Flat-plate collectors capture the energy from the sun. There are different designs, but for the most part they use heat transfer fluid. Once cold water has been collected in the storage tank, the flat-plate collector starts looping the fluid through the cold water and heats it up.
But this only happens after the radiation energy has been transformed into thermal energy through the collector. After the fluid has been heated it will start to circulate. The collector plate consists of a copper sheet and tubing. The surface is coated with a dark material, specifically to increase absorbency efficiency. The collector is also very well insulated in order to prevent heat from escaping.

The evacuated tube collector (ETC) consists of a number of sealed glass tubes which have a thermally conductive copper rod or pipe inside allowing for much high thermal efficiency and working temperature compared to the flat plate solar collectors.

The evacuated tube comprises a smaller glass tube suspended within a larger glass tube. The air is then pumped out of the space between the small inner tube and the larger outer tube creating a vacuum thermal insulation layer. This vacuum layer is absolutely key since it reduces heat loss from the solar collector.

An absorber plate absorbs the heat and transfers it to a heat transfer fluid. In passive systems, convection drives the movement of the heating fluid around the solar collector, when the transfer liquid gets heated it evaporates and turns to steam. This rises to the top of the evacuated tube solar collector the heat is transferred via a heat exchanger, the transfer liquid, having given up its heat to the heat exchanger then condenses and falls back down the evacuated tube where the process can start again