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A solar heating device is one which allows collection of a large amount of heat from the sunlight in a given region and restricts the loss of heat to the surrounding in the form of radiation.
The following facts help us to design an efficient solar cooker:
- A black body absorbs maximum heat
- Loss of heat by convection can be restricted in a closed system
- Radiation losses can be avoided by reflecting the rays inside the body with the help of reflecting object like a plane mirror
- Conduction can be reduced by using poor conducting materials
A solar cooker contains a wooden box having thermocol lining from inside. A box made aluminium is placed inside the wooden box. The inner side of the aluminium box is painted black. The top is closed by a glass sheet. A plane mirror is fixed to the hinged lid.
The box is closed such that the inner part does not come in contact with the outside air, there by reducing heat loss due to convection currents. The glass top reflects the light in to the box thereby reducing the heat loss due to radiation. The temperature inside the box is about 100 - 140o C.
A phenomenon similar to green house effect happens here. The glass top allows infrared radiations emitted by the sun into the box and traps the infrared radiations emitted by hot objects inside the cooker.
This type of solar cooker can be used for cooking. But the time of cooking depends on the intensity of the sunlight and hence the time taken is shorter in summer and longer in winter. Rice, pulses and vegetables can be boiled by keeping them inside the box. Baking and frying can be done in reflector type solar cooker. A huge spherical concave mirror is used as a reflector.
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