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Studies on a hot box solar cooker with transparent insulation materials
Energy Conrer.s. Mgmt Vol. 35. No. 9, pp. 787-791, 1994
Pergamon
0196-8904(93)E0035-J
Copyright ((3 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0196-8904/94 $7.00 + 0.00
ON A HOT BOX SOLAR COOKER WITH TRANSPARENT INSULATION MATERIALS
STUDIES
N. M. NAHAR,t R. H. MARSHALL and B. J. BRINKWORTH Solar Energy Unit, School of Engineering, University of Wales, Cardiff, U.K.
(Received 29 March 1993; received for publication 30 December 1993)
Abstract--A hot box solar cooker was tested in an indoor solar simulator with covers consisting of 40 and 100 m m thick Transparent Insulation Material (TIM). The stagnation temperature with the 40 m m TIM was found to be 158°C, compared with 117°C without the TIM. The corresponding ratios of U/~o were 7.13 and 10.3 W / m 2 K, respectively. The efficiency of the cooker is defined as the fraction of the incident energy retained within the cooking utensils at the time of first reaching 100°C to incident radiation. Using the 40 m m TIM this rose from 15.7 to 30.47% for test conditions representing severe cold at Jodhpur, India. Little additional benefit was found in using a 100 m m TIM instead of a 40 m m TIM in the hot box solar cooker. A variety of foods c o m m o n in India were successfully cooked in the hot box using the 40 m m TIM. Transparent Insulation Material
Solar cookers
Hot box
Solar thermal energy
NOMENCLATURE A = Cp = Cpw = G = m~ = m2 = t~ = t2 = tw, = tw2 = U = r/= r/o = 0 =
Area of window of cooker (m 2) Specific heat of cooking utensil (J/kg K) Specific heat capacity of water (J/kg K) Solar irradiance on cooker (W/m:) Mass of cooking utensils (kg) Mass of water in cooking utensils (kg) Initial temperature of cooking utensils ( C ) Final temperature of cooking utensils (°C) Initial temperature of water in cooking utensils (°C) Final temperature of water in cooking utensils (°C) Heat loss coefficient of cooker (W/m 2 K) Efficiency of cooker Optical efficiency of cooker Period of test
INTRODUCTION
Half of the total energy consumed in developing countries is used for cooking in the domestic sector [1]. Most of the energy requirements for cooking are met by firewood, agricultural waste and cowdung cake, the total consumption of these materials being 150, 75 and 50 million tonnes per year in India [2]. It has been predicted that there will be a great shortage in the supply of firewood if a suitable alternative is not found for the inhabitants of rural areas. Fortunately, India is blessed with abundant solar radiation [3]. The most arid parts of the country receive maximum radiation, i.e. 7600-8000 MJ/m 2 per annum, semi-arid areas receive 7200-7600 MJ/m 2 per annum, and the amount received in mountainous regions is also appreciable at 6000 MJ/m 2 per annum. Solar cookers would, therefore, seem to be a viable alternative to firewood for cooking. There are two broad categories of solar cooker: reflector [4-9] and hot box [10-15]. The reflector type solar cooker was developed in the early 1950s [4] and was manufactured on a large scale in India [16]. However, it was not popular because of its inherent defects, e.g. it required re-alignment every 10 minutes; cooking could be done only in the middle of the day and only in direct sunlight; tPresent and permanent address: Central Arid Zone Research Institute, Jodhpur-342 003, India. 787
788
NAHAR et al.: HOT BOX SOLAR COOKER
its performance was greatly affected by dust and wind; there was a danger of the cook being burned as it was necessary to stand very close to the cooker when cooking, and the design was complicated. Thus, the hot box type of solar cooker was found to be the better one [17]. Though performance of the solar oven [11-14] is very good, it also requires re-alignment every 30 minutes, it is too bulky and is costly. The hot box solar cooker with a single reflector [18] is, therefore, being promoted by the Department of Nonconventional Energy Sources, Government of India and by State nodal agencies. The hot box solar cooker performs very well during the summer, but very poorly during the winter, in the northern parts of India, and it cannot cope with the cooking of two main meals every day because of low solar radiation on the horizontal surface during the winter and heat loss due to the low ambient temperatures. Therefore, a hot box solar cooker with a cover consisting of Transparent Insulation Material (TIM) [19-25] has been designed, manufactured and tested in an indoor solar simulator at the University of Wales College of Cardiff. The use of a TIM reduces heat loss through the cover of the cooker so that higher temperatures and greater efficiency can be obtained. EXPERIMENTAL Three different designs of hot box solar cooker have been tested. The schematic diagrams are shown as Fig. 1. Design I The solar cooker is based on the hot box principle, having dimensions of 560 x 560 x 270 mm. The inner and outer trays are manufactured from 18 swg mild steel. The space between the trays is filled with foam insulation. Two glazings are provided over the cooker, the inner one being Teflon ~ and the outer Perspex (acrylic) sheet. The inner tray is painted black. The space between the two glazings can be changed by moving the wooden frame in such a way that a 40 mm and a 100 mm thick TIM can be placed between them. There is room in the chamber for four aluminium cooking utensils, each of 200 mm dia and 75 mm height, so that four dishes can be cooked simultaneously. Design H This is similar to the first design, except that a further 50 mm of insulation is provided outside the cooker around the bottom and sides. This was inadvertently loosely bound and, therefore, caused a slight air gap between the outer tray and extra insulation. Design III In this design the inner tray was manufactured from 20 swg aluminium and its dimensions reduced so that an extra 25 mm side and 50 mm bottom insulation could be provided inside the cooker. P E R F O R M A N C E AND T E S T I N G The performance and testing of all three designs were carried out using an indoor solar simulator with solar irradiance of about 950 W/m 2, ambient temperature 20°C and wind speed 2 m/s on the surface of the glazing and 4 m/s on the sides of the cooker. These represent extreme conditions for the coldest month (January) at Jodhpur, India. There is provision for a reflector to boost the incident radiation, but as the cooker was tested in an indoor solar simulator with radiation at the normal incidence it was tested without the reflector. The cooker was tested to determine (i) stagnation plate temperatures without load and with load (ii) efficiency by measuring the time of rise in water temperature up to the boiling point. Teflon": Registered trademark of Dupont Co.
789
NAHAR et al.: HOT BOX SOLARCOOKER j
Perspex
I ]
~ / T I M 40 mm
/ ~
_~
500
~]:~iteel
~/~//~/~//~/~.
"2!5~ / ~ / ~ S ~
'
Foam insulation
_~/TIM
~~////////////////////////~~i.jj///~~~ ~
~Mild steel
=
q
~
550 ]
]I I~
[ 470 400 ..
40 mm
[[
I f15 / 75~//
~Foam insulation
D] ] .~/TIM40mm
~ ~ - - I Aluminiumsheet ~.1 ~ / / / ~ / ~ j (20 swg) X . /Mild steel ~Foam insulation
Fig. 1. Schematicdiagramsof solarcooker.
Stagnation plate temperature Design I. The stagnation plate temperature inside the cooking chamber was measured without and with a 40 mm thick TIM, manufactured from Okalux '~' polycarbonate honeycomb, and was found to be 84.1 and 99.9°C respectively. U/rlo (U = heat loss coefficient and rio= optical efficiency of cooker) was also calculated from this measurement and found to be 12.98 and 10.43 W/m2 K for the cooker without and with the TIM, respectively. As the stagnation temperatures were still too low for boiling food, the bottom and side insulation were increased by 50 mm, as reported for Design II and described below. Design II. The stagnation plate temperature inside the cooking chamber was measured without and with a 40 mm TIM and a 100 mm TIM and found to be 106.2, 132.4 and 141.2°C, respectively. From this measurement, U/rio values were calculated and found to be 10.8, 8.6 and 8.0 W/m 2 K, respectively. Stagnation plate temperatures were also measured with load (250 g of water in each of the four cooking utensils) for the cooker without a TIM, with a 40 mm TIM and with a 100 mm TIM and found to be 86.7, 102.8 and 103.5°C, respectively. From the above measurements, it was concluded
Okalux": Registeredtrademarkof FlachglasGmbH.
790
NAHAR et al.: HOT BOX SOLAR COOKER
that there is not much benefit in using a 100 mm TIM instead of a 40 mm TIM in the hot box solar cooker. Design IlL To increase the stagnation temperatures further so that the boiling time could be reduced, an aluminium tray of reduced size was used and the bottom and side insulation was also increased, as reported above the Design II. The stagnation plate temperature inside the cooking chamber was measured for the cooker without and with a TIM of 40 mm and found to be 116.1 and 158.2°C, respectively. For these measurements, U/qo in these cases was found to be 10.30 and 7.13 W / m 2 K, respectively. By comparing the stagnation plate temperatures, it has been found that there is a decrease in U/rlo with an increase in insulation from 25 to 75 mm, both with and without TIM. By further increasing the insulation, there is not much decrease in U/qo for a cooker without TIM, but there is a significant reduction, i.e. from 8.3 to 7.1 W/m 2 K, for a cooker with a 40 mm TIM. Therefore, when using the TIM, it is considered that the thermal insulation should be at least 100 mm on the bottom and 75 mm on the sides.
Efficiency The efficiency of the solar cookers of Designs II and III was measured without and with a TIM by measuring the time taken for the water temperature to reach the boiling point. The efficiency was not measured for Design I because the temperature of the water never reached the boiling point. The efficiency was calculated by the following relation: . -
ml Cp(tl -
t2) + A
m 2 Cpw(tw2 -
;0'
two)
(1)
GdO
Design II. The efficiency of the cooker (Design II) was measured with and without wind (speed 2.0 m/s). With wind, the efficiency was found to be 13.0, 20.8 and 21.9% for the cooker without a TIM, with a 40 mm TIM and 100 mm TIM, respectively. From this, it was found that 60 and 68,5% more energy can be obtained by using a 40 and a 100 mm TIM, respectively. The efficiency of the cooker without wind and without a TIM and with a 40 mm TIM was found to be 18.7 and 22.5%, respectively. From these measurements, it was found that there is a larger reduction in efficiency because of the heat loss due to windy conditions for a cooker without a TIM, while there is very little reduction when the 40 mm TIM is used. Design III. This design used greater insulation and an aluminium tray. The efficiency of the cooker without a TIM is increased only to 15.7 from 13.0% (Design II), but for the cooker with a 40 mm TIM, there is a large increase in efficiency, i.e. from 20.8 to 30.4%. It is thought that the efficiency of the cooker will be improved if utensils with tight lids are used. In this test, the utensils used had loose-fitting lids, and it was noted that the evaporation loss was 50 ml during the test.
Cooking tests Cooking trials were also conducted on a cooker with a TIM (Design III). The cooker was fully loaded, i.e. four dishes were cooked simultaneously. The dishes were plain rice (200 g rice + 400 g water), fried rice (200g r i c e + c o o k i n g o i l + onion + peas), lentils (200g lentils + 500g water + cooking oil + spices + 50g tomatoes) and vegetables (650 g cabbage + 50 g tomatoes + 100 g potatoes + 50 g onion + cooking oil + spices). These were all cooked successfully. The duration of the cooking time for rice was 180 rain and for other dishes 240 min. The duration of the cooking time was longer than it would have been if the utensils used had had tight-fitting lids. This trial was conducted with solar radiation = 950 W / m 2 and wind speed 2.0 m/s on the surface of the outer cover and 4.0 m/s on the sides of the cooker; the ambient temperature was 20°C. It would be difficult to obtain a regime suitable for cooking without a TIM under these climatic conditions. The results indicate that a cooker using 40 mm TIM may be used throughout the year. The temperature does not rise above 110°C when cooking is taking place. This is below the melting
NAHAR et al.: HOT BOX SOLAR COOKER
791
p o i n t of 120°C for the p o l y c a r b o n a t e h o n e y c o m b T I M used in these tests. Moreover, the T I M is encapsulated between two glazings, a n d there is an air gap between the plate a n d the i n n e r glazing, resulting in a temperature of the inner glazing below ! 10°C. However, if a cooker were to be left in a s t a g n a t i o n c o n d i t i o n , the T I M would melt d u r i n g the s u m m e r in India. Therefore, the hot box solar cooker is equipped with a single reflector which acts as a lid which can be closed when the cooker is not in use. CONCLUSION The performance of the hot box solar cooker with a 40 m m T I M was very good, even in cold conditions. The s t a g n a t i o n temperature was as high as 158.2°C, c o m p a r e d with only 116.9°C for a cooker w i t h o u t a T I M . The c o r r e s p o n d i n g values of U/rlo were f o u n d to be 7.13 a n d 10.30, respectively. The efficiency of the cooker with a T I M a n d without a T I M was f o u n d to be 30.4 a n d 15.7%, respectively. The use of a T I M in the hot box solar cooker will be a b o o n to the p o p u l a t i o n of India, the only c o n s t r a i n t being the cost, mainly for the T I M . A reduction in the cost of the T I M for use in a solar cooker would help in the conservation of c o n v e n t i o n a l fuels, such as firewood, c o w d u n g cake a n d agricultural waste in the rural areas of India, a n d LP gas, kerosene a n d coal in the u r b a n districts. C o n s e r v a t i o n of firewood would help in preserving the ecosystem, a n d c o w d u n g cake could be used as a fertiliser, which would aid the increase of p r o d u c t i o n of agricultural products. Moreover, the use of the hot box solar cooker would result in a reduction of the release of CO2 to the e n v i r o n m e n t . Acknowledgements--Dr N. M. Nahar is grateful to the Commission of the European Communities in Brussels and the
Department of Science & Technology, Government of India, for providing funding of the Fellowship and Travel in connection with this study, and to the Director of the Central Arid Zone Research Institute, Jodhpur, India, for granting leave for this purpose.
REFERENCES 1. M. Fritz, Future Energy Consumption. Pergamon Press, New York (1981). 2. A. C. Pandya, Energy for agriculture in India. Central Institute of Agricultural Engineering, Bhopal, India. Tech Bull No. CIAE/81/20 (1981). 3. India Meteorological Department, Solar Radiation Atlas of India. India Meterological Department, New Delhi, India (1985). 4. M. L. Ghai, J. Sci. Indust. Res. 12A, 165 (1953). 5. J. A. Duffle, Trans. Conj. on Use of Solar Energy, Tucson, Arizona, 3(!I) 79 (1955). 6. G. O. G. L6f and D. A. Fester, Proe. UN Con£ on New Sources of Energy, Rome, 5, 347 (1961). 7. H. Tabor, Sol. Energy 10, 153 (1966). 8. U. Ya. Umarov, A. K. Alimov, D. N. Alavutdinov, A. Sh. Khodzhaev and A. M. Gafurov. Appl. Sol. Energy (Geliotekhnika) 12, 61 (1976). 9. M. Von Oppen, Appropriate Technol. 4, 8 (1977). 10. M. K. Ghosh, Sci. Culture 22, 304 (1956). 11. M. Telkes, Sol. Energy 3, 1 (1959). 12. H. P. Garg, Indian Farming 27, 7 (1976). 13. K. S. Malhotra, N. M. Nahar and B. V. Ramana Rao, Sol. Energy 31, 235 (1983). 14. 1. A. Olwi and A. M. Khalifa, Sol. Energy 40, 2 59 (1988). 15. N. M. Nahar, Energy Convers. Mgmt 30, 9 (1990). 16. M. L. Ghai, B. S. Pandhar and Harikishan Dass, J. Svi. Indian Res. 12A, 212 (1953). 17. H. P. Garg, H. S. Mann and K. P. Thanvi, Proc. ISES Congress, New Delhi (Edited by F. de Winter and M. Cox) Sun 2, 1491. Pergamon Press, New York (1978). 18. M. Parikh and R. Parikh, Proe. National Solar Energy Convention of India, 257. Central Salt & Marine Chemical Research Institute, Bhawnagar, India. 19. K. G. T. Hollands, Sol. Energy 9, 159 (1965). 20. A. Goetzberger, J. Schmid and V. Wittwer, Int. J. Sol. Energy 2, 289 (1984). 21. V. Wittwer, Int. J. Sol. Energy ll, 117 (1992). 22. H. X. Yang, R. H. Marshall and B. J. Brinkworth, Proe. 5th Int. TI Workshop, Freiburg, 109 (1992). 23. J. W. Twidell, Proc. 5th Int. T1 Workshop, Freiburg, 105 (1992). 24. A. Nordgaard and W. A. Beckman, Proc. 5th Int. TI Workshop, Freiburg, 21 (1992). 25. M. Rommel, J. Dengler and V. Wittwer, Proc. 5th Int. TI Workshop, Freiburg, 29 (1992).
Pergamon
0196-8904(93)E0035-J
Copyright ((3 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0196-8904/94 $7.00 + 0.00
ON A HOT BOX SOLAR COOKER WITH TRANSPARENT INSULATION MATERIALS
STUDIES
N. M. NAHAR,t R. H. MARSHALL and B. J. BRINKWORTH Solar Energy Unit, School of Engineering, University of Wales, Cardiff, U.K.
(Received 29 March 1993; received for publication 30 December 1993)
Abstract--A hot box solar cooker was tested in an indoor solar simulator with covers consisting of 40 and 100 m m thick Transparent Insulation Material (TIM). The stagnation temperature with the 40 m m TIM was found to be 158°C, compared with 117°C without the TIM. The corresponding ratios of U/~o were 7.13 and 10.3 W / m 2 K, respectively. The efficiency of the cooker is defined as the fraction of the incident energy retained within the cooking utensils at the time of first reaching 100°C to incident radiation. Using the 40 m m TIM this rose from 15.7 to 30.47% for test conditions representing severe cold at Jodhpur, India. Little additional benefit was found in using a 100 m m TIM instead of a 40 m m TIM in the hot box solar cooker. A variety of foods c o m m o n in India were successfully cooked in the hot box using the 40 m m TIM. Transparent Insulation Material
Solar cookers
Hot box
Solar thermal energy
NOMENCLATURE A = Cp = Cpw = G = m~ = m2 = t~ = t2 = tw, = tw2 = U = r/= r/o = 0 =
Area of window of cooker (m 2) Specific heat of cooking utensil (J/kg K) Specific heat capacity of water (J/kg K) Solar irradiance on cooker (W/m:) Mass of cooking utensils (kg) Mass of water in cooking utensils (kg) Initial temperature of cooking utensils ( C ) Final temperature of cooking utensils (°C) Initial temperature of water in cooking utensils (°C) Final temperature of water in cooking utensils (°C) Heat loss coefficient of cooker (W/m 2 K) Efficiency of cooker Optical efficiency of cooker Period of test
INTRODUCTION
Half of the total energy consumed in developing countries is used for cooking in the domestic sector [1]. Most of the energy requirements for cooking are met by firewood, agricultural waste and cowdung cake, the total consumption of these materials being 150, 75 and 50 million tonnes per year in India [2]. It has been predicted that there will be a great shortage in the supply of firewood if a suitable alternative is not found for the inhabitants of rural areas. Fortunately, India is blessed with abundant solar radiation [3]. The most arid parts of the country receive maximum radiation, i.e. 7600-8000 MJ/m 2 per annum, semi-arid areas receive 7200-7600 MJ/m 2 per annum, and the amount received in mountainous regions is also appreciable at 6000 MJ/m 2 per annum. Solar cookers would, therefore, seem to be a viable alternative to firewood for cooking. There are two broad categories of solar cooker: reflector [4-9] and hot box [10-15]. The reflector type solar cooker was developed in the early 1950s [4] and was manufactured on a large scale in India [16]. However, it was not popular because of its inherent defects, e.g. it required re-alignment every 10 minutes; cooking could be done only in the middle of the day and only in direct sunlight; tPresent and permanent address: Central Arid Zone Research Institute, Jodhpur-342 003, India. 787
788
NAHAR et al.: HOT BOX SOLAR COOKER
its performance was greatly affected by dust and wind; there was a danger of the cook being burned as it was necessary to stand very close to the cooker when cooking, and the design was complicated. Thus, the hot box type of solar cooker was found to be the better one [17]. Though performance of the solar oven [11-14] is very good, it also requires re-alignment every 30 minutes, it is too bulky and is costly. The hot box solar cooker with a single reflector [18] is, therefore, being promoted by the Department of Nonconventional Energy Sources, Government of India and by State nodal agencies. The hot box solar cooker performs very well during the summer, but very poorly during the winter, in the northern parts of India, and it cannot cope with the cooking of two main meals every day because of low solar radiation on the horizontal surface during the winter and heat loss due to the low ambient temperatures. Therefore, a hot box solar cooker with a cover consisting of Transparent Insulation Material (TIM) [19-25] has been designed, manufactured and tested in an indoor solar simulator at the University of Wales College of Cardiff. The use of a TIM reduces heat loss through the cover of the cooker so that higher temperatures and greater efficiency can be obtained. EXPERIMENTAL Three different designs of hot box solar cooker have been tested. The schematic diagrams are shown as Fig. 1. Design I The solar cooker is based on the hot box principle, having dimensions of 560 x 560 x 270 mm. The inner and outer trays are manufactured from 18 swg mild steel. The space between the trays is filled with foam insulation. Two glazings are provided over the cooker, the inner one being Teflon ~ and the outer Perspex (acrylic) sheet. The inner tray is painted black. The space between the two glazings can be changed by moving the wooden frame in such a way that a 40 mm and a 100 mm thick TIM can be placed between them. There is room in the chamber for four aluminium cooking utensils, each of 200 mm dia and 75 mm height, so that four dishes can be cooked simultaneously. Design H This is similar to the first design, except that a further 50 mm of insulation is provided outside the cooker around the bottom and sides. This was inadvertently loosely bound and, therefore, caused a slight air gap between the outer tray and extra insulation. Design III In this design the inner tray was manufactured from 20 swg aluminium and its dimensions reduced so that an extra 25 mm side and 50 mm bottom insulation could be provided inside the cooker. P E R F O R M A N C E AND T E S T I N G The performance and testing of all three designs were carried out using an indoor solar simulator with solar irradiance of about 950 W/m 2, ambient temperature 20°C and wind speed 2 m/s on the surface of the glazing and 4 m/s on the sides of the cooker. These represent extreme conditions for the coldest month (January) at Jodhpur, India. There is provision for a reflector to boost the incident radiation, but as the cooker was tested in an indoor solar simulator with radiation at the normal incidence it was tested without the reflector. The cooker was tested to determine (i) stagnation plate temperatures without load and with load (ii) efficiency by measuring the time of rise in water temperature up to the boiling point. Teflon": Registered trademark of Dupont Co.
789
NAHAR et al.: HOT BOX SOLARCOOKER j
Perspex
I ]
~ / T I M 40 mm
/ ~
_~
500
~]:~iteel
~/~//~/~//~/~.
"2!5~ / ~ / ~ S ~
'
Foam insulation
_~/TIM
~~////////////////////////~~i.jj///~~~ ~
~Mild steel
=
q
~
550 ]
]I I~
[ 470 400 ..
40 mm
[[
I f15 / 75~//
~Foam insulation
D] ] .~/TIM40mm
~ ~ - - I Aluminiumsheet ~.1 ~ / / / ~ / ~ j (20 swg) X . /Mild steel ~Foam insulation
Fig. 1. Schematicdiagramsof solarcooker.
Stagnation plate temperature Design I. The stagnation plate temperature inside the cooking chamber was measured without and with a 40 mm thick TIM, manufactured from Okalux '~' polycarbonate honeycomb, and was found to be 84.1 and 99.9°C respectively. U/rlo (U = heat loss coefficient and rio= optical efficiency of cooker) was also calculated from this measurement and found to be 12.98 and 10.43 W/m2 K for the cooker without and with the TIM, respectively. As the stagnation temperatures were still too low for boiling food, the bottom and side insulation were increased by 50 mm, as reported for Design II and described below. Design II. The stagnation plate temperature inside the cooking chamber was measured without and with a 40 mm TIM and a 100 mm TIM and found to be 106.2, 132.4 and 141.2°C, respectively. From this measurement, U/rio values were calculated and found to be 10.8, 8.6 and 8.0 W/m 2 K, respectively. Stagnation plate temperatures were also measured with load (250 g of water in each of the four cooking utensils) for the cooker without a TIM, with a 40 mm TIM and with a 100 mm TIM and found to be 86.7, 102.8 and 103.5°C, respectively. From the above measurements, it was concluded
Okalux": Registeredtrademarkof FlachglasGmbH.
790
NAHAR et al.: HOT BOX SOLAR COOKER
that there is not much benefit in using a 100 mm TIM instead of a 40 mm TIM in the hot box solar cooker. Design IlL To increase the stagnation temperatures further so that the boiling time could be reduced, an aluminium tray of reduced size was used and the bottom and side insulation was also increased, as reported above the Design II. The stagnation plate temperature inside the cooking chamber was measured for the cooker without and with a TIM of 40 mm and found to be 116.1 and 158.2°C, respectively. For these measurements, U/qo in these cases was found to be 10.30 and 7.13 W / m 2 K, respectively. By comparing the stagnation plate temperatures, it has been found that there is a decrease in U/rlo with an increase in insulation from 25 to 75 mm, both with and without TIM. By further increasing the insulation, there is not much decrease in U/qo for a cooker without TIM, but there is a significant reduction, i.e. from 8.3 to 7.1 W/m 2 K, for a cooker with a 40 mm TIM. Therefore, when using the TIM, it is considered that the thermal insulation should be at least 100 mm on the bottom and 75 mm on the sides.
Efficiency The efficiency of the solar cookers of Designs II and III was measured without and with a TIM by measuring the time taken for the water temperature to reach the boiling point. The efficiency was not measured for Design I because the temperature of the water never reached the boiling point. The efficiency was calculated by the following relation: . -
ml Cp(tl -
t2) + A
m 2 Cpw(tw2 -
;0'
two)
(1)
GdO
Design II. The efficiency of the cooker (Design II) was measured with and without wind (speed 2.0 m/s). With wind, the efficiency was found to be 13.0, 20.8 and 21.9% for the cooker without a TIM, with a 40 mm TIM and 100 mm TIM, respectively. From this, it was found that 60 and 68,5% more energy can be obtained by using a 40 and a 100 mm TIM, respectively. The efficiency of the cooker without wind and without a TIM and with a 40 mm TIM was found to be 18.7 and 22.5%, respectively. From these measurements, it was found that there is a larger reduction in efficiency because of the heat loss due to windy conditions for a cooker without a TIM, while there is very little reduction when the 40 mm TIM is used. Design III. This design used greater insulation and an aluminium tray. The efficiency of the cooker without a TIM is increased only to 15.7 from 13.0% (Design II), but for the cooker with a 40 mm TIM, there is a large increase in efficiency, i.e. from 20.8 to 30.4%. It is thought that the efficiency of the cooker will be improved if utensils with tight lids are used. In this test, the utensils used had loose-fitting lids, and it was noted that the evaporation loss was 50 ml during the test.
Cooking tests Cooking trials were also conducted on a cooker with a TIM (Design III). The cooker was fully loaded, i.e. four dishes were cooked simultaneously. The dishes were plain rice (200 g rice + 400 g water), fried rice (200g r i c e + c o o k i n g o i l + onion + peas), lentils (200g lentils + 500g water + cooking oil + spices + 50g tomatoes) and vegetables (650 g cabbage + 50 g tomatoes + 100 g potatoes + 50 g onion + cooking oil + spices). These were all cooked successfully. The duration of the cooking time for rice was 180 rain and for other dishes 240 min. The duration of the cooking time was longer than it would have been if the utensils used had had tight-fitting lids. This trial was conducted with solar radiation = 950 W / m 2 and wind speed 2.0 m/s on the surface of the outer cover and 4.0 m/s on the sides of the cooker; the ambient temperature was 20°C. It would be difficult to obtain a regime suitable for cooking without a TIM under these climatic conditions. The results indicate that a cooker using 40 mm TIM may be used throughout the year. The temperature does not rise above 110°C when cooking is taking place. This is below the melting
NAHAR et al.: HOT BOX SOLAR COOKER
791
p o i n t of 120°C for the p o l y c a r b o n a t e h o n e y c o m b T I M used in these tests. Moreover, the T I M is encapsulated between two glazings, a n d there is an air gap between the plate a n d the i n n e r glazing, resulting in a temperature of the inner glazing below ! 10°C. However, if a cooker were to be left in a s t a g n a t i o n c o n d i t i o n , the T I M would melt d u r i n g the s u m m e r in India. Therefore, the hot box solar cooker is equipped with a single reflector which acts as a lid which can be closed when the cooker is not in use. CONCLUSION The performance of the hot box solar cooker with a 40 m m T I M was very good, even in cold conditions. The s t a g n a t i o n temperature was as high as 158.2°C, c o m p a r e d with only 116.9°C for a cooker w i t h o u t a T I M . The c o r r e s p o n d i n g values of U/rlo were f o u n d to be 7.13 a n d 10.30, respectively. The efficiency of the cooker with a T I M a n d without a T I M was f o u n d to be 30.4 a n d 15.7%, respectively. The use of a T I M in the hot box solar cooker will be a b o o n to the p o p u l a t i o n of India, the only c o n s t r a i n t being the cost, mainly for the T I M . A reduction in the cost of the T I M for use in a solar cooker would help in the conservation of c o n v e n t i o n a l fuels, such as firewood, c o w d u n g cake a n d agricultural waste in the rural areas of India, a n d LP gas, kerosene a n d coal in the u r b a n districts. C o n s e r v a t i o n of firewood would help in preserving the ecosystem, a n d c o w d u n g cake could be used as a fertiliser, which would aid the increase of p r o d u c t i o n of agricultural products. Moreover, the use of the hot box solar cooker would result in a reduction of the release of CO2 to the e n v i r o n m e n t . Acknowledgements--Dr N. M. Nahar is grateful to the Commission of the European Communities in Brussels and the
Department of Science & Technology, Government of India, for providing funding of the Fellowship and Travel in connection with this study, and to the Director of the Central Arid Zone Research Institute, Jodhpur, India, for granting leave for this purpose.
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