At the heart of a solar thermal system is the solar collector. It absorbs solar radiation, converts it into heat, and transfers useful heat to the solar system. There are a number of different design concepts for collectors such as flat-plate collectors and evacuated-tube collectors.
Flat-plate collectors
The main components of these are a transparent front cover, collector housing and an absorber. The absorber, inside the flatplate collector housing, converts sunlight to heat and transfers it to water in the absorber tubes. The absorber is usually made of metal materials such as copper, steel or aluminium. The collector housing can be made of plastic, metal or wood, and the glass front cover must be sealed so that heat does not escape, and dirt, insects or humidity do not get into the collector itself.

Evacuated-tube collectors
An evacuated-tube collector comprises a closed glass tube, inside which is a metal absorber sheet with a heat pipe in the middle, containing a temperature-sensitive medium such as methanol. The sun heats up and vaporizes this heat pipe fluid, and the vapour then rises to the condenser and heat exchanger at the end of the pipe. There, the steam condenses, and transfers heat to the heat carrier of the solar cycle, water with antifreeze agent. The condensed fluid flows back to the bottom of the heat pipe where the sun begins heating it up again. To work properly, the pipes must have a minimum angle of inclination, in order for the steam to rise and the fluid to flow back.