The German solar thermal power plant industry
Solar thermal power plants utilise the sun’s energy to generate electricity in industrial-scale systems.
The technology at a glance
Current technologies can above all be distinguished in the way they concentrate solar radiation. With parabolic trough, solar tower and solar dish systems, the direct radiation is concentrated using reflectors. The energy concentrated in this way is transformed into steam, which is used to drive conventional electricity generators.
The solar array of a parabolic trough power plant consists of several parallel rows of collectors, 20 to 150 metres in length, which are made of parabolic reflectors. These concentrate the sunlight onto an absorber tube that runs along a caustic line. The solar radiation concentrated in the absorber tubes heats water via a heat exchanger to temperatures of around 400 degrees Celsius. The resulting water vapour drives a generator, as with conventional steam or gas turbine power plants. Parabolic trough power plants have already been producing electricity at competitive prices for 15 years, e. g. in the Mojave Desert. To date, these 9 plants have generated more than 12 TWh of solar power. The first European plant, in Andalusia in the south of Spain, will go into operation in 2009. Three power generation units, each with a 50 megawatt capacity and a collector surface area of 512,000 square metres, will supply environmentally-friendly electricity to up to 200,000 people. German firms have been instrumental in planning and implementing this project.
So-called Fresnel collectors are also undergoing practical trials as part of the development of parabolic troughs. With these collectors, reflectors arranged in facets concentrate the solar energy, which directly heats and vaporises the water in the absorber tube. Due to a secondary reflector above the absorber tube, these systems require a smaller reflector surface area for the same output.
In solar tower power plants, the solar radiation is concentrated onto a central heat exchanger/absorber by hundreds of reflectors that position themselves automatically. Temperatures can reach up to 1,300 °C – significantly higher values than with parabolic trough collectors. Process heat can be generated to practically any temperature and also be used for chemical processes. However, the heat created inside the absorber is generally used to generate electricity via a steam or gas turbine power plant. Europe’s first commercial solar tower power plant was constructed near Seville, Spain in 2006, and achieves an output of 11 MW. Another 20 MW tower is planned for construction by 2008. Other international projects led by German companies are currently in the planning stage. In mid-2006, in Jülich, Germany, building work was started on a solar tower power plant. This power station will employ energy storage to balance out fluctuations in available solar energy from the sun. This means that electricity generation can become more independent of available solar radiation and therefore more consumer-oriented. Experience gained during the construction and operation of this power station, which is the only one of its type worldwide, will form the basis for the optimisation of future projects.
With solar dish power plants, a parabolic reflector that can turn on two axes reflects the sun’s rays onto a thermal receiver positioned at the focal point. This can generate temperatures of up to 1,000 degrees Celsius. Oil is a typical medium used to transfer these high temperatures. Its heat is used to generate water vapour to drive the turbines in the electricity generator. The electrical output of individual reflectors varies from 10 to 50 kilowatts per system, and the same applies to larger arrays.
With the so-called dish-stirling systems, a Stirling engine is connected downstream of the thermal receiver, which, in this case, is a dish. The engine converts the thermal energy directly into mechanical work or electricity. With these systems, efficiency levels of more than 30 % can be reached. There are example prototypes installed at the Plataforma Solar in Almería, Spain. These systems are suitable for stand-alone operation. They also offer the possibility of connecting up several individual installations to create a solar farm, and can thus cover an electricity requirement of between 10 kW and several MW.
Outlook
The German government has been supporting the development of solar thermal power plants for several years, with the result that Germany is now global leader in the research and development of this technology. German companies supply all of the essential components, such as the precision reflectors for parabolic trough power plants. Valuable experience has been gained in the construction and operation of various pioneer solar thermal power plants, which were either German-led projects or projects with German involvement. Thanks to sophisticated heat storage systems, solar thermal power plants can now be used around the clock. Currently, different methods of heat storage are undergoing testing in Germany, as well as efficient methods of combining applications with conventional power plants.
Solar thermal power plants will play an important role in global energy supply in the future. Studies have shown that by 2050, approx. 15 % of European electricity demand could be covered by solar power plants in North Africa and the Middle East. The right network infrastructure for the transport of electricity as well as clearly defined regulations for supplying public electricity grids with electricity are imperative.
Solar thermal power plants without concentration of direct solar radiation represent another technical variation. In a so-called solar chimney power plant, air is heated by direct solar radiation beneath a large roofed surface, which has an air-tight connection to a chimney situated at its centre. The heated air flows upwards through the chimney via air ducts at the bottom. These convection currents drive one or more wind turbines and the generator attached to them, which converts kinetic energy into electrical energy. A German-planned solar chimney power plant with an output of 200 MW will be completed in Australia by 2008.
Many new projects, under German leadership or with German support, are currently in the planning and construction phases, in particular in the south-western United States, North Africa and Spain.
