Design, development and testing of a double reflector hot box solar cooker with a transparent insulation material

Renewable Energy 23 (2001) 167–179 www.elsevier.nl/locate/renene

Design, development and testing of a double reflector hot box solar cooker with a transparent insulation material N.M. Nahar

*

Central Arid Zone Research Institute, Jodhpur 342 003, India Received 21 July 2000; accepted 23 August 2000

Abstract A double reflector hot box solar cooker with a Transparent Insulation Material (TIM) has been designed, fabricated, tested and the performance compared with a single reflector hot box solar cooker without TIM. A 40 mm thick honeycomb made of polycarbonate capillaries was encapsulated between two glazing sheets of the cooker to minimise convective losses from the window so that even during an extremely cold but sunny day two meals can be prepared, which is not possible in a hot box solar cooker without TIM. The use of one more reflectors resulted in an avoidance of tracking towards sun for 3 h so that cooking operations could be performed unattended, as compared to a hot box solar cooker where tracking ahead of the sun is required every hour. The efficiencies were 30.5% and 24.5% for cookers with and without a TIM respectively, during the winter season at Jodhpur. The energy saving by use of a solar cooker with TIM has been estimated to be 1485.0 MJ of fuel equivalent per year. The payback period varies between 1.66 and 4.23 y depending upon the fuel it replaces, and is in increasing order with respect to the following fuels: electricity, firewood, coal, LPG and kerosene. The estimated life is about 15 y. Therefore, the use of a solar cooker is economical. The double reflector hot box solar cooker with TIM will be a boon in popularising solar cookers in developing countries.  2001 Elsevier Science Ltd. All rights reserved. Keywords: Solar thermal energy; Solar cookers; Payback periods; Energy conservation

* Fax: +91-291-740706. E-mail address: [email protected] (N.M. Nahar). 0960-1481/01/$ - see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 1 4 8 1 ( 0 0 ) 0 0 1 7 8 - 6

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Nomenclature a A b c C Cp Cw E H m1 m2 M N t1 t2 q h

Compound interest rate per annum Absorber area, m2 Inflation rate in energy and maintenance per annum Concentration ratio Cost of the cooker, Rs. Specific heat of cooking utensils, J kg⫺1 °C⫺1 Specific heat of water, J kg⫺1 °C⫺1 Energy savings per year, Rs. Solar radiation, J m⫺2 h⫺1 Mass of water in cooking utensils, kg Mass of cooking utensils, kg Maintenance cost per annum, Rs. Payback period, y Initial temperature of water in the utensils, °C Final temperature of water in the utensils, °C Period of test, h Efficiency of the solar cooker

1. Introduction Cooking accounts for a major share of energy consumption in developing countries. Fifty per cent of the total energy consumed in India is for cooking [1]. Most of the cooking energy requirement is met by non-commercial fuels such as firewood (75%), agricultural waste and cow dung cake (25%) in rural areas. The fuel wood requirement is 0.4 tons per person per year in India. In rural areas, firewood crisis is far graver than that caused by a rise in oil prices. Poor villagers have to forage 8–10 h a day in search of firewood as compared to 1–2 h 10 years ago. One third of India’s fertiliser consumption can be met if cow dung is not burnt for cooking and is used instead as manure. The cutting of firewood causes deforestation that leads to desertification. Fortunately, India is blessed with abundant solar radiation [2]. The arid parts of India receive maximum radiation i.e., 7600–8000 MJ m⫺2 per annum, followed by semi arid parts, 7200–7600 MJ m⫺2, per annum with least amount on hilly areas where solar radiation is still appreciable i.e. 6000 MJ m⫺2 per annum. Therefore, solar cookers seem to be a good substitute for cooking with firewood. The first solar furnace was fabricated by naturalist Georges Louis Leclerc Buffon (1707–1788). But Horace-de-Saussure (1740–1799) was the first in the world to use the sun for cooking. Augustin Mouchot, a french physicist, described a solar cooker in his book “La Chaleur Solaire” published in Paris, in 1869. He also reported in the same book earlier work on solar cooking by English astronomer, Sir John Herschel, in South Africa, between 1834 and 1838. Adams, an army officer, made

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India’s first solar cooker in 1878 and he cooked food in it at Bombay, India [3]. Since then different types of solar cookers have been developed all over the world. The solar cookers can be classified into three broad categories (i) reflector/focusing type, (ii) heat transfer type, and (iii) hot box type. These are described below. 1.1. Reflector/focusing type The reflector type solar cooker was developed in the early 1950s [4] and was manufactured on a large scale in India [5]. Attempts were also made in the 1960s and 70s to develop a reflector type solar cooker [6–9]. However a reflector type solar cooker did not become popular due to its inherent defects, e.g., it required tracking towards the sun every 10 min, cooking could be done only in the middle of the day and only in direct sunlight, 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. 1.2. Heat transfer type In the heat transfer type solar cooker, the collector is kept outside and the cooking chamber is kept inside the kitchen of the house [10–12]. But this type of solar cooker also did not become popular because of its high cost and only limited cooking can be performed. 1.3. Hot box type The third type of cooker is known as a hot box in which most of the defects of the above two types of cookers have been removed [13–18]. Different types of solar cookers have been tested and the solar oven [19–22] has been found best. Although the performance of the solar oven is very good, it also requires tracking towards the sun every 30 min, it is too bulky and is costly. Therefore, the hot box solar cooker with a single reflector [23] has been promoted at subsidised cost by the Ministry of Non-Conventional Energy Sources, Government of India and the state nodal agencies in India since 1981–82, and 462,000 solar cookers were sold upto the 31 December 1998 [24]. From 1 April 1998 to December 1998 only 5000 solar cookers were sold [24]. This shows that the popularity of solar cookers is declining due to its defects: it also requires tracking towards the sun every 60 min. Therefore, its operation also becomes cumbersome and the performance of the hot box solar cooker is very poor during winter when solar radiation and ambient temperatures are very low. Considering this, a two reflector hot box solar cooker was developed by Gupta and Purohit [25] so that tracking could be avoided for 3 h, but the problem of poor performance during winter still remains with this solar cooker. Therefore, attempts were also made by Nahar et al. [18] to improve performance of the hot box solar cooker during extreme cold weather by using Transparent Insulation Material (TIM) between two glazings, and a hot box solar cooker with a TIM was tested in an indoor solar simulator of the University of Wales, College of Cardiff. In this paper both defects of

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the hot box solar cooker have been removed by providing one more reflector, and convective heat losses have been suppressed by using TIM as suggested by Hollands [26], Goetzberger et al. [27,28], Hollands et al. [29], Nordgard and Beckman [30], Platzer [31,32] and Nahar et al. [33]. The cooker is kept in such a way that one reflector is facing south and the other is facing east in the forenoon so that tracking is avoided for 180 min. In the afternoon, one reflector is facing south and other is facing west so that again tracking is avoided for 180 min. The maximum time taken for cooking a dish is less than 3 h.

2. Design The device (Fig. 1) consists of a double walled hot box. The outer and inner boxes are made of aluminium. The dimensions of the outer box are 560×560×180 mm3

Fig. 1.

Schematic diagrams of a double reflector TIM solar cooker.

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and of the inner box are 460×460 mm2 at the top and 400×400 mm2 at the bottom, with 80 mm height. The space between them is filled with glass wool insulation and separated by a wooden frame. The inner box is painted black with blackboard paint. Two clear window glass panes of 4 mm thickness have been fixed over it with a wooden frame which can be opened. A 40 mm thick TIM (KAPIPANER)1 honeycomb made of polycarbonate has been inserted between the two glass panes to minimise convective heat losses. Two 4-mm thick plane mirror reflectors are fixed over it. These reflectors can be put one over the other on the cooker and act as a lid. The tilt of the reflector can be varied from 0° (closed lid) to 120° from the horizontal plane depending upon the season. The absorber area is 0.16 m2. Four cooking utensils of 200-mm diameter can be kept inside it for cooking four dishes simultaneously. The overall dimensions of the cooker are 560×560×230 mm3 and its weight is 20 kg. Fig. 2 depicts field installation of the double reflector hot box solar cooker with TIM. 3. Performance and testing The cooker was tested extensively. The solar cooker with double reflectors was first tested without TIM and stagnation temperatures were compared with a hot box

Fig. 2.

Double reflector TIM solar cooker.

1 KAPIPANER: registered trade mark of OKALUX, Kappillarglass GmbH, Marktheidenfeld-Altfeld (Germany).

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solar cooker with a single reflector (Fig. 3). The stagnation temperatures were measured with the aid of copper–constantan thermocouples and recorded in the potentiometric strip chart temperature recorder; the glazing was not opened during the measurements for either of the solar cookers, i.e., hot box solar cooker with double reflectors and without TIM, and hot box solar cooker with a single reflector (Fig. 4). The solar cooker with double reflectors was kept in such a way that one reflector is facing south and the other is facing east in the forenoon and in the afternoon the cooker was rotated 90°, so one reflector is facing south and the other is facing west, i.e., the cooker was not tracked, while the hot box solar cooker was tracked towards the sun every hour. From the Fig. 4 it is clear that the performance of both the solar cookers is similar. This suggests that by using one more reflector, tracking towars the sun is avoided for 3 h. At this point, 40 mm thick TIM was encapsulated between the two glazings of the double reflector hot box solar cooker. The stagnation air temperature inside the cooking chamber of the double reflector hot box solar cooker with TIM was measured and compared with the hot box solar cooker without TIM. Stagnation temperatures were measured without load, i.e., nothing was kept inside the cooking chamber, and temperatures measured are shown in Fig. 5. The four cooking utensils with 250 g of water in each, were left inside both the solar cookers, and stagnation temperatures were measured (Fig. 6). The double reflector hot box solar cooker with TIM is kept in such a way that one reflector is facing south and the other is facing east in the forenoon so that tracking is avoided for 180 min. In the afternoon, one reflector is facing south and the other is facing west so that again tracking is avoided for 180 min. The hot box solar cooker without TIM was tracked towards the sun every hour. From Figs. 5 and 6 it is clear that the performance of the double reflector hot box solar cooker with TIM is better than that of the hot box solar cooker with a single reflector. Cooking trials have also been conducted successfully with rice, lentils, kidney

Fig. 3.

Hot box solar cooker.

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Fig. 4. Diurnal variations in stagnation temperatures of a double reflector solar cooker without TIM and a hot box solar cooker with a single reflector, both without load.

beans, cauliflower, and baking of bati (local preparation made of wheat flour). It takes about 2 h for soft food and 3 h for hard food. The cooker is capable of cooking 1.0 kg of dry food at a time. The four cooking utensils each containing 250 g of water were kept inside the cooking chambers of the solar cookers for ±1.0 h local noon time, and rise in water temperatures was measured. The duration of time required for reaching water temperatures near to boiling point were also measured. The efficiencies of the solar cookers were obtained by the following relation: (m1Cw+m2Cp)(t2−t1) h⫽ q

冕

cA Hdq 0

(1)

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Fig. 5. Diurnal variations in stagnation temperatures of a double reflector TIM solar cooker and a hot box solar cooker, both without load.

The efficiencies of the solar cookers with and without TIM have been found to be 30.5% and 24.5% respectively. The performance studies on the double reflector hot box solar cooker with TIM suggests that the cooker can be used throughout the year. The temperature does not rise above 110°C when cooking is carried out. This is below the melting point of 120°C for the polycarbonate honeycomb TIM used in this cooker. Moreover, the TIM is encapsulated between the two glazings, and there is an airgap between the absorber and the inner glazing, resulting in the temperature of the inner glazing being below 110°C. However, if a cooker were to be left in a stagnation condition, the TIM would melt down under Indian conditions, where even during the winter, stagnation temperatures as high as 161°C have been observed. Therefore, when a solar cooker with TIM is not being used for cooking, both reflectors should be put over the cooker like a lid so that solar radiation will not enter the cooker.

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Fig. 6. Diurnal variations in stagnation temperatures of a double reflector TIM solar cooker and hot box solar cooker, both with load.

4. Energy conservation and payback periods The cooking time for a dish is between 2 and 3 h, and cooking operation can be carried out between 9 a.m. and 5 p.m. on clear sunny days. The available data on the duration of sunshine hours is not adequate for finding the number of days with bright sunshine from 9 a.m. to 5 p.m. Therefore, it is presumed that the double reflector hot box solar cooker with TIM will be suitable for both meals if the duration of bright sunshine exceeds 6 h/day, and only one meal per day if bright sunshine is between 6 and 4 h per day. It will not be able to cook on days when bright sunshine is less than 4 h/day. Based on the above assumption, the minimum energy that can be saved by using the solar cooker has been estimated. By analysing the duration of bright sunshine hours, it has been found that the cooker will cook both meals for about 321 days and one meal per day for about 12 days in a year at Jodhpur. The energy for cooking per person is about 900 kJ of fuel equivalent per meal. The solar cooker is capable of cooking for about 5 persons, and it will save 50% of cooking

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fuel per meal. Therefore, it will save 2.25 MJ of energy per meal and 1485.0 MJ of fuel equivalent per year. The payback periods have been computed by considering the equivalent savings in alternate fuels, viz. firewood, coal, kerosene, liquid petroleum gas (LPG) and electricity. The payback periods have been calculated by considering the compound annual interest rate, maintenance cost and inflation in fuel prices and maintenance cost per year. The economic evaluation and payback periods have been computed by the following relation [34,35] N⫽

log[(E−M)/(a−b)]−log[(E−M)/(a−b)−C] log[(1+a)/(1+b)]

(2)

The economic evaluation and payback periods have been computed by considering the following annual cost: interest rate a=10%; maintenance M=5% of cost of the solar cooker; inflation rate b=5%. The cost of the cooker is Rs.1800.00 (US$1.0=Rs.43.50, Stg£1.0=Rs.70.50). The break up of the cost of the double reflector TIM solar cooker is shown in Table 1. The cash flow of the cooker with respect to different fuels has been carried out and is shown in Table 2. From Table 2, it is clear that the cash flow is greater with respect to electricity and least with respect to kerosene. The exact payback periods have been computed from Eq. (2) with respect to different fuels and are shown in Table 3. The payback period is least, i.e., 1.66 y, with respect to electricity and Table 1 Break down of the cost of a double reflector hot box solar cooker with TIM Item

Quantity

Aluminium sheet (22 gauge) Glass wool Plane glass (460×460×4 mm3) Plane mirror (460×460×4 mm3) Wood Plywood Stainless steel channel Castor wheel 50 mm TIM Miscellaneous handle, blackboard paint, screws, etc.

1.5 m2 4 kg 2 2 0.02 m3 510×510×4 mm3 460×460×50 mm3 4 460×460×40 mm3

Approx. cost (Rs.) 350.00 50.00 75.00 150.00 200.00 50.00 50.00 60.00 450.00 50.00 1485.00

Labour charges 20% of material cost

297.00 Total cost

1782.00 ⬇1800.00

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Table 2 Economic analysis of double reflector TIM solar cooker Year

Cash flow (Rs.) Interest (Rs.)

1

2

(a) Alternate fuel firewood 0 ⫺1800.00 1 ⫺991.09 2 ⫺51.84 3 1033.25 (b) Alternate fuel coal 0 ⫺1800.00 1 ⫺1095.44 2 ⫺276.19 3 671.41 (c) Alternate fuel kerosene 0 ⫺1800.00 1 ⫺1475.70 2 ⫺1093.75 3 ⫺647.14 4 ⫺128.06 5 471.11 (d) Alternate fuel LPG 0 ⫺1800.00 1 ⫺1446.77 2 ⫺1031.56 3 ⫺546.84 4 15.76 (e) Alternate fuel electricity 0 ⫺1800.00 1 ⫺767.37 2 129.15

3

Maintenance (Rs.) 4

Energy savings Net savings (Rs.) (Rs.) 5 6=5⫺4⫺3

— 180.00 99.11 5.18

— 90.00 94.50 99.23

— 1078.91 1132.86 1189.50

— 808.91 939.25 1085.09

— 180.00 109.54 27.62

— 90.00 94.50 99.23

— 974.56 1023.29 1074.45

— 704.56 819.25 947.60

— 180.00 147.57 109.38 64.71 12.81

— 90.00 94.50 99.23 104.19 109.40

— 594.30 624.02 655.22 687.98 722.38

— 324.30 381.95 446.61 519.08 600.17

— 180.00 144.68 103.16 54.68

— 90.00 94.50 99.23 104.19

— 623.23 654.39 687.11 721.47

— 353.23 415.21 484.72 562.60

— 180.00 76.74

— 90.00 94.50

— 1302.63 1367.76

— 1032.63 896.52

Table 3 Payback periods of double reflector TIM solar cooker Sr. No.

Type of fuel

Calorific value

Efficiency (%)

Cost (Rs.)

Payback periods (y)

1 2 3 4 5

Firewood Coal Kerosene LPG Electricity

19.89 MJ kg⫺1 27.21 MJ kg⫺1 45.55 MJ kg⫺1 45.59 MJ kg⫺1 3.6 MJ kWh⫺1

17.3 28.0 48.0 60.0 76.0

2.50 kg⫺1 5.00 kg⫺1 7.00 L⫺1 11.48 kg⫺1 2.40 kWh⫺1

2.05 2.31 4.23 3.97 1.66

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maximum, i.e., 4.23 y, with respect to kerosene. The payback periods are in increasing order with respect to fuel: electricity, firewood, coal, LPG and kerosene. The estimated life of the cooker is about 15 y.

5. Conclusion The performance of the double reflector hot box solar cooker with TIM is very good even in extreme winter, and two meals can be prepared even on extremely cold days as compared to the hot box solar cooker without TIM in which it is difficult to cook even one meal on an extremely cold day. The double reflector hot box solar cooker does not require any tracking for 3 h as compared to the hot box solar cooker that requires tracking towards the sun every hour. The time required for cooking any dish does not exceed 3 h. The payback period varies between 1.45 and 3.86 y depending upon the fuel it replaces and is in increasing order with respect to the following fuels: firewood, coal, electricity, kerosene and LPG. The estimated life is about 15 y. Therefore, the use of a solar cooker is economical. The double reflector hot box solar cooker with TIM will be a boon in popularising solar cooker in developing countries. The use of this novel solar cooker would help in conservation of conventional fuels, such as firewood, cow dung cake and agricultural waste in rural areas of India, and LPG, kerosene, electricity and coal in the urban districts. Conservation of firewood helps in preserving the ecosystems, and cow dung cake could be used as fertiliser, which could aid in the increase of production of agricultural products. Moreover, the use of the solar cooker would result in the reduction of the release of CO2 into the environment.

Acknowledgements The author is grateful to the Director, CAZRI, Jodhpur and the Head of the Division of Agricultural, Engineering and Energy for providing necessary facilities and constant encouragement for the present study.

References [1] [2] [3] [4] [5]

Fritz M. Future energy consumption. New York: Pergamon Press, 1981. IMD. Solar radiation atlas of India. New Delhi: India Meteorological Department, 1985. Adams W. Cooking by solar heat. Sci Am 1878;38:376. Ghai ML. Design of reflector type direct solar cooker. J Sci Ind Res 1953;12A:165–75. Ghai ML, Pandhar BS, Dass H. Manufacture of reflector type direct solar cooker. J Sci Ind Res 1953;12A:212–6. [6] Duffie JA, Lof GOG, Beck B. Laboratories and field studies of plastic reflector solar cookers. In:, 1961;5:339–46, [Paper S/87]. [7] Lo¨f GOG, Fester DA. Design and performance of folding umbrella type solar cooker. In:, 1961;5:347–52, [Paper S/100].

N.M. Nahar / Renewable Energy 23 (2001) 167–179

[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]

[24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35]

179

Tabor H. A solar cooker for developing countries. Solar Energy 1966;10:153–7. Von Oppen M. The sun basket. Appropriate Technology 1977;4:8–10. Abot CG. Smithsonian Misc. Cells 1939;98. Alward R. Solar steam cooker. In: Do it yourself leaflet L-2. Quebec (Canada): Brace Research Institute, 1972. Garg HP, Thanvi KP. Studies on solar steam cooker. Indian Farming 1977;27(1):23–4. Ghosh MK. Utilisation of solar energy. Sci Culture 1956;22:304–12. Telkes M. Solar cooking ovens. Solar Energy 1959;3:1–11. Garg HP. A solar oven for cooking. Indian Farming 1976;27:7–9. Nahar NM. Performance and testing of an improved hot box solar cooker. Energy Conv Mgmt 1990;30:9–16. Grupp M, Montagne P, Wackernagel M. A novel advanced box-type solar cooker. Solar Energy 1991;47:107–13. Nahar NM, Marshall RH, Brinkworth BJ. Studies on a hot box solar cooker with transparent insulating materials. Energy Conv Mgmt 1994;35:784–91. Garg HP, Mann HS, Thanvi KP. Performance evaluation of five solar cookers. In: De Winter F, Cox M, editors. Proc. ISES Congress; New Delhi. New York: Pergamon Press, 1978:1491–6, [SUN 2]. Malhotra KS, Nahar NM, Ramana Rao BV. Optimization factor of solar ovens. Solar Energy 1983;31:235–7. Nahar NM. Performance studies on different models of solar cookers in arid zone conditions of India. In: New York: Hemisphere Publishing Corporation, 1986;1:431–9. Olwi IA, Khalifa AH. Computer simulation of solar pressure cooker. Solar Energy 1988;40:259–68. Parikh M, Parikh R. Design of flat plate solar cooker for rural applications. In: Proceedings of the National Solar Energy Convention of India; Bhavnagar. Bhavnagar (India): Central Salts and Marine Chemical Research Institute, 1978:257–61. MNES. Annual Report 1998–99. New Delhi: Ministry of Non-conventional Energy Sources, Government of India, 1999. Gupta JP, Purohit MM. Role of renewable energy sources for mitigation of cooking fuel problem. Trans Indian Soc Desert Technol Univ Centre Desert Stud 1986;11(1):7–17. Hollands KGT. Honey comb devices in flat-plate solar collectors. Solar Energy 1965;9:159–64. Goetzberger A, Schmid J, Wittwer V. Transparent insulation system for passive solar energy utilization in buildings. Int J Solar Energy 1984;2:289–308. Goetzberger A, Dengler J, Rommel M, Gottsche J, Wittwer V. A new transparently insulated bifacially irradiated solar flat-plate collector. Solar Energy 1992;49:403–11. Hollands KGT, Iynkaran K, Ford C, Platzer W. Manufacture, solar transmission, and heat transfer characteristics of large-celled honeycomb transparent insulation. Solar Energy 1992;49:381–5. Nordgaard A, Beckman WA. Modelling of flat-plate collectors based on monolithic silica aerogel. Solar Energy 1992;49:387–402. Platzer WJ. Total heat trasport data for plastic honeycomb-type structures. Solar Energy 1992;49:351–8. Platzer WJ. Directional-hemispherical solar transmittance data for plastic honeycomb-type structures. Solar Energy 1992;49:359–69. Nahar NM, Marshall RH, Brinkworth BJ. Investigations of flat plate collectors using transparent insulation materials. Int J Solar Energy 1995;17:117–34. Bo¨er KW. Payback of solar systems. Solar Energy 1978;20:225–32. Duffie JA, Beckman WA. Solar engineering of thermal processes. New York: Wiley, 1980.