Flat collectors, and even concentrator is a tool used to collect solar radiation energy so that thermal energy produced can be used in more practical for a variety of processes. Flat solar collector consists of a transparent cover, absorber and insulator. Solar radiation that falls on the surface of the transparent material in the short wave will be forwarded by transparent material and then absorbed by absorber. Black color have an ability to absorb a larger radiation so that most of the sun radiation will be absorbed. Absorption of this radiation will create a high temperature absorber. Heat radiation emanated by absorber but in the form of long waves. Most of the transparent material has an opaque nature of long wave radiation and therefore part of the long wave radiation reflected back by this transparent material to the absorber. Some radiation is absorbed will be reflected back and the rest will experience the same process that is part reflected back to the absorber. Thus, the loss of heat due to radiation can be minimized by flat collector. In addition, the transparent cover also functions as a heat loss barrier carried by the air above the absorber to the environment.
Heat from the absorber is used through heat exchanger to the media of heat carrier. Media of heat carrier commonly used can be air or water. When using water as the media, absorber will conduct heat to the surface of the outside pipes. Then take heat conduction from the outside surface to the inside surface. With the process of convection, heat will move from the inside surface to the water that flows in the pipes, so the water temperature will increase. Water with a high temperature and then used in the other parts of the outside collectors flat. A similar process occurs when air is used as a medium of heat, but in this case the pipe is rarely used. The air above (or below) the absorber heated through convection due to direct contact with the absorber. High temperature air is then flowed out of collectors than used in processes that require hot air.
The performance of a solar collector will depend on the absorptivity characteristics of the absorber, transmissivity of transparent material, overall heat transfer coefficient of the insulator, the transparent material and the absorber.
Absorptivity is the portion of light that is absorbed by an object; transmissivity is the portion of light that is forwarded by an object, while the overall heat transfer coefficient is a power of heat transfer or the opposite of heat resistant.
Check out my other guide on solar house