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

Functional principle of the solar dish

There is a basic distinction between solar thermal power plants that concentrate direct solar radiation and those that don't. 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 rows of collectors, 20 to 150 metres in length, which are made of parabolically curved 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, as the currently least expensive variant of solar electricity generation, are the only type of solar thermal power plants operating competitively anywhere in the world. They have been producing electricity at competitive prices for more than 15 years. The last of a total of nine power plants constructed in the Mojave desert in southern California was completed in 1991. To date, these plants have generated 10 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 200,000 people. Several German firms are instrumental in planning and implementing the project.

Functional principle of the parabolic trough

Functional principle of the solar tower

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. Several hundred megawatts of electricity can be produced in this way.

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 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 that will be operational in 2008 and provide an output of
1.5 megawatts.

With solar dish power plants, a parabolically curved 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 up to 1,000 degrees Celsius. Oil is a typical medium used to transfer these high temperatures, by means
of which water vapour is generated, driving the turbines in the electricity generator. The electrical output of individual reflectors varies between 10 and 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.

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. This updraft drives one or more wind turbines and the generator attached to them, which converts kinetic energy into electrical energy. The low level of technical complexity of such systems is reflected in a comparatively low efficiency level of around one percent. A German-planned solar chimney power plant with an output of 200 MW will be completed in Australia by 2008.

Regulatory framework

Electricity cost of solar thermal power systems as a function of cumulative installed capacity
SEGS stands for 9 parabolic trough power plants built between 1984 and 1991 in the California desert. Total power: 354 MWe
AndaSol: Parabolic trough power plant project in Spain with 50 MW power each (beginning of construction in 2005)

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 are supplying all 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. The funding of solar thermal power plant technology is currently on the increase in many other parts of the world. Thanks to sophisticated heat storage systems, solar thermal power plants can now be used around the clock. Further research on heat storage tanks will therefore be worthwhile. A further advantage is that these systems can easily be combined with conventional power plants.

Outlook

Solar thermal power plants will play an important role in global energy supply in the future. By 2050, 15 % of European electricity demand could be covered by solar power plants in North Africa and the Middle East. With the right pipe infrastructure, more efficient electricity transmission would also be possible. The energy could be distributed all over the world in the form of hydrogen. There are already many new projects in planning or underway, led by or involving German firms, particularly in south-west USA, North Africa and Spain.