Solar cooker
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A solar cooker or solar oven is a way of harnessing the sun's power to cook food. A metal box forms the simplest solar cooker. A set of large mirrors or a large Fresnel lens to focus sunlight to a single point may also be added. On a sunny day a black baking tray or cooking pot can convert thousands of watts of light into heat, in addition to any infra-red. Temperatures in a typical oven can reach 400 degrees F (200°C).
Apart from the obvious need for sunlight and the need to aim the solar oven before use, using a solar oven is not substantially different from a regular oven. However since they use no fuel they are free to run, humanitarian organizations are promoting their use worldwide to help slow deforestation and desertification caused by the need for firewood with which to cook.
The first known western solar cooker was built by Horace de Saussure in 1767.
Solar box cooker
A solar box cooker is an insulated box with a transparent top and a reflective lid. It is designed to capture solar power and make use of the greenhouse effect to cause heat to accumulate inside. The top can usually be removed to allow dark pots containing food to be placed inside. The box usually has one or more reflectors with aluminum foil or other reflective material to bounce extra light into the interior of the box. Cooking containers and the inside bottom of the cooker should be dark-colored or black. The inside walls should be reflective to reduce radiative heat loss and bounce the light towards the pots and the dark bottom, which is in contact with the pots. A good example of this type of cooker is the "Minimum" Solar Box Cooker.
The inside insulator for the solar box cooker has to be able to withstand temperatures up to 150° C 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 tray cannot be located, these can be darkened either with flat-black spray paint (one that is non-toxic when dry) or black tempera paint.
The solar box cooker typically reaches a temperature of 150° C; not as hot as a standard oven, but still hot enough to cook food over a somewhat longer period of time. It should be remembered that food containing moisture cannot get hotter than 100° C in any case, so it is not necessary to cook at the high temperatures indicated in standard cookbooks. Because the food does not reach too high of 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 is also used to warm food and drinks but 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.
Solar panel cooker
The solar panel cooker is the simplest form of solar cooker yet developed. Solar panel cookers have multiple simple reflectors arranged to focus solar radiation onto a covered black pot enclosed in a clear heat-resistant plastic bag or other clear enclosure. The original design by Dr. Roger Barnard of France used an open, foiled box with a dark, lidded pot of food in the focal point insulated by an inverted glass salad bowl. From Dr. Barnard’s basic design, Barbara Kerr, an Arizona housewife, developed a foldable model suitable for backpacking. Working from that model, Bev Blum, the director of Solar Cookers International, joined with volunteer engineers to develop a mass-producible solar panel cooker, the CooKit. The CooKit has found acceptance in refugee camps as well as for recreational and teaching uses and in households in the United States. Thousands of families in the Kakuma Refugee Camp in Kenya and the Aisha Refugee Camp in Ethiopia have taken up this form of solar cooking to save both firewood and money.
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.
Popularity and rejection
Diffusion of innovations theory, discussed in the book of the same name by Everett Rogers, discusses adoption of solar ovens in Lesotho. Although solar ovens were safer and cheaper to use than oil burning ovens, Lesotho women resisted using them. Agencies such as the Peace Corps have tried to use "change agents" to influence the women. But these change agents have differed from the women in class and norms and were not effective in spurring change. Researchers have found that the diffusion process works better if the change agent trains a local aide who belongs to local social networks and can better influence locals.