Box cookers
The inside insulator for the solar box cooker has to be able to withstand temperatures up to 150°C (300 °F) without melting or off-gassing. Crumpled newspapers, wool, rags, dry grass, sheets of cardboard, etc. can be used to insulate the walls of the cooker, but since most of the heat escapes through the top glass or plastic, very little insulation in the walls is necessary. The transparent top is either glass, which is durable but hard to work with, or an oven cooking bag, which is lighter, cheaper, and easier to work with, but less durable. If dark pots and/or bottom trays cannot be located, these can be darkened either with flat-black spray paint (one that is non-toxic when warmed), black tempera paint, or soot from a fire.
The solar box cooker typically reaches a temperature of 150 °C (300 °F). This is not as hot as a standard oven, but still hot enough to cook food over a somewhat longer period of time. Food containing a lot of moisture cannot get much hotter than 100 °C (212 °F) in any case, so it is not always necessary to cook at the high temperatures indicated in standard cookbooks. Because the food does not reach too high a temperature, it can be safely left in the cooker all day without burning. It is best to start cooking before noon, though. Depending on the latitude and weather, food can be cooked either early or later in the day. The cooker can be used to warm food and drinks and can also be used to pasteurize water or milk.
Solar box cookers can be made of locally available materials or be manufactured in a factory for sale. They range from small cardboard devices, suitable for cooking a single meal when the sun is shining, to wood and glass boxes built into the sunny side of a house. Although invented by Horace de Saussure, a Swiss naturalist, as early as 1767, solar box cookers have only gained popularity since the 1970s. These surprisingly simple and useful appliances are seen in growing numbers in almost every country of the world.
Panel cookers
Panel solar cookers are very inexpensive solar cookers that use shiny panels to direct sunlight to a cooking pot that is enclosed in a clear plastic bag. A common model is the CooKit. Developed in 1994 by Solar Cookers International, it is often produced locally by pasting a reflective material, such as aluminum foil, onto a cut and folded backing, usually corrugated cardboard. It is lightweight and folds for storage. When completely unfolded, it measures about three feet by four feet (1 m by 1.3 m). Using materials purchased in bulk, the typical cost is about US$5. However, CooKits can also be made entirely from reclaimed materials, including used cardboard boxes and foil from the inside of cigarette boxes.
The CooKit is considered a low-to-moderate temperature solar cooker, easily reaching temperatures high enough to pasteurize water or cook grains such as rice. On a sunny day, one CooKit can collect enough solar energy to cook rice, meat or vegetables to feed a family with up to three or four children. Larger families use two or more cookers.
To use a panel cooker, it is folded into a bowl shape. Food is placed in a dark-colored pot, covered with a tightly fitted lid. The pot is placed in a clear plastic bag and tied, clipped, or folded shut. The panel cooker is placed in direct sunlight until the food is cooked, which usually requires several hours for a full family-sized meal. For faster cooking, the pot can be raised on sticks or wires to allow the heated air to circulate underneath it.
High-temperature plastic bags (oven roasting bags) can be re-used for more than a month, but any plastic bag will work, if measures (such as sticks or wires) are taken to keep the bag from touching the hot cooking pot and melting to it. The purpose of the plastic bag is to trap heated air next to the pot; it may not be needed on very bright, windless days.
A recent development is the HotPot developed by US NGO Solar Household Energy, Inc. The cooking vessel in this cooker is a large clear pot with a clear lid into which a dark pot is suspended. This design has the advantage of very even heating since the sun is able to shine onto the sides and the bottom of the pot during cooking. An added advantage is that the clear lid allows the food to be observed while it is cooking without removing the lid. The HotPot provides an alternative to using plastic bags in a panel
