Solar ovens operate on the principles of collecting sunlight, trapping heat from that sunlight and putting the heat on the food. Though the principles are simple, implementing them effectively can be a very sophisticated prospect. Engineers and inventors in recent years have been refining the process of collecting, trapping and conveying heat from sunlight, creating the modern technology of solar ovens.
Fresnel Lenses
One way of gathering sunlight and putting it on the food is the Fresnel lens. Edmund Optics, an optical component supply store, describes the Fresnel lens: It "replaces the curved surface of a conventional lens with a series of concentric grooves, molded into the surface of a thin, lightweight plastic sheet." The cross section of a Fresnel lens is thin, making it easy to move and aim, thereby making it ideal for use in solar ovens. The lens is mounted on a frame, and the cooking container is mounted at the focal point of the lens. Ideally the container easily absorbs and holds the heat, transferring it to the food.
Rotisseries Paired with a Parabolic Reflector
Parabolic reflectors are one of the most common ways of gathering sunlight for cooking. One of the problems of using parabolic reflectors, however, is that these reflectors focus the light to a very small focal point. The focal point heats a small area on the cooking pot, causing burning at that spot and underdone food everywhere else. Anderson Manufacturing solves this problem using rotisserie technology paired with a parabolic reflector. The Anderson cooker places the cooking pot in front of the reflector so food doesn't boil over on to the reflector. The light hits the side of the pot, and a turntable rotates the pot so the focal point heats all sides evenly.
Reflectors for Box Cookers
The box cooker, a kind of solar oven that uses light reflected into a box and trapped there by insulation and a glass lid, relies on its reflectors to gain the temperatures necessary for cooking. Modern solar oven technology has boosted the power of these reflectors by using highly reflective materials. One of the most basic is common household aluminum foil. It is glued shiny-side-out onto the reflectors. Mirrors, mylar, reflective tape, and recycled lithograph printing plates have also been tested and found to be good reflective material.
Solar Plane Mirrors
Another technology used to put the sunlight on the food is the solar plane mirror. For example, the NELPA cooker, which was invented by French inventor Roger Bernard, is a solar oven built on solar plane mirror technology. In the NELPA cooker, several rectangular mirrors are attached to a rectangular array that is curved along the short axis. This array points upward to catch the sun and to reflect it to a point above the array. At the the focal point, a pot rests on a stand and catches the heat from the below. The array swings from a pivot above, allowing the mirrors to be directed at the sun. The advantage of this technology is that it can be used either as the "burner" of a solar oven or, if enough mirrors collect sufficient heat, as a stove-top burner for frying and sauteing.
Indirect Solar Cooking
Rather than putting sunlight on the cooking pot, indirect solar cooking collects heat using collectors, and then brings that heat to the food. One type of indirect solar cooker, the Fraber cooker, collects the heat using a trough, which heats oil inside a pipe. The oil then heats the solar oven. One of the advantages of indirect solar cooking is that the heat can be collected outdoors and then brought indoors for cooking.
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