Heat transfer and losses
As explained earlier the easiest way of solar radiation collection is for low temperature heating purposes. It is well known that black surfaces absorb solar radiation more than any other color, and therefore, when a surface is blackened it will absorb most of the incident solar radiation. Continuous pour of solar radiation on such a surface will increase its temperature. This will continue until the heat gain from the solar radiation will be in equilibrium with the heat loss from the collector. There are two types of heat losses, namely, natural unavoidable losses and losses due to human uses. The heat can be transmitted to consumption site through pipes soldered to the metal plate, which is heated due to solar radiation exposition. The heat balance of collector will have three components in general as follows [8,35].
Absorbed heat — lost heat = removed heat by coolant
It is possible to define the coefficient of efficiency for the collector as
= (absorbed heat — lost heat)/incident solar radiation
In practice, the collectors must be designed in such a manner, that the efficiency becomes high. In order to achieve such a goal there are two methods either to reduce the heat losses or to increase the incident solar radiation, and hence, the heat absorbed per unit area. For low temperature collectors, heat loss reduction methodology is suitable. It is possible to reduce heat loss by using transparent cover plates specially treated absorber surfaces, and by evacuating the space between the cover plate and the absorber surface. For high temperature solar collectors, the efficiency must be increased by increasing the incident radiation through the concentrators. For this purpose only the direct radiation is considered.
It is necessary in any solar radiation collector efficient work to reduce the heat losses or to minimize them. As a material is heated by solar radiation it seeks to reach equilibrium with its surrounding by conduction, convection and radiation processes. Let us examine these processes one by one in more detail.