There are different types of solar thermal collectors. At present flat plate and vacuum tube collectors are most often used. The main differences are the efficiency and the cost. The efficiency of solar thermal collectors is a function of the fluid temperature inside the collec­tor (working temperature), the solar insolation and the ambient temperature. In Figure 1 the efficiency curves of some sample collectors are shown.

There are big differences between different collectors. Especially the efficiency of the cheaper flat plate collector decreases sharply with rising working temperatures of the col­lector. To reach a high efficiency of the complete system consisting of flat plate collectors and the absorption chiller it is important that the absorption chiller is able to work with rela­tively low driving (heating) temperatures.