David M. Delaney, December 4, 1999.
A greenhouse pot integrates a black metal cooking pot and a transparent greenhouse into a convenient assembly for use in a solar panel cooker.
Roger Bernard was the first to hang a black metal pot in a glass bowl with the lid of the pot exposed to allow vapor to escape, and to facilitate access by the cook. [Bernard1]. The resulting pot and greenhouse can be made more convenient by designing the glass bowl and metal pot specifically for each other, putting handles on the glass bowl and no handles on the metal pot [Delaney1].
An additional greenhouse built on the lid of the pot [Delaney1] has been shown to perform no better than simple glass and metal lids [Delaney2]. This is a fortunate result, because the simple lids are less expensive to make, to design, and to tool for manufacture. While doing these experiments, I found the use of a greenhouse pot to be as convenient and advantageous as I had hoped. Greenhouse pots do avoid condensation, and do allow easy access to the cooking food. I also found that greenhouse pots do not need greenhouse lids to be effective.
Greenhouse pots are not currently manufactured, and will not be widely available for some time. This article describes some improvisations for the meantime.
In experiments in the summer of 1999, [Delaney2] I built greenhouse pots by searching in shops for plain steel and glass bowls that fitted well together. The result was usable, and distinctly more convenient than plastic oven bags. I used bowls, both glass and steel, that had no handles. I would have liked small handles on the glass bowls, but I believe that having no handles at all is to be preferred to handles on the steel bowls, at least for smaller pots. There should be at least 5 cm ( 2 inches) of air space between the bottom of the steel bowl and the bottom of the enclosing glass bowl, to allow for light to bounce from the reflective floor of the cooker and strike the bottom of the metal pot. A shallower space will still work, if less well, but there should be no contact between the bottom of the steel pot and the bottom of the glass bowl which would allow conductive heat loss.
I do not recommend any of the specific lid designs that I compared last summer [Delaney2]. All of these lid designs were too complicated. Although the complexity was necessary for accurate thermal comparison of the different lid principles, much simpler lids are more suitable for cooking.
If you find a metal pot that has its own lid and fits an available glass bowl, you don't have a lid problem. If the only suitable metal bowls you can find have no lids, (plain stainless steel mixing bowls are good candidates) you can improvise a lid, either from a disk cut from a flat piece of metal, or from a disk of window glass. Window glass can be cut into disks with inexpensive tools. Businesses that cut glass for windows often have the capability to cut glass disks to a desired diameter, and usually quite inexpensively.
Ernst Goetz, [Goetz1] has proposed a neoprene or silicone rubber ring to be manufactured as a separate item to provide a soft bumper for the edges of lids for solar pots made from glass disks, whether manufactured or improvised. The ring is slotted all the way around its length to fit onto the edge of the glass disk, and is cemented in place with rubber cement.
What if you cannot find metal and glass bowls that fit each other well? Roger Bernard has recently published a useful little book, La Cuisson Solaire Facile, [Bernard2] that describes a good technique for constructing a greenhouse for a metal pot that is a little too small for the best available glass bowl. Since the book is in French, I give fairly complete description of his solution. (If you are interested in solar cooking and read French, you should definitely read this book.)
Bernard introduces an adapter ring to suspend the metal pot in a glass or plastic bowl that is slightly too large. He cuts the adapter ring from cardboard. The central hole of the ring is just large enough to suspend the metal pot. See the diagram below. The difference between the inner and outer diameters of the cardboard ring should be as small as possible, of course, to keep its shadow as small as possible. The diagram shows a thick ring for clarity. You won't have much control over this, however, unless you have a choice of pots and bowls of different diameters.
The adapter ring does not have to be made of cardboard if some more durable material is available. You could attach some sort of lip, or circular guide, to the adapter to center it in the glass bowl.
If the adapter ring could be made of clear glass, there would be no shadow. I do not recommend trying to cut such a ring from window glass. Even if you succeeded in forming the ring, it would be too fragile for use. A suitable ring could be easily cut from clear plastic sheet with simple and inexpensive tools, however.
A clear plastic adapter ring for a specific combination of a pre-existing
pot and a glass bowl could be manufactured by simple methods from flat
plastic sheet stock. Using an annular "cookie cutter" heated on a
stove, a worker would cut rings from the sheet stock and pass them to another
worker to finish the edges. Disks for lids could be produced
in the same way. The disk cut out to form the center hole of the
adapter ring could form the lid for the pot for which the ring was intended.
[Bernard2] Bernard, Roger, "La Cuisson Solaire Facile", Editions Jouvence, 1999, ISBN 2-88353-172-2. 91 pages.
[Delaney1] Delaney, David M., Pot with Integrated Greenhouse for Solar Panel Cooker, March 28, 1999, http://www.geocities.com/~dmdelaney/intgrnhouse/conv-pot.htm
[Delaney2] Delaney, David M., Comparison
of Lids for Greenhouse Pots for Solar Panel
Cookers, November 15, 1999, http://www.geocities.com/~dmdelaney/lidcompare/lidcompare.htm
[Goetz1], Impact resistant glass lids for pots
for solar cookers, September 1999, Ernst Goetz, Herrenmattstrasse
11, 4132 Muttenz, Switzerland, firstname.lastname@example.org