REVIEW OF SOLAR COOKERS

 

 

 

Parabolic Concentrators 2

Collapsible Parabolas/Fresnel reflectors/Cylindro Parabolic/Plane Mirrors

Collapsible parabolas

Parabolic reflectors were bulky and were difficult to transport and hence, some inventors thought of collapsible concentrators.  A Swedish design, called Umbryoler, was probably one of the first designs to emerge on the scene (PC 1, Figure 14). It was mainly designed with campers in mind, and the unit would open out as an umbrella. The reflector was made from sheets of aluminized polyester or sheets. Fluttering focus and unstable reflector were the chief drawbacks of this otherwise good looking designs ( Annon 1981a ). Chinese ( Fang, Susan 1979 ) and Japanese designs (PC 1b) are variations of this design. In 1961, the VITA group tried out another interesting design, the famous inflatable concentrator (VITA 1961). Though it had a checkered history the design is still in vogue and, recently, it has appeared again with more firm materials in solar water pump application (Beale 1981) . Basically, the reflector is a large round aluminized polyester sheet of slightly thicker gauge, over which another clear polyester sheet was fused all along the edge. When this assembly was tightly but evenly stretched and tied between a frame and inflated, a Perfect parabolic reflector was formed (Figure 15). Fluttering focus was the main drawback.

 

 

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Sobako (Type PC 3, Figure 16) was an ingenious design from Germany. It was a foldable parabolic trough, with a stainless steel reflector. When opened, the reflector focused the sun’s rays at the base of a copper plate housed in a doubly glazed glass box. The area of the reflector was about 0.75 sq. m. Kanua (1979) has worked extensively on this design and found that the glass box broke often, and difficulties were encountered while folding the reflector. He suggests many modifications but it is felt that unless the reflector area is increased the unit may not perform well. Egypt cooker is another design based on similar principles (Type PC 3b, Figure 17), Here, the reflector was a cylindrical parabolic mirror. A specially designed 5 liter box was fixed at the center for holding the cooking vessels but the German group found the unit rather bulky, and maximum temperature attainable was only 180oC. This German group evolved a similar but smaller cooker where the triangular box is hosed in the glass box.

 

Another recent entry to this group is the design evolved by an American Inventor. The device is called as Go Sun Cooker. It has a central doubled walled evacuated cooker tube.  Cooking vessel which is also in the form of a half tube, sliding in and out of this evacuated tube and easy to handle. The Cooking power comes from parabolic trough streaching out on either side of the cooking tube. The parabolic reflector folds to form a compact pack for easy transport. Cooking power appears to be good. Solar cooker enthusiasts like Bruce, of USA have tried it successfully and are happy with its performance. However, I would have been happy if a larger cooker of this type is evolved with a bigger cooking area.

 

Go Sun Cooker

A young inventor Aniketh too seems to have arrived at similar design. I could not get the details of this design. Not sure if he is also using evacuated tube.

An evacuated tube will definetly perform better, but I feel a double glazed tube would also perform well and it could be less costly. A rectangular box of glass should also work fine. A larger design to house bigger vessels would be more useful.

The modified umbrella (PC 4) is, in fact, nothing but an umbrella with an aluminized polyester sheet stuck to the concave side of its cloth. Though modified design, the reviewer still considers it as a main type for it converts an ordinary umbrella into a solar cooker. Recently, there were some reports that this design was getting popular in north India (Figure 18). As seen in Figure 18, instead of keeping the vessel on the grill fitted to the handle, the vessel could be placed on a fixed platform and the umbrella adjusted to focus the sun rays which may give a better result but cooking on a windy day could be difficult and tricky.

Umbrellas have always enthused Solar innovators. Recent entry is by Juan Francisco Paredes. (Email: juanfranciscoparedes@gmail.com) His very illustrative web site is a must visit for  all. (http://club.telepolis.com/elcatamaran/barbacoa-solar/).   He has barbecued meat on it and thus showing its effectiveness. Marc Ayats  ( http://cuinessolars.iespana.es ) has also presented this idea in a slightly different manner.

Barbacoa Cooker

The Russian folding cooker (Type PC 5, Figure 19) was probably the largest folding cooker ever designed. It had a reflector of 1.5 sq. m in diameter. It has been designed with lot of care to avoid flaws, as it was meant for use of Military personnel. The rotating screw indicated that mirror adjustment was easy and the cooking pot could be kept in an insulated receiver. It is claimed that the cooker could double as a rain shade or tent at night.

A Swedish designer (cited by Venkatesam 1980) had come out with a very neat package of a solar cooker that fits into a specially designed suitcase (Figure 20). It is ideal for campers, but for day to day use the reflector has to be larger.

VITA (1961) announced another interesting design from China (Type PC 7, Figure 21). The foldable reflector, evidently a section of the parabola, made up of small mirror pieces, opens out at the base of a tripod stand holding the vessel. To some extent, the design resembles the Sobaco type described earlier.

Recently Brett White has taken up this very useful concept. He is the first contributor to this web page. He has infact used a Beach umbrella and this appears to perform better. The cooker can double as a regular beach umbrella. For more information visit his site. URL is provided under other links.

You can buy a folding cooker from Brett White, from Australia.

Deris, a prolific inventor from USA has brought out another interesting Parabolic Cooker. It is very interesting in various ways. It is 2'x2' square Parabola, collapsible at that. It does not focus the light at the bottom of the cooking vessel, but on the sides as indicated in the photo. He claims that it cooks food very fast. For more details and prices you should visit his web site( please see under links ~solar16.htm )

 You can buy a folding cooker from him, from USA

Mr Li-Yan Zhu and Kim have tried their hands in designing non tracking Parabolic Concentrator. In their beautiful site they describe the logic behind the design and present several picturers too. Their site could be visited through Solarcooking.org. I have mentioned about this cooker in previous section as well.

Fresnel reflectors

Though the parabolic reflector was a perfect design, even good technicians, more so the villagers, found it difficult to construct one. This was how the Fresnel reflectors got invented. The very first design in this category came from VITA in 1961 (Type F 1, Figure 22). Three to four rings of masonite (hardboard like material) were cut from a 4’ x 4’ sheet. Aluminized polyester was stuck to this. After which the rings were nailed to specially notched wooden reapers to form a Fresnel concentrator. VITA provided a template of these materials as it was a do-it-yourself project (VITA 1971). The cooking pot was supported on a rod projecting from the center. Adjustments, once in 30 minutes, were considered sufficient. Ease of construction and focusing characteristics made this design better than the regular parabolic reflector (VITA 1961). Prof. Garg, an Indian designer (Garg et al 1978), suggested further improvements by making the reflector with five to six rings.

 

 

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In a comparative study, the VITA design emerged as a top model (Garg 1978). Attracted by the simplicity of the Fresnel geometry, Steenbleek (Type F 2, Figure 23) developed an ingenious way of making Fresnel concentrator (strips of reflector materials, with the width determined by using a computer) were wound in concentric circles on a suitable support. Solar stalwarts like Edmondson (1981) claim that this design was a boon to solar designers, reflectors of any size could be easily fabricated. The author tried out this technique but encountered considerable difficulty in cutting and fixing strips.

Recently Mr Ed Norman of Peru has taken keen interest in the VITA design and has revived it. I am yet to get the latest details from him as to the performance.

Bernhard Muller of Germany who visited my site recently, March 2003 to be precise, has directed me to www.mueller-solartechnik.de. Here I found some more interesting variation of Fresnel design. The Photo taken from his site gives good details. The linearly arranged Fresnel lens focuses light on to a pot kept at the focal point, about 50 cm away from the mirrors. The design is supposed to deliver ~300W power. The one which could deliver ~600 W has reflector array in  the form of cross, claims   Muller. He recommends unit with Glass mirrors. The standard Cooker comes with Aluminium Foil Reflector.

Fatangre's ( 1992 ) design is actually a Box type design wherein he incorporates a transparent Fresnel lens in the lid of the Gosh type box.

Another interesting Fresnel arrangement has been described by Bob Culbertson,of US, in which an array of flat mirrors are lined up which direct sun rays on the lower side, in such a way that they focus the  sun rays at the bottom of assembly. The configuration could be described with the help of forward and backward slashes. This I guess works and it may be easy to divert the focused rays to the base of a cooking pot as well.

         O ( Sun )

\\\\\\\\\///////////

            + ( focus)

The above concept has been presented by an Unknown Inventor(at least till now for me).At the International Conference on Solar cookers held at Granada in 2006. The cooker has circular arrangement of reflectors which direct the focus below the umbrella like holder. I would call it Fresnel Umbrella. The focus is being used directly, but I feel it would be better and much more convenient to divert the focus to the bottom of the vessel. In which case the Fresnel Umbrella can be 'hung' over suitable 'sunny' window, and focus diverted inside home to cook food from the comforts of kitchen/room. This confugaration has been used in Solar Poer generation as well, especially in Australia. Such a confugaration of long mirror strips are used to focus sun rays on to water carring pipes in evacuated tubes to generate very hot steam, at high pressure, which is then used for turning turbines.

Magnar Totland, hailing from beautiful Norway, but now settled in Congo and helping out there, has found time to launch a excellent site on Solar cookers with superb rendering. His site has Parabolic reflectors of various types, Conical Cookers and Box cookers. It is one of the best sites ever, and I urge the reader to visit his site at (http://solarcookers.ning.com. His design discussed here can be classified as Semi-fresnel Cocentrator. His other designs will be discussed under respective sections. In the design mentioned here, the concentrator proper is rectangular parabolic reflector, and probably to get a better focus concentric Fresnel arrangement has been incorporated in the center. Th reflector is mounted on a sturdy stand and stable provision is made for cooking vessel as well. (More on his wonderful work on Conical cookers later)

Cylindro Parabolic

These are an interesting class of concentrators used mostly in water heating applications, but the very first design using such mirrors was that of Prata (1961) (Type CP 1, Figure 24). The cylindro-parabolic concentrator focused the rays into an insulated cylindrical box in which two or more cooking vessels could be accommodated (Fraber et al. 1975). Bowman, who built and tested the design, encountered several difficulties (Bowman, Blatt 1978), and he tried to improve upon the design. This resulted in a series of new concepts. The FIT concept No. 1, (Type CP la, Figure 25) is almost similar to Prata’s design but it has only a single cylindro-parabolic swinging reflector.

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Another improvement  was the incorporation of a bigger and easy-to-use insulated box to house the cooking vessel. Results were slightly better. Reddy (1980) has presented a similar design.

The Edmond Scientific Supplies Company (Type CP2) was selling yet another design of this category, mainly meant for warming food; it was basically a styrofoam parabola with aluminized polyester (Warenham 1995)

Another very recent cooker design  presented here has been taken from BBC online. ( BBC Online . This is being classified by me under this section of Cylindro parabola. I quote from their site as follows: "The solar sausage sizzler is a bit more tricky to make and watch out for all of those cocktail sticks. However, once built it makes short work of a sausage or two (we used Frankfurters sausages). Have a go, and let us know if you succeed" . Further details can be obtained from their site

Once again  Bernhard's site, mentioned above in case of  Fresnel design, has another design to offer under this category. Called as CRADLE Cooker, is coated with  Aluminium' Sheets, houses 13 lit capacity box in the center of the Unit. The unit measures about 110 x 41.5 cms. This works out to an area of 0.4565 Sq.m. It is supposed to deliver ~450W of power capable of boiling 1 lit of water in one hour at Germany.

Plane Mirrors

Plane mirrors were also in vogue in solar cooking but it was Adams who, in 1870, built a cooker at Bombay using plane mirrors. (Type MP 1, Figure 26) (Bowman, Blatt 1978). He was able to cook food for several soldiers. The cooker consisted of sixteen flat mirrors arranged to form a cone around a flat black surface meant for the cooking vessel. Further, it was covered with a glass dome with a wooden handle. Such a design was available till recently in the market. A cooker very similar to this design , called ‘Solar chef’ (Type MP la) was marketed by Sedona Solar Shop, ( PO 3072, West Sedona, Arizona 86340, USA). Prof. Bowman found the performance of the cooker satisfactory. He suggested some improvements like fixing the mirror assembly on to a stand such that it could be oriented to the sun esasily, increasing the number and size of the assembly as well as the central dome to accommodate bigger cooking vessels. I (Type MP 1b). This variation gave good results. Faber (1975) describes a similar design where the mirror cone is comparatively larger (Figure 27).

The Chinese designers (Fang, Susan 1979) have another interesting variation (Type MP 2, Figure 28). Here, the 15-20 cm wide and 25-30 cm flat long mirror strips were arranged in a row of 14-16 pieces. Two such rows of mirrors were hinged at the centre. These mirrors focused the rays on to a cooking pan held on top of the grill projecting from the center of the hinged panels. There must have been some mechanism to tilt the mirrors, but the details were not available.

Bowman created a new design called FIT design (Type MP. 3, Figure 29) which was an offshoot of his studies on cookers especially like CP la. He felt that flat-mirror assemblies were the best for villagers. The cooker consisted of a solid frame to hold an insulated box with a door to house the cooking vessels. The box did not have insulation at the bottom but a weld – mesh. Light was focused by 15 strips of mirrors held in a row at the base of this stand. Provision was made to tilt the mirrors to aid in focus and this was affected with one single lever, and the frame assembly could also be rotated to face the sun.

For domestic use, Bowman suggests a mirror assembly of about 19 mirrors with a length of 120 cm held at an angle of 100 to the horizontal (Bowman, Blatt 1978). FIT concept 3 had a mirror assembly of 3 m and it could melt even lead (MP 327.50 C ). The FIT concepts described were excellent performers and with some more adaptations like folding the reflector for easier transportation and storing, mounting the entire assembly on bigger wheels for easy mobility, and orientation would have made these cookers very popular. Interestingly, such a cooker was fabricated here at the Manipal Institute of Technology (MIT), Manipal, under a student project, but the cooker did not perform as expected. The Indian Institute of Technology (IIT), New Delhi, suggests the use of such a row of mirrors (Type MP 2a) to be kept outside the house so as to concentrate the rays on to the vessel kept inside the house. Concept III proposed here by the author (Type 3c) envisages the use of two rows of mirrors as in the Chinese design and the entire assembly kept at the window. However, depending on the location and orientation of the house, it may be essential to shift the cooker from one window to another, in which case the design has to incorporate features which would enable easy mobility (Type MP 3 a, Figure 30). Concept IV proposed in this review by me,  is a variation of Bowman’s design. In this variation, the use of a larger window covered with glass is suggested at the base of the insulated cooking box (Figure 31) such that a larger reflector array could be incorporated without increasing the height of the stand, but if a 3 or 4 m long array is to be incorporated for faster cooking then the height of the stand has to be more. In such a case, three or four steps may be added to the stand for easy access to the cooking vessels inside the insulated box.

Prof. Bernard's 'NELPA' cooker appears to be a  variation from Prof. Bowmans design, but evidently an independent design of this type.

The photo above, was sent by Prof. Bernard, note that the cooker has provision for two pots. ( I have added a couple sketches so as to enable the reader to build a Nelpa. I thank Editions-Jouvence.fr, Publishers from France for permitting to use these drawings.

Prof. Bernard's 'NELPA' cooker, though I find the design very close to that of Prof. Bowman's FIT design presented above,  I feel 'NELPA' has been designed independently. For some reason Prof. Bowman's designs had not become popular.

'NELPA' when folded forms a neat little 'packet'.

 Bernhard Muller of Germany has fine tuned Prof Roger Bernard's design of Nelpa and is in fact manufacturing and selling it from Germany. Presented here is his design of Nelpa, called by him as "Primrose cooker" Mr Muller can be contacted through his excellent site in Five languages at http://www.mueller-solartechnik.com

Any person who visits this review site would feel that it will not be possible to evolve a new design, but new design keep on appearing and two such useful designs, and I feel very promising one at that, are by Xavior Devos and Atouts Soleil of France another design by Alexander of Germany. Xavior Devos and Atouts Soleil of France ( tablesol@yahoo.fr) present a new variation of Bowman's variation. It could be regarded as improvement over Roger Bernard's NELPA. Variation is in the form of a larger concentrator below the table. The two photos furnish the design details and effectiveness of the setup. They have detailed the design in pdf format. (http://www.solarcooking.org/Devos-cooker-description.pdf)

Alexander Safronov of Germany has proposed a very innovative design called as Romaschka Cooker. The cooking vessel is incorporated in the table. A Flat reflector of about one meter long reflects the sun light into the cooker box. Two additional strips of reflectors augment sun light. The setup appears to be quite effective as the designer has been able to grill meat.(http://www.solarcooking.org/Romaschka.htm)

         

Prof. Ajay Chandak, working at Dhule, is a versatile inventor. It is a pity I did not know about him earlier. That I guess is the problem with independent Inventors and workers. They have problem or are not so keen writing a Technical paper, and naturally they remain unknown for a long time. It could be otherwise as well, independent reviewer like me, with out easy access to scientific journals, could miss a published scientific work. It is my wish to list all such workers and their work in my site.  

If our Government or some other philanthropic Agencies  decide to sponsor my work, I wish to test each and every such solar cooker especially designed by ‘non technical’ workers, and maintain a detailed Data Base along with this site.

Prof Ajay is the founder president of an NGO at Dhule. The organization is called as PRINCE (Promoters and Researchers In Non Conventional Energy, http://www.princeindia.org ) I strongly urge the reader to visit to their excellent site.

Prof Ajay has developed Hybrid Solar Cooker, which I have classified as an independent Design, and designate it as MP4. It is similar to Prof. Bowman’s FIT Design, but different and one step ahead it, or for that matter Prof. Bernard’s Nelpa. I saw the design for the first time in Dev’s site. ( listed under links ~~~/solar16.htm ). Ajay writes that the performance is as good as or even better than that of Parabolic cooker. The temperature developed was so high that they had problem with the Glazing used at the bottom of the box. Further technical details are awaited.

 

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