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Plane mirrors were also in vogue in solar cooking but it was Adams who, in 1870, built a cooker at Bombay using plane mirrors (Bowman, Blatt 1978). He was able to cook food for several soldiers. The cooker consisted of sixteen flat mirrors strips 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. The cooker very similar to this, called ‘Solar chef’ was marketed by Sedona Solar Shop, (PO 3072, West Sedona, Arizona 86340, USA.) Professor 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, increasing the number and size of the assembly as well as the central dome to accommodate bigger cooking vessels. Insulating the flat plate at the bottom was also suggested. This variation gave good results. Farber (1975) describes a similar design where the mirror cone is comparatively larger .The Chinese designers (Fang, Susan 1979) have another interesting variation. 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 center. 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 which was an offshoot of his studies on cookers especially Compound Parabolic type of cooker. 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 sheet of 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 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 well. The Indian Institute of Technology (IIT), New Delhi, suggests the use of such a row of mirrors to be kept outside the house and to concentrate the rays on to the vessel kept inside the house. Concept III proposed here by the author 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.
Concept IV proposed in this review 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 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 increase. 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.
Professor Bernard's 'NELPA' cooker appears to be slight variation from Professor Bowmans design, but evidently an independent design of this type.
The photo presented below was sent by Professor Bernard, note that the cooker has provision for two pots. (This reviewer has added a couple sketches so as to enable the reader to build a Nelpa. He thanks Editions-Silence.fr, publishers from France for permission to use these drawings. The picture does not show the NELPA but the enhanced version Primrose with a greenhouse compartment underneath the pot and 9 instead of 3 mirrors to form a more narrow light bar to obtain a better concentration of the solar radiation. The bi-lingual plans and instruction manual in French and German language can be ordered from mueller-solartechnik.com.
Professor Bernard's 'NELPA' cooker, though author finds the design very close to that of Professor Bowman's FIT design presented above he feel 'NELPA' has been designed independently. For some reason Professor Bowman's designs had not become popular.'NELPA' when folded forms a neat little 'packet'.
Roger Bernard's first prototype of the solar cooker "Primrose", a more sophisticated model of the "Nelpa". (http://www.mueller-solartechnik.com)
Xavier Devos of France has presented a new type of solar cooker, close to a solar barbecue. It is different from Roger Bernard's Nelpa, since the concentrator below the table focus the solar rays in two directions. The concentrator is large and made of little plane mirrors. This solar cooker is as concentrative as a parabolic cooker and allows to grill and fry. The two photos furnish the design details and effectiveness of the setup. For more information, look at the web site of ATOUTS SOLEIL.([mail to:firstname.lastname@example.org email@example.com]) (http://tablesol.free.fr/)
Alexander Safronov of Germany has proposed an 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)
Professor Ajay Chandak, working at Dhule, is a versatile inventor and 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 Author strongly urge the reader to visit his excellent site.
Professor Ajay has developed Hybrid Solar Cooker, which reviewer has classified as an independent Design under new category of mixed types. It is similar to Professor Bowman’s FIT Design, but different and one step ahead it, or for that matter Professor Bernard’s Nelpa. 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.
There could be slight confusion as to the classification. Because in the Box cooker section Deve's Inverted Cooker has been described. Ajay's cooker has been developed after this inverted cooker concept. The design is included here because it uses multiple plane mirror reflectors.
Mike Pool has designed and built the Solar Fryer. It will grill steaks, cook a pizza, or cook about anything that you would normally put on an electric grill.He has a piece of low iron glass cut to fit a motorcycle wheel. It works great for covering a 16 inch pizza.
With the sun at an elevation of 59 degrees and the ambient temperature at 19 degrees Celsius, the Solar Fryerwill cook a pizza in about 30 minutes.
Carroll Hampelman of US has designed another new solar gadget, called 'Hamp Hi Tracker'. It is actually a Solar water heater, used by the inventor to pre heat water. But I find it to be a promising design as a cooker as well. The reflector array of mirrors can be kept out side the kitches, and all of them aiming at cooking vessel inside the kitchen. Carroll uses 28 mirrors, and all of them are linked to two 35 W motors which track the sun throughout the day. Carroll has not published any data as to amount of water heated.
Independent inventors and workers have problem or are not so keen writhing a technical paper, and naturally they remain unknown for a long time. It is with this wish to list all such workers this review has been evolved. This review enables an inventor to add his work directly to this site.
If an agency decides to sponsor the work of the reviewer, he wishes to test each and every such solar cooker especially designed by ‘non technical’ workers, and maintain a detailed database along with this site.
The Mirror Array Cooker project was started in the summer of 2004. The idea is very simple, several small mirrors reflecting the sunlight for the same focal point. I resolved to make a small prototype of only 100 mirrors of 5 x 5 cm each, thus would have a final mosaic of 0.5 x 0.5 m, and be light and portable.
Plane glass mirrors, silvered on the back, are liable to crack if exposed to prolonged direct sunlight. The light passes through the glass twice as it is reflected, which heats a mirror more than a simple glass window through which light passes only once. The present writer, David Williams, has tried using bathroom-type mirrors, about 50x50 cm, in heliostats, and has had them crack several times. Usually, the crack starts from a small chip, drilled hole, or tiny crack, then spreads across the mirror until it breaks apart.
I would make the following suggestions:
- Make sure that your mirror has no chips or cracks. Don't drill any mounting holes in it.
- Use several small mirrors instead of one large one. Small mirrors develop smaller stresses, and are cheaper to replace if necessary.
- Use mirrors with glass that is as thin as possible. Thin glass heats up less than thick glass.
- If possible, use a mirror made of borosilicate glass ("Pyrex"), acrylic plastic ("Perspex" or "Plexiglass"), or metal, instead of window glass. These materials have some disadvantages, but are much less likely to crack.
- If possible, use curved, e.g. parabolic, mirrors instead of plane ones. Curved mirrors can flex, taking up thermal expansion without developing large stresses.
- Try to arrange that nothing casts a shadow onto your mirror at any time of day. If part of the mirror is in sunlight and part in shadow, large differences in temperature are produced, often causing cracking.
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