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It was the desire of the authors to build and test all the important models, especially the newer ones. When data available, the selection process becomes easy.
As regards the box-type cookers, the general contention is that if they are manufactured on a large scale they would be cheaper, but then that would mean manufacturing several other related components such as steel sheets, etc. and although it could generate several jobs, cost of cookers may not be reduced, and subsidies would not support them for long.
This does not mean that Author recommends only Box Cookers. The title of most popular cooker has been captured by Bernard's CooKit]]. Though there are lot of do-it-yourself plans, low as well as high cost designs are available in the market. Dars' Diamond cooker along with specially designed Glass bowl has been supported by World Bank. Solar cooker International is supplying low cost ready made CooKit]] Author has presented a easy plan to fabricate Cookit.
Author feels that parabolic cookers can also be designed using small plane mirrors. Fresnel cooker plans were presented by VITA way back in 70s. But they have not become very popular.
The author strongly recommends do-it-yourself designs for villagers,and sites like this should put more such plans.
Before that, there is a need to standardize the materials. One can not suggest ordinary Aluminium foil or Aluminized polyester sheets for reflectors. They lose their shine very soon. It is felt that the most likely candidate for reflectors is polished stainless steel, but user data for this is not available. For example, how long will the shine last? When the shape is lost, then can the sheet be polished locally ? If yes, then can it be done by the housewife or at least by a traditional knife sharpener with his slightly modified leg operated grinder ?
It is of common knowledge that even good glass mirrors do not last long in open sunshine (Buckwalter, McVay 1980; Howe 1981). In fact, there exists a long list of materials which need testing under the sun, that is their sun worthiness. Which glass mirror is to be recommended?
As regards glazing, the author had recommended ‘UV stablized’ transparent polyester materials. But they did not last for more than three months (Ashok Kundapur1980). One cannot insist on tempered glass for the box-type cookers as it is very costly, low iron glass is not easily available, and 4 mm glass would be costly too. Which is the best alternative? What if one uses ordinary 3 mm glass? What will be the difference in performance between ordinary and iron-free glass? Data is not available easily. A small difference will not matter, and the author has used only 3 mm glass, and got satisfactory results.
Similarly, glass wool or mineral wool insulation cannot be recommended for the villagers, the material is hazardous. One has to standardize and suggest composites made from agricultural wastes like rice husk or straw or any other locally available material. Insulating materials like foam glass is a good alternative (with a k of 0.036 and a weight of 137 kg/m3, it is as good as mineral wool) and for urban cookers too one can definitely recommend the same. This could make the cookers cost less. Further, the problem of broken glass could also be solved to a certain extent as broken glass could be used in the manufacture of foam glass.
Absorptive coating calls for special attention. Is there anything better, safer, and cheaper than common blackboard paint ? What about the special absorptive stickers, especially for the cooking vessels, or certain special treatments to darken the surface ? This is the area which requires serious attention and research. Only when the scientists have definite answers for aspects like insulation, absorptive coating, etc. can they think of a massive popularization program. This, by itself, will boost the use of solar cookers.
As regards testing, the wisdom of some of the tests conducted by Prof Bowman of Florida Institute of Technology, or German Appropriate Technology (GATE) to be detailed later in this paper, are not seen in some of the modern test procedures. The new tests concentrate on thermal efficiency. But other aspects need to be looked into and reported. To this effect author had presented series of recommendations.
User comment: Another useful and low cost material for transparent covers with good absorption of long wave radiation from the hot pot is Laserlite 2000 Polycarbonate produced by Bayer (definitely sold in Sydney, Australia, but I don't know if available elsewhere). It costs approximately AU$18/sqm (2006). It uses curves (corrogation) to give strength and has a 10yr warranty as a roofing material (suitable for high UV exposure). It can handle hailstones up to 2.5cm in diameter. However, at high temperatures (>100oC), I do not know if it breaks down or not and this would be the most significant potential problem for concentrating collectors. For absorbers, black polyethylene sheet is very cheap (around AU$2/sqm) and quite durable, even under moderate UV exposure. Spray paints are also an option but they may peel off over time - Bany
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