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In this type of solar cookers, the light is concentrated from above. Though this mode of concentration of energy from the top is not very desirable for cooking, there are several old designs which have used the method. Many new designs have been developed. One of the most interesting and popular designs in this category is the 'CooKit' developed by Professor Roger Bernard of France.
A lens formed by water is an old idea which has been tried successfully in metallurgy. Antonine Lavosier (1743 – 1794) had used an alcohol lens to melt Platinum (MP 1768 °C) (349 °F). The lens was made of two curved glass sheets joined to form a bi-convex lens and the space was filled with alcohol. The lens measured 130 cm in diameter and, as its refractive power was rather insufficient to form a sharp focus, an additional lens of smaller diameter was used (Meinel, Meinel 1977).
More information on water lenses is available here.
Salaria, Singh (1978) advocated this idea for cooking, but felt that there was too much heat coming from the top so they advocated the use of a deflector to direct the focus to the bottom of the vessel. Instead of one large lens, several designers have proposed the use of a dome of multiple lenses, but De Witt C Maine was probably the first to apply for a patent (US Patent No. 4057048 of 12 November 1975). Here again the heat coming from the top, that is focused sunlight, would be too much on top of the vessel and it would be essential to divert the focus suitably to the bottom. Besides, it may be difficult to fabricate an assembly of lenses to have a long focal length. Of late, a multiple lens dome has been tried successfully in the case of power generation.
To offset some of the problems associated with such multiple lenses, the use of transparent Fresnel lenses was suggested (Mathur, Bansal 1981). The IIT, Delhi group was working on various aspects of large Fresnel lenses. Two configurations were recommended. In one, the vessel moves and in another the position of the lens is altered. If the cost of the Fresnel lens is low then these designs may become popular. There would still be the problem of durability; plastic lenses would blur very soon. Recently, Fatangare (1992) has also recommended these types of Fresnel lenses.
Concept VI (Type LCA 3a) proposed in this compilation by the author suggests the use of a suitable large water lens incorporated in the roof, and the focus diverted to the base of the vessel. Provision has to be made to prevent the glare and also to cover the lens when it is not used. The cheapest solar cooker in this category of light coming from above was designed by VITA (1961)(See in the figure below). The reflector was made of disposable cardboard cones covered on the inside with reflective material like Aluminium Foil. The three cones concentrated the light on to the top of the cooking vessel. The cones telescoped into one another for easy packing. VITA claimed that it could be used for warming the food or cooking some dry food.
One more diagram illustrates the concept-VI in a better manner. Large Fresnel lenses available, at least in USA may find use for such applications. This arrangement is better than Scheffler's Cooker, because, it can be used by people living in flats. All they would require is a south facing window. But this may be difficult in some cases, in which case method should be evolved to channel the focus.
Recently Bing Gu of California has designed a Solar Oven using Fresnel Lens, he has two models, 32x32" and 42x42". The ovens also come with PV operated tracking device. The design appears very sophisticated but vesry simple to work with. For more details visit his web page (http://california-sunlight.com/10401/index.html)
Professor Mann (1981) has improved on this design. He used only two cones, but with better reflection and concentrating characteristics. The light was focused on top of the vessel kept in an insulated circular box underneath. This design could be a good alternative to the Telkes type of oven. Recently, El-Sebail (1994) too has proposed a similar design.
Another Figure in the above illustration was an interesting model which appeared in Popular Science 219 (6): P. 84, 1981. Here, the cooking vessel was placed on a small platform covered with a glass jar, and two row of plane mirror assembly above this platform, so that the light is focused from top to the cooking vessel.
The simplest cooker that one reviewer found could be that described by the US innovator Deris Janette, the Cleardome Cooker. It consists of a semicircular row of plane mirrors around a cooking pot kept under two transparent domes. In emergencies, or even during normal periods for that matter, this type of configuration can be used by anyone, anywhere. This cooker would work with modified Diasana type of plastic enclosure also.
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