Types of Solar Ovens
Solar ovens are simple devices with four basic parts. They have a stand, a cooking pot, a greenhouse device, and a reflector panel. Well, they should be simple, but some have made it much fancier than it needs. We will start with the most basic and show how one can really mess it up, oops, I mean improve on it.
Probably the simplest device is to fill a clear 2L Soda bottle with water and put it on a piece of corrugated Iron. You can use a south facing roof (whenever I say south, you in the southern hemisphere think north) or put the corrugated iron on a southern slope. The bottle sits in a valley in the iron and is held in place with a rock. The sun reflects off the shiny Al-Zn coating and is concentrated in the bottle. This effectively heats and sterilizes the water.
Some of the best improvements on this simple oven are cheap and practical even for a mobile family. Each of the four parts can be changed as the ovens become fancier. Instead of cheap ovens for third world countries, you might want to build a solid oven for your own use. You can even attach it to a south facing wall and access the pot from inside your house. Thus we start simple and get fancy.
The simplest change for the stand, our first part, is to use a plastic bucket half filled with rocks. The bucket can serve double duty for other functions. Others use a simple wire stand, while the fanciest have a deluxe wooden stand. Many can be rotated 180 degrees so the oven sits at a 30 degree angle to face the low winter sun and a 60 degree angle to face the high summer sun. http://solarcooking.org/plans/inclined-box-cooker.htm
The next section is the Pot which can be a simple affair. A cheap pot (the thinner the better for quick heating) is blackened. One can paint it black or use a black enamel pot, but rubbing it with charcoal is simple and effective. A black pot absorbs the most solar energy and any blackening will work effectively. Some places sell stacking pots. http://www.angelfire.com/80s/shobhapardeshi/ParvatiCooker.html
Since most of the heat comes from the side, these work better than pots side by side especially when several items are cooked simultaneously. A very inflexible system is to have a board with a circular hole in it that fits only one pot. If you lose or ruin the pot, the whole system is useless. http://www.mueller-solartechnik.com/manuals/primrose.htm If you want speed of cooking and durability, it is one to be considered.
Most systems use the third part, a greenhouse device. Greenhouses work on the principle of letting light in where it is converted to heat, but keeping the heat from going back out. One can simply put the pot in an oven bag. http://solarcooking.wikia.com/wiki/Reflective_Open_Box The bag is then reusable for a month. Some use regular plastic, but it would melt if it touched the hot pot. To solve this problem, a wire frame is used to keep the pot away from the plastic. Still, a new bag is needed frequently.
The quality of the greenhouse device is especially important when a small reflector is used, an important consideration in windy locales. Instead of a plastic bag, one can line a cardboard box with aluminum foil, put the pot in and cover the box with plastic. Fancier greenhouse devices use two cardboard boxes with insulating material in between them (newspaper or pieces of cardboard) http://solarcooking.wikia.com/wiki/Minimum_Solar_Box_Cooker . Some cover the top with glass. Glass, though fragile, traps heat better. A double paned glass top is really the cat’s meow for the greenhouse effect. http://solarcooking.org/plans/tire_eng.htm
The heart of the system is the reflector. Here we want the opposite of the black pot, the shiniest material. The device must cope with a sun that moves position constantly during the day and a sun that is at a different angle in winter than in summer. http://www.solarovens.org/
A very simple approach is to use more cardboard and aluminum foil. A back reflector and two side reflectors can be used with the boxes just mentioned or with the pot in a bag that sits on a bottom reflector. The angle of the back and side reflectors need to be adjustable for winter or summer. The side reflectors need to be spread wide enough so one position can be used for the cooking process. If you are cooking a rice dish while working in the yard, the dish might finish cooking in an hour, but stay hot for another 2-4 hours. This setup will not burn the food and you eat at your convenience. http://www.backwoodshome.com/articles/radabaugh30.html
Of course, the best system would be a parabolic system. Imagine a car headlight. The light bulb sits at the focal point of the parabola. This focuses most of the light into a beam straight ahead. The sun’s light works in reverse. It comes straight and if it hits a parabola, automatically concentrates at a focal point. http://solarcooking.org/espanol/cocina%20parab%C3%B3lica%20plegable.pdf (in Spanish, but the picture shows the principal) A pot at the focal point can get hot quickly. The problem is that this style reflector must be repositioned twice an hour. There are several simple portable devices that can approximately make a parabolic reflector. One just uses one of those silvery car windshield reflectors. http://solarcooking.wikia.com/wiki/Windshield_shade_solar_funnel_cooker With some Velcro, it can be bent into a funnel/parabolic shape. Another system uses an umbrella coated with aluminum on the inside. http://solarcooking.org/plans/paracuina.pdf
More permanent systems cut flat sheets of aluminum into wedges and bolt them together. A well made system can heat a pot so fast that no greenhouse device is needed around the pot. Of course, the food can burn as can anything else at the focal point such as your hand or your eye.
A system that uses a wooden stand also uses glass mirrors. It is so efficient that they warn you to protect your eyes when you remove or replace the pot. It also warns you that the wood stand can catch fire if not protected. This might be a tad too efficient.
Most systems are promoted for third world country use to save forests and to save people money. As we become more environmentally conscious, run out of oil, and want to save money, we might also want to consider a more durable oven than one built out of cardboard. There are several good options to consider even if you do not live in a desert.