Sunny weather makes ovens out of more than just cars.
The same physics that make a hot car interior a hazard to children or pets left inside can be used to heat food in solar cookers. In Sacramento and abroad, people use solar cookers as an alternative to conventional ovens or open fires.
Linda Hayward, a retired elementary school teacher, has been using solar cookers for over 20 years, starting with a box cooker she made at a workshop. In the summer, when the amount of available light for cooking is greatest, she cooks with solar "just about every day."
There are three common types of solar cookers.
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▪ The box solar cooker is most similar to an overheated car interior. It has a transparent, heat-resistant top, often glass, with black material for the sides and the pot holding the food. Black is preferred because it absorbs all wavelengths of visible light.
▪ As light from the sun passes through the glass warming the oven, it emits increasing amounts of invisible infrared light, which reflects off the glass lid and is reabsorbed by the oven - the same greenhouse effect that heats up cars.
Box cookers are usually insulated to hold in heat, and often include extendable reflective surfaces to catch more light. They can get as hot as a conventional oven, well over 300 degrees Fahrenheit.
Julie Greene, president of Solar Cookers International, said that she regularly uses one to bake cookies in the parking lot outside her office in midtown Sacramento.
▪ Panel cookers are similar but lack an insulated box and tend to be less expensive. Panels of reflective material direct sunlight onto a black pot. A plastic bag may be used to help produce a greenhouse effect and provide some insulation, but the temperature will be less stable than in a box cooker. Panel cookers work best with slow cooker-style recipes.
▪ Parabolic cookers use a mirror shaped like an umbrella or a satellite dish, which focuses the collected sunlight to a single point above the mirror. The cooking pot or pan is suspended at this focus. The large amount of energy in a small area allows some parabolic cookers to fry items like a stove. "I've grilled a steak" with this kind of cooker, Greene says.
But there are drawbacks: The chef has to move the pot every 15 or 20 minutes because the focused hot spot will move with the sun. Larger cookers will include a mechanism for tracking automatically. The hot spot can also cause burns or start fires, similar to the focused light of a magnifying glass.
Solar cookers range greatly in price and size, from panel cookers to large-scale parabolic cookers with automated sun tracking that are capable of cooking meals for hundreds of people. Solar Cookers International sells solar cookers ranging from $30 to $400.
Small-scale solar cookers can also be constructed at home, either from materials on hand or using commercially available kits.