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THERMAL PERFORMANCE OF A SOLAR COOKER HAVING PHASE by epn13773

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									THERMAL PERFORMANCE OF A SOLAR COOKER HAVING PHASE
    CHANGE MATERIAL AS TRANSPARENT INSULATION

                                    V.V.S. Murty & Preeti Kanthed,
                                      Solar Energy Laboratory,
                              Physics Department, Holkar Science College,
                                        Indore-452017, INDIA

ABSTRACT

       Efforts were made to study the Thermo-physical properties and to enhance the effective thermal
conductivity of the Phase Change Materials (PCMS). Some Phase Change Materials are white, like water
in physical appearance in the liquid state and can transmit solar radiation. Efforts have been made recently
to study the transmittance of these materials. Latent heat thermal energy storage materials usually have low
thermal conductivity and good transmittance and therefore these materials can be used as transparent
insulation and can also trap the heat. During the sunshine hours, phase change material in liquid phase
would allow the transmission of solar radiation.
       The main objective of the present study is to measure the transmittance of the solar radiation
through a PCM in the liquid and the semi–liquid phases. Commercial grade Phase change material, Lauric
Acid (M.P.42.2 °C, Latent heat of fusion 181 KJ/Kg) was used as a latent heat storage material.


INTRODUCTION

       Utilisation of solar energy for thermal applications like cooking, heating and drying is well
recognized in tropical and semi-tropical regions. Different types of solar cookers developed for cooking are
Concentrator type and Box type solar cooker. The box type solar cookers are more popular due to their
simplicity of handling and operation. The detailed design, test procedure and theory are well developed
[1-5]. Utility of box type solar cooker is limited because cooking of food is not possible due to frequent
clouds in the day or in the evening.
       Buddhi and Sahoo [5] designed and tested a solar cooker with latent heat storage for cooking food
in the late evening. Buddhi et al [ 6] and Sharma et al [7, 8] designed and developed cylindrical PCM
storage unit for a box type solar cooker to cook the food in the late evening. Mullick et al [5] developed a
thermal test procedure for box type solar cookers. P. Funk [9] specified a test procedure for thermal
performance evaluation of a box type solar cooker in terms of effective cooking power (P), which accounts



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for the heat gain rates. This test procedure has been developed as ASAE standards for testing the thermal
performance of a box type solar cooker [10]. Buddhi et al [2] has reported the effect of load on the second
figure of merit.
       This paper presents the thermal performance of a box type solar cooker with phase change material
(PCM) filled in the glass cover to use as transparent insulation during low solar radiation or off sun–shine
hours, it would not allow the loss of heat from of the solar cooker. The thermal performance of solar
cookers is evaluated by performing the stagnation temperature test called as First figure of Merit (F1) of
the solar cooker.   Various parameters like plate temperature, load temperature, temperature of the PCM,
intensity of solar radiation and the ambient temperature were measured for the PCM solar cooker. A
similar solar cooker was taken as a reference cooker. The thermal performance of PCM solar cooker was
compared with the reference solar cooker.
       The experiments for thermal performance were conducted with two pots inside the solar cooker.
Using empirical relations, the standard cooking power of the PCM solar cooker was also calculated and
compared with the reference cooker to study the effect of phase change material (PCM) on the
performance.



DESIGN OF THE PCM SOLAR COOKER WITH PCM IN THE GLAZING

       The PCM solar cooker and reference solar cooker were having 37 cm x 37 cm as aperture area. A
cooking pot made up of aluminum with internal diameter 16cm, depth 4cm and thickness 0.5cm were used
for the experiments. The top aperture of the solar cooker is covered with a double walled toughened glass
of thickness 0.3mm with a separation distance of 10mm. The space between the glass covers of a solar
cooker was filled with the PCM. This cooker is referred as ‘PCM solar cooker’.


EXPERIMENTS AND MEASUREMENTS

       Lauric acid is used as the phase change material. The physical and thermo-physical properties of
the Lauric acid are given in the table 1 .To study the insulating behavior of the phase change material and
hence the thermal performance of the PCM cooker, various experiments are carried out at outdoors in the
months of December 2002, January 2003 and February 2003 at Indore (Latitude 22.7° N) with and
without loads and evaluated the following:




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i.     First Figure of Merit of both the solar cookers.
ii.    Comparative study of Thermal performance of the PCM solar cooker and reference solar cooker.
iii.   Evaluation of standard cooking power of the PCM solar cooker and its comparison with the
       reference solar cooker.


Table 1.: Thermophysical properties of the phase change material Lauric acid.

Chemical Formula                   Laboratory Grade           Commercial grade
CH3 (CH2)10 COOH

Melting Temperature (°C)           49                         42.2 (43-46)*
Latent Heat of Fusion (kJ/kg)      177                        181*
Specific Heat (kJ/kg °C)           1.6                        --
Thermal Conductivity (w/m °K)      0.147 ( at 50 °C)          --
Density           Solid            1007                       --
(kg/m3)           Liquid           862                        --
Cost (US$)                         195/ kgb                   32/kgb
                                   --                         3.0/ kgc

.*Measured through Differential Scanning Calorimeter (DSC)
b      From the price list of Fluka Chemica-Bio Chemical, Switzerland, 1993-1994
c      From the local market of India, 2003
1 US$ ≈ Rs 48/-


       During all the experiments, the intensity of solar radiation is measured using Solarimeter [make
C.E.L, India] with an accuracy of 0.2 mw/cm2. Calibrated (copper- constantan) thermocouples are used
with Digital Multimeter [Make MIC-3300 A] with an accuracy of 0.2 % digit and resolution of 0.1 mV for
the measurement of temperatures such as the absorber plate, the cooking fluid, lower and upper surfaces of
the glass plates, intensity of solar radiation and the ambient temperature were measured continuously for a
specific interval of time during the experiment.




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RESULTS AND DISCUSSIONS

Box type solar cooker with PCM in the glazing: - Evaluation of its First Figure of merit.

        Experiments are conducted on PCM solar cooker and Reference solar cooker of the same
dimensions without load and without reflector and various measurements like plate temperature, intensity
of solar radiation and the ambient temperatures are measured.
        The First Figure of Merit of the solar cooker is given by

                                      F1 = (Tps -Tas)/Hs
        The First Figure of Merit (F1 ) of the PCM solar cooker and Reference solar cooker is observed to
be 0.0615 3 and 0.09573 respectively. The variation of the First figure of merit of the reference cooker
at the various starting exposure times are studied and it is observed that the value of F1 increases as the
starting exposure time increases and but stagnation reaching time decreases and the variation of F1 for
various starting exposure times are shown in the Table 2


Table 2.: Variation of F1 at different exposure times on different dates

Date         Texp      Tps      Tas      Hs       Stagnation reaching Time (mts)     F1

                       °C       °C       (W/m2)


7th Jan 03   8:30     75       28.3      610                     285               0.076
 nd
2 Jan 03     10:05    77.5     28.2      690                     135               0.084
 th
4 Jan 03     11:05    77.5     23.0      570                     130               0.096
5th Jan 03   12:00    75.0     25.5      520                     95                0.096
6th Jan 03   14:10    57.5     28.2      320                     60                0.092




Box type solar cooker with PCM in the glazing: - Insulation behavior of the PCM cooker during
charging and discharging processes.
       Various experiments were conducted to study the thermal performance during charging and
discharging processes of the PCM solar cooker having an aluminum container with water as a fluid. The
variations of the load temperature with time of Reference solar cooker and PCM solar cooker during


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charging and discharging are shown in the fig. 1 and fig. 2 .It is observed that the effect of the PCM in the
glazing has a significant importance during the charging mode and the discharging mode as well.



                                                                                                70
                   120
                                                                                                                                                                                                                  Twr (°C)
                                                                                                                                                                                                                  Twp(°C)
                                                                                                60
                   100



                                                                                                50
                    80
Temperature (°C)




                                                                             Temeprature (°C)
                                                                                                40

                    60

                                                                                                30


                    40

                                                                                                20


                    20                                 Twr(°C)
                                                       Twp(°C)                                  10



                     0
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                                                                                                     16:00 16:15 16:30 16:45 17:00 17:15 17:30 17:45 18:00 18:15 18:30 18:45 19:00 19:15 19:30 20:00 20:15 20:30 21:30 21:45
                      :0

                      :1

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                    16




                                        Time (hrs)                                                                                                          Time (hrs)




Fig. 1. Comparison of load temperature of reference                         Fig. 2. Comparison of load temperature of
solar cooker and PCM solar cooker during charging                           reference solar cooker and PCM solar cooker
process (Date 21.01.2003)                                                   during discharging process (Date 21.01.2003)

                         During the charging mode, the plate temperature and load temperature of the reference solar cooker
is higher throughout the process. It has a maximum difference of temperature above the PCM solar cooker
around 55°C and 45°C during the charging process respectively. The rate of increase of plate temperature
and the load temperature in the PCM cooker is less compared to the similar change in their values in the
reference cooker. Hence during the charging process, while PCM is melting, the presence of PCM medium
does not allow the increase of temperature. Hence the PCM acts as an insulator for the outflow of thermal
energy.
                         During discharging mode, the plate temperature of the PCM cooker maintains a higher value
throughout the process. It has a maximum difference of temperature above reference cooker are around 15
°C and 45°C respectively during the discharging process. The rate of increase of plate temperature and the
load temperature in the PCM cooker is less compared to the similar change in their values in the reference
cooker. Hence during the charging process, while PCM is melting, the presence of PCM medium does not
allow the increase of temperature. Hence the PCM acts as an insulator for the flow of thermal energy.
                         During the discharging process, the rate of decrease in the plate temperature of the PCM cooker is
less compared to the reference cooker for the same change of temperature. Hence the rate of discharge of
energy from the PCM during solidification process helps in maintaining the higher plate temperature for a
longer time. Hence the rate of decrease of plate temperature is less during discharge process. During


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discharging mode, the load temperature of the PCM solar cooker maintains higher value throughout the
process. It has a maximum higher difference of temperature above the reference cooker of around 20 °C
during discharging process. The rate of decrease of load temperature of the PCM solar cooker is less
compared to the reference cooker for the same change. Hence the rate of discharge of heat energy from the
PCM during solidification process helps in maintaining the load temperature for a longer time. During
solidification process, as the thickness of the PCM layer increases, insulation behaviour dominates and
hence the rate of decrease in the load temperature is less in the PCM solar cooker.


       Box type solar cooker with PCM in the glazing: - Evaluation of standard cooking power and the
coefficient of regression and their comparison with the reference solar cooker and PCM solar cooker.
       Various experiments were conducted by measuring the variables like temperature of water, ambient
temperature and the intensity of solar radiation at a fixed interval of time for a known mass of fluid (water).
The standard cooking power (P s) was calculated for the load of 1.0 kg equally distributed in two pots for
an interval of 10 minutes is given by the equation,

                            Ps = M* C* dT* 700 / 600 Havg
       The variation of cooking power of the reference solar cooker and PCM solar cooker with the
difference of average of the temperatures of water in two pots and the ambient temperature is shown in fig.
3 and 4. The linear regression of the plotted curve was used to find the relationship between the cooking
power and the temperature difference in terms of intercept and the slope.


       According to the international standards, the coefficients of determination (r2) should be better than
75%. The cooking power regression equation of the reference solar cooker and PCM solar cooker were
presented in the table 4.


Table 4.: Variations of Cooking Power regression Equations (Intercept and Slope)
Quantity of water (kg)       Cooking Power Regression Equation in Cooking               Power      Regression
                             PCM Cooker                                     Equation in reference Cooker
                             Intercept (W)              Slope (W/°C)        Intercept (W)     Slope (W/°C)
1.0                          23                         -1.5773             68                -1.3317




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        30                                                                                          80

                                                                                                                       y = -1.3317x + 79.821
                 y = -1.5773x + 47.722                                                                                         2
                        2                                       Power
                                                                                                    70
                                                                                                                             R = 0.937
        25            R = 0.7239                                Linear (Power)
                                                                                                                                                    Power
                                                                                                                                                    Linear (Power)
                                                                                                    60

        20
                                                                                                    50
Power




                                                                                            Power
        15                                                                                          40



                                                                                                    30
        10


                                                                                                    20

        5
                                                                                                    10


        0                                                                                           0
             0           5               10    15     20   25                    30                      0   10   20        30           40    50                60   70
                                              Tw-Ta                                                                              Tw-Ta




Fig. 3. Variation of standard cooking power (Ps )                                           Fig.4. Variation of standard cooking power (P s )
        of the PCM solar cooker with (T w -T a )                                                   of the PCM solar cooker with (T w -T a )



CONCLUSION
                    In addition to the Physical and Thermo-physical properties, PCM is observed to possess a good
transmittance and insulating property and hence they are useful during the charging and the discharging
processes. During charging process, as the phase change material melts, it allows the solar radiation into
the solar cooker and improves the cooking performance and during discharging process, as it solidifies, the
thickness of the insulating medium increases with time and the rate of heat loss from the system also
decreases and hence the heat energy retains for a longer time inside the PCM solar cooker and hence it can
be used as a hot case.
                     It is also necessary to evaluate the Heat loss coefficient and the Thermal resistance of the PCM
medium of a required thickness to evaluate thermal performance of the PCM solar cooker during charging
and discharging processes.




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NOMENCLATURE

C              Specific heat of water (kJ/kg°C)
dt             T w -T a
F1             First figure of merit
Havg           Average value of solar radiation (w/m2)
Hs             Solar radiation at stagnation (w/m2)
M              Mass of water (kg)
Ps             Standardized cooking power
Ta             Ambient Temperature (°C)
Texp           Time of exposure (hrs)
T as           Stagnation ambient temperature of the cooker (°C)
T ps           Stagnation plate temperature of the cooker (°C)
Tw             Average load temperature(°C)


REFERENCES

1. Telkes M., Solar cooking ovens, Solar Energy, 1959: 3 (1):1-11.
2. Buddhi D ., Sharma S.D. and Sawhney R.L.,’ Performance test of a box type solar cooker : effect of
    load on the second figure of merit’, International Journal of Energy Research , 1999; 23: 827-830.
3. Solar cooker specifications: part 1-3 of IS 13429 (192). ‘ Bureau of Indian Standards’, Manak Bhavan ,
    9 ,Bahadur Shah Zafar Marg, New Delhi, India.
4. Tulsi Das , T.C.,Karmarkar , S. and Rao, D.P., Solar Box cooker: Part II –Analysis and simulation ,
    Solar Energy, 1994; 52, 265-272.
5. Mullick. S.C., Kandpal T.C., and Saxena. A.K., ‘Thermal test procedure for box type solar cooker’,
    Solar Energy ; 1987, 39, 353-360.
6. Buddhi.D and Sahoo, L.K., ‘ Solar cooker with latent heat storage: Design and experimental testing’,
    Energy Conservation and Management, 1997; 38; 493-498.
7. Buddhi .D & Sharma .S.D. , ‘Design, development and performance evaluation of a latent heat
   storage unit for solar cooker’ , Proceedings of International conference on energy storage technologies
   and systems, Indore, India, 1-4, February 1994; 46-52.
8. Sharma. S.D. , Buddhi D., Sawhney R.L. , Atul Sharma ‘ Design, development and performance
    evaluation of a latent heat storage unit for evening cooking in a solar cooker’, Energy conservation and
    management, 2000; 41; 1497-1508.
9 . Paul A. Funk ‘Evaluation of the international standard procedure for testing solar cookers and
    reporting performance’, 2000, vol.68, No.1, pp.1-7.
10. Paul A. Funk ‘ASAE standards; Drafted paper solar energy committee SE-414’




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