Solar panels are used to convert the incident solar radiation into thermal energy. The solar panels are divided into flat solar collectors and vacuum collectors.


The flat or black semi-selective dye collector is a simple type of collector for converting solar energy into heat. It consists of copper piping through which the heat transfer medium (water or a mixture of water-glycol) circulating in contact with a sheet of copper or aluminum of a black surface (absorber) is circulating. To reduce heat losses, the absorber is positioned inside an insulated housing, which on one side has a transparent glass pane to allow the entrance of the solar radiation. Flat collectors are used for heating water as well as space heating.






Source: CRES, TRASOL, Training material in the field of solar thermal systems



The flat selective collector is a flat collector, using a selective surface absorber instead of a simple black. The black dye has been replaced by a high absorption material (~ 95%) and low radiation (~ 5%). By using a selective absorber, higher yields and higher water temperatures are achieved. Flat selective collectors are used for heating water as well as for space heating and air conditioning.



Vacuum collectors consist of series of parallel glass tubes. Each individual pipe is discharged to reduce thermal losses. Tubular geometry is statically necessary due to the pressure difference between atmospheric pressure and the vacuum in the interior. Inside each pipe there is a flat or curved aluminum plate attached to a metallic (usually bronze) or glass tube. The aluminum plate is coated with a selective material. The heat transfer fluid is water and circulates in the piping. Some vacuum tube collectors have external mirrors behind the vacuum tubes or in the glass pipes. Exterior mirrors increase the radiation collected by the collector, since usually the radiation passing through the empty space between the tubes is led back to the absorber. The use of the vacuum reduces the heat losses of the collectors and aims to further increase the efficiency and water temperature. Vacuum collectors are used for space heating and air conditioning and under special conditions for water heating.