Design, development and performance evaluation of a latent heat storage unit for evening cooking in...

Design, development and performance evaluation of alatent heat storage unit for evening cooking in a solar

cooker

S.D. Sharma, D. Buddhi*, R.L. Sawhney, Atul Sharma

Thermal Energy Storage Laboratory, School of Energy and Environmental Studies, Devi Ahilya University,Takshashila Campus, Khandwa Road, Indore 452017, India

Received 19 June 1999; accepted 8 December 1999

Abstract

In this paper, a PCM storage unit for a solar cooker was designed and developed to store solarenergy during sunshine hours. The stored energy was utilised to cook food in the evening. Commercialgrade acetamide was used as a latent heat storage material. Cooking experiments were conducted withdi�erent loads and loading times during the summer and winter seasons. The thermal performance of asolar cooker with the PCM storage unit was compared with a standard solar cooker. The experimentalresults showed that evening cooking is possible with a solar cooker having the PCM storage unit and isnot possible in a standard solar cooker. 7 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Solar energy; Solar cooker; Latent heat storage; Evening cooking; Phase change material

1. Introduction

Utilisation of solar energy for thermal applications, like cooking, heating and drying, is wellrecognised in tropical and semitropical regions. The di�erent types of solar cookers developedfor cooking are (i) concentrator type (ii) box type and (iii) indirect type Box type solar cookersare more popular due to their simplicity of handling and operation. The detailed design, test

Energy Conversion & Management 41 (2000) 1497±1508

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* Corresponding author. Tel.: +91-731-460309; fax: +91-731-470372.E-mail address: [email protected] (D. Buddhi).

Nomenclature

A outer surface area of solar cooker (m2)Cw speci®c heat of water (kJ/kg 8C)CPCM speci®c heat of phase change material (kJ/kg 8C)F fraction of incident solar energy stored by PCM in time tF1 ®rst ®gure of merit (m2 8C/W)F2 second ®gure of meritG average global solar radiation over time period Dt1 (W/m2)Gaa average global solar radiation during batch cooking process (W/m2)Gs average global solar radiation at stagnation temperature (W/m2)L latent heat of PCM (kJ/kg)Mf mass of food (rice + water) (kg)Mw mass of water (kg)MPCM mass of PCM (kg)Qstored energy stored by PCM (kJ)r re¯ectivity of re¯ector (%)S incident solar radiation at horizontal surface (W/m2)t time taken to achieve maximum temperature of PCM (s)Ta average ambient temperature over time period Dt1 (8C)Tas ambient temperature at steady state (8C)Taa average ambient temperature during batch cooking process (8C)Tps absorbing plate temperature at steady state (8C)Tw1 lower value of water temperature for evaluating F2 (8C)Tw2 upper value of water temperature for evaluating F2 (8C)Dt1 time taken for heating from Tw1 to Tw2 (s)Dt time interval (s)ti time at which food is loaded (h)to time at which food is taken out (h)Tpi absorbing plate temperature when food is loaded (8C)Tpo absorbing plate temperature when food is taken out (8C)Tf food temperature during cooking process (8C)Tfi food temperature when food is loaded (8C)Tfo food temperature when food is taken out (8C)Tm melting temperature of PCM (8C)TPCM PCM temperature (8C)TPCMi PCM temperature when food is loaded (8C)TPCMo PCM temperature when food is taken out (8C)TPmax maximum plate temperature during batch cooking process (8C)TPCM max maximum PCM temperature during batch cooking process (8C)Tfmax maximum food temperature during batch cooking process (8C)UL heat loss coe�cient of cooker (W/m2 8C)a absorptivity of cooking trayt transmitivity of glass

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procedures, theory and utility of box type solar cookers are well developed [1±4]. The use of abox type solar cooker is limited because cooking of food is not possible due to frequent cloudsin the day or in the evening.If storage of solar energy can be provided in a solar cooker, then there is a possibility of

cooking food during clouds or in the evening, and the storage will increase the utility andreliability of the solar cookers. Buddhi and Sahoo [5] designed and tested a solar cooker withlatent heat storage for cooking food in the late evening. In their design, the phase changematerial (PCM) was ®lled below the absorbing plate. In such type of design, the rate of heattransfer from the PCM to the cooking pot during the discharging mode of the PCM is slow,and more time is required for cooking the evening food. If the cooking vessel is surrounded bya PCM unit, then the rate of heat transfer between the PCM and the food will be higher, andcooking can be faster.Experiments with solar cookers indicate that foods are cooked at temperatures between 95

and 978C. No appropriate and promising PCM, having a melting temperature between 95 and1058C is available in the literature. Therefore, in the present case, commercial grade acetamide(melting point 828C) is used as a latent heat storage material, which has the nearest meltingtemperature out of the quoted materials in the literature. In the present paper, a PCM storageunit for the cooking pot was designed, fabricated and tested. Cooking of three batches a dayduring summer and two batches a day during winter were made successfully.

2. Designing of the storage unit for cooking pot

To estimate the required mass for the latent heat storage material for an evening cookingcapacity of 0.75 kg during the winter season of the composite climate in India, the followingassumptions have been made:

(i) Average solar radiation (9 AM to 3 PM) = 600 W/m2

(ii) Average ambient temperature = 208C(iii) PCM temperature at 3 PM (before evening loading) = 1208C(iv) Food temperature at 3 PM (before loading) = 208C(v) Evening food is loaded at 3 PM(vi) Solar cooker is closed to the solar radiation at 4 PM(vii) PCM and food temperature at 4 PM = 958C(viii) PCM and food temperature at 6.00 PM = 828C

To have the food temperature at the solidifying point of the PCM at 6.00 PM, the energyreleased by the PCM and food should be equal to the energy loss from the cooker, i.e.

MfCw�Tf ÿ Tm� �MPCMCPCM�TPCM ÿ Tm� �MPCML � ULA�Tf ÿ Ta�Dt �1�The thermophysical properties of acetamide and various cooker parameters are given inTable 1. Using Eq. (1), the required storage mass of PCM is 2.0 kg. The amount of heatrequired to melt the PCM is 805 kJ and part of the available solar radiation (12900 kJ/m2) willbe used for the same.The volume required for 2.0 kg of acetamide is 1725 cm3. One of the accepted sizes of the

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cooking vessel is 18 cm in diameter and 6 cm in height. A PCM container to hold the cookingvessel was designed and fabricated as shown in Fig. 1. It has two hollow concentric aluminiumcylinders of diameter 18 cm and 25 cm and 8 cm deep of 2 mm thickness. The space betweenthe cylinders was ®lled with PCM. The dimensions of the vessel used for cooking were 17.5 cmand 10 cm in diameter and height, respectively, and it can be inserted inside the PCMcontainer for cooking purposes. To enhance the rate of heat transfer between the PCM and theinner wall of the PCM container, 8 ®ns (1 � 3 cm) were welded at the inner wall of the PCMcontainer.

Table 1Thermophysical properties of commercial grade acetamide and heat loss coe�cient used in design calculation

Melting temperature of acetamide (commercial grade) 828Ca

Latent heat of fusion of acetamide (commercial grade) 263 kJ/kga

Density of solid acetamide 1.159 g/cm3

Density of liquid acetamide 0.998 g/cm3

Speci®c heat of the acetamide 1.94 kJ/kg 8Ca

Heat loss coe�cient from top, bottom and side of solar cooker (when solar cooker is closed) 1.00 W/m2 K

a Measured using DSC.

Fig. 1. Schematic diagram of latent heat storage unit.


3. Experimental setup and measurements

To conduct the cooking experiments with the PCM container and cooking vessel, a doubleglazed box type solar cooker with re¯ector having 50 � 50 cm aperture area and 15 cm deepwas used. To compare the thermal performance of this `PCM Cooker', a similar box type solarcooker of 50� 50 cm aperture area and 10 cm deep with standard cooking vessel was used as a`Reference Cooker'.Four K type thermocouples were used to measure the temperature of the absorbing plate,

PCM, ®n and food of the `PCM Cooker', and two K type thermocouples were used tomeasure the absorbing plate and food temperature of the `Reference Cooker'. Thesemeasurements were made using an HP 3852A data acquisition system. Solar radiation andambient temperature were also recorded during the course of the experiments. The experimentswere conducted at Indore (Latitude, 22.78 N), India, and the orientation of the solar cookerwas adjusted towards the sun after every 15 min.

4. Performance test

The thermal performance of the `PCM Cooker' (without storage unit) and `ReferenceCooker' were determined by conducting the stagnation temperature test (®rst ®gure of merit:F1) and by sensibly heating a known amount of water (second ®gure of merit: F2) [6]. Thevalue of F1 for both solar cookers was found to be 0.14. To determine the second ®gure ofmerit, each cooker was loaded with 1 kg of water, and the values of F2 were found to be 0.34and 0.37, respectively. The equations used to calculate the values of F1 and F2 are given in Ref.[6], and the experimental values of the various parameters used to calculate F1 and F2 are givenin Table 2. The values of F1 for both cookers are the same, and a minor di�erence is found inthe values of F2, i.e. the thermal performance of both cookers can be considered the same. Thee�ectiveness of the re¯ector in enhancing the incident solar radiation on the glass cover (thetop one) is shown in Fig. 5. The average re¯ectivity of the re¯ector was found to be 77%.

5. Calculation of fraction (%) of incident solar energy stored by the PCM

The fraction of incident solar energy stored by the PCM is

Table 2

Comparative thermal performance tests of `PCM Cooker' (without storage unit) and `Reference Cooker'

Type of Cooker Tps

(8C)Tas

(8C)Gs

(W/m2)

F1

(8C m2/W)

Tw1

(8)Tw2

(8C)G

(W/m2)

Ta

(8C)Dt1(s)

Mw

(kg)

F2

PCM Cooker 167.4 38.0 912 0.14 60.4 95.6 874 38.3 3000 1.0 0.34

Reference Cooker 172.3 37.5 922 0.14 60.8 94.9 916 38.2 2520 1.0 0.37


f � Qstored=�ta�A�t0

S dt �2�

and the energy stored by the PCM is

Qstored �MPCM

�CPCM�Tm ÿ Ta� � L� CPCM�TPCM max ÿ Tm�

� �3�It has been assumed that the PCM initially is at ambient temperature, and the speci®c heat isthe same for both the solid and liquid phases. Using Eq. (2), the fraction (%) of incident solarenergy stored by the PCM on di�erent days was calculated and is tabulated in Table 3.

6. Results and discussions

Cooking experiments were conducted using the `PCM Cooker' and `Reference Cooker' withthe same quantity of load. On June 8, 1998, both solar cookers were exposed to the sun at 9.50AM and were loaded with 0.15 kg rice and 0.30 kg water at 10.25 AM. The rice of both solarcookers was taken out at 11.40 AM and found to be well cooked. For the second batchcooking, both solar cookers were loaded again with 0.15 kg rice and 0.30 kg water at 3.00 PM

Fig. 2. Variation of temperatures and solar radiation with time of `PCM Cooker' and `Reference Cooker' with

cooking load, 8 June, 1998.


Fig. 3. Variation of temperatures and solar radiation with time of `PCM Cooker' and `Reference Cooker' with cooking load, 30 June, 1998.

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and closed for solar radiation. The rice was taken out at 4.00 PM, and it was found that therice of the `PCM Cooker' was well cooked but the rice of the `Reference Cooker' wasuncooked. For the third batch cooking, the PCM cooker was exposed to solar radiation at4.15 PM again and loaded at 5.00 PM with the same quantity. Because of the low solarradiation, the `PCM Cooker' was closed for solar radiation at 5.00 PM. Before loading the

Fig. 4. Variation of temperatures and solar radiation with time of `PCM Cooker' and `Reference Cooker' withcooking load, 6 October, 1998.

Table 3

Fraction (%) of incident solar energy stored by the PCM

Date First batch loading (kg) Total solar radiation (exposure to PCMmax temperature achieved) (kJ) f (%)

8.6.98 0.45 2385 2330.6.98 0.30 2294 25

5.10.98 1.5 2649 246.10.98 1.5 3399 207.10.98 1.5 2316 28

8.10.98 1.5 2602 24


cooker, the plate and PCM temperature at 5.00 PM were 79.6 and 84.88C. The `PCM Cooker'was opened at 6.00 PM, and it was found that the rice was not cooked. The recordedtemperature pro®les of the plate, ®n, PCM, food and ambient temperatures and solar radiationwith time are plotted in Fig. 2.On June 30, 1998, both solar cookers were exposed to solar radiation at 10.30 AM and

loaded with 0.10 kg rice and 0.20 kg water at 11.00 AM. The rice of both solar cookers wastaken out at 2.00 PM and found to be well cooked. The day was cloudy and the time taken forcooking was about 3 h. For the second batch of cooking, both solar cookers were loaded againwith 0.10 kg rice and 0.20 kg water at 4.00 PM and taken out at 4.35 PM, and it was foundthat the rice was well cooked. The plate and PCM temperatures at the loading time were 136.1and 128.58C, respectively.For the third batch and evening cooking, the solar cooker was loaded with 0.15 kg rice and

0.25 kg water (less quantity as compared to June 8, 1998) at 6 PM, and the plate and PCMtemperatures at the loading time were 127.8 and 130.68C, respectively, and the solar cookerswere closed for solar radiation after loading. The status of the rice was checked at 6.50 PM,and it was found that the rice of the `PCM Cooker' was well cooked and that of the`Reference Cooker' was not cooked. The temperature pro®les of the absorbing plate, ®n, PCM,food and ambient temperatures and solar radiation are plotted in Fig. 3. The temperatureswere recorded up to midnight, and one can see from the ®gure that the PCM and food

Table 4

Comparative temperatures of `PCM Cooker' and `Reference Cooker' with load, 30 June, 1998

I Batch cooking II Batch cooking III Batch cooking

PCM Cooker ReferenceCooker

PCMCooker

ReferenceCooker

PCMCooker

ReferenceCooker

ti 11.00 11.00 16.00 16.00 18.00 18.00to 14.10 14.10 16.35 16.35 18.50 18.50Tpi 87.7 107.9 136.1 167.7 127.8 127.9

Tpo 92.9 99.4 131.5 126.9 76.2 69.2TPCMi 56.4 ± 128.5 ± 130.6 ±TPCMo 96.2 ± 125.9 ± 96.2 ±

Tfi 35.4 35.6 35.2 34.8 42.1 38.2Tfo 95.4 106.4 116.9 116.5 92.1 75.9TPmax 131.9 129.8 136.1 167.7 127.8 127.9TPCM max 96.8 ± 128.5 ± 130.6 ±

Tfmax 95.4 115.4 116.9 116.5 94.5 77.5Taa 31.1 31.1 32.5 32.5 30.7 30.7Gaa 320 329 610 610 155 155

Mf 0.30 0.30 0.30 0.30 0.30 0.30Remarks1 Cooked Cooked Cooked Cooked Cooked Not cooked

2 Exposed tosolarradiation

Exposed tosolarradiation






Table 5Comparative temperatures of PCM cooker and reference cooker with load, 7 and 9 October, 1998

07.10.98 09.10.98

I Batch cooking II Batch cooking I Batch cooking II Batch cooking

PCMCooker

ReferenceCooker

PCMCooker

ReferenceCooker

PCMCooker

ReferenceCooker

PCMCooker

ReferenceCooker

ti 9.50 9.50 16.00 16.00 10.25 10.25 16.25 16.25to 12.45 12.45 19.00 19.00 13.00 13.00 17.50 17.50

Tpi 85.7 123.9 125.6 87.4 109.3 170.2 132.3 116.5Tpo 143.3 144.7 55.2 46.5 149.9 144.7 89.9 73.0TPCMi 66.4 ± 133.9 ± 124.6 ± 142.3 ±

TPCMo 119.9 ± 64.2 ± 142.3 ± 99.9 ±Tfi 31.8 31.1 31.6 30.5 29.6 30.3 36.8 39.9Tfo 101.6 117.3 66.9 50.4 102.1 117.1 98.7 71.3TPmax 143.3 144.7 125.6 87.43 149.9 170.2 132.3 116.5

TPCM max 119.9 ± 133.9 ± 124.6 ± 142.3 ±Tfmax 101.6 117.3 91.6 67.1 102.8 117.2 98.9 69.8Taa 31.2 31.2 32.4 32.4 31.5 31.5 31.2 31.2

Gaa 488 488 137 137 747 747 159 159Mf 1.5 1.5 0.75 0.75 2.3 2.1 0.60 0.60Remark

1 Cooked Cooked Not cooked Not cooked Cooked Cooked Not cooked Not cooked2 Exposed to

solar radiationExposed tosolar radiation

Exposed tosolar radiation






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temperatures (about 508C) were almost the same during the discharging period of the PCM inthe night. The comparative temperatures of the `PCM Cooker' and `Reference Cooker' withtheir maximum values and average values of ambient temperature and solar radiation duringthe cooking process are tabulated in Table 4.On October 6, 1998, the solar cookers were exposed to the sun at 9.15 AM and were loaded

with 0.50 kg rice and 1.0 kg water at 9.50 AM. The rice of the `Reference Cooker' was takenout at 11.15 AM and found to be well cooked. The cooked rice of the `PCM Cooker' wastaken out at 12.00 Noon. For evening cooking, both solar cookers were loaded with 0.25 kgrice and 0.50 kg water at 3.30 PM. The PCM and absorbing plate temperatures at the loadingtime were 157.9 and 146.48C, respectively. The rice of the `PCM Cooker' was taken out at 4.20PM and found to be well cooked. The rice of the `Reference Cooker' was taken out at 5.45PM and found to be uncooked. The absorbing plate, PCM and food temperatures at this timewere 102.3, 107.1 and 99.68C, respectively. The recorded temperature pro®les of the absorbingplate, ®n, PCM, food and ambient temperatures, and solar radiation are plotted in Fig. 4.From Figs. 2±4, it can also be noticed that the rise in plate temperature of the `ReferenceCooker' is higher than that of the `PCM Cooker' during the charging period of the PCM.Similar experiments with di�erent loads were also conducted on October 7 and 9, 1998. A

comparative tabulation of the experimental data are given in Table 5. From Tables 4 and 5, itcan be seen that during the month of June (summer), there is a possibility of cooking threebatches (two noon and one evening) a day, while during the month of October (winter),cooking of two batches (noon and evening) of food are possible in the `PCM Cooker'.However, evening cooking is not possible in the `Reference Cooker'. From Figs. 2±4 andTables 4 and 5, it can also be observed that for evening cooking, the temperatures of the PCM

Fig. 5. E�ectiveness of the re¯ector.


and absorbing plate should be higher than 1208C at the time of loading for a PCM having amelting point of 828C. During cooking experiments with the `PCM Cooker', it was found thatthe time taken for evening cooking is about one hour. It was also observed that the rice waswell cooked if maintained at about 958C for 15 min.

7. Conclusion

A cylindrical latent heat storage unit for the cooking pot of a solar cooker was designed andfabricated to store solar energy during sunshine hours. The stored energy was used to cook riceduring the evening. From the experimental results, one can conclude that (i) the storage ofsolar energy does not a�ect the performance of the solar cooker for noon cooking and (ii) if aPCM having a melting temperature between 105 and 1108C is used, the cooking with thepresent design will be possible even during the night.

Acknowledgements

The authors are grateful to Dr. A.K. Sharma, Deputy Director, C.E.S.R., D.A.V.V., Indorefor his valuable discussions and suggestions.

References

[1] Telkes M. Solar Energy 1959;3(1):1±11.[2] Hoda MM. In: Proceedings of ISES, UK Conference, London. 1977. p. 817.

[3] Mullick SC, Kandpal TC, Saxena AK. Solar Energy 1987;39:353.[4] Thulasi Das TC, Karmakar S, Rao DP. Solar Energy 1994;52:265.[5] Buddhi D, Sahoo LK. Energy Convers and Mgmt 1997;38:493±8.

[6] Buddhi D, Sharma SD, Sawhney RL. International Journal for Energy Research 1999;23:827±30.


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