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Spherical mirrors are the simplest type of concentrator and are easy to build and use. It is easy to focus sun rays, and if one opts for a moving vessel to meet the focus, cooking can also be done very easily. Such a design was suggested for the first time in the year 1961 by Stam (1961). He suggested a large reflector of 4.0 m diameter made of local material which could even include mud, and the reflector surface suitably smoothed with fine mud/cement and coated with aluminized polyester. An appropriate technology handbook describes a simple method of construction of the spherical mirror in the ground (a tall tripod with a long string to which a stone is attached at the tip, will act as a guide for excavating a hollow in the ground) and after finishing and stabilizing the interior, the reflector material could be stuck to make it into a spherical mirror. Such a mirror, of about 2.0 m in diameter, would do useful work for at least five to six hours a day.
The cooking vessel could be hung from the tripod or a suitable stand and positioned to meet the focus. Dan Halacy (1974) suggests a similar design. He uses two full and several half cardboard ribs to fabricate the base and attaches mylar film as a reflector. This device was meant mainly for campers. Bamboo and/or other locally available materials could be used to fabricate such hemispherical baskets.
Recently Prof. Quintone of the United Kingdom has taken up this design and is trying to popularize it in places like Peru. In his beautifully designed and illustrated site he presents detailed instructions on fabricating the design and using it.
The cooker below is a simple steel bowl used for carrying sand, is coated with a reflector foil and a blackened cooking vessel is put in it. The entire assembly is covered over by a flat sheet glass. The design is very similar to Suryakund cited by Kuhnke et al in their book Solar Cookers in the third world. In Suryakund, the vessels are kept in a inverted glass jar. Like Suryakund, this cooker too would suffer from limitation of size.
Unfortunately, this simple design has not attracted much attention, but on a very big scale, like in power generation (as in Marseilles, France), such a hemispherical mirror is being used (Jet Propulsion Laboratory 1981). Scientists of Australia (Anon. 1979) have presented a similar design.
Margaret Koshoni developed the Cone Cooker to suit the needs of Nigerian women. Most people live in flats with balconies; the structure of the balconies will shade the CooKit and make a shadow. The Cone Cooker being placed on a stand has the advantage of elevation and the stand can be moved about without disturbing the cooking.
Medved et al., propose an interesting design (1996) called a 'SOLAR BALL'. It is an inflatable plastic ball with lower part of reflective material. The cooking vessel is kept at the base. It is an interesting variation but there appears to be some serious limitations with reference to size of the ball as well as size and handling of the cooking vessel.
Recently, the spherical geometry seems to have made a come back, and we see that at Auroville in India a 15 meter diameter mirror cooks food for over 1500 persons. A similar large solar bowl was built at the University of Mexico.
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