Green Energy – Solar Heating Devices Explained!
Solar Heating Devices Explained
A solar heating device is a device which allows collection of a large amount of heat from the sunlight in a given region and restricts the loss of heat from that region.
Any object placed in the sun will absorb heat from it. Absorption increases if the surface of the body is painted black. This is because black surfaces absorb more radiation than white or light-colored surfaces. However, just painting an object black and placing it in the sun will not make a good solar heating device. Arrangement has to be made to restrict the loss of the heat collected. A hot object loses heat to the surrounding bodies by three processes: conduction, convection and radiation. An object placed in the sun will ultimately achieve a constant temperature which will be the same as that of the surrounding bodies. Any further heat absorbed will be equal to the heat lost in the same time.
A simple solar heating device can be constructed taking a box made of a poorly conducting material, painting its inner surfaces black and closing it at the top by a glass sheet. The bottom surface receives most of the heat and is the main collector of sunlight.
In such an arrangement loss of heat by conduction is minimized because the walls are made of poorly conducting material. Convection loss is reduced because the box is closed and the air inside cannot take the heat out of the box. Radiation loss is also reduced because large part of the heat radiated by the blackened surfaces is reflected back into the box by the glass top. Thus the temperature inside the box becomes much higher than that of the surrounding.
The role of the glass top in reflecting heat radiation is interesting. When sunlight falls on the glass top from outside, the glass top allows the radiation to go into the box. Only a small part of the radiation is reflected. But when radiation from inside the box falls on the glass top, the top reflects a large part back into the box. This is because of the different natures of the radiations emitted by the sun and by the blackened surfaces of the box.
The outer surface of the sun has a temperature of about 6000°C and the radiations emitted by it are dominated by wavelengths in the visible region. It also radiates some infrared radiation (wavelength close to that in the visible region and higher than it). The inside surfaces of the box are, on the other hand, at about 100°C—140°C and emit mainly infrared radiation of larger wavelengths. The glass top can reflect infrared radiation more effectively than it can reflect visible radiation. Therefore, it reflects most of the radiation coming from inside the box back into the box, whereas, it allows large part of the radiation coming from the sun to go into the box. The glass acts like a traffic controller allowing only one-way traffic.
A similar thing happens in a greenhouse. Greenhouse is a building with glass walls and is used to grow certain plants which need warm environment. Because of the glass walls, sun’s radiation enters the greenhouse and the heat is trapped within. Carbon dioxide and some other gases present in the atmosphere also have the property of trapping infrared radiation. These gases help to keep the earth warm. This phenomenon is similar to the process in a greenhouse arid is, therefore, known as the greenhouse effect.
The same effect is seen when a car is parked in the sun with its windows closed. The glass windows allow a large part of the sun’s radiation to go in. But a good fraction of the radiation coming from inside, which is mostly infrared, is reflected back into the car’ As a result the inside becomes quite warm.
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