Solar thermal power

Solar thermal power is an approach where energy from the sun is used directly to heat an energy carrier, most often water. The hot water produced can then be used for domestic and industrial purposes, and there are almost no carbon emissions because no fuel is used to heat the water. More sophisticated solar thermal systems are able to trap and concentrate the sun’s energy to produce steam, which can be used to drive turbines and produce electricity.

Solar water-heating systems are the most popular form of solar energy currently used in the UK. A solar water-heating system can be easily fitted as an addition to an existing domestic hot water system. They can provide over half of a household’s hot water requirements over the course of a year. Two different types of solar water-heating collectors are in use:

• Flat plate collectors are made from a sheet of metal painted black, which absorbs the sun’s energy. Water is fed through the panel in pipes attached to the metal sheet and picks up the heat in the metal. For the UK climate the pipework contains non-toxic anti-freeze. The pipes are often made of copper for better conduction. The metal sheet is embedded in an insulated box and covered with glass or clear plastic. The system is usually installed on a roof.
• Evacuated tubes consist of a series of glass heating tubes grouped together. The tubes are highly insulated, provided by a vacuum inside the glass. They therefore tend to be both more efficient but also more costly than the flat plate collectors.

A related but distinct approach is Passive solar heating, where buildings are designed in such a way that they are able to absorb and retain heat from the sun. This then leads to a greatly reduced need for heating, increasing the energy efficiency of the building. In hot countries similar techniques can be used to help keep buildings cool and thus reduce the need for the use of electricity in air conditioning systems. Passive solar design is a proven design approach that can greatly reduce energy costs for buildings.

A wide variety of designs are currently in operation or testing for producing electricity through solar thermal power. One dramatic example has recently been constructed just outside Seville in southern Spain. Referred to as a solar power tower, this extraordinary power plant consists of a huge 40-storey tower surrounded by a large area covered in some 600 huge mirrors, each the size of a bus. The mirrors focus sunlight onto the uppermost parts of the tower, creating temperatures of up to several hundred degrees Celsius. The heat is used to drive a boiler and create steam. The steam is then used to drive turbines and create electricity in much the same way as a conventional power plant. Large tanks on the ground can store the heat generated in the tower so that the plant can even produce electricity at night-time.

This system is able to produce large amounts of electricity; the cost is currently around three times that of a conventional coal power station but this is a first attempt and costs will reduce as further plants are built. There are plans to build many more such plants in southern Spain, and even talk that one day such power plants could be built across the Sahara and used to generate enormous amounts of electricity from solar thermal power. (Source British Council, report August 2008)