Research ArticleCharacterization and Experimental Investigation ofNaNO3 : KNO3 as Solar Thermal Energy Storage for PotentialCooking Application

Elias Wagari Gabisa and Abdulkadir Aman

Bahir Dar Institute of Technology, Bahir Dar University, P.O. Box 26, Bahir Dar, Ethiopia

Correspondence should be addressed to Elias Wagari Gabisa; eliaswa@bdu.edu.et

Received 14 March 2016; Revised 7 May 2016; Accepted 8 May 2016

Academic Editor: Charles M. Drain

Copyright © 2016 E. W. Gabisa and A. Aman. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

Household cooking is a major energy intensive activity in most of the Ethiopian households. Replacing the existing inefficientcooking stoves and the polluting energy source with a renewable source of energy plays a paramount role in conserving theenvironment and reducing the indoor pollution. In this study an energy storage phase change material is proposed to store solarthermal energy for a potential household cooking application.The selected phase change material has a melting point range whichis well fitted to the operating range of temperatures for most of the household cooking activities. The solar energy source issimulated with electrical heating for experimental investigation of the thermal characteristics. Also it is intended to study thethermal characteristics of the mixture using deferential scanning calorimeter to identify at which mass ratio the mixture showsbetter thermal characteristics. From the laboratory analysis it is found that the 60% NaNO

3and 40% KNO

3by mass have shown

promising thermal characteristics. For applying the selected salt mixture for cooking application, an experiment was conducted ontwo Ethiopian local meals, shiro wet and potato meal, to know howmuch energy is required to cook them and what amount of thePCM is required to store the required energy.The result reveals that 2.38 kWh energy is required for cooking the two meals for fivefamily members for lunch and dinner. To store the energy required 4 kg of the PCM was required. Experiments were conducted tosee the charging and discharging time of 60%NaNO

3and 40% KNO

3by mass. From the experimental result for 1.4 kg of the PCM,

charging time of 50 minutes up to 300∘C and a discharging time of 4.5 hours (from 300∘C to 100∘C) are required.

1. Introduction

Energy storage systems have an enormous potential toincrease the effectiveness of energy conversion equipmentuse. Thermal energy storage (TES) deals with the storage ofenergy by cooling, heating, melting, solidifying, or vaporiz-ing amaterial; the energy becomes available when the processis reversed. Solar thermal energy can be stored by elevatingor lowering the temperature of the substance (sensible heatstorage), by changing the phase of the substance (latent heatstorage), or through combination of the two. In this studythe thermal characteristics of sodium nitrate and potassiumnitrate mixture as a phase change material with a differentmass ratio are going to be investigated to know at whichmassratio the best thermal characteristics will be found. In orderto know the solar thermal energy requirement experiment

was conducted on two local meals, shiro wet and potatowet, and experiment was conducted at laboratory level todetermine the charging and discharging time of the phasechange material.

As it is knownworldwide energy demand and supplymis-match are greater problem, especially in developing countrieslike Ethiopia, which have solar thermal energy for about 13months. But to fulfill themismatch between demand suppliesin such countries they are using wood energy for cooking andbaking, which has an adverse effect on health of the societyespecially Women.

And this encourages deforestation highly. In order to alle-viate such a problem it is important to develop technologieswhich utilize a renewable energy as source of energy. Solarcook stove which has a storage media in it is one exampleof such technology. However, solar energy is available during

Hindawi Publishing CorporationJournal of Solar EnergyVolume 2016, Article ID 2405094, 6 pageshttp://dx.doi.org/10.1155/2016/2405094

2 Journal of Solar Energy

day time and the demand for cooking in those countries isduring off shine time [1] during which the sunshine availabil-ity is very law.

According to Ethiopian Rural Development and Pro-motion Center (2006) 77% of total energy consumption iscovered by fire wood and charcoal and the other 15% isfrom agricultural residue, and the other 6% was met bymodern electricity and kerosene [2]. Foong et al. (2011)have developed a solar box cooker by using stearic acid andmagnesium nitrate hexahydrate as a phase change materialfor storing Energy. They have conducted an experiment toevaluate the thermal performance of the developed solarcooker for its charging and discharging time. Finally theycome up with the fact that the cooker has an efficiency of 82%and maximum temperature within the cooker was rangingfrom 78 to 84∘C [3]. Zhang and Fang (2006) used a mixtureof nitrate salts as thermal energy storage material.The nitratesalts are sodium nitrate and potassium nitrate with 60 : 40mole percent ratio; they have conducted the thermal behaviorof the salts by using differential scanning calorimeter (DSC)[4]. Having the above and other related studies, in thisstudy studying the thermal characteristics of NaNO

3: KNO

3

mixture is intended which is not clearly addressed withscholars for different mass ratios, so that different scholarsmy pick one which is important for their application. TheDSC method which is employed here is the world standardto investigate phase transition of phase change materials(PCM) and also to determine their thermal characteristics.The method followed during experimental investigation ofthermal energy requirement was designed locally as well asfor the charging and discharging time determination.

The obtained result shows that it is possible to use thePCM for high temperature cooking application and it isbetter to use the 60 : 40 mass ratio since it has promisingthermal characteristics. The thermal stability of the PCMwith a required range (200–300) of temperature is also verypromising.

There are large numbers of phase change materials thatmelt and solidify at a wide range of temperatures, makingthem attractive in a number of applications. Paraffin waxesare cheap and have moderate thermal energy storage densitybut low thermal conductivity and, hence, require large surfacearea. Hydrated salts have larger energy storage density andhigher thermal conductivity but experience super coolingand phase segregation, and, hence, their application requiresthe use of some nucleating and thickening agents. The mainadvantages of PCM encapsulation are providing large heattransfer area, reduction of the PCMs reactivity towards theoutside environment, and controlling the changes in volumeof the storage materials as phase change occurs [4].

TES systems have also been suggested for storing ther-mal energy at medium (38–304∘C) and high temperatures(120–566∘C). For instance, systems in an oil-rock systemfor hot water and heat-recovery applications are examplesof medium-temperature applications, while those in moltennitrate salt systems (an excellent storage medium) for steamproduction for process applications are for high temper-atures. Oil-rock TES, in which the energy is stored in amixture of oil and rock in a tank, is less expensive than

molten nitrate salt TES but is limited to low-temperatureapplications. However, this oil-rock TES has been provensuccessful for solar thermal applications. The selection ofthe type of TES depends on various factors such as thestorage period (diurnal or seasonal), economic viability, andoperating conditions [5].

According to Sulaiman and Inambao [6], based on theirstudies on development of thermal energy storage and cookermodule, they come up with the conclusion that to allowadequate heat flow during rapid cycles of heat charging anddischarging the heat transfer coefficient between the thermalmedium and PCM needs to be increased.

Tesfay [7] investigates thermal storage wit solar saltexperimentally as well as numerically and simulates it inCOMSOL. According to Tesfay’s study the PCM (solar salt) of2 kg melts within about 4.5 hours by applying 650W averagepower and Tesfay comes up with the possibility of storingthermal energy with solar salt for more than one day sincethe salt mixture has the melting point in the range of itsapplication, 180–220∘C, of Injera baking.

Zhang et al. [8] investigated the thermodynamic evalua-tion of phase equilibria in NaNO

3: KNO

3system and came

up with the conclusion that the solid solutions in the inter-mediate phase appeared to be a mixture of the NaNO

3based

solid solution and KNO3based solid solution depending on

the X-ray diffractometry result at 473K. Finally they under-stand that solidus behavior indicates that NaNO

3: KNO

3is

better regarded as a systemwith limited solid solutions ratherthan as a continuous series of solid solutions.

Greis et al. [9] have conducted a study on phase diagramof the binary system NaNO

3: KNO

3using DSC. They come

up with the result that liquidus curve has its minimum at 494and 50% mole KNO

3with a very flat solidus curve.

2. Methods

In this section, the materials used for conducting the char-acterization of PCM and experimental investigation withthe employed methodologies are discussed in detail. Forthe characterization of the PCM, different standard methodswere followed to insure the accuracy of the results. Theequipment used to analyze the thermal characteristics of thePCM is differential scanning calorimeter (DSC) which is themethod adopted all over the world. Differential scanningcalorimeter (DSC) measures the melting point, specific heatcapacity, and latent heat of fusion of the PCM. The densityof the liquid PCM is determined by the well-known standardmethod using pycnometer.

Melting point, specific heat capacity, and latent heat offusion were analyzed by using standard method of ASTM-D4419-90(2005) by using differential scanning calorimeter.The prepared salt mixtures were 12 g : 8 g (NaNO

3: KNO

3),

10 g : 10 g (NaNO3: KNO

3), 8 g : 12 g (NaNO

3: KNO

3),

4 g : 16 g (NaNO3: KNO

3), and 16 g : 4 g (NaNO

3: KNO

3).

Differential scanning calorimeter (DSC) analysis wasperformed using PerkinElmer DSC8000 instrument. Heatflow and temperature were recorded in the instrument withan accuracy of 0.01mW and 0.05∘C, respectively, withintemperature ranges of 100–500∘C. The measurements were

Journal of Solar Energy 3

made under purified nitrogen atmosphere with a flow rate of20 cm3/min and at a heating rate of 5∘C/min. The mixturesof the salts are taken from the furnace and kept in oven at30∘C to protect them from moisture and then after a daythe samples were taken out from the oven and measuredon electrical balance of accuracy of 0.0001 g. The alreadyweighed samples are put in the aluminum pans of DSCand closed with the lid for analysis. The lid is crimped by asample press. The crimped sample pan was immediately putinside the sample chamber of DSC after preparation. In theexperimental procedure, a temperature range from 100∘C to500∘C was set with a heating rate of 5∘C/min followed bya cooling cycle at the same rate. The density of the liquidNaNO

3: KNO

3mixture was determined using standard

method for density and relative density by using pycnometer(ASTM-148-02(2007)).

The experimental investigation for charging and dis-charging was conducted in the research grade laboratory ofFaculty of Chemical and Food Engineering. The PCM usedis a laboratory grade solar salt (NaNO

3: KNO

3) which has a

purity of 99.9%.Thermal energy requirement for cooking the two local

mealswas conducted electrical stovewhich has a powermeterwithin it and its rating power is 1000W. A known amountof shiro wet and potato was cooked by the stove; the powerconsumption is recorded when the wet becomes maturedenough sensibly.

The charging and discharging time were conductedexperimentally by selected PCM during DSC analysis andputting it in a known volume stainless steel jar and 600Wpower is applied from the bottom of the jar and the increasein temperature of the PCM was recorded within 2-minuteinterval until the PCM reaches slightly above its meltingpoint.

3. Result and Discussion

In this subsection, the detailed description of the results isdiscussed thoroughly. The results discussed are the charac-terization of the PCM, which are the melting point, latentheat of fusion, specific heat capacity and the density, thethermal energy requirement for potential cooking applica-tion, and modeling of the heat transfer characteristics duringcharging.

3.1. Characterization of Storage Material. Differential scan-ning calorimetry (DSC) was used to determine the meltingpoint. A low scanning rate was chosen to record the heatflow curve as function of temperature in order to improvethe sensitivity of detection. It helps to pick up any smallendothermic peaks and also avoids the thermal resistancebetween the internal furnace and sample. All the selectedsystems are composed of alkaline nitrate; all of them have twobasic components which are sodium nitrate and potassiumnitrate. DSC plots for each ratio were collected for three runsto ensure the reproducibility. Peak temperatures, enthalpy offusion for melting peaks, and the solid phase transformationtemperatures are shown in Figure 1 for deferent ratios of thesample.

Figure 1: DSC result for sample ratio of 30 : 70.

75.58

70

60

50

40

30

20

10

−2.676−2.552

Temperature (∘C)146.9

108.9

200 300 400 500 555.9

100

80

60

40

20

Spec

ific h

eat (

J/g∘C)

Hea

t Flo

wEn

do u

p (m

W)

Heat FlowEndo up (mW)Specific heat (J/g∘C)

Figure 2: DSC result for sample ratio of 40 : 60.

As indicated in Figure 1, the melting point of 30 : 70sample ratios by mass is 225.36∘C, the enthalpy of fusion is44.8320 J/g, and specific heat capacity at the solid state (at200∘C) is around 10 J/g∘C and above the melting point itincreases steeply with temperature. At the melting point theabsorbed heat becomes very high and also the specific heatat this point is in order of 40 J/g∘C. Even though the meltingpoint of this sample is very promising, the enthalpy of fusion(the latent heat absorbed) is very low. This implies that forstoring energy per a given amount of sample it requires largercontainer.

As indicated in Figure 2, the melting point of 40 : 60sample ratios by mass is 225.38∘C, the enthalpy of fusionis 95.7905 J/g, and specific heat capacity at the solid state(at 200∘C) is around 5 J/g∘C and above the melting point itincreases with temperature. It is about 10 J/g∘C in the liquidregion at temperature range of 350∘C. At the melting pointthe absorbed heat becomes very high and also the specificheat at this point is in order of 40 J/g∘C. Here in this samplethe melting point is promising and is in order of required

4 Journal of Solar EnergyH

eat F

low

Endo

up

(mW

)

−20

−40

−66.29

100

150

200

250

300

350

409.4

50

−12.16

147

120

100

80

60

40

20

147 200 300 400 500 555.9

0

Temperature (∘C)

Spec

ific h

eat (

J/g∘C)

Specific heat (J/g∘C)Heat Flowendo up (mW)

Figure 3: DSC result for sample ratio of 50 : 50.

range. But the absorbed latent heat per a given mass of thesample is not very promising and the additional sensible heatabsorbed after the melting point as the temperature increasesis relatively low.

Figure 3 shows the DSC results of the 50 : 50 mass ratiosamples. This shows that the melting point is still promisingand is in the required range for potential cooking applica-tion. The melting point is 225.10∘C; enthalpy of fusion is54.9971 J/g and the specific heat capacity in the solid regionseems uniform at about 3 J/g∘C. In the liquid region it isaround 25 J/g∘C in the required temperature range of cookingapplication which is required actually in the liquid region.Having good melting point and specific heat in the liquidregion did not guarantee the use of this sample for utilizationfor the required application.The absorbed energy per a givenamount of sample should be in order of 100 J/g and above forconsideration as good thermal energy storage.

In Figure 4 it is indicated that the melting point of thesample 60 : 40 ratio is 225.38∘C, the heat absorbed per gramof the sample as a latent heat is 120.9100 J/g, and specificheat capacity of the sample in the solid region is around6 J/g∘C. The specific heat increases with the temperature inthe liquid region and it is around 10 J/g∘C in the requiredtemperature range (200–300∘C) for cooking application; itabsorbs large amount of energy as the peak value whichmakes it more preferable and highly promising for thermalenergy storage. The area under the peak is the amount ofenergy absorbed per a given mass of the sample, and itshows that the melting process requires a large amount ofenergy.

From Figure 5 the DSC result of the 70 : 30 mass ratiosamples is shown with two peeks. The first peak indicates themelting point of the mixture, while the second peak showswhen theDSC scans the sample and found some pureNaNO

3

salt alone. The melting point of the mixture is 224.30∘C,enthalpy of fusion is 29.3720 J/g which is lower. The specific

146.9 200 300 400 500 555.9

Temperature (∘C)

20

30

40

50

60

70

75.58

10

−2.676

Spec

ific h

eat (

J/g∘C)

Hea

t Flo

wEn

do u

p (m

W)

Heat FlowEndo up (mW)

−2.552

108.9

100

80

60

40

20

Specific heat (J/g∘C)

Figure 4: DSC result for sample ratio of 60 : 40.

Temperature (∘C)

20

30

40

50

60

70

10

Hea

t Flo

wEn

do u

p (m

W)

Heat FlowEndo up (mW)

−20

−10

−40

−30

−55.17

50.61

40

30

20

10

0

147 200 300 400 500 555.9

74.75

Spec

ific h

eat (

J/g∘C)

−2.651

Specific heat (J/g∘C)

Figure 5: DSC result for sample ratio of 70 : 30.

heat capacity, 𝐶𝑝, in the solid region looks lower, 3 J/g∘C, as

compared to other samples and it is still lower in the liquidregion, 8 J/g∘C.

3.2. Charging andDischarging TimeDetermination. The tem-perature distribution in the stainless steel PCM containerjar is recorded during charging and discharging process.The increase in the temperature of PCM during chargingprocess and the decrease in temperature of the PCM duringdischarging process are recorded.

Figure 6 shows the charging process of the PCM. Initiallythe temperature of the PCM was at 25∘C and the thermostatof the stove on which the jar is put is set to 300∘C, which is

Journal of Solar Energy 5PC

M te

mpe

ratu

re

350

300

250

200

150

100

50

00 10 20 30 40 50 60

Time

Figure 6: PCM charging process for experimental setup.

the maximum temperature required for cooking application.The charging process is continued until the PCM temperaturebecomes in order of 300∘C, which is about 70∘C greater thanthat of the PCM melting point. The temperature of the PCMat the top of the jar was recorded at interval of two minutes.From Figure 6 it is observed that PCM temperature increasesgradually until it reaches its melting point and it increasessteeply until it reaches the melting point within 25 minutesand stays there at almost constant temperature for some timesince it is a phase change process.

The discharging process was conducted with load appli-cation and without load application. 500mL of tap water wasput on the jar containing charged PCM and the decrease intemperature of PCM and the increase in water temperaturewere recorded with an interval of two minutes when load isapplied. Meanwhile for the case with no load application thePCM is taken from the heat source and the bottom part isput on insulator prepared by the size and then the decrease intemperature is recorded by interval of 2 minutes.

The discharging process is continued until the PCM tem-perature reduces to be in order of 100∘C, though it containshigh amount of usable energy. Figure 7 shows the variation ofload temperature and PCM temperature during dischargingprocess. The load temperature increases gradually, while thePCM temperature decreases. As it is clearly seen in the figurethe load starts to absorb the heat rapidly until it becomesabove 50∘C and starts to decline; this is because there was agap between the PCM and the load and the heat transfer wasby convection in the gap and conducted to the load throughthe air in the gap rather than pure conduction between thePCM and the load.

Figure 8 shows the variation of PCM temperaturewithoutload and with load during discharging process. The PCMtemperature decreases gradually and reaches 100∘C within4.53 hrs, while it takes about 4.9 hrs with the case of no load.When the load is applied the PCM temperature starts todecrease to its phase change temperature at about 25minutes,while in the no load case it starts to decrease to its phasechange temperature after 60 minutes. The PCM stays at itsphase change temperature for about 100 minutes when thereis no load application. Both becomedecreasing slowly beyondthe phase change process to 100∘C.

Discharging time

PCM

tem

pera

ture

Load temperaturePCM temperature

250200150100500

350

300

250

200

150

100

50

0

Figure 7: Discharging processes with load.

PCM

tem

pera

ture

350

PCM temperature with no loadPCM temperature (with load)

300

250

200

150

100

50

00 50 100 150 200 250 300

Time (minutes)

Figure 8: Discharging of the PCM with no load and with load.

4. Conclusion

This paper presents the characteristics of different ratioby mass of the PCM (NaNO

3: KNO

3) mixture and the

numerical modeling of the PCMwas simulated using ANSYSmechanical APDL 15.0 to see the temperature distributionin the PCM. And the potential energy requirement forpotential local cooking application was conducted. Finallyexperimental investigation was conducted to see the validityof the model. Generally from the whole study we have thefollowing conclusions.

6 Journal of Solar Energy

After characterizing the different ratios of the PCM theone which is 60%m/m of NaNO

3and 40% of KNO

3m/m

ratio was selected as promising one which has the thermalcharacteristics of melting point of 225.38∘C, latent heat offusion of 120.91 J/g, and specific heat capacity of <6 J/g∘C inthe solid region and >10 J/g∘C in liquid region. The thermalcharacteristic of the PCM shows that there is possibility ofusing the PCM for a wider high thermal application.

From the experimental analysis of the energy require-ment for cooking of two local meals it was found that2.38 kWhof power is required to cook “shirowet” and “potatomeal” for five family members for lunch and dinner. To store2.38 kWh of energy 4 kg of PCM and 2.19 liters of PCMcontainer are needed.Having PCM in small volume containerwill provide a large amount of energy and therefore it ispossible to use the PCM for cooking application.

On evaluating the model by experimental work to fullycharge the PCMof 1.4 kg consumes about 50minutes to bringit to 300∘C, while it needs about 4.9 hours for discharging to100∘C without any load application and 4.53 hour when 500liters of water is used as a load. From this experimental resultit is concluded that it is possible to use this PCM by chargingit with a short period of time as it is also seen in the numericalmodeling, and it can be used for a longer time; thereforethe model and the experiment reveal the same conclusion ofshort charging time.

Generally, standing on all conclusions above, the PCM(NaNO

3: KNO

360%m/m : 40%m/m) is the appropriate

choice for potential cooking application by storing a cheapestsolar thermal energy.

5. Recommendation

Even though different parameters are investigated and ana-lyzed during the study still there are other remaining worksto be investigated by different scholars in the area.The obser-vations for further investigation are summarized as follows.Since solar energy is the most available and promising sourceof thermal energy, it is better to design a system comprisingthe PCM for solar thermal energy storage system, so that theenergy will be available at any time required storing it in suchmaterials.

Industries are using thermal energy for different appli-cation in the process. During the time of utilization there iswaste thermal energy which can be used for the same processor other processes by storing the energy that is going to bewasted. Here, for industries that are using high temperaturethermal energy, recovering the thermal energy waste usingNaNO

3: KNO

3(60 : 40%m/m) is recommended allowing

further study for the mechanism of utilization and recoveryoptions in their waste recovery strategy.

The effect of insulation on heat transfer is a very criticalthing in order to minimize loss of thermal energy. In the caseof such a PCMmaterial it is the case which is very critical andhas a very large effect on the efficiency of the designed system.For such matter it is highly recommended to investigatehow it affects a storage process and efficiency of a certaindesigned system during charging and discharging of thePCM.

Competing Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper.

Acknowledgments

Elias Wagari is very pleased to thank all the people who inone way or another helped him to successfully accomplishthis M.S. thesis work. In particular, he would like to expresshis profound and sincere gratitude to his supervisor Dr.Abdulkadir Aman for his supervision, advice, and inspirationfrom the early stage of the thesis work to the final level. Hisvaluable guidance and immense interest in the thesis topicwere prime mover for his daily activities. He is very gratefulfor his all-round assistance and family type relationship. Heis very grateful for the help he received from the technicalpersons in the Faculty of Chemical and Food Engineering,particularly Adisu Wondimeneh, for his help during labo-ratory analysis and experimental investigation by providinghim the necessary equipment and technical help. Finally, hewould like to thank his lovely wife, Khayriya Yenus, for herefforts and encouragement all the way during his work.

References

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[2] Ethiopian Rural Development and Promotion Center Report,2006.

[3] C. W. Foong, J. E. Hustad, J. Løvseth, and O. J. Nydal, “Numer-ical Study of a High Temperature Latent Heat Storage (200–300∘C) Using Eutectic Nitrate Salt of Sodium Nitrate andPotassium Nitrate,” 2011.

[4] Z. Zhang and X. Fang, “Study on paraffin/expanded graphitecomposite phase change thermal energy storage material,”Energy Conversion and Management, vol. 47, no. 3, pp. 303–310,2006.

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[6] A. S. A. Sulaiman and F. L. Inambao, Development of thermalenergy storage and cooker module for integrated solar energyproject [M.S. thesis], University of Kwatulu-Howerd CollegeCampus, Durban, South Africa, 2008.

[7] A. H. Tesfay, Experimental investigation of a concentrating solarfryer with heat storage [Ph.D. thesis], NTNU, 2015.

[8] X. Zhang, J. Tian, K. Xu, and Y. Gao, “Thermodynamic evalu-ation of phase equilibria in NaNO

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