Cookers which concentrate light from above

In this type of solar cooker the light is concentrated from above. Though for cooking this mode of concentration of energy from the top is not very desirable there are several old designs which have used the method.

Water lens

Type LCA 1 (Figure 31) is an old idea which has been tried successfully in metallurgy. Antonine Lavosier (1743 – 1794) had used an alcohol lens to melt Platinum (MP 1760⁰ C). 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).

Source:- TIDE., March 1998, 8-1, pp 1-37, For Comments, suggestions,contributions contact < ashok_kundapur@yahoo.com>

Salaria, Singh (1978) also 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 (Figure 32). 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. 4 057 048 of 12 November 1975) (Figure 33). Here again the heat coming from the top, that is focused sunlight, would be too much and it would be essential to divert the focus. 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 V (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 (Figure 34).

Source:- TIDE., March 1998, 8-1, pp 1-37, For Comments, suggestions,contributions contact < ashok_kundapur@yahoo.com>

The cheapest solar cooker in this category was designed by VITA (1961). The reflector was made of disposable cardboard cones covered on the inside with reflective material (LCA 4, Figure 35). 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.

Prof. Mann (1981) has improved on this design. He used only two cones, but with better reflection and concentrating characteristics (Type LCA 4a, Figure 36). The light was focused on tot he 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 (LCA 4b)

Type LCA 5 (Figure 37) 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 projected above this platform, so that the light came from the top (Anon. 198l b).

Recently, another very interesting but very simple design, and naturally very low cost design of solar cooker has been presented by a Bernard et al. (1995). Brand named ‘Cookit’ (Type LCA 6, Figure 38), it is just a packet of interconnected reflectors. When open, they fold out into a small bracket of reflectors around a cooking vessel kept in a transparent container or a heat resistant plastic. The cooker, probably the simplest among the lot, cheaper than that of VITA design, should work well in tropics.

As was indicated earlier, the cookers which concentrate the light from below stand a better chance in the field. The best candidate in the field is the Murthy type of asymmetrical reflector among the parabolic reflectors. In the cookers using plane mirror geometry the winner could be Prof. Bowman’s FIT design or Concepts III or IV proposed by the author in this review. Another proposal of the author, Concept V, which envisages the use of a large roof-top lens, could be ideal in smaller towns, with single-storied buildings, and, when the system is not used for cooking the concentrated heat could be used for heating water or to turn on a water pump or a generator.

Source:- TIDE., March 1998, 8-1, pp 1-37, For Comments, suggestions,contributions contact < ashok_kundapur@yahoo.com>

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