Cooking with Sunlight: Learn How to Cook Food With Solar Cookers

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A parabolic cooker, such as the one above, cooks nearly as quickly as a conventional oven.
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This box cooker, made of wood, is used in Sri Lanka.

When Louise Meyer and Dar Curtis invite guests over for dinner on a late summer afternoon, all the cooking is done outdoors. There are no charcoal grills or other fossil fuel-burning stoves to heat up and clog the air. Instead, appetizers, entrée, and dessert are all prepared with abundant, free, clean energy from the sun.

Find out how to build and use your own solar oven.

Meyer and Curtis use special ovens called solar cookers to prepare their food. With eight people expected for dinner, they have four solar cookers at work on Curtis’s patio. The food is ready one or two hours before dinner but stays warm in the cookers until the chefs transfer it to a buffet table where guests help themselves. The evening’s offerings: chicken baked in its own juices, ratatouille, Italian peppers–“slightly overdone,” Curtis allows–and to top everything off, banana cake. “Perfect!” Meyer proclaims, noting the dessert is moist on top yet browned around the edges.

Meyer and Curtis, both of the Washington, D.C., area, are partners along with Minneapolis sociologist Barbara Knudson in Solar Household Energy (SHE), a nonprofit organization that offers an alternative to wood-burning fuel in developing countries where trees are overharvested. Solar cookers, or ovens, also are useful in the United States as easy, practical, and economical ways to bake, braise, or stew food. Not only are they an earth-conscious cook’s best friend, but their outdoor location conserves energy. “In summer, you don’t have the air conditioner fighting with the oven to keep the house cool,” Meyer says.

Solar Cookers International (SCI), a nonprofit organization assisting communities to use the power of the sun to cook food and pasteurize water for the benefit of people and environments, traces solar cooking to the late 1700s, when European naturalist Horace de Saussure set out to show that a place is hotter when the sun’s rays pass through glass. He built a miniature greenhouse out of five glass boxes stacked inside each other on a black table. The outermost box stayed cooler, and the smallest box recorded the hottest temperature, 189.5 degrees Fahrenheit. “Fruits… exposed to this heat were cooked and became juicy,” he wrote. De Saussure experimented with other “hot boxes” and was able to achieve cooking temperatures that reached 230 degrees Fahrenheit regardless of the outside climate. He couldn’t explain how the hot boxes cooked the food, but physicists today say the phenomenon will work in any glass container exposed to the sun.

A solar cooker is a device that directs sunlight onto a dark-colored cooking pot, maximizing the amount of light energy that reaches the pot and minimizing the amount of heat loss. The essential elements of a solar cooker are sunlight, a black pot, reflectors, and a glass-covered box or a clear plastic bag. The black pot attracts the sun’s rays while the panel reflectors amplify them. Meanwhile, the glass or plastic prevents the heat from escaping, thus creating a “greenhouse” inside the solar cooker. As inventors and ecologists have experimented with hot boxes over the years, a variety of solar cookers have become available.

The three basic types are:

1. Panel: The most affordable solar cookers rely on foil-covered panels or reflectors that concentrate sunlight onto a pot surrounded by a plastic bag. SCI, which has been selling panel cookers for $20 each since about 1994, markets its products internationally, with about 100 buyers per month. The “CooKit” includes plastic oven bags, a single sheet of foil-covered cardboard that folds to form the base and four reflective panels, and an instruction pamphlet that includes recipes. The panel cooker folds to the size of a three-ring binder and weighs less than two pounds. Cooking pots are not included; SCI sells them for ten dollars each, or they can be found at many stores. Lightweight, dark Granite Ware generally works best. Metal pots painted black on the outside or blackened in a fire also work well.

2. Box: More elaborate, box-shaped cookers are made from a variety of materials including cardboard, wood, bamboo, metal, stone, fiberglass, clay, tree bark, and cloth stiffened with glue. Insulation, which is essential for the cooker to reach hot temperatures, can be made from foil-lined cardboard, down feathers, spun fiberglass, crumpled newspapers, or any number of other materials. A glass panel covers the box, and reflectors on one to four sides direct the sun’s rays to the inside of the cooker. Illinois-based Sun Ovens International has manufactured a common box cooker, the Global Sun Oven, for the past fifteen years. About thirty dealers around the world, including SCI, sell the Global Sun Oven for about $229 each. Sales have remained steady, says Paul Munsen, president of Sun Ovens International, but tend to peak during energy brownouts or blackouts. The Sun Oven box cooker is sturdier than the cardboard panel cooker and gets hotter, so food cooks faster, which means it can be used during more months of the year.

3. Parabolic: Consisting of a concave disk that focuses light onto the bottom of a pot, parabolic cookers cook about as quickly as a conventional stove. In fact, Curtis says the temperature inside the pot on a good parabolic cooker can actually fry food, which is not possible with the other cookers. The reflectors, he explains, direct the sun’s energy onto a small focal area within the cooker with such intensity that a rolled-up newspaper would burst into flames. Parabolic cookers can cause burns if used incorrectly, and, because the concentrated sunlight heats the pot quickly, food must be stirred and watched carefully. Parabolic cookers cost $100 or more and can be ordered through E.G. Solar, a German company.

Solar cooking made easy

Catch some rays. On a clear day with strong sun, food can be put in the cooker in the morning and left all day until it’s time for the evening meal. Just place the cooker so that it faces the sun’s position in the early afternoon sky, and the food will cook slowly and at an average temperature of about 225 to 275 degrees Fahrenheit, depending on the quality and type of cooker. “Some higher-quality cookers can even cook on partly cloudy days or during the winter months, but temperatures will generally be somewhat lower, and it is best to rotate the cookers more frequently to track the sun,” says Kevin Porter of Solar Cookers International.

Cloudy days. The cookers’ efficiency depends more on the intensity of the sun than on the outside air temperature. Food will not cook on days when clouds completely block out direct sunlight, yet Dar Curtis, director of Solar Household Energy (SHE), cooked a chicken at his Washington, D.C.-area home one Christmas. “It was one of those cold, bright days with no passing clouds,” he says.

Timing is everything. It’s not always easy to tell when food is ready, and people new to solar cooking will have to test food and rely on trial and error. As a guideline, Curtis and his one of his partners in SHE, Louise Meyer, say it takes about 50 percent longer to cook food in a solar cooker than in a conventional oven. And like a gas or electric oven, a solar cooker cooks some foods faster than others. A three-pound chicken, for example, may be ready in two hours, whereas stews may need five or six hours. Bacteria growth in the food is not a concern as long as the cooker stays hot. Food is pasteurized at 160 degrees Fahrenheit, Curtis explains, and begins cooking at 180 degrees.

Here’s the dish. Panel cookers usually hold only one pot, which can cook food for up to five or six people. Many box cookers, though, will hold several pots, allowing you to cook more than one dish at a time. The pots should preferably be thin metal to allow for the most efficient transfer of heat; they should also have a dark-colored, tight-fitting lid.

Keeping it warm. Even after you finish cooking in your solar cooker, you can still keep it warm. In a box cooker, the heat is trapped inside and retained even after cooking is finished. A CooKit, in which the black pot is contained by a plastic bag, will not retain its heat for very long. You can, however, transfer the cooked dish into a Hay Box or Heat Retention Box that will keep the food warm. A Hay Box is a box insulated with hay, straw, wool, or feathers and will insulate for two to four hours, depending on how hot the dish was originally and how well the box is insulated. Kevin Porter at SCI also suggests adding rocks or a brick alongside the pots in a box cooker. This will keep the food warm even longer but will cause the cooker to heat more slowly.

On the menu. Curtis and Meyer advise newcomers to solar cooking to start with a panel or box cooker and prepare something simple such as rice, cereals, egg dishes, fish, poultry, fruits, and vegetables. Other easy-to-cook foods include cornbread, gingerbread, medium-size roasts, quick breads, yeast rolls, and buns. More advanced solar chefs may want to try cooking whole turkeys or large roasts. Dried beans are difficult to cook in a solar cooker unless they have been presoaked for at least eight hours.

Hot, hot, hot. Always use potholders when removing lids or pots from a solar cooker.