Pergamon 1. INTRODUCTION - Science Direct

Renewable Energy, Vol. 12, No. 2, pp. 151-155, 1997

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Pergamon PIh S0960-1481(97)00037-2

~', 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain

0960--1481/97$17.00+0.00

THE PERFORMANCE OF A BOX-TYPE SOLAR COOKER WITH AUXILIARY HEATING M. HUSSAIN, K. C. DAS and A. HUDA Department of Physics and Renewable Energy Research Centre, Faculty of Science, University of Dhaka, Dhaka-1000, Bangladesh

(Received 6 October 1996; accepted 25 April 1997)

Abstract--InBangladesh

it is very difficult to use solar cookers during the months when the days are generally cloudy and at times solar cooking becomes impossible. To overcome such problems for a box-type solar cooker, we have used an auxiliary source of energy inside it. This is done with the help of a built-in heating coil inside the cooker or a retrofit electric bulb in a black painted cylinder. It is found that the use of auxiliary sources allows cooking on most cloudy days. © 1997 Elsevier Science Ltd.

1. INTRODUCTION Around 65% of the energy supply in Bangladesh [1] comes from biomass, agricultural residues, twigs and branches, fuel wood and cowdung. About 80% of the population live in villages [2] and they depend solely on biomass for cooking their food. A serious fuelwood shortage has led to the use of all conceivable agricultural residues in village kitchens. Indiscriminate felling of trees continues unabated. In such a situation solar cooking should offer a big promise. Two types of solar cookers may be used in our country: the focussing-type and box-type. The focussing-type cooker can attain a high temperature very quickly but the temperature of the cooking vessels falls rapidly when clouds appear while for the box-type cooker it is not so. The effect of occasional clouds during cooking is not serious for the box-type solar cookers. From different experiments, we find [3] that solar cooking is possible on sunny days as well as on semi-cloudy days of the year by a box-type solar cooker. But cooking is not possible under overcast conditions. From the sunshine data of the Meteorological Department of Bangladesh, it is seen that sunny days are very common for 8 months of the year. The other 4 months of the year (June-September) are often cloudy. We have made an attempt to modify the solar cooker to accept a small amount of electrical energy in addition to solar energy so that cooking becomes possible on cloudy days. For this purpose, auxiliary energy is supplied by either built-in electrical heating elements under the absorber 151

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70 c m Fig 1. Sectional elevation of solar cooker-1. 1,3,4: wooden frame; 2,5,6: insulation (glass wool and soft foam); 7,8: rubber gasket; 9: clamp; 10,11: glass covers (3 and 5 mm thickness, respectively); 12: absorber plate (black painted G. I. sheet); 13: black painted utensils; 14: booster mirror reflector; 15: mirror hinges; 16: electrical heating elements.

plate (Fig. 1) or by converting light energy into heat from a 100 W electric bulb in a black painted cylinder placed on the absorber plate. The performance of the solar cooker-1 having either types of auxiliary energy input has been compared with that of another cooker, cooker-2 without auxiliary heating arrangements. It is learnt that solar cookers integrated with electric bulbs, as well as electric resistance, have been studied and reported in the literature at least by authors from India and Costa Rica, respectively, and cookers with electric resistance are sold in the Indian market. We are reporting the performances of these types of cookers for the first time in the climate of Bangladesh.

2. DESCRIPTION OF THE SOLAR COOKER Figure 1 shows the sectional elevation of the solar cooker- 1. The second cooker is almost similar to cooker-1 except that the outer casing is of G.I. sheet in place of teak wood used in cooker-1 and the dimensions are slightly different. Six heating elements used in electric irons (each element is of 900 W nominal) are connected in series to generate 150 W heat from 220 V a.c. and are placed below the absorber plate. The other type of supplying auxiliary energy is done by placing an electric bulb inside a cylindrical bulb holder on the absorber tray of the cooker. The cylindrical

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bulb holder has a diameter of 10 cm and a length of 20 cm. An electric bulb of 100 W is kept inside the holder. The inside surface of the holder is black painted. The light energy is converted to heat energy which increases the temperature of the cooker. There are holes in the bulb holder for the circulation of hot air from inside the bulb holder.

3. RESULTS AND DISCUSSION

Figure 2 shows a comparative study of two cookers without any auxiliary energy. It is evident that the cookers perform more or less similarly. The figure of merits [4] F~ and F2 have been determined to be F~ = 0.16 and F2 = 0.27 for cooker-l, while for cooker-2, F~ = 0.17 and F2 = 0.32. Figure 3 gives the curves for the variation of water temperature of the solar cooker-1 with auxiliary electric heater inside and cooker-2 (with load of 1 kg of water in each case) for a cloudy day on 14 N o v e m b e r 1994. The global radiation was very poor, i.e. overcast conditions prevailed. When the temperature of cooker-1 with auxiliary electric power reached 90°C, cooker-2 reached around 45°C. We find that solar cooking is not possible by using cooker-2 on an overcast day but for cooker- 1 with auxiliary electric power, cooking was clearly possible.

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