solar6.htm

Cookers which concentrate light from above

In this type of solar cookers 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.Many new designs have been evolved and one of them the 'COOKIT' evolved by Prof.Roger Bernard of France, has become very popular.

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 < arkundapur@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 < arkundapur@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). Both of these designes are shown in the following figure.

There seems to be renewed interest in this type of Solar cookers. Mr Teong H.Tan fabricated what he calls as DATS meaning Double Angled Twelve Sided Solar cooker. He thought about this design mainly to avoid the transparent plastic enclosure/guard around the cooking pots to avoid heat loss. The DATS saw the light of the day through Solar cooker Review published by the Solar Cooking International.( Photo from SCI site )

In India, Mr. Ravendra Pardeshi and his wife Mrs. Shobha, of Pune developed a similar Cooker. They call their cooker as 'PARVATI Solar Cooker'( Named after inventors Grand Mother ). It is clear from perusal of their site that they took the clue from Prof. Mannanís improvement of VITA Design. But to start with they fabricated Prof. Stevens Funnel Cooker and then settled upon the Parvati design . Vist to their site is a must, and their URL is http://www.angelfire.com/80s/shobhapardeshi. The two photograph/drawing presented here introduces this type of cooker. It is commendable that this concerned and higly motivated husband and wife team have done lot of experiments and have suggested that staking the vessel one above the other gives better results than keeping them side by side.Following illustrations are form Ravidra Pardesi's site.

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.

Fig.38a, 'COOKIT' as it appears in SCI site.

Popularity of this design has compelled me to include more details regarding the same. Prof. Bernard, the desiner of this unique and most simple design has brough out a book in French where he has presented many more simple designs. The Cookit is to be cut out from a cardboard of about 1 x 1 meter.Other material also can be used, for example Polished Aluminium or Stainless steel. After cutting the cardboard as directed, aluminised Mylar or aluminium foil is stuck to it. It is then folded and as shown and prop-it-up, and lo, you have the cooker ready.

Fig. 38b, design details as in SCI site.

The above figure is from Prof. Bernards little book on solar Cookers. It shows more, but slightly different way in which Cookit can be cut out.

When I tried to make one 'Cookit' some difficulty was experienced, especially with reference to round cuts at the top and other details. I tried to make the whole design simpler. The central part would be about one to one-and-a-half inche ( 5 to 7.5 cm) larger than the cooking vessel.Rest of the design iseasier to draw on the flat cardboard and cut out the Cookit. It would be possible to calculate precisely the angle at which the reflector at the back should be, for various latitudes. The anagle thus established will have to be incorporated in the design, and once it is cut, it can not be altered easily. THE CORRECT ANGLE OF THE REFLECTOR CAN BE OBTAINED BY INCREASING OR DECREASING THE DISTANCE BETWEEN THE TOP OF THE PANEL AND CUTTING ANGLE. IN THE DIAGRAM SHOWN, IT IS ON THE YOUR RIGHT IN THE RANGE OF 50 CM FROM TOP.IF THE DISTANCE IS SET AT, SAY 55, THEN THE REFLECTOR WILL RECLINE. /font>

The cooking pots have to be kept inside a suitable transparent cover. Simple transparent plastic bag of even Polyethelene has been found to work. Mr.Diasanna from Africa deviced a simple enclosure of transparent plastic to cover the cooking vessel from the top. To retain the shape of such a cover the ingenious local inventor,used thick copper wire, or the like.( See the diagram in the composite sketch bellow)

But when the cover is lifted to inspect the cooking material, the vessel gets exposed too and considerable heat is lost. To offset this many have devised what is popularly called as 'Green House Enclosure'. Mr David from Canada has come out with wonderful idea and I have detailed the same. I have suggested some improvement over Mr Diassana's Cover. The design of Transperent enclosure I suggest comes in two parts. First part is a transperant cover, cylindrical is shape, and a size bigger than the cooking vessel, slightly taller too. This is suitably supported by thin but strong sticks or Copper or Aluminium wire. The top of this enclosure has a Chinese hat like transperent cover, which isalso supported by a wire at the edge only. I have sent the details to the Solar Cooker Review, and soo the design should see the light of the day. I shall present the plans and drawing for the construction

'COOKIT' seems to have become very popular especially after the introduction of the transperent bag around the cooking pot as suggested by Mrs. Barbera Kerr. Prof. Bernard himself had suggested use of a inverted transperent bowl around the black cooking pot. Later considering severl problems associated with such a transperent cover around the cooking pot. He had suggested use of upright transperent bowl of a size larger than the cooking pot and had also suggested several other modifications for easy cooking. David ( see under other links) also reccommends this and in addition has tried to improve this idea.

The diagram above gives a clear picture as to the assembly of black cooking pot in 'cookit'

This diagram shows as to how a smaller pot could be held in a larger transperent upright glass bowl

All the above diagrams are from David Delaneys site ( see under other links for URL )

There appears to be innumerable variations of the famous and most popular Cooker the "COOKIT" designed by Prof Bernard of France. Some of the easily available ones have been presented here, if you feel like adding any other variation, please send it over to me through E-mail.

Prof. Bernard's Cookit is becomming more and more popular. A group from Florida Solar Energy center has developed and tested what they call it as 'HIGH BACKED COOKER'. Their test indicate that it performs better than the regular Cookit,at least at Florida.Incidentally this group has done excellent work on various aspects of Solar Cooking, like selecting good reflectors, transparent material for 'Green house' enclosure, back support for the Cookit Cooker. A visit to their site,http://www/fsec.ucf.edu, is a must.>

The following diagrm gives a comparative view of all the three types of panels

Mr. Teong H Tan working at Malasia has evolved another variation to Cookit. Called 'Duel Setting Panel Cooker'( DSPC)was specifically designed to offset the high altitue sun angle near to the tropics.>

Mr. Deris has presented several similar cooking gadgets. Please visit his sites for further details ( under other links )

 

All the above illustrate different types of very simple cookers designs of which are very close to that of 'COOKIT'.>

Prof. Steven has another interesting variation which he calles a 'FUNNEL COOKER' and his following photo presents his innovation. Prof Stevens funnel has several advantages, it is simple to construct, and also store, there is hardly any wastage of sheet, supporting the funnel may pose problem in the beginning but one can easily solve the problem, like for example a somall hole in the ground to hold the base of the funnel. Two small sticks, or stones could be proppedup at the back to hold the funnel in the right position. Here at India I wish to use Stainless steel of say 28 gz thickness. I feel it would give good result. Prof. Steven has a new site, also linked to solarcooking site.

Mr Ravindra Pardesi and his wife Parvathi have evolved another new design, an improvement over Stevens funnel, it is a double funnel. I request you to visit his site for more details. THey have introduced a new kind of Line Trace diagram, which calles for immediate attention by all the Solar Cooker Designers and I request them to make such diagrams for all the the type of Cookers they evolve. Their URL is http://www.angelfire.com/80s/shobhapardeshi

This is a very interesting and useful drawing, my special compliments to Mr Ravindra ans his wife Parvathi

Most of such cookers would work very well, there are reports indicating that they have worked even in Nepal. But they all look rather small for me, but bigger reflectors should also work.

One of the most simple cookers, rather the simplest, could be that described by Deris Janette( URL under other links ), called as Cleardome Cooker it is just a semicircular row of plane mirrors around a cooking pot kept under two transperent domes.

Deris's Clear dome solar cooker

Photo of Deris's Clear dome solar cooker

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 < arkundapur@yahoo.com>


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