Solar energy. Diagram showing the principles of solar thermal energy generation. The system comprises an array of troughs made from mirrors that are parabolic in cross section, with pipes running along the troughs at the focus of the mirrors.. The pipes contain a transfer fluid, such as mineral oil or molten salt, that is heated by the sunlight concentrated on them. The transfer fluid passes through a heat exchanger where it generates steam, in turn this is passed through a turbine which powers a generator to create electricity.