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)
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.
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).
NEW Dr
S D Sharma( now 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.
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,
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