Solar concentrators that concentrate light from above

Template:Compendium TOC 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 evolved and one of the most interesting and popular design in this catogory is the 'COOKIT' evolved by Professor Roger Bernard of France.

Water lens

Water lens 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).

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. 4 057 048 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 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. 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-V in a better manner. Lage 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.

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.

COOKIT

In the year 1999, another very interesting but very simple and effective design, but a very low cost solar cooker has been presented by Prof Roger Bernard (1995), of France. Brand named ‘Cookit’ (Figure 38), it is just a packet of interconnected reflectors. When open, they fold out into a small bracket of reflectors around a central space where cooking vessel kept in a transparent container of a heat resistant plastic. The cooker, probably the simplest among the lot, cheaper than that of VITA design, work well , and is one of the most popular solar cooker on the planet.

'COOKIT' as it appears in SCI site.

Popularity of this design has compelled the reviewer to include more details regarding the same. Professor Bernard, the designer of this unique and most simple design has brought 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 1.25 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.

Plan details of COOKIT Design as in SCI site.

When reviewer tried to make one 'Cookit' some difficulties were experienced, especially with reference to round cuts at the top reflector and other details. Hence he tried to make the whole design simpler. The central part would be about one to one-and-a-half inch (5 to 7.5 cm) larger than the cooking vessel. Rest of the design is easier 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 angle thus established will have to be incorporated in the design, and once it is cut, it can not be altered easily.

The cooking pots have to be kept inside a suitable transparent cover. Simple transparent plastic bag of even Polyethylene has been found to work, called as Green house enclosure. Such an enclosure was suggested by legendry Mrs. Barbera Kerr. Professor Bernard himself had suggested use of a inverted transparent bowl around the black cooking pot. Later considering several problems associated with such a transparent cover around the cooking pot, he had suggested use of upright transparent bowl of a size larger than the cooking pot and had also suggested several other modifications for easy cooking.

The glass bowl concept has been developed by David and recently by Darwin Curtis of 'SHE'.

Mr. David from Canada has come out with wonderful idea detailed here. David uses a glass bowl larger than cooking vessel. To keep the vessel inside the bowl, a thick card board coller, with a hole to accomodate the bowl, is used as shown in the diagram. To keep a smaller bowl for cooking coller with smaller hole can be used.

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

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

Carrying this concept of Glass Bowl, recently Mr. Dar Curtis and Louise Meyer of Solar Household Energy (SHE) have come out with a specially designed Hot pot of Glass sourounded by foldable sparkling reflector panels. They have won a grant from World Bank to distribute this type of cookers.

Glass bowls are costly. In India they cost about Rs 500/-. In Africa too glass bowls would be costly, so recently Mr. Diasanna, the ingenious down to earth inventor, from Africa devised a simple and effective enclosure of transparent plastic to cover the cooking vessel from the top. To retain the shape of such a cover, used thick copper wire, or the like. (See the diagram in the composite sketch bellow). Reviewer has suggested some improvement over Mr. Diassana's Cover. The design of Transparent enclosure I suggest comes in two parts. First part is a transparant 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 transparent cover, which is also supported by a wire at the edge only. I have sent the details to the Solar Cooker Review, and so the design should see the light of the day. I shall now present the plans and drawing for the construction.

As regards the popular 'COOKIT' there appears to be innumerable variations. Only some of the easily available variations have been presented here, reader can add other variations.

Professor Bernard's Cookit is becoming more and more popular. To increase the efficieny of such cooker, the angle of the reflectors has to be altered to suit latitude. there are many attempts to achieve this.

Florida Solar Energy center has developed what they call it as 'HIGH BACKED COOKER'. Their test indicate that it performs better than the regular Cookit,at least at Florida. It is a special adaptation for regions beyond 35 N latitude. 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.

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

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

In the year 2006, Derris has come out with one more interesting variation called Pyramid cooker. For the sake of convenience it can be classified under Panel Cookers. (http://home.att.net/~cleardomesolar/SolaReflexPyramidcooker.html)

The Pyramid Cooker, and interesting and apt name indeed. It is supposed to perform better than regular 'Cookit'. As shown in the photos the two sides of the pyramid are made of reflector material, and the front is covered by a transparent plastic. Third, adjustable reflector is in the front.

There seems to be renewed interest in this type of Solar cookers, where light is focused from top. 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 which are used to avoid heat loss.

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 the inventor's grandmother). It is clear from perusal of their site that they took the clue from Professor Mannan’s improvement of VITA Design. But to start with they had fabricated Professor Stevens Funnel Cooker and then settled upon the Parvati design. Visit 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 highly 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.

FUNNEL COOKER

Professor Steven has another interesting variation which he calls as '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 small hole in the ground to hold the base of the funnel. Two small sticks, or stones could be propped up at the back to hold the funnel in the right position. Professor 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 calls for immediate attention by all the Solar Cooker Designers and reviewers request them to make such diagrams for all the the type of Cookers they evolve. Ravindra's URL is http://www.angelfire.com/80s/shobhapardeshi.

This is a very interesting and useful drawing, author's special compliments to Mr. Ravindra and his wife Parvathi on creating such wonderful drawing.

Ms. Molly Baker, has presented another interesting variation of Funnel Cooker. The cooker is made from stiff cloth and the inner lining is a reflector material, may be like the one produced by Derris of Clear dome Solar. When the fold is opened and plastic tubes are inserted at the top and bottom the cooker forms a stiff cone ready to cook. The innovator has not shown and green house enclosure, but the cooker will perform better with one. (http://mollybakersolaroven.com/home.html)

Innovator like Ms. Anna Pearce had modified the Cone shaped hat of the far east regions into a multipurpose device which can be used as Hat, as a vessel to cook food, carry water, an also as a Cone type solar cooker called Anahat. The following diagram is self explanatory.

Most of such cookers would work very well, there are reports indicating that they have worked even in remote areas like Nepal. But many appear to be rather small for the reviewer, bigger reflectors should work better.

One of the most simple cookers, rather the simplest, the reviewer found, could be that described by the US innovator Mr. Deris Janette, called as Cleardome Cooker it is just a semicircular row of plane mirrors around a cooking pot kept under two transparent domes. In an emergencis or even during normal periods for that matter, this type of confugatation can be used by any, anywhare! The cooker would work even with modified Diasana type of plastic enclosure.

Deris's Clear dome solar cooker