An article on solar cookers, how they work and their various types...
With the continuous rise in the prices of basic commodities and the increasing threat posed by the use of fossil fuel, the need to harness free and renewable energy has gained much prominence. A very promising source of such energy is the sun which virtually cost nothing and pollution-free. One of the best technologies derived from the utilization of solar energy is the solar cooker.
A solar cooker, also known as solar oven, is a device that uses heat by concentrating or intensifying the heat from sunlight. Solar cookers are generally used for cooking, drying, and pasteurization, and are a form of outdoor cooking where sunlight is available. Many organizations are advocating on the use of such device, which is usually low-cost, because it helps slow down deforestation rate as gathering of firewood for cooking is avoided.
There are several types of solar cookers based on design. There are least 65 major designs and hundreds of variations of them. They differ on the mechanism of collecting and reflecting sunlight, but the principles of operation are similar. All these devices must be able to do perform three basic functions: to concentrate sunlight, to convert light to heat, and to trap heat.
Reflective mirrors of polished glass, metal, or metalized film are used to concentrate sunlight to a small cooking area, thereby increasing the heat power. On the inside of the solar cooker, a black surface which is low in emissivity is provided to further improve the conversion of light to heat. Lastly, a sealed glass cover or related mechanism is used to trap hot air inside the solar cooker. With these factors considered, solar cookers can effectively work.
Some of the more common types of solar cookers are the following:
This is the simplest cooker which is first built in 1767 by Horace de Saussure. A basic box cooker consists of an insulated container with a transparent lid normally made of glass or plastic. The lid can be removed to allow dark pots containing food to be placed inside. The box cooker can work effectively with partially overcast skies and will typically reach temperatures of 90 – 150 deg. C.
The panel cooker uses a reflective panel to direct sunlight onto an insulated container, usually enclosed in a clear plastic bag. A very common model of this type of solar cooker is the CooKit developed in 1994 by Solar Cookers International. The CooKit is made of aluminum pasted on a corrugated cardboard which is cut and folded to shape. It is lightweight and when folded for storage only measures about three feet by four feet. In general, panel cookers can reach temperatures comparable to box cookers.
The reflector cooker uses various concentrating geometries (dish, trough, Fresnel mirrors) to focus light on a cooking container. These cookers reach temperatures of 315 deg.C and above but require direct light to function properly and must be repositioned to track the Sun.
a. Solar Kettle
One example of a reflector cooker is the solar kettle. Solar kettles, used to heat water to boiling point, can generate temperatures as high as 220 deg. C.
b. Parabolic Solar Cooker
This technology uses parabolic reflectors to concentrate sunlight. Its design is somewhat complex and difficult to fabricate. It also requires more precision to focus sunlight on the cooking vessel. Hence, the cooker must be frequently adjusted to catch sunlight and supervised for safe operation. Accordingly, it has the fastest cooking time for all cookers.
c. Cookers with Spherical Reflectors
The Solar Bowl in Auroville, India is this type of technology. Its spherical reflector focuses light along a line perpendicular to the sphere’s surface and a computer control system moves the receiver to intersect this line. It can reach temperatures up to 150 deg. C.
The hybrid cooker integrates conventional electrical heating elements in its layout for used during cloudy days or night time.