Solar Thermal Power PlantsSolar thermal power plants use the sun’s energy to generate electricity in industrial-scale systems. Germany is the world leader in the research and development of this technology. Valuable experience has been gained in the construction and operation of various pioneering solar thermal power plants, which were either German-led projects or projects with German involvement.

The technology at a glance

The common basic principle of solar thermal power plants is the use of concentrating parabolic reflector systems in large-scale versions of what are known as solar fields, which direct the solar radiation onto a receiver. The concentrated radiation is then transformed into thermal energy at temperatures ranging from around 200 to over 1,000 degrees (depending on the system). As in a conventional power plant, this thermal energy can then be converted into electricity via steam- or gas-powered turbines, or it can also be used for other industrial processes such as water desalination, cooling or, in the near future, the production of hydrogen.

Due to this principle, solar thermal power plants excel in their ability to store the thermal energy generated in a relatively simple and cost-effective manner, allowing them to generate electricity even during hours of darkness. Consequently, they can make a key contribution to planned, demand-oriented electricity production in a future electricity mix with high proportions of renewable energies.

There are four different types of concentrating reflector systems:
linear concentrating systems such as parabolic trough and Fresnel collectors and point focus concentrating systems such as solar towers and dishes (paraboloid). All systems must track the sun in order to be able to concentrate the direct radiation. The various types of power plants are briefly described below.

The solar field of a parabolic trough power plant consists of numerous parallel rows of collectors, which are made of parabolic reflectors. These concentrate the sunlight onto an absorber tube that runs along the focal line, generating temperatures of approximately 400 °C. Circulating thermo-oil serves as a heat transfer medium to conduct the thermal energy to a heat exchanger, where water vapour is generated with a temperature of around 390 °C. This is then used to power a steam turbine and generator, the same as in conventional power plants.

What are known as Fresnel collectors are also undergoing practical trials. With these collectors, long, only slightly curved reflectors concentrate the solar radiation onto a fixed absorber tube, where water is directly heated and vaporised. As the basic concept of these collectors is simpler in comparison
to parabolic troughs, lower investment costs for the reflectors can be expected. However, the comparable annual efficiency will be somewhat lower.

In solar tower power plants, solar radiation is concentrated onto a central heat exchanger /absorber by hundreds of automatically positioned reflectors. The significantly higher concentration in comparison to parabolic trough collectors, for example, allows higher temperatures in excess of 1,000 °C to be achieved. This enables greater efficiency, particularly when using gas-powered turbines, thereby resulting in lower electricity costs.

For what are known as dish-Stirling systems, a parabolic reflector mirror concentrates the solar radiation onto the receiver of a connected Stirling engine. The engine then converts the thermal energy directly into mechanical work or electricity. These systems can achieve a degree of efficiency in excess of 30 %. Prototype systems are undergoing trials at the Plataforma Solar centre in Almería, Spain. Although these systems are suitable for stand-alone operation, they also offer the possibility of interconnecting several individual systems to create a solar farm, thus meeting an electricity demand from ten kW to several MW.