February 9, 2003
The glazing in the top of a solar box cooker (SBC) lets a great deal of heat escape. Since the glazing is difficult to insulate economically, it is wasteful and pointless to greatly improve the insulation in other parts of the SBC. But what if we turned an SBC upside down? Since the inverted SBC, or ISBC, would take light energy in at the bottom, several heat transfer principles would work better for us than they do in the SBC, making better insulation worth while.
The glazing of an ISBC would be in the coolest part of the cooker rather
than in the hottest part, greatly reducing thermal loss through the glazing.
Most of the light could be made to strike the coolest part of the pot,
the bottom, rather than the hottest part, the top, greatly reducing thermal
loss. The top of the cooker could be well insulated, greatly reducing thermal
loss. Although all light entering an ISBC would have to be reflected,
while much of the light entering an SBC comes directly from the sun, an
ISBC's better insulation and lower heat losses would allow it to heat faster
and hotter on less received light energy.
With appropriate insulation, an ISBC should heat much faster than an SBC which admits the same amount of light, even much faster than an SBC that admits much more light. The reduction of thermal loss would be limited by the necessary thermal radiation loss from the glazing, which is just as hard to insulate well as the glazing of an SBC. There will, however, be much less radiation and other loss from the glazing of an ISBC. In an ISBC the glazing is in the coolest part of the cooker. In an SBC the glazing is in the hottest part of the cooker. There is no significant radiation loss from the glazing of an ISBC until the whole mass of the food is hot. Conduction and convection loss from the glazing can be greatly reduced by recessing the glazing up inside the body of the cooker as shown above. The recess protects the layer of hot air adjacent to the outside of the glazing from exterior air currents. This layer of hot air is held in place against the lower surface of the glazing by the tendency of the the less dense hot air to rise.
The following diagram shows what the reflector system of an ISBC might look like. The reflector has the form of a rough approximation of a parabolic trough. When the trough is wider than the body of the ISBC, the lower opening of the ISBC will be well illuminated for two or three hours. With such a reflector, the ISBC will need to be re-oriented to the sun no more than most SBCs.
An unglazed lower aperture of the cooker would see the cold sky through the reflector system. The thermal radiation loss from the lower aperture of an unglazed ISBC would be greatly reduced if some elements of the reflector system were composed of ordinary glass mirrors instead of aluminum reflectors. Glass mirrors, unlike aluminum reflectors, are black at thermal wavelengths, and would have a radiative temperature intermediate between the hot interior of the ISBC and the cold sky, cutting the radiation losses by almost as much as a glazing would. It seems likely that it would be possible to arrange a modest reflector system, possibly composed of both glass and aluminum reflector elements, that would provide sufficient input power to exceed all the thermal losses of an unglazed ISBC up to very high temperatures. Getting rid of glazing altogether would significantly simplify the construction of the ISBC.
Without a well sealed lower glazing, any air leaks between the lid and the edge on which the lid sits would encourage a continuous flow of air rising up through the cooker from below, being heated, and finding its way out of the leaks in the lid system. To avoid the loss of heat these thermosyphon air flows would produce, the flows must be prevented. Thermosyphon air flows can be prevented by a well sealed lid, or by a well sealed lower glazing, or both. Another way to prevent thermosyphon flows is to make the lid into a large inverted cup, as shown above. Such a shape is easy to make air tight. With this configuration, there will be no thermosyphon losses through the lid, even if the lower glazing is loose (unsealed) or absent.
Although I knew about and referred to Roger Bernard's Nelpa solar panel cooker when I wrote the first version of the above description of the ISBC, a reader, Darwin Curtis, sent me a photograph of a drawing from a book by Roger Bernard that shows that he had thought of something that is more similar to the ISBC than the Nelpa is, although it is still closer to the Nelpa than to the ISBC. The drawing clearly shows a flat tilted glazing (vitre) on the bottom of the housing of the pot (récipient) and insulation (isolation) on the sides of the pot-housing. Although there is no insulation over the top of the pot (récipient), and the glazing is slanted and unprotected from breezes, most of the components of the ISBC are there. This design, and the Nelpa, seem to have resulted from a different goal than the one I followed to get to the ISBC, resulting in several differences of detail and emphasis. Bernard's goal seems to have been primarily ergonomic, with easy access to the pot, being a primary requirement. My goal was better thermal performance. In fact, the real genesis of my ISBC was my curiosity about whether it would be possible to build an effective solar cooker without a glazing, a question that might be answered in the affirmative through an experiment with an ISBC. Also, my design was generated by mentally inverting an SBC, whereas the Nelpa-like designs seem to have been generated by elevating a greenhouse pot in a panel cooker. To do away with the glazing, something more similar to the ISBC described here than to the Nelpa-like designs would seem necessary. More generally, the ISBC described here has possibilities for the design of large community ovens that the Nelpa-like designs do not.
Prof. Ajay Chandak has developed a solar box cooker with glazing and reflectors for the top and bottom. He may be seen with his cooker here.
Roger Bernard, "Construisez Votre Cuisinière Solaire," (Lyons: Editions Silence, 1997). Page 6
Photograph by Prof. Ajay Chandak, PRINCE (Promoters & Researchers In Non Conventional Energy), Jankibai Trust, Shamgiri, Agra Road, Deopur, DHULE: 424 005. INDIA. email@example.com