and Thermal Energy

Storage, which improves the overall plant efficiency and lowers the cost.

These first systems employed molten salts in an indirect two-tank design (see Figure 9). For example

the Solar Two receiver was comprised of a series of panels (each made of 32 thin-walled, stainless

steel tubes) through which the molten salt flowed in a serpentine path. The salt storage medium was a

mixture of 60 percent sodium nitrate and 40 percent potassium nitrate [R 10].

The thermal storage system was a two-tank system designed to deliver thermal energy at full rated

duty of the steam generator for three hours at the defined hot and cold salt temperatures of 565 °C and

288 °C, respectively. So after the first cycle, the salt from the cold tank is heated from 288 °C to the

hot tank temperature of 565 °C during charging, and the process is reversed during discharging.


Contract n. 325320



Figure 9: Two-tank indirect thermal energy storage system

More recently, the Torresol, Gemasolar Power Tower in Seville, Spain uses a molten salts storage

system in a direct two-tank design (see R 10). The storage medium acts also as heat transfer fluid,

removing the need for a heat exchanger and hence redusing cost and increasing overall efficiency [R

11]. The storage capacity is 15 hours for a plant capacity 12 MWe [R 12].

Figure 11 shows molten salts storage tanks of Gemasolar.

Figure 10: Two- tank direct storage system

Figure 11: Gemasolar molten salts storage tank

Determining the optimum storage size to meet power-dispatch requirements is an important part of the

system design process. Storage tanks can be designed with sufficient capacity to power a turbine at

full output for up to 15 hours.

The heliostat field that surrounds the tower is laid out to optimize the annual performance of the plant.

The field and the receiver are also sized depending on the needs of the utility. In a typical installation,

solar energy collection occurs at a rate that exceeds the maximum required to provide steam to the


Contract n. 325320



turbine. Consequently, the thermal storage system can be charged at the same time that the plant is

producing power at full capacity. The ratio of the thermal power provided by the collector system (the

heliostat field and receiver) to the peak thermal power required by the turbine generator is, as said, the

solar multiple. With a solar multiple of approximately 2.7, a molten-salt power tower located in the

California Mojave desert can be designed for an annual capacity factor of about 65%. Consequently,

a power tower could potentially operate for 65% of the year without the need for a back-up fuel

source. Without energy storage, solar technologies are limited to annual capacity factors near 25%.

The dispatchability of electricity from a molten-salt power tower is illustrated in Figure 12, which

shows the load dispatching capability for a typical day in Southern California. The figure shows solar

intensity, energy stored in the hot tank, and electric power output as functions of time of day. In this

example, the solar plant begins collecting thermal energy soon after sunrise and stores it in the hot

tank, accumulating energy in the tank throughout the day. In response to a peak-load demand on the

grid, the turbine is brought on line at 1:00 PM and continues to generate power until 11 PM. Because

of the storage, power output from the turbine generator remains constant through fluctuations in solar

intensity and until all of the energy stored in the hot tank is depleted. Energy storage and

dispatchability are very important for the success of solar power tower technology, and molten salt is

believed to be the key to cost effective energy storage.

Figure 12: Dispatchability of molten-salt power towers [R 5]

High temperature heat storage with steam

For cloud transients, the PS 10 solar power tower plant has a 20-MWh thermal capacity saturated

water thermal storage system (equivalent to 50 minutes of 50% load operation). The system is made

up of 4 tanks that are sequentially operated in order of their charge status. During full-load plant

operation, part of the 250ºC/40 bar steam produced by the receiver is employed to load the thermal


Contract n. 325320



storage system. When energy is needed to cover a transient period, the energy is recovered from the

saturated water at 20 bar to run the turbine at 50% load.

Figure 13 represents the steam storage plant concept.

Figure 13: Plant concept with steam storage [R 10]

Figure 14 shows the steam storage tanks for the PS10 power tower.

Figure 14: heat storage tanks for the PS10 power tower

PS 10 and PS 20 have each a storage capacity of 60 minutes [R 13].

High temperature heat storage with concrete

In the air-cooled solar tower from Jülich, Germany, a thermal storage is connected parallel to the

boiler. The storage system consists of a rectangular housing of 7 m*7 m*6 m size. The storage is

partitioned into four chambers of identical size, connected in parallel through a dome and connecting


Contract n. 325320



pipes (see Figure 15). Each of the chambers is filled with a ceramic storage material formed to provide

a large heating surface. The total volume of the inventory amounts to 120 m3. The mild steel

containment is protected from the storage heat and kept at a surface temperature below 60°C through

an inner insulation made of ceramic fibre blankets [R 7].

When charging, hot air passes through the vessel filled with ceramic material, which develops a

temperature profile between the hot and the cold end of the storage. The cool air exiting the steam

generator and/or storage is returned to the receiver. To discharge the storage, cool air from the steam

generator is circulated in reverse flow through the storage vessel and back to the steam generator.

Figure 15: Storage subsystem used in Jülich solar tower

Table 3 resumes the specifications of the storage system.

Storage design specifications

Inlet temperature (charge/discharge) 680 °C/120 °C

Outlet temperature (charge/discharge) 680–640 °C/120–150 °C

Charge mass flow 9.4 kg/s

Discharge heat rate 5.7 MWth

Full load discharge period 1.5 h

Pressure loss < 1500 Pa

Table 3: Storage specifications for the Jülich solar power tower

High Temperature thermal storage development

Recently in 2011 the Texas A&M University, has explored modifications to molten salt, in order to

determine if the specific heat of thermal energy storage materials could be improved by adding

nanoparticles. The researchers are pursuing parallel paths: One path is to develop low melting point

composites based on nitrate eutectics, which could be both a HTF and TES. The other is to develop

high melting point composites based on a carbonate eutectic for TES, where the melting point is near

the upper end of the power system interface temperature for extended power production time. The

chosen molten salt was a sodium nitrate and potassium nitrate eutectic. Two nanoparticle types were

chosen, alumina and silica. The results suggest that the addition of the nanoparticles using the given

manufacturing technique increased the specific heat of the molten salt by approximately 20%. The

http://www.tamu.edu/


Contract n. 325320



silica and the alumina improved the specific heat by nearly the same amount over the base material [R

14].

A more long-term research programme is underway at the National Renewable Energy Laboratory

(NREL), where researchers are looking to develop a new class of TES materials and increase the heat

capacities by a factor of 5. The researchers at NREL are developing nano-scale ‘encapsulated’

structures of 50 to 500 atoms that can then be suspended in fluids. These nano-clusters have

embedded metals with a ceramic-like outer coating. The research direction is to have clusters which

can provide tuneable parameters to improve the TES functionality and at the same time withstand the

repeated cycling between states. The overall concept is to use a family of nano-particles with different

transition points to smooth out the phase transitions and achieve a simple linear dependence between

energy and temperature. The most success was obtained with noble metals, but those are also the most

expensive materials. They are looking to other less expensive metals too [R 14].

Presently the development of TES materials is very much in an R&D phase, and lagging the collection

hardware development. Although there are very few commercial suppliers, there are now several

strong R&D programmes in place including the USA, Spain, Germany and France.

Table 4 shows a representative layout of the current development activities:

TES

Now Steam accumulator

Molten salts

Near term Phase change

Improved molten salts

Nanoscale molten salts

Solid materials storage

Emerging concepts Thermo-chemical

Nano fluids

Table 4: current TES development activities [R 14]

2.4.2 Medium temperature storage

The realization of thermal storage in CSP type solar trough, in the version adopted by ENEA, foresees

both in the field that in the solar storage system the use of binary salts.

In this way there is "direct" storage. Unlike, systems that use oil as thermal fluid and salt as storage

mean show an indirect storage because an heat exchanger is interposed between the two circuits. Since

the binary mixture of the salts cannot fall below 238 ° C for freezing problems, a minimum

temperature of the cold reservoir equal to 290 ° C is generally set; the temperature of the hot reservoir,

instead, is fixed at 550 ° C, upper working limit of the binary salts. A boiler integration based on fossil

fuels is expected to compensate for the loss of the solar field.


Contract n. 325320



Because of the upper limit on the operating temperature of the binary salts, it will be necessary to

control the flow within the solar field as a function of the intensity of the radiation.

Figure 8 shows the trend of the salts mass flow rate in the loop of the solar field; it was limited

inferiorly to a value equal to 2 kg/s in such a way as to reach 550 ° C even in the presence of low solar

radiation equal to about 320 W/m2. The loop is a series of collectors where the warm up of thermal

fluid happens. Generally 6 collectors in series are necessary to reach 550 °C from 290°C. In order to

better manage the flow variations and efficiency, 8 collectors can be used for each loop.

To describe the system working, first hours of the day have low solar radiations, the hot tank is empty

and the cold one is full; so the flow leaving the solar field is equal to the minimum value required by

circuit and it is sent to the cold tank even if it has a temperature above 290 °C.

During the day, solar irradiation increases and consequently also molten salt flow in the solar field;

when the molten salts output temperature is greater than the threshold vale (e.g. 520 °C), the salts are

sent to the hot tank to be stored and/or to produce thermal energy.

Otherwise, if the fluid temperature is inferior than the threshold, the salt is sent back to the cold tank

to store solar energy to be used for the thermal losses compensations.

During the storage loading phase, the level of the hot tank is controlled in such a way that the inlet

flow rate is not greater than the capacity available in the reservoir.

In this way, during the solar irradiation time, the hot tank fills up, a part of collected energy directly

feeds the users while the remaining fraction of the stored heat will be used when there is not enough

solar irradiation. In fact, in this last case, the hot stored fluid is withdrawn and sent to the users,

emptying the hot tank and filling up the cold one.

A possible problem that may arise during the annual operation of the plant is related to the summer

months when there is a very high solar radiation, so it can happen that by recirculating hot salts (Tref

<520 ° C) in the cold tank there is an excessive raising of its temperature with consequent rising in the

temperature of the fluid entering the solar field. In these circumstances the output salts would reach

temperatures above 550 ° C, with harmful consequences for the integrity of the system. To avoid this

dangerous condition it is necessary to put out fire one or more collectors, losing energy.

From the above, it is clear the storage size significantly affects the productivity of the plant, in fact, if

the mass of stored salt is lower than the amount able to absorb the power that may be accumulated

during the day, part of this should be wasted. On the other hand, it is reasonable to arrive at a

compromise between storage costs and lost power.

So it is possible to establish the storage size and make an assessment of the thermal inputs.

From the curve of the daily time solar radiation, using the total collecting surface of the solar field and

the efficiency curve of the string manifolds, it is possible to evaluate the thermal energy absorbed by

the fluid (energy consumption).

This, purified by thermal losses represents the energy that can potentially be stored in the storage

system (energy accumulated).

ENEA has been also studying an alternative way to store thermal energy by molten salts through a

system based on a single tank containing a thermal storage liquid material (HSM: heat storage

material) where energy is stored (during the charging phase) as sensible heat, and, during thermal


Contract n. 325320



discharging, the energy is transferred to another fluid by a heat exchanger immersed into the HSM

bulk. The working concept is based on maintaining a vertical thermal stratification profile in the

HSM fluid (thermocline), which is due to the layers difference of density with temperature.

At the ENEA solar energy test facility (PCS), a binary mixture of nitrate salts (60% NaNO3 and 40%

KNO3 in weight percentage) has been tested as HSM at this aim. This mixture, in general known as

“solar salt”, because its large employment in CSP plants, presents a solidus point (that is, the

temperature where the liquids starts forming a solid phase by cooling) of about 240 °C, but, for

practical reason is employed above 290 °C (cold temperature); the upper temperature limit can be

considered at around 600 °C, though, at the ENEA test rig 550°C is the higher (hot) temperature

value.

The experimental work has shown that the thermal stratification of the molten salts mixture can be

maintained quite constant for several hours and the presence of the integrated steam generator actively

guarantees and maintains the stratification during the operation time, avoiding mixing of the stratified

layers. This behaviour is also favoured, in absence of perturbative phenomena, by the low temperature

value of the solar salt thermal conductivity (about 0.5 W/K m). Another characteristic of the nitrate

mixture which enhances the thermocline stability against the homogenization due to convection is

related to the dynamic viscosity trend respect to temperature: this value increases of about 60% from

550°C to 290°C, hindering the mass transfer from the cold layers towards the hotter ones [R 15].

In Figure 16 is shown a scheme of the ENEA test facility, consisting of a 300 kWth SG inserted in a 8

m³ molten salts storage tank. Also the position of the thermocouples, positioned at different heights, is

indicated.


Contract n. 325320



Figure 16 : Scheme of the experimental set-up

Medium temperature storage alternatives

Thermal energy can be stored as well as sensible heat, also by means of latent heat fusion (using

PCM, phase change material as storage media) and as thermo-chemical reactions or a combination of

these in order to increase the total amount of stored heat [R 16].

In terms of storage media there are a wide variety of organic and inorganic pure compounds, mixtures,

and eutectic mixtures, depending on the temperature range and application. In literature [R 17-R 20] a

classification of PCM (paraffin waxes, fluorides, chlorides, hydroxides, alkali nitrates, carbonates,

other salts, and metal alloys) is made according to the melting temperature (°C), heat of fusion (kJ/kg)

and the thermo-physical properties such as density (kg/m3), specific heat (kJ/kg K), and thermal

conductivity (W/m K).

There are many different types of paraffins, fatty acids, and inorganic salt hydrates that could be used

like PCM for domestic uses with temperatures below 120°C, salt-hydrated eutectic mixtures for solar

water heaters [R 21], and the sodium nitrates for industrial application in the temperature range 120-

350° C, such as direct steam-generating parabolic troughs [R 22, R 23].


Contract n. 325320



The latent heat storage (LHTES) concept using phase change materials (PCM) provides for a high

energy storage density at a constant temperature corresponding to the phase transition temperature of

the heat storage media.

The criteria for choosing PCM are:

- a melting point in the desired operating temperature range;

-high latent heat of fusion per unit mass, so that a lesser amount of material stores a given amount of

energy;

-high density, so that a smaller container volume holds the material;

-high specific heat to provide for additional significant sensible heat storage effects;

-high thermal conductivity, so that the temperature gradients required for charging and discharging the

storage material are small;

- congruent melting : the material should melt completely so that the liquid and solid phases are

identical in composition, otherwise the difference in densities between solid and liquid cause

segregation, resulting in changes in the chemical composition of the material;

-small volume changes during phase transition, so that a simple containment and heat exchanger

geometry can be used;

-chemical stability, no chemical decomposition;

-non-corrosiveness to construction materials;

-the material must be non-toxic, flame-safe and fire-safe;

-available in large quantities and inexpensive.

There are also some uncertainties about PCM for the solar application [R 23]:

-It is not clear to what extent thermo-physical properties of PCMs were investigated and how reliable

any data is;

- There is no commonly accepted criteria on the magnitude of thermal conductivity of commercially

available PCMs to decide whether a PCM could be used for solar applications;

-It is not clear whether the heat and mass transfer process investigations were performed for a

commercially available PCM so that results obtained could be used in designing the solar energy

device;

-It is difficult to determine whether it is feasible to use a certain PCM for a solar energy application

even if there is information on its cost.

In the sensible heat storage, the temperature of the PCM changes during charging /discharging of the

storage. During the discharging process, the energy released by solidification of the PCM must be

transported from the solid–liquid interface through the growing solid layer to the heat exchanger

surface. Hence the heat transfer coefficient is dominated by the thermal conductivity of the solid

PCM. Most PCMs usually provide a low thermal conductivity around 0.5-0.8 W/ m K which results in

a low amount of heat transfer between the HTF and the storage material itself.

One of the techniques to enhance the heat transfer, is increasing the thermal conductivity of the PCM

by embedding high conductive structures into the storage media [R 24-R 26], so as to create a


Contract n. 325320



composite material using nanoparticles (SiO2, TiO2, Al2O3, graphite) embedded in a molten salt base.

The addition of nanoparticles to a base salt results in an increase of PCM specific heat, thermal

conductivity and thermal diffusivity. ENEA characterized the thermo physical properties of the

composite material made of the binary mixture base (64NaNO3/36KNO3, %mol) by adding

nanoparticles in order to improve the solar contribution in terms of sensible heat storage (2-tank

storage trough systems) [R 27].

In the following are shortly described the experiments carried out, at laboratory scale, at the ENEA

Casaccia Labs.

Two different sized nanoparticles were dispersed in binary mixture base (64NaNO3/36KNO3, %mol),

the nitrates salts mixtures with the addition of nanoparticle are all prepared in the same manner. About

1 g of binary mixture base was dissolved in 13 ml of water (7ml) and ethanol (6ml) which contains a

specific amount (% by weight) of silica (SiO2) and titanium dioxide (TiO2) nanoparticles, the amount

of nanoparticles for each mixture is list in the Table 5. This solution was sonicated using a ultra

sonicator at 20 Hz for 30 min to obtain homogeneous dispersion of the nanofluids, after that the

water-ethanol solution was then rapidly evaporated. The SiO2 and TiO2 nanoparticles have dimensions

5 nm and 21 nm respectively. The heat capacity value of all the nanofluids and mixture base in the

liquid state were measured using a Differential scanning calorimeter (DSC), see Figure 17, with a

heating rate of 10 °C/min, the sample mass was about 50 mg and aluminium cruicibles with a lid were

used. The results in terms of heat capacity value are collected in table, in the temperature range 280-

400°C for binary mixture base and 260-400°C for the nanofluids. The measurement uncertainty in the

experiments is estimated to be in the range 1–17%. This shows further studies are needed.

In the case of samples with TiO2 (Figure 18) on the grain surface of salt, nanoparticles appear

relatively dispersed with aggregates of a few hundred nm.

Mixtures Heat capacity [J∙g∙°C] Enhancement

binary 1.6 ±0.17 /

binary base 1%TiO2 2 ±0.077 25%

binary base 3 %TiO2 1.6 ±0.01 0.6%

binary base 1 %SiO2 1.9 ±0.064 19%

binary base 3 %SiO2 1.9 ±0.038 19%

binary base 2%SiO2 2%TiO2 1.8 ±0.018 13%

Table 5: Average heat capacity values of the binary mixture base and nanofluids with SiO2 and TiO2.


Contract n. 325320



Figure 17: Average heat capacity values vs. temperature, on the left side nanofluid with 1%TiO2 (blu dots)

and nanofluid with 3% di TiO2 (black dots) in the temperature range 260-400°C, on the right side binary

mixture base in the temperature range 280-400°C.

Figure 18: Sem images of TiO2 nanoparticles on the grain surface, the dimension of aggregates are around

hundreds nm as order of magnitude, and the nanoparticles appear to be relatively dispersed.

Nevertheless some theoretical evaluations carried out by AALTO using FACTsage SW showed

addition of particles such as TiO2 might cause extra reactions, and it also shifts the liquidus line

(Figure 19), while addition of CaO particles should not present this advantage (Figure 20).


Contract n. 325320



Figure 19: phase diagram of NaNO3-KNO3 mixtures with TiO2 inclusions.

Figure 20: phase diagram of NaNO3-KNO3 mixtures with CaO inclusions.

Heat storage with hydrides

Hydrogen forms metal hydrides (MHn) with some metals and alloys leading to solid-state storage

under temperature and pressure, in general form the reaction is


Contract n. 325320



where M is metal, metal alloy, intermetallic compound.

The majority of metals, metal alloys and intermetallic compounds react directly with gaseous

hydrogen (reaction 1 from left to right) to form metal hydrides in an exothermal way, that is, with

release of heat (ΔH<0, heat recovery phase), backward there is the dissociation into metal and

hydrogen by heat absorption (ΔH>0, heat storage phase). This way thermal energy can be reversibly

stored as the heat of reaction of reversible chemical reactions. In order to apply this thermochemical

method to heat storage application [R 29] the magnesium hydride/magnesium system is particularly

suitable for this purpose, thus the reaction is

MgH2 + 75 kJ*mol-1

Mg+ H2

The reversible hydrogen storage capacity is the highest among reversible binary hydrides (7.6wt.%)

and because upon the reaction of magnesium with hydrogen a relatively large amount of heat (75

kJ·mol-1

H2, (0.9 kWh kg-1

Mg) at a temperature level between ≈ 200 and 500 °C is set free. The

knowledge of the H2 dissociation (equilibrium) pressure as a function of temperature (Figure 21) is of

fundamental importance for the operation of the MgH2 heat and hydrogen storage system, choosing

the hydrogen pressure it is possible to determine at which temperature level the stored heat will be

delivered. Moreover Felderhoff et al. [R 29] presented the cycle stability of MgH2 / Mg (basis) doped

with Ni (4 wt%), and referring to the Ni-doped Mg-powder system, after 650 cycle test , at 500°C

with an hydrogen pressure in the system about 100 bar, it reveals reversible and irreversible hydrogen

capacity losses after 45 and 135 min hydrogenation times.

Figure 21: Dissociation pressure curve of MgH2 [R 29]

Another aspect of this thermochemical method is that during the endothermic process (heat storage),

hydrogen is liberated and until the reverse process of heat recovery occurs must be temporarily stored.

heat storage

heat recovery


Contract n. 325320



Felderhoff et al. [R 29] presented a schematic representation of construction of MgH2/Mg heat stores,

one is with temporary storage of hydrogen in a pressure container; the other is temporary storage of

hydrogen in a low-temperature (LT) metal hydride, such as Mg2FeH6.

This last metal hydride/metal system (Mg2FeH6) presents higher stability in test cycle and a lower

hydrogen dissociation pressure (at 500°C 60 bar). While the MgH2/Mg system under different HT

conditions already loses capacity after 200-250 cycles (due to sintering), the Mg-Fe-H-systems, under

comparable conditions, show no signs of loss of capacity and reaction rates even after 600 or more

cycles.

Each configuration could be applied to warm water supply, production of ice or climatisation, they

also reported experimental results concerning the MgH2/Mg system for different fields of applications,

such as steam generator (400°C) and the combination of a solar-thermal power generator with a MgH2

heat accumulator.

In this case the main components were a “fixed focus concentrator” for solar radiation, a cavity

radiation receiver, a heat pipe system for heat transfer, a Stirling engine, a MgH2 store, a hydrogen

pressure or LT hydride store.

The concentrated solar heat is used to drive the Stirling engine and produce power. Simultaneously, a

more or less part of the solar heat is absorbed by the MgH2 heat store and the released hydrogen

streams in the pressure container or, alternatively, in the LT hydride store. In periods without or weak

solar irradiation the MgH2/Mg system cools down and the hydrogen flows back to the MgH2 store and

produces there high temperature heat and via Stirling engine electric power, see Figure 22.

An interesting comparison of the MgH2/Mg system with the existing heat storage system of Andasol 1

was made.


Contract n. 325320



Figure 22: Solar thermal-Hydrogen: on top during the day, on the bottom at night.

Andasol 1 is a solar thermal power plant located in Andalucia (Spain) with a 50 MWe steam turbine

for electricity production. To run the plant during the dark a heat storage system is installed for

additional 7.5 h electricity production at night. The storage system consists of two insulated storage

tanks each of these is 14 m high and 36 m in diameter. They are filled with 28,000 tons of a molten

salt mixture of sodium and potassium nitrate. The tanks are kept at different temperatures, the cold

tank at 260 °C and the hot tank at 390 °C. The overall storable heat amount in this system is 1,000

MWh.

The amount of heat released upon reaction of 1 kg of Mg with H2 is 0.9 kWh, thus it can be roughly

estimated that an amount of about 1,100 tons of Mg-metal would be able to store the same amount of

heat as the Andasol storage system. In addition to the storage of the Mg/MgH2 mixture, a second

container is necessary for the temporary storage of the hydrogen released during the dissociation of

MgH2. Furthermore Felderhoff et al. [R 29] as an example, assumed working temperatures of the heat

storage between 370 and 400 °C. At 370 °C the dissociation pressure of MgH2 is 10 bar and at 400 °C

roughly 20 bar (Figure 21). In this pressure and temperature ranges, a simple calculation shows a

volume of 109,000 m3 of H2 gas resulting from decomposition of MgH2 must be stored. In order to

better understand the benefit of this Mg/MgH2 storage it should be important to have an economic

evaluation of this system, but it is not so easy to find in literature the exact cost of this process,

because it is not yet applied at industrial scale.


Contract n. 325320



2.5 Electrical energy production

2.5.1 Electrical energy production by solar trough at medium temperature

An advantage of the use of Solar Trough technology to collect solar energy is the possibility to use the

same technology to transform heat to electricity used for the most classical and spread systems with

the only peculiarity to exchange heat by molten salts and water/steam. Also, in this way the generated

electricity does not give problems to the grid.

The power block, indeed, includes a conventional thermodynamic cycle consisting of steam turbines

(high-pressure and low-pressure ones), the exhaust steam cooling system (condenser), the electricity

generator (alternator), the electric transformer, and the electric connection to the grid. The molten salt

steam generator is the interface between the power block and the solar field including the heat storage

system.

The super-heated steam produced by the steam generator drives the high pressure (HP) turbine and

subsequently the medium-low pressure (MP-LP) turbine, after a re-heating phase.

The steam exhaust from the LP turbine enter the condenser and the resulting water is pumped into the

feed water pre-heating system to be recirculated into the steam generator. The feed water pre-heating

system includes the extraction pump from the condenser, low and high-pressure water pre-heaters,

water outgassing system, and the feed water circulation pump. An auxiliary steam generator is needed

to generate vacuum in the condenser and for the turbines' leak-tights during the plant start-up phase.

The heat sink of the condenser can be sea-water or fresh water from a river or lake. Wet or dry cooling

towers can be used if insufficient or no water is available. The design choice depends on the

characteristics of the site. The use of water from lakes, rivers and sea is the most convenient option

from the economic point of view. Cooling towers are significantly expensive, particularly dry cooling

towers.

The cycle is basically an Hirn one and considering a scheme shown in Figure 23 [R 28], it has been

constituted by:

-an exchanger ECON1 (economizer), which preheats water to be sent to the evaporator EVAP1. This

warming takes place using the molten salt residual heat.

- an evaporator EVAP1, which brings the water to saturation and vaporizes at a pressure of 100 bar;

- Two exchangers, SPHT1 SPHT2 (respectively: super-heater and re-heater), that overheat the steam

up to the maximum possible temperature (540 ° C) using the molten salt exiting the solar field at 550

°C. SPHT1 warms up the steam for the high-pressure turbine, while SPHT2 thinks to the steam for the

medium pressure turbine (ST2).

- two steam turbines, ST1 and ST2, for the power production and having intermediate steam

extractions, one in the case of ST1 and 4 for ST2. The first tapping of ST2 provides heat to the

degasser; the others, along with the outgoing stream from turbines provide the heat that preheat the

exchangers FWSH.

- 5 water exchangers (liquid / vapor) FHW. FHW2, FHW3 and FHW4 receive the heat from the ST2

steam extractions and warm the water coming from the condenser CND1 to be sent to the degasser.

FHW1 and FHW5, instead, receives the heat from the ST1 steam tapping and warm the water coming


Contract n. 325320



from the degasser to be sent to the economizer ECON1. The cold current from FHW1 and FHW5 goes

to the degasser, while the cold FHW2, FHW3 and FHW4 goes to the condenser.

- 1 capacitor CND1, which condenses the outgoing current from ST2.

- 2 pumps PMP1 and PMP2, that lead the water to the operating pressure of the heat exchangers steam

/ water.

- a valve V1, which provides for reducing the water pressure from the outgoing FWH2 until the

pressure in the condenser.

The simulations have shown that, as regards the efficiency of transformation heat / electric, a value of

40.7% in nominal conditions can be considered.

Figure 23 : solar throw power production scheme [source: R 28]

2.1.3 Steam solar power tower

The PS 10 solar power tower (Figure 24), works on a saturated steam concept: it stores heat in tanks

as superheated and pressurized water at 50 bar and 285°C. The water evaporates and flashes back to

steam, when pressure is lowered.

The receiver produces saturated steam at 40 bar 250ºC from thermal energy supplied by the

concentrated solar radiation flux. This receiver has a cavity design to reduce radiation and convection

losses. The steam is sent to the turbine, where it expands, producing mechanical work and electricity.

The turbo-generator output goes to a water-cooled 0.06 bar pressurized condenser. The condenser

output is preheated by 0.8 bar and 16 bar turbine extractions. The output of the first preheater is sent

to a degasser fed with steam from another turbine extraction. A third and last preheater is fed with


Contract n. 325320



steam coming from the receiver. This preheater increases the water temperature to 245ºC. This flow is

mixed with the flow of water returning from the drum, raising the temperature of water fed to the

receiver to 247ºC.

Figure 24: Solar plant PS10 in Spain

PS10 and PS20, are producing electricity from saturated steam since 2007 [R 4]. New projects by

Abengoa Solar, Brightsource, and eSolar demonstrate the feasibility of producing superheated steam

at above 500 °C with dual receivers [R 30].

Electrical efficiency of solar power towers

Characteristics of solar power towers are given in Table 6 taken from Romero et al. [R 4]. The values

are projections based on experiences with early commercial projects assuming future development.

Accordingly, central receiver systems can reach a solar-to-electricity conversion efficiency of up to

23 % and a net annual efficiency of 7–20 %, which is very much dependent on the location of the

power plant. Currently, a next generation of solar tower systems is under development with higher

process temperatures of more than 1000 °C applying new receivers with volumetric absorbers making

use of porous structures or particles. These power plants have the potential to reach higher electrical

efficiencies and are, moreover, applicable to processes for soar fuel production (e.g. hybrid sulphur

cycle). By this, the performance is expected to further increase.

Power unit 10-200 MWa

Temperature operation 565 °C

Annual capacity factor 20-77%a

Peak efficiency 23%

Net annual efficiency 7-20%a

Commercial status Early projects

Technology risk Medium

Thermal storage Yes

Hybrid schemes Yes a Data interval for the period 2010–2025

Table 6: Characteristics of central receiver systems [R 4]


Contract n. 325320



2.6 Concepts on size and thermal storage to couple the solar plants to the chemical plant.

In order to obtain a desired amount of hydrogen by the process, which SOL2HY2 has been working, a

definite quantity of energy is needed. This energy has to be provided in form of:

- daily continuous heat at medium temperature to feed the H2SO4 concentration;

- continuous heat for a specified number of hours for the vaporization of H2SO4;

- continuous heat for a specified number of hours for the H2SO4 cracking;

- continuous electrical energy for the H2 production by SDE (Sulphur dioxide depolarized

electrolysis/electrolyser) and for the BOP (Balance of Plant) equipments.

The amounts and modalities of these energy requests are being studied in the WP1 (Development of

the plants design), WP2 (SDE design), WP3 (Sulphuric acid concentrator and cracker) and WP5 (BOP

units).

In this WP, instead, the optimal energy integration for these different equipments has to be studied.

In particular there are some constrains concerning the modality and the kind of energy required:

- H2SO4 concentration must work in continuous for the whole day, due to the cost of equipments,

which have to work as long as possible for amortization optimization and mostly for the long time

of start-up and shut-down operations. Nevertheless the involved equipments should have

considerable drops in efficiency working with variable or intermittent streams in the place of the

nominal ones.

In this case the best option seems to be the MT heat production by the Solar Troughs CSP plants

supplied with a proper thermal storage able to assure the daily continuity of the H2SO4

concentration step. The requested temperature of 500 °C makes the best option for the use of

molten salts as thermal fluid.

- H2SO4 vaporization requires medium temperatures and continuity during the H2SO4 cracking step,

so it is not necessary the daily continuity. Since it has foreseen for this step a maximum

temperature of 500 °C, the Solar Troughs CSP plant seems to be the best solution for its better

efficiency.

- H2SO4 cracking requires high temperature and continuity during the SO2 production time, but it is

not necessary the daily continuity. In this case a CSP plant using Solar Central Receiver

technology is desirable. It should be supplied with a thermal storage able to assure the heat

continuity collection at least during the cracker working.

- SDE and BOP require electrical energy continuously for the whole time. In this case the best

options should be the use of the grid. Nevertheless, since the solar plants will produce, excess of

energy during the high irradiated periods (e.g. in summer), it could be considered the option to

produce electrical energy by this surplus to be sent in the grid in order to compensate the


Contract n. 325320



withdrawals of SDE and BOP. In this way it will be possible to produce an hydrogen produced by

an percentage of renewable source.

In order to optimize and verify the feasibility of this energy integration, different options concerning

location, choice of basic design and thermal storage size have to be considered.

First of all four different locations have been analyzed: Almeria (Spain, 36.84 N; 2.47 W), Larnaca

(Cyprus, 34.91 N 33.63 E); San Diego (USA, 28.13 N 15.43 W) and Alghero (Italy, 40.56 N 8.32 E)

and their DNI data for every hour of the year have been considered (source: METEONORM).

For each location and for each month it has been assessed the average of the all data corresponding at

the same hour of the day in order to have a typical day for every month of the year in every considered

location.

In Figure 25 some of these daily irradiation trend are shown and it is possible to verify as in South of

Europe the irradiation is very different in winter and in summer both in intensity and in number of

hours. This phenomenon is less evident in South California (San Diego) and it means large heat

collection in summer and only a few in winter for the South Europe in comparison of the South of

California.

0

100

200

300

400

500

600

0 5 10 15 20

DN

I (W

/m2

)

Hour

Almeria-January

0

100

200

300

400

500

600

0 5 10 15 20

DN

I (W

/m2

)

Hour

Almeria-June

0

200

400

600

800

0 5 10 15 20

DN

I (W

/m2

)

Hour

Larnaca-January

0

100

200

300

400

500

600

0 5 10 15 20

DN

I (W

/m2

)

Hour

Larnaca-June


Contract n. 325320



Figure 25: some examples of average daily DNIs

Choosing the daily requested irradiation collection (e.g: 20 MWh/d) and the desired percentage of

energy at medium temperature (e.g. 60 %) in continuous (e.g. 45 %) and in discontinuous, it is

possible to assess, for every location, the size of the solar field, the volume of the storage, the number

of collectors and for each hour of the day and for each month the amount of stored energy and the

quantity of energy to be sent to other users or to produce by a backup coming from other source. This

last case happens when the heat storage is not able to guarantee for whole day the requested energy

and it is mostly true during the winter time. Obviously it is possible to oversize the plant, but also the

cost increase in this way so it is necessary to find the best compromise and it is also depending on the

location. With this aim three different configurations can be selected, depending on the solar field

design, they are based on:

- Peak irradiation, this is the smaller configuration plant, because it is able to assure the requested

energy only for the high irradiated times, but it is also the cheapest solution.

- Average irradiation, this configuration is able to guarantee the heat continuously in yearly average

irradiation times.

- Minimum irradiation, this configuration is able to guarantee the heat continuously during the

whole year. Obviously, during some period having irradiation particularly low can be necessary a

backup integration. This solution is the most expensive, but can assure also a large production of

electrical energy during the summer.

0

200

400

600

800

0 5 10 15 20

DN

I (W

/m2

)

Hour

San Diego-January

0

200

400

600

800

0 5 10 15 20

DN

I (W

/m2

)

Hour

San Diego-June

0

100

200

300

400

500

0 5 10 15 20

DN

I (W

/m2

)

Hour

Alghero-January

0

200

400

600

800

0 5 10 15 20

DN

I (W

/m2

)

Hour

Alghero -June


Contract n. 325320



It is worth to notice that the MT solar plant using the solar trough technology is based on the use of a

series of collectors (generally six) to warm up the molten salt from 290 °C to 550 °C, so it should be

necessary to design the plant for at least about 3400 m2, which is the surface of six collectors. This

surface is able to produce energy of about 10 MWh/d in condition of yearly average irradiation in

South Europe. If the daily requested energy is lower, the plant will be oversized or will be necessary

to apply an artifice consisting in the following strategy.

During the start up, molten salts is warmed by its recycling into the solar field up to reach the desired

temperature of 550 °C. In order to guarantee the correct inlet temperature during the working part of

the cold molten salt from the cold tank (290 °C) has to be mixed with part of the outlet molten salt

stream at 550 °C as Figure 26 shows.

This warking way, obviously, introduces delays, better controls and higher costs so it is not desirable

to scale up the SOL2HY2 plant under the size to have a MT solar plant having too small solar field.

Obviously, if the size of the MT plant is under the size of one collector (about 566 m2) it is necessary

to build a collector different from the commercial ones with very high cost increasing.

Figure 26: small solar through plant concept

Table 7 shows four different designs based on the same daily energy, collection, medium temperature

energy fraction and its percentage request in continuous. The case studies 1, 2 and 3 for the same

location (Sardinia) have been sized basing the mirror surface on minimum, average or peak irradiance

respectively. It is possible to see the first case, able to guarantee the complete heat availability also in

December (Table 8), has the largest theoretical mirror field area, 4846 m2, so, in practice, includes 2

loops of 6 collectors for a real surface of 6797 m2

. In July (Table 9) it produces a lot of energy, which

is important to use for other applications and users. Case study 2, instead, is based on average yearly

irradiance and it does not assure energy for the plant for whole the day in the all months of the year, in

fact, in December, for example, it can provides the whole necessary heat only from 8 am to 17 pm


Contract n. 325320



(Table 10). Nevertheless it has a theoretical mirror field area of 2405 m2 and includes only one loop.

Case study 3, based on peak irradiance can provide MT heat to the H2 production plant only in the

most irradiated periods (i.e.: July), but also in May its collection is not enough to assure the continuity

(Table 11) and in December guarantees the continuity of the requested energy only from 8 am to 11

am. In this last case the surface has to be smaller than a complete loop, so 3 collectors are sufficient

and it is necessary to use the strategy shown in Figure 26. Obviously storage tanks volumes will be

bigger for the first cast and smaller in the third one. To establish the best solution it is important to

confront the energy price (surely higher in the first case) and the possibility to use the excess of

energy. If other users can utilize the heat excess, probably the first solution is the best. The third

solution, instead, is more convenient if only a percentage of renewable energy is requested, because

heat from other sources is available. Finally, the last case study has been chosen to compare a plant,

able to assure the same energy production (based on minimum irradiance), built in Sardinia or in San

Diego. In the last case the plant can use only a loop and a thermal storage is an half. It shows how

much can be convenient to build this kind of plant in high irradiated counties.

Scenario 1 2 3 4

Total Thermal daily energy collection 20 20 20 20 MWh/d

Direct solar radiation time 8 8 8 8 h

Total Thermal Power Direct irradiance 2.5 2.5 2.5 2.5 MW

MT Thermal storage 16 16 16 16 h

HT Thermal storage 0 0 0 0 h

Medium Temperature energy fraction 60 60 60 60 %

MT Heat Collection efficiency 0.52 0.52 0.52 0.52

HT Heat Collection efficiency 0.4 0.4 0.4 0.4

MT Thermal production 6.24 6.24 6.24 6.24 MWh/d

MT Thermal production in continuous (%) 45 45 45 45 %

MT Thermal production in continuous 2.808 2.808 2.808 2.808 MWh/d

HT Thermal production 3.2 3.2 3.2 3.2 MWh/d

Total Thermal daily Energy Production 9.4 9.4 9.4 9.4 MWh/d

Location Sardinia Sardinia Sardinia San Diego

Mirror surface based on Minimum Average Peak Minimum

DNI - selected 2.48 4.99 7.63 4.63 kWh/m2/d

MT Theoretical Mirror field area 4845.7 2404.6 1573.0 2593.8 m2

Collection surface of the collector 566.4 566.4 566.4 566.4 m2

Number of collectors for a loop 6 6 3 6

Loop surface 3398.4 3398.4 1699.2 3398.4 m2

Number of loops 2 1 1 1

Mirror surface 6796.8 3398.4 1760 4821 m2

Max heat stored 10971 5816 2601 5523 kWh

Volume of storage hot solar tank 77 37 16 35 m3

Volume of storage cool solar tank 71 33 15 32 m3

Start discontinuous MT production 11 11 11 11 h


Contract n. 325320



End discontinuous MT production 18 18 18 18 h

Table 7: Different MT solar plant sizes varying mirror surface modality or location

h Plant production Ch Plant

consumption Heat stored

Heat

available(st+prod)

Other

users

Wh Wh Wh

for plant

consumption Wh

0 0 117000 1551812 117000 104687

1 0 117000 1330125 117000 104687

2 0 117000 1108437 117000 104687

3 0 117000 886750 117000 104687

4 0 117000 665062 117000 104687

5 0 117000 443375 117000 104687

6 0 117000 221687 117000 104687

7 0 117000 0 117000 104687

8 559907 117000 338220 117000 104687

9 1075122 117000 1191655 117000 104687

10 1232343 117000 2202310 117000 104687

11 1252067 546000 2803690 546000 104687

12 1225731 546000 3378733 546000 104687

13 1164051 546000 3892096 546000 104687

14 1071474 546000 4312883 546000 104687

15 827377 546000 4489572 546000 104687

16 344427 546000 4183311 546000 104687

17 0 546000 3532624 546000 104687

18 0 546000 2881936 546000 104687

19 0 117000 2660249 117000 104687

20 0 117000 2438562 117000 104687

21 0 117000 2216874 117000 104687

22 0 117000 1995187 117000 104687

23 0 117000 1773499 117000 104687

Table 8: Energy production, storage, consumption and availabilities for case 1 in December.



Heat

available(st+prod)

Other

users

Wh Wh Wh

for plant

consumption Wh

0 0 117000 4645417 117000 863406

1 0 117000 3665011 117000 863406

2 0 117000 2684606 117000 863406

3 0 117000 1704200 117000 863406

4 0 117000 723794 117000 863406

5 170902 117000 0 117000 863406

6 1066115 117000 85710 117000 863406

7 1717003 117000 736597 117000 863406

8 2064280 117000 1820472 117000 863406


Contract n. 325320



9 2204742 117000 3044808 117000 863406

10 2322401 117000 4386803 117000 863406

11 2379178 546000 5356575 546000 863406

12 2379634 546000 6326803 546000 863406

13 2347711 546000 7265109 546000 863406

14 2329925 546000 8185629 546000 863406

15 2351816 546000 9128038 546000 863406

16 2167574 546000 9886207 546000 863406

17 1934422 546000 10411223 546000 863406

18 1237930 546000 10239747 546000 863406

19 288105 117000 9547446 117000 863406

20 0 117000 8567040 117000 863406

21 0 117000 7586635 117000 863406

22 0 117000 6606229 117000 863406

23 0 117000 5625823 117000 863406

Table 9: Energy production, storage, consumption and availabilities for case 1 in July.



Heat

available(st+prod)

Other

users

Wh Wh Wh

for plant

consumption Wh

0 0 117000 0 0 0

1 0 117000 0 0 0

2 0 117000 0 0 0

3 0 117000 0 0 0

4 0 117000 0 0 0

5 0 117000 0 0 0

6 0 117000 0 0 0

7 0 117000 0 0 0

8 279954 117000 162954 117000 0

9 537561 117000 583515 117000 0

10 616172 117000 1082686 117000 0

11 626034 546000 1162720 546000 0

12 612865 546000 1229585 546000 0

13 582025 546000 1265610 546000 0

14 535737 546000 1255347 546000 0

15 413688 546000 1123036 546000 0

16 172213 546000 749249 546000 0

17 0 546000 203249 546000 0

18 0 546000 0 203249 0

19 0 117000 0 0 0

20 0 117000 0 0 0

21 0 117000 0 0 0

22 0 117000 0 0 0

23 0 117000 0 0 0

Table 10: Energy production, storage, consumption and availabilities for mirror surface based on average DNI

in Sardinia, in December.


Contract n. 325320





Heat

available(st+prod)

Other

users

Wh Wh Wh

for plant

consumption Wh

0 0 117000 0 0 0

1 0 117000 0 0 0

2 0 117000 0 0 0

3 0 117000 0 0 0

4 0 117000 0 0 0

5 62164 117000 0 62164 0

6 278985 117000 161985 117000 0

7 404710 117000 287710 117000 0

8 446352 117000 617062 117000 0

9 490133 117000 990195 117000 0

10 484147 117000 1357342 117000 0

11 459578 546000 1270920 546000 0

12 468756 546000 1193675 546000 0

13 472746 546000 1120421 546000 0

14 469981 546000 1044402 546000 0

15 472774 546000 971177 546000 0

16 409071 546000 834248 546000 0

17 325159 546000 613407 546000 0

18 211519 546000 278925 546000 0

19 0 117000 161925 117000 0

20 0 117000 44925 117000 0

21 0 117000 0 44925 0

22 0 117000 0 0 0

23 0 117000 0 0 0

Table 11: Energy production, storage, consumption and availabilities for case 3 in May.



Heat

available(st+prod)

Other

users

Wh Wh Wh

for plant

consumption Wh

0 0 117000 0 0 0

1 0 117000 0 0 0

2 0 117000 0 0 0

3 0 117000 0 0 0

4 0 117000 0 0 0

5 0 117000 0 0 0

6 0 117000 0 0 0

7 0 117000 0 0 0

8 139977 117000 22977 117000 0

9 268781 117000 174757 117000 0

10 308086 117000 365843 117000 0

11 313017 546000 132860 546000 0


Contract n. 325320



12 306433 546000 0 439293 0

13 291013 546000 0 291013 0

14 267868 546000 0 267868 0

15 206844 546000 0 206844 0

16 86107 546000 0 86107 0

17 0 546000 0 0 0

18 0 546000 0 0 0

19 0 117000 0 0 0

20 0 117000 0 0 0

21 0 117000 0 0 0

22 0 117000 0 0 0

23 0 117000 0 0 0

Table 12: Energy production, storage, consumption and availabilities for case 3 in December.

2.7 Materials and structures.

The STH system is comprised of several materials. Depending on the design, an external metal case, a

quartz window, an internal structural material for holding in place the absorber material, and the

absorber material itself are the main components of a receiver. Of these materials, the most critical is

likely the absorber material, as it is exposed to the highest thermal stresses and critical for the

performance of the STH unit.

The absorber material for the designs considered in Sol2hy2 needs to be of porous nature in order to

have a flow passing through it. It is exposed to concentrated solar radiation that generate high

temperatures, and the collected heat needs to be transferred to the medium flowing through it in an

efficient manner without creating critical pressure drops. Considering these basic requirements, the

absorber material should be high temperature stable, demonstrate good resistance to thermal shocks,

and have a high thermal conductivity. Moreover, to obtain high efficiencies, the material needs to

have high absorptivity in the visible and near infrared range and large surface area for convective heat

transfer from the solid to the fluid [R 31].

Several materials have been tested in the past for their performance as volumetric absorbers. A good

overview is given by Fend et Al. (2004), where four main material classes are tested and ranked

against each other. Silicon carbide foams, SiC fibre meshes, ceramic and metallic catalyst carriers

(honeycombs) have been investigated for their efficiencies and usability in a volumetric receiver. The

study reports the ceramic foams and fabrics as the most promising materials because of their potential

towards large specific surface combined with beneficial pressure loss characteristics.

Erbicol’s standard material is a 10PPI silicon-infiltrated ceramic foam used in porous burners, as

depicted in Figure 27. The high temperature stability and thermal shock resistance has been

demonstrated for porous burners [R 32], and with a material thermal conductivity of about 110 W/mK

(@ 20°C) the characteristics are suitable for the material to be used as solar absorber.


Contract n. 325320



Figure 27: Standard 10 PPI SiSiC foam used in a porous burner. Right: SiSiC foam at RT; Left: SiSiC foam at

1350°C.

Concerning the architecture of the porosity, a standard foam is a random distribution of Kelvin cells.

The pore geometry is derived from the polyurethane template used in the replica technique to

manufacture the foams. While this geometry offers several benefits, one drawback is the fixed

porosity for each foam. For solar absorber materials, it has been shown [R 31] that a gradient porosity

can be beneficial to increase the performance of the receiver while keeping the pressure drop below

critical values. To solve this issue with random foams Erbicol joined several foams slices with

different porosity during the green stage of the ceramization process (Figure 28). This results in a

stepwise porosity gradient that should demonstrate behavior similar to a continuous porosity gradient.

Future work will concentrate on the design and manufacturing of regular lattice structures with

continuous porosity gradient.

Figure 28: Left: SiSiC foam assembly showing a stepwise gradient from 10 ppi to 30 ppi. Right: SiSiC

elongated Kelvin cell lattice comparable to 10 ppi.


Contract n. 325320



2.8 MCDM of different options.

Given a set of alternatives (solutions) and a set of decision criteria, it is important to establish what is

the best option. In multiple-criteria design problems (multiple objective mathematical programming

problems) the alternatives are not explicitly known. An alternative can be found by solving a

mathematical model. The number of alternatives is either infinite and not countable (when some

variables are continuous) or typically very large if countable (when all variables are discrete).

As in the case of the SOL2HY2 project, it is very uncommon to tackle problems concerning only a

single objective when dealing with real-world industrial applications: generally multiple, often

conflicting, objectives arise naturally in most practical optimization problems; thus several multi-

objective optimization methods have been developed. Many of these algorithms are based on a

stochastic search approach such as evolution algorithms, simulated annealing and genetic algorithms.

Optimizing a problem means finding a set of decision variables which satisfies constraints and

optimizes simultaneously a vector function. The elements of the vector represent the objective

functions of all decision makers. This vector optimization leads to a non-unique solution of the

problem.

For example, as shown in the figure, for the selection of the best hydrogen production process one has

to maximize efficiency and availability, minimize costs, environmental threats, complexity and

process feasibility at the same time.

Figure 29: example of selection of the best hydrogen production process.

Here we consider, without loss of generality, the minimization of two objectives all equally important,

where no additional information about the problem is available.

A solution of the problem can be described by a “decision vector” ⃗ of the form ⃗ ( ) lying

in the design space . The evaluation of the two objective functions on ⃗ produces a solution ⃗

( ) in the objective space , i.e. is a vector map of the form: .


Contract n. 325320



Figure 30: multiobjective optimization according to Pareto criteria.

Comparing two solutions ⃗ and ⃗ requires to define a dominance criteria. In modern multiobjective

optimization the Pareto criteria is the most used. This criteria states:

- An objective vector ⃗ is said to dominate another objective vector ⃗ (i.e., ⃗ ⃗ ) if no

component of ⃗ is greater than the corresponding components of ⃗ and at least one

component is greater;

- accordingly, the solution ⃗ dominates ⃗ , if ( ⃗ ) dominates ( ⃗ );

- all non-dominated solutions are the optimal solutions of the problem, solutions not dominated

by any others. The set of these solutions is named Pareto set while its image in objective space

is named Pareto front.

A generic multiobjective optimization solver searches for non-dominated solutions that correspond to

trade-offs between all the objectives. The utopia (or ideal) point corresponds to the minimal of all the

objectives and typically is not a real and feasible point.

Figure 31: utopia point concept.

When optimization has been performed and the Pareto front has been computed it is necessary to

choose the design point of the system, namely how the system should be made. Many different design

points exist each for a different point of the Pareto front and moreover two closed points in the Pareto

front does not mean that have the corresponding design points closed.

Genetic algorithms [R 33] are search methods based on the mechanics of natural evolution and

selection. These methods are widely used for solving highly non-linear real problem because of their


Contract n. 325320



ability of being robust against noisy and local optima. Genetic algorithms are largely used in real-

world problems as well as in a number of engineering applications that were hard to solve with

“classical” methods.

EnginSoft is creating a mathematical model of the plant considering the different options to couple

solar electricity and solar heat to the cycles and to cover the electricity demand of the electrolyser as

well as of the BoP components. Multi-objective optimization will be performed on such a model,

taking advantage of the NSGA-II, the second version of the famous “Non-dominated Sorting Genetic

Algorithm” based on the work of Prof. Kalyanmoy Deb [R 34]. NSGA-II is a fast and elitist multi-

objective evolutionary algorithm.


Contract n. 325320



3. Conclusions

In order to select the optimized STH/STE configurations, different concepts have been proposed and

evaluated. A literature research on solar tower technologies to provide high temperature heat >600 °C

for sulphuric acid cracking with special focus on thermal storage options has been performed.

Concerning medium temperature technology devoted to power the sulphuric acid concentration step in

continuous and its vaporization up to 500 °C, the solar trough technology has been considered using

also the large experience acquired by ENEA on R&D of solar trough plant using molten salts as

thermal vectors and thermal storage mean.

Some innovations have been also considered for the thermal storage trough this technology: system

based on a single tank designed to use the vertical thermal stratification profile in the HSM fluid; the

utilization of nanoparticles inclusions in the salts (CaO, TiO2, etc), which could improve its heat

capacity but need more experimentation to demonstrate the absence of secondary reactions. PCM

might be another option, if the latent heat and materials costs were beneficial vs. molten nitrates and

the case of MgH2 has been discussed.

Furthermore the methods and the efficiency to generate electricity by CSP both at high temperature

and at medium has been described.

Concerning the medium temperature different scenarios have been assessed based on the following

input: thermal power production, hourly direct normal irradiance (kWh/m2/d) throughout the year,

basic design option based on peak, annual average or minimum DNI values. Some of the above listed

input are site specific, therefore 4 specific locations have been considered, namely: Almeria (Spain),

Larnaca (Cyprus), Alghero (Italy) and San Diego (USA).

The main outputs obtained for each scenario are: mirror field surface, thermal storage size, salt

consumption, backup energy integration and thermal energy supply exceeding SAC needs.

From the analysis, it has been deduced that at low total solar energy input low total solar energy input,

and therefore low mirror surfaces (< 3400 m2), the conventional configuration of parabolic trough

CSP plant cannot be used. Therefore, a specific design to be used for low capacity CSP plant has

been proposed.

This assessment will serve as the basic input for Task 4.2 and techno economic assessment in WP6.

Relating to HT, volumetric receivers applying porous or particle absorbers were identified as

candidate designs. Both receiver concepts can be equipped with storage systems to compensate for up

to several hours without solar radiation. In addition, to storage tanks for sulphuric acid and sulphur

dioxide a more continuous operation could be achieved.

For solar heat collection different receiver designs have been analyzed and porous SiC manufacturing

technology has been evaluated with different framework topologies. These ceramic structures were

compared with known ceramic materials and the advantages of SiC have been demonstrated.

Finally MCDM concepts have been presented as useful instrument to manage the large number of

variables related to SOL2HY2 virtual plant and to know its best optimization.


Contract n. 325320



4. Acronyms

ANI Aperture Normal Irradiance

BOP Balance of Plant

CSP Concentrated Solar Power

DC Direct Current

DNI Direct Normal Irradiance

DSC Differential Scanning Calorimeter

EOR Enhanced Oil Recovery

FR Fresnel Reflector

HP High Pressure

HSM Heat Storage Material

HT High Temperature

HTF Heat Transfer Fluid

LHTES Latent Heat Thermal Energy Storage

MCDM Multiple-Criteria Decision Making

MENA Middle East and North Africa

MH Metal Hydrides

MP Medium Pressure

MT Medium Temperature

PCM Phase Change Material

PCS Solar Collectors Proof

PT Parabolic Trough

SCA Solar Collector Assembly

SD Solar Dish

SDE Sulphur dioxide depolarized electrolysis/electrolyser

SEGS Solar Energy Generating Systems

SG Steam Generator

SOL2HY2 Solar To Hydrogen Hybrid Cycles

ST Solar Tower

STE Solar to Electricity

STH Solar to Heat

SW Software

TES Thermal Energy Storage

WP Work Package


Contract n. 325320



5. References

R 1 Kuravi, S., et al., Thermal energy storage technologies and systems for concentrating solar

power plants. Progress in Energy and Combustion Science, 2013. 39(4): p. 285-319.

R 2 Richter, C., S. Teske, and R. Short, Concentrating Solar Power, Global outlook 2009.

R 3 Barlev, D., R. Vidu, and P. Stroeve, Innovation in concentrated solar power. Solar Energy

Materials and Solar Cells, 2011. 95(10): p. 2703-2725.

R 4 Romero, M. and A. Steinfeld, Concentrating solar thermal power and thermochemical fuels.

Energy & Environmental Science, 2012. 5(11): p. 9234-9245.

R 5. www.solarpaces.org/csp-technology.

R 6 Romero, M. and A. Steinfeld, Concentrating solar thermal power and thermochemical fuels.

Energy & Environmental Science, 2012. 5(11): p. 9234.

R 7 Zunft, S., et al., Jülich Solar Power Tower—Experimental Evaluation of the Storage

Subsystem and Performance Calculation. Journal of Solar Energy Engineering 2011. 133(3): p.

0310193-1:031019-5.

R 8 Schmitz, M., et al., Assessment of the potential improvement due to multiple apertures in

central receiver systems with secondary concentrators. Solar Energy, 2006. 80(1): p. 111-120.

R 9 Kolb, G.J. and R.B. Diver, Screening Analysis of Solar Thermochemical Hydrogen Concepts,

2008, Sandia National Laboratories.

R 10 Pitz-Paal, R., J. Dersch, and B. Milow, ECOSTAR - European Concentrated Solar Thermal

Road-Mapping.

R 11 European Academies Science Advisory Council (EASAC), Concentrating solar power: its

potential contribution to a sustainable energy future2011.

R 12 Dunn, R.I., P.J. Hearps, and M.N. Wright. Molten-salt power towers: newly commercial

concentrating solar storage. in IEEE. 2012.

R 13 http://www.nrel.gov/csp/solarpaces/project_detail.cfm/projectID=38.

R 14 http://www.renewableenergyfocus.com/view/17095/csp-developments-in-heat-transfer-and-

storage-materials/.

R 15 Gaggioli et. Al - An innovative concept of a thermal energy storage system based on a single

tank configuration using stratifying molten salts as both heat storage medium and heat transfer fluid,

and with an integrated steam generator – Solar Paces (2013).

R 16 A. Felix Regin, Heat transfer characteristics of thermal energy storage system using PCM

capsules: A review, Renewable and Sustainable Energy Reviews 12 (2008) 2438–2458.

R 17 Murat Kenisarin, Khamid Mahkamov Solar energy storage using phase change materials,

Renewable and Sustainable Energy Reviews 11 (2007) 1913–1965.

http://www.nrel.gov/csp/solarpaces/project_detail.cfm/projectID=38

http://www.renewableenergyfocus.com/view/17095/csp-developments-in-heat-transfer-and-storage-materials/

http://www.renewableenergyfocus.com/view/17095/csp-developments-in-heat-transfer-and-storage-materials/


Contract n. 325320



R 18 A. Abhat, Low Temperature latent heat thermal energy storage: heat storage materials, Solar

Energy Vol. 30, No. 4. pp 313-332 1983.

R 19 Murat M. Kenisarin, High-temperature phase change materials for thermal energy storage,

Renewable and Sustainable Energy Reviews 14 (2010) 955–970.

R 20 Belen Zalba, José Ma Marın, Review on thermal energy storage with phase change: materials,

heat transfer analysis and applications, Applied Thermal Engineering 23 (2003) 251–283.

R 21 Anant Shukla, D. Buddhi Solar water heaters with phase change material thermal energy storage

medium: A review, Renewable and Sustainable Energy Reviews 13 (2009) 2119–2125.

R 22 Marc Medrano, Antoni Gil, State of the art on high-temperature thermal energy storage for

power generation. Part 2—Case studies, Renewable and Sustainable Energy Reviews 14 (2010) 56–

72.

R 23 Rainer Tamme,y, Thomas Bauer, Latent heat storage above 120°C for applications in the

industrial process heat sector and solar power generation,Int. J. Energy Res. 2008; 32:264–271.

R 24 S. Pincemina, R. Olives, Highly conductive composites made of phase change materials and

graphite for thermal storage, Solar Energy Materials & Solar Cells 92 (2008) 603–613.

R 25 Hani Tiznobaik, Donghyun Shin, Enhanced specific heat capacity of high-temperature molten

salt-based nanofluids, International Journal of Heat and Mass Transfer 57 (2013) 542–548.

R 26 Donghyun Shin, Debjyoti Banerjee, Enhanced Specific Heat Capacity of Nanomaterials

Synthesized by Dispersing Silica Nanoparticles in Eutectic Mixtures.

R 27 Miliozzi A, Veca E, Sau S, Grena R, Celino M, Falconieri M, Rondino F, Individuazione e

caratterizzazione di miscele di materiali a cambiamento di fase e nanoparticelle da impiegare come

sistemi alternativi di accumulo termico. Report RdS/2013/079.

R 28 Crescenzi T, Fontanella A, Liberatore R, Metelli E, Russo V. Analisi tecnico-economica di

impianti solari a collettori parabolici lineari con differenti fluidi di lavoro. Report RdS/2013/077

R 29 Felderhoff M, Bogdanovic’ B. High temperature metal hydrides as heat storage materials for

solar and related applications, International journal of molecular sciences , 10: 325-344.

R 30 Romero, M. and J.G. Aguilar, Energy & Power Generation Handbook, A. K. R. Rao, Editor

2011: Three Park Avenue, New York, NY 10016-5990, USA.

R 31 Fend et Al., Porous materials as open volumetric solar receivers: Experimental determination of

thermophysical and heat transfer properties, Energy, 29, 5-6, pp. 823-833, 2003, doi: 10.1016/S0360-

5442(03)00188-9.

R 32 Ortona et Al., Aging of reticulated Si–SiC foams in porous burners, Advances in Applied

Ceramics, Volume 109, Issue 4, pp. 246-251, 2010, doi: 10.1179/174367510X12663198542586.

R 33 David E. Goldberg. Genetic Algorithms in Search, Optimization & Machine Learning. Addison-

Wesley, 1989


Contract n. 325320



R 34 Deb, K., Pratap. A, Agarwal, S., and Meyarivan, T. (2002). A fast and elitist multi-objective

genetic algorithm: NSGA-II. IEEE Transaction on Evolutionary Computation, 6(2), 181-197


Contract n. 325320



Coordinator: Stefano Odorizzi, ENGINSOFT SpA

Via della Stazione,27 38123 – Trento (ITALY)

Tel: +39 0461 915391 Fax: +39 0461979201

e-mail: [email protected]

Coordinator Main Contact: Carla Baldasso, ENGINSOFT SpA

Via Giambellino, 7 35129 – Padova (ITALY)

Tel: +39 049 7705311 Fax: +39 0461979201


Responsible partner for this deliverable: Raffaele Liberatore, ENEA

Via Anguillarese, 301 00123 – Rome (ITALY)

Tel: +39 06 30484035 Fax: +39 0630486779


mailto:[email protected]



Selected Solar-to-heat/Solar-to-electricity · PDF fileD4.8 Selected...

Documents

Transcript of Selected Solar-to-heat/Solar-to-electricity · PDF fileD4.8 Selected...