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Solar Cooker Design and Construction Guide

Why Solar Cooking?

Solar cooking is used widely in developing countries as an alternative to wood, charcoal, coal, and dung fires, all of which create environmental problems for the country, and health problems for the cook, and they are expensive.

Solar cookers are also "environmentally-friendly" and non-polluting; food will not burn; fewer problems with boil-over; keeps house cooler and reduces summer cooling costs; economical — cookers can be made from recycled material and cost nothing to operate; safe — no risk of fire

Solar cookers are easy to make and even easier to use. The simplest design can be made in less than 1/2 hour and will cook a hot-dog or tray of cookies in under an hour! A larger panel cooker or a box cooker will cook a pot of dried beans in less than a day, and there is no need to "watch the pot" — just place it in the cooker in the morning and return in the evening.

With an understanding of basic principles of solar energy and access to simple materials such as cardboard, aluminum foil, and glass, one can build an effective solar cooking device. Whether the need is to cook food, pasteurize water, or dry fish or grain, the basic principles of solar, heat transfer, and materials apply. We look forward to the application of a wide variety of materials and techniques as people make direct use of the sun's energy.

Heat Principles

A solar box cooks because the interior of the box is heated by the energy of the sun. Sunlight, both direct and reflected, enters the solar box through the glass or plastic top. It turns to heat energy when it is absorbed by the dark absorber plate and cooking pots. This heat input causes the temperature inside of the solar box cooker to rise until the heat loss of the cooker is equal to the solar heat gain. Temperatures sufficient for cooking food and pasteurizing water are easily achieved.

Given two boxes that have the same heat retention capabilities, the one that has more gain, from stronger sunlight or additional sunlight via a reflector, will be hotter inside. Given two boxes that have equal heat gain, the one that has more heat retention capabilities — better insulated walls, bottom, and top — will reach a higher interior temperature.

Materials Requirements

Structural material. Structural materials are necessary so that the box will have and retain a given shape and form, and be durable over time. Structural materials include cardboard, wood, plywood, masonite, bamboo, metal, cement, bricks, stone, glass, fiberglass, woven reeds, rattan, plastic, papier mache, clay, rammed earth, metals, tree bark, and cloth stiffened with glue or other material. Many materials that perform well structurally are too dense to be good insulators. To provide both structural integrity and good insulation qualities, it is usually necessary to use separate structural and insulating materials.

Insulation. In order for the box to reach interior temperatures high enough for cooking, the walls and the bottom of the box must have good insulation (heat retention) value. Good insulating materials include: aluminum foil (radiant reflector), feathers (down feathers are best), spun fiberglass, rockwool, cellulose, rice hulls, wool, straw, and crumpled newspaper. When building a solar cooker, it is important that the insulation materials surround the interior cooking cavity of the solar box on all sides except for the glazed side — usually the top. Insulating materials should be installed so that they allow minimal conduction of heat from the inner box structural materials to the outer box structural materials. The lower the box heat loss, the higher the cooking temperatures.

Transparent material. At least one surface of the box must be transparent and face the sun to provide for heating via the "greenhouse effect." The most common glazing materials are glass and high temperature plastics such as oven roasting bags. Double glazing, using either glass or plastic, affects both the heat gain and the heat loss. Depending on the material used, the solar transmittance — heat gain — may be reduced by 5-15%. However, because the heat loss through the glass or plastic is cut in half, the overall solar box performance is increased.

Moisture resistance. Most foods that are cooked in a solar box cooker contain moisture. When water or food is heated in the solar box, a vapor pressure is created, driving the moisture from the inside to the outside of the box. There are several ways that this moisture can travel. It can escape directly through box gaps and cracks or be forced into the box walls and bottom if there is no moisture barrier. If a box is designed with high quality seals and moisture barriers, the water vapor may be retained inside the cooking chamber. In the design of most solar box cookers, it is important that the inner-most surface of the cooker be a good vapor barrier. This barrier will prevent water damage to the insulation and structural materials of the cooker by slowing the migration of water vapor into the walls and bottom of the cooker.

Design and Proportion

Box Size of a Solar Box Cooker. The size should allow for the largest amount of food commonly cooked. If the box needs to be moved often, it should not be so large that this task is difficult. The box design must accommodate the cookware that is available or commonly used.

Solar Collection Area to Box Volume Ratio. Everything else being equal, the greater the solar collection area of the box relative to the heat loss area of the box, the higher the cooking temperatures will be. Given two boxes that have solar collection areas of equal size and proportion, the one that is of less depth will be hotter because it has less heat loss area.

Solar Box Cooker Proportion. A solar box cooker facing the noon sun should be longer in the east/west dimension to make better use of the reflector over a cooking period of several hours. As the sun travels across the sky, this configuration results in a more consistent cooking temperature. With square cookers or ones having the longest dimension north/south, a greater percentage of the early morning and late afternoon sunlight is reflected from the reflector to the ground, missing the box collection area.

Reflector. One or more reflectors are employed to bounce additional light into the solar box in order to increase cooking temperatures. This component is optional in equatorial climates but increases cooking performance in temperate regions of the world.

Solar Box Cooker Operation

One of the beauties of solar box cookers is their ease of operation. For midday cooking at 20 N - 20 S latitude, solar box cookers with no reflector need little repositioning to face the sun as it moves across the midday sky. The box faces up and the sun is high in the sky for a good part of the day. Boxes with reflectors can be positioned toward the morning or afternoon sun to do the cooking at those times of day. Solar box cookers used with reflectors in the temperate zones do operate at higher temperatures if the box is repositioned to face the sun every hour or two. This adjustment of position becomes less necessary as the east/west dimension of the box increases relative to the north/south dimension.

Cooking Tips

Most foods can be cooked in a solar cooker. They will cook more slowly (similar to a crock pot), but will cook just as well. Deep-frying does not work well (the oil does not reach a critical temperature). As with any stove or oven, you should be sure that the food is cooked through before eating. This is especially true for meats and eggs.

Hints on Specific Foods:
·  Dry cereals, grains — barley, corn, millet, oats, quinoa, rice, wheat: Start with usual amount of water. Next time adjust to your taste. Some people heat water and dry cereal/grain in separate pots before putting together to get the preferred texture.
·  Fresh vegetables — add no water. Most green and yellow vegetables — 1 to 1-1/2 hours. If cooked longer they lose their nice green color. Try cooking corn on the cob with husks in a clean black sock. Beans (dried) — 3 to 5 hours. Add usual amount of water. Presoaking shortens cooking time. Some types of beans, especially in large quantity, may take two days. Most root vegetables — 3 hours.
·  Eggs: Cook in shells without water 1-2 hours. With longer cooking whites may turn brownish, but flavor is fine.
·  Meats: Add no water. The longer they cook, the more tender they become.
·  Pastas: Heat water in one pot and dry pasta with a bit of cooking oil in another pot. Heat both until water is near boiling. Add hot pasta to hot water, stir, and cook about 10 minutes more.
·  Baking: is best done in the middle of the day (between 10 am and 2 pm). Cookies don't need a cover. Bake bottom crusts (pies, pizzas) alone and heat fillings separately, adding just before eating.
·  Water, milk — pasteurization: 1 liter takes about 1 hour (4 liters/1 gallon about 4 hours). With a WAPI (water pasteurization indicator) there's no guesswork.
·  Sauces/gravies (made with flour or starch): Heat juices and flour separately, with or without a bit of cooking oil in the flour. Then combine and stir. It will be ready quickly.
·  Roasting nuts: Bake uncovered. Almonds — 1 hour, peanuts (groundnuts) — 2 hours.

Suggestions for Making your Design Hotter

To make your cooker hotter you can (1) increase the amount of energy reaching the food, and (2) reduce the amount of heat energy lost.

Add reflectors. Additional reflective surfaces around the cooker can increase the amount of solar energy reaching the food.

Adjust orientation. Keep the cooker oriented toward the sun. Rotating most cookers every two hours is good enough for normal cooking. An orientation guide can help (such as the shadow produced by a nail placed at the front of the cooker).

Insulate. Insulation helps reduce heat loss. This is most important for partly-cloudy days when the food will continue to cook without sun if the cooker is well-insulated. Well-insulated box cookers can even continue cooking at night: simply place a pillow on top of the window. In the panel cooker design, the insulation is the layer of air between the plastic bag and the pot or jar.

Suggestions for Making your Design Cheaper

To make your cooker more cheaply, you can (1) use recycled materials, (2) use cheaper materials, and (3) keep it simple. Many designs can be made completely from recycled materials.

Panels and walls: cardboard boxes, wood scraps from construction sites, discarded containers (5 to 50 gallon drums).

Windows: glass and oven bags are best; clear plastic bags, food wraps (Saran) and soda bottles can also be used if the plastic will not come into contact with the pot or jar.

Reflectors: foil liners from some cracker, cookie and breakfast cereal boxes, interior of Pringles cans and juice boxes.

Insulation: crumpled newspaper or scrap paper, dry leaves, air space between window and food.

Glue: white (school or carpenters) glue or flour paste (mix 1/4 cup white flour with 1/2 cup cool water; pour into 1 cup boiling water and stir until thick).

Mark Aalff, Solar Box Cookers Northwest
523 18th Avenue East, Seattle WA 98112
ph 206-328-0832

Solar Cookers on the World Wide Web: Solar Cooking Archive —

Most of the information presented here was provided by Solar Cookers International (SCI), "a nonprofit organization spreading solar cooking to benefit people and environments worldwide." SCI has a wide variety of educational materials (including teachers' guides, cookbooks and cooker plans and kits) available in English, Spanish and French. The organization is run almost entirely by volunteers and materials are provided at cost. To order materials or learn more about SCI, contact:
Solar Cookers International
1724 1lth St., Sacramento CA 95814
ph 916-444-6616, fax 916-444-5379

A profusely-illustrated and much larger printed version of this publication, with sample plans and construction tips, is available from El Paso Solar Energy Association; PO Box 26384, El Paso TX 79926;


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