Radiated energy of the sun reaches the earth to the extent of about a few hundred watts per square meter, a figure which varies with the location and the time of day. With the use of an appropriately designed absorber, this energy can be collected as heat energy. Depending on the temperature at which this heat energy is available, it can find different applications. If this heat energy is available at sufficiently high temperatures, it can be converted into electrical power through the use of turbines and generators.
For achieving higher temperatures, sunlight must be concentrated onto the absorber. Domestic solar hot water systems work without concentrators and can provide hot water at around 60º C. Small devices generating temperatures of 100º C or more can be used for cooking, and larger devices for drying. Steam in larger volumes generated at 100º C or more can be used as process heat in industry.
With higher degrees of concentration, if steam is available at around 400º C, it can be used to generate power using the standard steam turbines and generators. If power is to be generated for longer hours, i.e. during periods of reduced or no sunlight, then some means of thermal energy storage are also needed.
The basic physics underlying solar thermal devices and systems is quite simple. However the costs of using solar thermal energy are today higher than those of conventional fuels, and therefore research effort must focus on bringing down these costs using better materials and technology.