Concentrating sunlight

SOLAR thermal technologies concentrate sunlight and convert the sun's heat into electricity. In these systems, solar reflectors concentrate sunlight onto a receiver, which contains liquid. The heated liquid drives a turbine, producing electricity with an overall efficiency of about 20 per cent. An array of collectors that collects 5 KW of heat is, therefore, required to produce 1 KW of electricity.

There are two types of solar thermal systems in use -- the central receiver type and the distributed receiver type. In the central receiver system, the sun's heat is focussed onto a receiver, perched atop a large tower, by a field of mirrors. In the distributed receiver system, the reflector mirror and the heat receiving unit are contained in a single unit. A parabolic trough focusses sunlight onto a tube that runs the length of the trough. The liquid in the tube conducts the heat to a central power plant where steam is raised to drive a turbine.

Because of the large number of mechanical devices involved, solar thermal systems tend to be far more expensive and clumsier than solar photovoltaic systems and suffer many breakdowns.