A solar cooker, or solar oven, is a device which uses the energy of sunlight to cook, heat, or sterilize food and drink. Electric ovens powered by solar cells are possible but are very unusual today because they are expensive. Most solar cookers presently in use are relatively cheap, low-tech devices. Many nonprofit organizations are promoting their use worldwide to help reduce fuel costs for low-income people, reduce air pollution and slow deforestation and desertification, caused by use of firewood for cooking.
There are various types of low tech solar cookers. Some concentrate sunlight by using reflective mirror of polished glass, metal or metallised film to concentrate energy and increase its heating power. Others convert light to heat using black or low reflective surface on a food container or the the inside of a solar cooker. Some trap the heat by isolating the air inside the cooker from the air outside by using a plastic bag or tightly sealed glass cover. Other cookers use the greenhouse effect using glass to transmit visible light while blocking infrared thermal radiation from escaping to increase the heat trapping effect.
The most commonly used ones are box cookers, panel cookers, solar kettle, cookers with paraboloidal reflectors, Scheffler cookers, hybrid cookers to name a few.
Box cookers have a transparent glass or plastic top, have insulated sides, and may have additional reflectors to concentrate sunlight into the box. The top can be removable to allow dark pots containing food to be placed inside. One or more reflectors of shiny metal or foil-lined material may be placed to bounce extra light into the interior of the oven chamber. Cooking containers and the inside bottom of the cooker should be dark-colored or black. Metal pots and/or bottom trays can be darkened either with non-toxic flat-black spray paint, black tempera paint, or soot from a fire. Inside walls should be reflective to reduce radiative heat loss and bounce the light towards the pots and the dark bottom, which is in contact with the pots. Thermal insulation for the solar box cooker must be able to withstand temperatures up to 150 °C (300 °F) without melting or out-gassing. Crumpled newspaper, wool, rags, dry grass, sheets of cardboard, etc. can be used to insulate the walls of the cooker. The solar box cooker typically reaches a temperature of 150 °C (300 °F) — not as hot as a standard oven, but still hot enough to cook food over a somewhat longer period of time. Because the food does not reach too high a temperature, it can be safely left in the cooker all day without burning. The cooker can be used to warm food and drinks and can also be used to pasteurize water or milk.
Panel solar cookers use reflective panels to direct sunlight to a cooking pot that is enclosed in a clear plastic bag. A common model is the CooKit developed in 1994 by Solar Cookers International. Produced locally by pasting a reflective material, such as aluminum foil, onto a cut and folded backing, usually corrugated cardboard. It is lightweight and folds for storage. When completely unfolded, it measures about three feet by four feet (1 m by 1.3 m). Using materials purchased in bulk, the typical cost is about US$5. However, CooKits can also be made entirely from used materials like cardboard boxes and foil from the inside of cigarette boxes.
Solar tea kettle in Tibet
A parabolic mirror can take light from a very distant source such as the sun, so the rays of light are effectively parallel, to a point focus. The diagram shows parallel rays QP being reflected and then converging to the focus (or focal point), marked F. The point V, where the curvature of the parabola is greatest, is called its vertex. For the focussing to be accurate, the incoming rays have to be parallel to the axis of symmetry of the parabola, the line that passes through V and F. The distance between V and F is the focal length of the parabola. If the axis of symmetry is aimed at the sun, any object that is located at the focus receives highly concentrated sunlight, hot enough to cook food.
Paraboloidal solar cookers can cook as well as conventional ovens but are difficult to construct. Paraboloids’ compound curves are more difficult to make with simple equipment than single curves. They generate high temperatures and cook quickly but are not user friendly. Large-scale institutional cooking, however, use this technology. Several hundred thousands are used in China.
Hybrid Cookers
A more independent type of solar cookers are the hybrids. They are solar box cookers fitted with conventional electrical heating parts for cloudy days or nighttime cooking. However, they are more expensive than some other types of solar cookers.
A hybrid solar grill is fitted with an adjustable parabolic reflector suspended in a tripod and a movable grill surface. These outperform solar box cookers in temperature range and cooking times. The design uses any conventional fuel such as gas, electricity, or wood when solar energy is not available.
High-tech Solar Cookers
Although various ideas for high-tech solar cookers have been proposed, such as the electric oven powered by solar cells very few of them have progressed past the experimental stage to the point of producing pilot batches, let alone mass production. They are generally much more expensive than low-tech cookers.
Integrated Solar Cooking
It has been recognized that solar cookers are limited to cooking on clear days. Moreover, most people want to eat hot food late in the day, when the sun is low or has already set. For these reasons, solar cooking advocates are recognizing the need for combining three devices for a total cooking solution: a) some type of solar cooker; b) a fuel-efficient cookstove; c) an insulated storage container such as a basket filled with straw to store heated food. Hot food will continue to cook for hours if it is stored in a well-insulated container. With this three-part solution, fuel use is minimized while still providing hot meals reliably. This concept is referred to as “integrated solar cooking” or the “integrated cooking method”.