A Solar Cooker is a device to cook food only by the sun light and it is a kind of solar thermal applications like solar water heater.
More than Three billion poeple in the world (about a half of the global population) rely on firewoods as a heat source. Most of such people use the energy to cook foods. From the energy balance, roughly around 2.5 billion people are not allowed to use conventional energy such as gas or oil and of course the firewoods are not enough to provide energy to all of them. More than 1.5 million poeple dies every year due to the indoor air polution by using the open fire in their houses.
In Africa, woods/charcoals as fuel are very expensive and some poeple are exchanging them with foods or money and that accelerates the deforestation in Africa. And some tree planting volunteers realized that the dissemination of solar cookers is a must to protect the planted trees. Nearly for 20 years, the most powerful NPO in the world, the Solar Cookers International has been introducing the SolarCookit, the panel type solar cooker to the poeple in Africa. Now it is started to circulate creating the jobs among them.
In China, they have a concentrator type solar cooker called "Sangli" which was produced 1.8 million units in number. They are not only using it inside China but also they export it to other countries including Africa.
In India several hundred thousand box-type cookers are in use. And they have several large scale solar cooking system which can provide more than 30 thousand foods a day.1
In the world, there are energetic NPO, NGO's such as SCI, EG Solar, ULOG and so on. Please check them on internet.
The history of solar cooking is very old and a box type solar cooker was found and invented more than 240 years ago in Switzerland. Even in Japan, more than 60 years ago, Mr. Goto invented Goto rice cooker and he introduced it in USA at the 1st International Congress on Solar Energy in Arizona.
Japanese started to use it as educational material several years ago it is mentioned in the science text books or English text books now.
You have to control "Radiation", "Conduction", and "Convection"
Sunlights are reflected by booster mirrors or concentrators to hit inside the box or the pot. The light absorbed changes to the heat, so the outside color of the pot should be black (or the color near to black). If we are allowed to ask for more, even the inside of the pot should be black because heated pot will emit the infrared light to the food inside. That's why you have to control the "Radiation".
Most people think that heat always conduct along the materials and that is the only way for the heat to travel, but in the case of solar cooking, the conduction does not play a very important role. In the case of panel type solar cookers, the light will hit the pot from above mainly and usually the food stays near the bottom of the pot, so the conduction of heat seems important but actually the heat conductivity of the pot does not affect the time to cook foods very much. Do not touch any part of the pot with bare hand after you started solar cooking. For box type solar cookers many people employ black aluminum plates for the inner bottoms of the bodies. On the other hand, for the panel type solar cookers, you'd better put something such as thin wood sticks or three nuts between the pot and the cooker to prevent the loss of heat by condution. And when you cook something like eggs or vegetables, you can cook the food faster by putting a little amount of water in the pot with the food. So, those are the tips concerning "Conduction".
Lastly, the convection. If you do not prevent the convection of the air around the pot, you will lose 35 to 40% of the heat created by the solar cooker. It is a considerable amount of heat especially when the heating power of the cooker is not very high as box type or panel type cooker. For that reason, we use glass lids or double glass rids for the box type cookers and plastic (PP) bags or transparent covers for panel type solar cookers.
All the solar cook in the world agree that there are at least three types of solar cookers, that is, the concentrator type, box type and panel type.
Some people think that there is 4th category for the box cookers which have the reflectors with high concentration as the one invented by Dr. Telkes. They call the category Telkes type after the inventor's name.
3.1 The concentrator type
You can concentrate the light with lenses or concave mirrors. For the solar cookers, there are ones with lenses but most solar cookers have concave mirrors. Usually concentrator type solar cookers have high concentration rates. You do not have to stick to the parabolic shape of the mirror. Spherical mirrors or assembly of planes or cones can have very high concentration rates because pots have finite sizes. For the material of the reflectors, you can use glass mirrors, metal mirrors, aluminum foils, coated aluminum foils, aluminum metallized plastic films, silver cardboards, etc.
Typical concentrator type solar cooker by Mr. Numata.
3.2 The box type
Box type solar cooker has a long history. It was found and invented by swiss scientist, Saussure about 240 years ago. They are relatively easy to make. The main body of a box type solar cooker is a box with high heat insulation with a glass (or double glass) or a transparent plastic lid. According to the designs, some have black insides and others have reflective insides. Most of them have booster mirrors to increase the amount of light to take into the box.
A typical box type solar cooker designed by us.
3.3 The panel type
Panel type solar cookers are made from panels or warped panels. Its simple design makes it possible to be made at low cost. The SCI（Solar Cookers International）boasts their SolarCookit can substitute for a ton of firewoods a year! Also most of panel type solar cookers have relatively high robustness, or simply, the length of time you can leave it without readjustment to the sun. I liked the panel type very much and designed the Educooker003 which you can use all the year in Japan.
A typical panel type solar cooker、SolarCookit by SIC.
3.4 The Telkes type
As mentioned above, it is categorized into box type but has both merits and demerits of concentrator type and box type. The following figure shows the transition from box type to Telkes type.
The most famous Telkes type solar cooker is of course, the cookers by SunOven Co. in USA.
The above photo shows the Telkes type solar cooker made by high school students at our Science Camp.
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