SOLAR COOKERS USING CHEMICALS

Solar cookers using chemicals, exist since 1961. The need was felt much earlier as solar energy is very intermittent in nature. Many ideas have been proposed but none of them appear to be very promising.

The VITA group (1961) and Huxtable (1976) have proposed another system with H2SO4 and water (Type IDT 8b). When water is mixed with acid, heat would be liberated and later the acid could be concentrated by driving away the water using solar heat. Though very simple, it did not meet the safety standards.

Hall et al. (1977) proposed the use of simple salts like MgC1₂ or Cacl₂ (Type IDT 8a). Ammoniated MgCl₂ and CaCl₂ are kept in separate but interconnected boxes. Solar heat was used to drive away the ammonia from MgCl₂ which would then combine with CaCl₂ in the other box. Now, the system could be considered as charged. When heat is required the box containing CaC1₂ is slightly heated, this releases the ammonia which would then move to the other box, combine with MgCl₂, and release heat at 300⁰C. This apparently simple system has not been studied in detail.

Japanese scientists had announced a chemical system (Anon. 1981 a) which would store solar energy and release it at very high temperature when Silver salts are sprinkled over it. The cost of these chemicals and other details are not known.

Certain salt mixtures melt on heating and then they would release the heat at fairly high temperature when they solidify. The phenomenon is known as the latent heat of fusion. Salt mixture of NaNO2 and NaOH melts at 240⁰C and 1 g of this mixture releases 58 cal at 240⁰C before it solidifies. If the same quantity of heat has to be liberated from oil then about 120 ml of oil will be required (Walton et al. 1977).

Dr S D Sharma (working at Japan) and his group, working at Indore, ( India ) along with Dr Buddhi seem to have answer for people who were keen on storing Solar Heat for late evening, if not late night Cooking. As mentioned earlier there were many attempts earlier but this one uses a commercial grade Acetanilide ( melting point 118.90 C and Latent Heat of Fusion at 222 kJ/kg. ). It was heartening to see that they have used Box type of Cooker of 50 x 50cm and 19 cm deep, with three reflectors ( in fact I too had suggested three reflector design way back in 1980, but did not build and cook in it ). The group use ball & socket joint to the reflectors to turn them suitably to direct Sun light into the box. The stagnant temperature they attain is in their Box was in the range of 204 C. They use a hollow cylindrical container to keep the Phase change material PCM. The inner diameter of the cylinder was 20 cm , and the outer was 30 cm, and the height being 125 cm. They load about 4 kg of PCM . Using suitable Aluminium ( ? ) vessels they were able to cook nearly 1.6 kg of food ( 0.40 kg Rice and 1.2 kg water along with water ). The food was loaded at about 19.30 hrs and was found well cooked at about 21.30 hrs. The data has been acquired using HP 3852 Data Acquisition system and based on what has been presented, I would say the trio have done a commendable work. Now it has to be seen as to the cost of the unit, its durability especially the part which holds the PCM, and the things like it. If you are interested in knowing more about it you may visit their site http://www.sciencedirect.com/science/journal/01968904.

Though attractive, and apparently simple, extracting useful heat from this type of system has posed several difficulties. For example, the molten salt would release heat and solidify at the surface, act as insulation, and impair further transfer of heat. Further, the cost of salt to store sufficient heat to cook two square meals a day may also be prohibitive.

There are many more claims, about heat storage, (in fact, some users of Scheffler Cooker, have stored heat in molten Tin, but such systems evidently can not be useful for domestic applications). but details, especially the cost, here , are not available. The system has been designed by Prof. David Wilson of MIT along with his students. If I get details, I will share it with the readers. Some information is available with reference to a system developed at MIT, US. The image presented here appears to be very promising. (The image has been created photoshop the source says.(http://solarcooking.wikia.com/wiki/Wilson_Solar_Grill) (http://www.onlinetes.com/mit-news-molecular-approach-solar-power-energy-41414.aspx)

Under IDT 9, the author has included the biogas digester. Plant product of any type could be used here and as plants use and store solar energy. The inclusion of this type of cooker should not be out of place. The Government of India has supported this type of cooker and it has solved fuel problems in many a household.

IDT 10, is solar hydrogen, the fuel of the future. When solar research work was launched here way back in 1979, the author had thought that solar hydrogen would be available at least in developed countries by 1990-95. But this has not been the case even by the end of 1997. There are many hurdles and some of the basic problems are: a) cost of production is still considered not competitive enough with conventional fossil fuels, and b) difficulties with storing the highly explosive gas have not been sorted out.

IDT 11 is the popular solar water heater, and those who find it inconvenient to cook directly in the sun can think of using the solar water heater. Solar heated water could reduce the cost of cooking by about 40%, and at present, it is probably the cheapest method to store solar heat. Conventional thermosiphon heaters are a bit costly but a modified box in the box-type solar water heaters could become popular. Several scientists have suggested very promising improvements in the passive box-type solar water heaters.

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For details regarding testing standards for Solar Cookers ( International Standards ) please click here

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Dr. Ashok Kundapur

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