The solar thermal industry has used low-tech technology until relatively recently and been largely concerned with small domestic and building applications for heating space or water, or cooking. However, the industry is now taking a more sophisticated direction and progressing to higher-tech applications involving relatively large electricity generation projects in a number of countries. Some of these schemes have been in existence for a number of years on a trial basis. Solar cooling, although still a very small application with around 80 solar cooling systems in the world is making rapid strides.
Solar thermal collectors are divided into three categories, according to temperature, with low, medium, or high temperature collectors. Low temperature collectors are flat plates generally used to heat swimming pools directly. Medium-temperature collectors are also usually flat plates and are used directly for creating hot water for residential and commercial use. High temperature collectors concentrate sunlight using mirrors or lenses and are generally used for electric power production. These are known as CSP (concentrating solar power) units. In use described as ‘direct’ the solar energy or heat is used to heat water or buildings, or for factory process, and not transformed into electricity.
All technologies operating through solar heating come under the category of solar thermal. These include non-grid solar thermal technologies; water heating systems, solar cookers and solar drying applications etc. These technologies help conserve energy in heating and cooling applications. Solar thermal appliances can be manufactured with a low level of technology and are ideally suited for developing countries. In industrialised countries, solar thermal technology has more advanced applications such as solar thermal building designs. All of these solar thermal devices use heat directly from the sun. They are cheap to manufacture and cost nothing to use.
The more advanced use of solar thermal energy, employing high temperature collectors, involves conversion from heat into secondary energy, electricity. Several technologies have been developed and tested to generate power from solar thermal energy and where some of these technologies are classified as mature, others are in their infancy. Current trends show that two broad pathways have opened up for large-scale delivery of electricity using solar thermal power: ISCC-type hybrid operation of solar collection and heat transfer, combined with a state-of-the-art, combined-cycle gas-fired power plant: Solar-only operation, with increasing use of a storage medium such as molten salt, enabling solar energy collected during the day to be stored and then dispatched when demand requires. Solar thermal power is up to four times as expensive as fossil fuel power.