Solar cooker
A solar cooker is a device which uses the energy of direct sunlight to heat, cook or pasteurize drink and other food materials.
In ancient times, the use of solar energy was believed to have existed in civilizations amidst the Greeks, Romans and the Chinese, though not for cooking.
[3] The first academic description of the principles of a solar cooker is by the Swiss geologist, meteorologist, physicist, and Alpine explorer Horace-Bénédict de Saussure, in 1767.
[4] A mirrored surface with high specular reflection is used to concentrate light from the sun into a small cooking area.
Depending on the geometry of the surface, sunlight could be concentrated by several orders of magnitude producing temperatures high enough to melt salt and metal.
Simply using a glass lid on your pot enhances light absorption from the top of the pan and provides a greenhouse effect that improves heat retention and minimizes convection loss.
This "glazing" transmits incoming visible sunlight but is opaque to escaping infrared thermal radiation.
[6] For very simple cooking, such as melting butter or cheese, a lid may not be needed and the food may be placed on an uncovered tray or in a bowl.
Unlike cooking on a stove or over a fire, which may require more than an hour of constant supervision, food in a solar oven is generally not stirred or turned over, both because it is unnecessary and because opening the solar oven allows the trapped heat to escape and thereby slows the cooking process.
If the food is to be left unattended for many hours during the day, then the solar oven is often turned to face the point where the Sun will be when it appears highest in the sky, instead of towards its current position.
Food cooks faster in the two hours before and after the local solar noon than it does in either the early morning or the late afternoon.
One or more reflectors of shiny metal or foil-lined material may be positioned to bounce extra light into the interior of the oven chamber.
[9] If a reflector is axially symmetrical and shaped so its cross-section is a parabola, it has the property of bringing parallel rays of light (such as sunlight) to a point focus.
If the axis of symmetry is aimed at the Sun, any object that is located at the focus receives highly concentrated sunlight, and therefore becomes very hot.
Although paraboloids are difficult to make from flat sheets of solid material, they can be made quite simply by rotating open-topped containers which hold liquids.
The top surface of a liquid which is being rotated at constant speed around a vertical axis naturally takes the form of a paraboloid.
[citation needed] This rotation technique is sometimes used to make paraboloidal mirrors for astronomical telescopes, and has also been used for solar cookers.
Paraboloidal reflectors generate high temperatures and cook quickly, but require frequent adjustment and supervision for safe operation.
The cooking vessel is located at the focus which is on the axis of rotation, so the mirror concentrates sunlight onto it all day.
The mirror has to be occasionally tilted about a perpendicular axis to compensate for the seasonal variation in the Sun's declination.
It is often made up of a large number of small plane sections, such as glass mirrors, joined by flexible plastic.
[citation needed] Sometimes the rotating reflector is located outdoors and the reflected sunlight passes through an opening in a wall into an indoor kitchen, often a large communal one, where the cooking is done.
They can be easily turned to follow the Sun's motions in the sky, rotating about any axis that passes through the focus.
Two perpendicular axes can be used, intersecting at the focus, to allow the paraboloid to follow both the Sun's daily motion and its seasonal one.
It is possible to use two parabolic troughs, curved in perpendicular directions, to bring sunlight to a point focus as does a paraboloidal reflector.
Also, the light that reaches the targeted cooking pot is directed approximately downward, which reduces the danger of damage to the eyes of anyone nearby.
The central cooking tube is made from borosilicate glass, which is resistant to thermal shock, and has a vacuum beneath the surface to insulate the interior.
The inside of the tube is lined with copper, stainless steel, and aluminum nitrile to better absorb and conduct heat from the Sun's rays.
[13] Cardboard, aluminium foil, and plastic bags for well over 10,000 solar cookers have been donated to the Iridimi refugee camp and Touloum refugee camps in Chad by the combined efforts of the Jewish World Watch, the Dutch foundation KoZon, and Solar Cookers International.
