Solar cookers using chemicals have been popular since 1961. However, 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 promising.
Hall et al. (1977) propose 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.
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.
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).
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.
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.
Source:- TIDE., March 1998, 8-1, pp 1-37, For Comments, suggestions,contributions contact < ashok_kundapur@yahoo.com>
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