Figure out what you will normally want to cook.
Figure out how quickly you need to cook the food.
Figure out how many btu's you need to cook the food. (As a very rough rule of thumb, figure on needing about 150 btu's per hour per pound of food to cook food in "normal" time periods.)
Depending on your latitude, the cooker will receive between 200-300 btu's per square foot per hour. It's about 300 btu's near the Equator and closer to 200 btu's near 45 degrees North and South Latitude. Only approximately 30% of this energy will actually cook the food.
Size your solar cooker accordingly. For example: A solar cooker with a total of 10 square feet of sunlight directed into the box,
has about 2,000 btu's per hour entering the box when used near Aprovecho. The cooker is only about 30% efficient so about 660 btu's will actually cook the food per hour. 660 btu's divided by 150 equals 4.4. Therefore, about 4.4 pounds of an easily-cooked food (including the weight of the water) can be made per hour in this cooker. Hard-to-cook items will require more time. To accomplish this type of powerful cooking, make sure that:
A.) The glass is as perpendicular to the average position of the sun as possible. With the Telkes cooker, aiming the glass directly at the sun will optimize the effectiveness of the reflectors. The maximum amount of sunlight will be reflected down into the box. Take into account whether more cooking is done during the summer or winter.
To figure out the angle of the sun above the horizon, subtract the local latitude from 90. Then add 23.5 degrees to the result. This will give you the highest sun position in the summer. Subtract 23.5 degrees from the result to find the sun's lowest position in winter. For example, the latitude at Aprovecho is 44 degrees north. 90 minus 44 equals 46.46 plus 23.5 is 69.5 degrees. This is the highest point reached by the sun in summer.
46 minus 23.5 is 22.5. The sun only reaches 22.5 degrees above the horizon in the dead of winter. Since Aprovecho does most of its solar cooking in the middle of the summer, we place the glass in the Telkes cooker at 60 degrees. Aim the glass in your cooker at the most advantageous angle.
B.) The reflectors are at 120 degrees to the glass. Another way to say the same thing is that the reflectors are positioned at 30 degrees to the incoming rays of light. The reflectors can be about as long as the glass surface they face. Longer reflectors will only aim sunlight at the opposite reflector. (See diagram A.)
C.) Make the box well insulated, as airtight
How to Construct a Telkes Solar Cooker
Make the box first. Begin by cutting out all
of the pieces. In this case we used 1/2 inch
plywood.
as possible and, if economically feasible, use double panes of glass with a small air gap between the two panes to increase the insulative value of the glass cover. This is especially important in cold, windy climates. Insulation made up of cardboard and aluminum foil works well. Good insulators also include wood ash, charcoal, rigid foam, etc. (See notes on insulation in the Rocket Stove section, pp. 20-23.)
D.) In a Telkes type cooker, paint the inside flat black. Have the black pot rest on a black metal floor so that we take advantage of conduction and convection. (Any part of the interior of the box can be shiny instead of black, if sunlight will hit the surface and be directed at the black pot. Then the pot is most efficiently absorbing the solar energy. In some situations, however, shiny interior walls reflect sunlight out of the box through the glass.)
E.) Since hot air rises, the best theoretical place for the door is on the bottom of the cooker. However, this may be difficult. The Telkes cooker has a back entrance door.
F.) Remember that heat and temperature are two different things. Heat is the amount of btu's available for cooking. The internal temperature of the box is only the point in degrees where the amount of heat gained and lost equalizes. It's important to have a high enough temperature and enough heat energy for cooking to occur. A large Telkes model will be a powerful solar cooker. Solar cookers with less square footage of reflector area will cook at lower temperatures, over a longer period of time.
G.) The square footage that is multiplied by 200 to 300 btu's per hour is not the square footage of the reflectors! It is, instead, the square footage of the intercepted sunlight. (See diagram A.) The Telkes cooker is only one of many. But all designs work in the same manner. Each receives the same 200 to 300 btu's per hour depending on latitude. It is easy to figure out in advance how much energy is available in any design for cooking.
The accompanying diagrams show how to build a medium to large Telkes cooker. This cooker is made out of plywood or sheet metal. In the U.S., it's easiest to make the stove from these materials. It is important to remember that a high mass stove body will absorb a lot of heat that could have initially gone into the cooking process. (A heavy pot will do the same. See "Insulation and High Mass in Stoves (and Houses)", pp. 22-23.) A lower mass oven will heat up much more quickly. The higher mass stove will only assist in the cooking process when the pot is cooler than the stove body. It's important to insulate the stove body from the stove interior by using low mass, highly insulative or reflective materials. A solar cooker made from bricks or earth without interior insulation will waste energy in heating up the stove instead of the food. A solar cooker is also relatively low powered so it's usually better to use the energy directly without storing heat for later use.
It is also possible to make the body of the stove out of scrap metal, such as metal containers. We are confident that it is quite possible to make both the stove body and reflector from any sort of wired together cans or found metal.
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