Technologies and applications
The common basic principle of solar thermal power plants is the use of concentrating parabolic dish systems in large-scale solar fields that concentrate the solar radiation onto a receiver. All systems must track the sun in order to be able to concentrate the direct radiation. This radiation is first converted in a special absorber system (receiver) into thermal energy at temperatures in the range of about 200 to over 1,000 °C (depending on the system). The thermal energy can then be converted to electric power, as in a conventional power plant, using steam or gas turbines; if needed, it can also be used in other industrial processes, for example, water desalination, cooling or – in the near future – for hydrogen production.
Owing to this principle, solar thermal power plants are characterised by the fact that the heat generated can be stored in a relatively easy and inexpensive way, and can thus be used to generate electricity in the evening and at night. In this way, they can make a decisive contribution to predictable, needs-based electricity generation in a future electricity mix with a high proportion of renewable energy.
A distinction is made between linear and dish concentrator systems; within these systems there are four different configurations:
Linear concentrator systems
Parabolic trough plants
The solar field of a parabolic trough plant contains numerous parallel rows of collectors that comprise parabolic curved dishes and concentrate sunlight onto an absorber tube that runs along a focal line, thus producing temperatures of about 400 °C. The heat carrier here is circulating thermal oil which absorbs the generated heat and creates steam at an approximate temperature of 390 °C in a heat exchanger; the steam is then used to drive a steam turbine and a generator to generate electricity as in conventional power plants. The principal share of solar thermal power generation in Spain is currently supplied by numerous parabolic trough plants each with a capacity of 50 MW, the majority of which have thermal storage for about seven hours of operation without sun.
Long, only slightly curved, flat mirrors concentrate the solar radiation onto a fixed absorber tube, thus directly heating and vaporising water. In comparison with the parabolic trough, the investment outlay in terms of the reflecting surface is lower due to the simpler basic concept; on the other hand, the comparative annual efficiency is lower. Two Fresnel power plants with a total capacity of 31 MW have been put into operation in the Spanish province of Murcia.
Dish concentrator systems
In solar tower power plants, the solar radiation from hundreds of automatically positioned dishes is concentrated on a central absorber at the top of the receiver. The significantly higher concentration of sunlight than in parabolic trough collectors, for example, also allows for higher temperatures of up to about 1,000 °C. This allows for greater efficiency, particularly when using gas turbines, and is therefore likely to lead to lower electricity costs.
The first commercial solar tower power plant in Europe, the PS10, which has an installed capacity of 10 MW, was commissioned in 2007 in Seville, Spain; it was supplemented in 2009 with the PS20, a twin solar tower power plant. In mid-2011, the Gemasolar solar tower power plant was connected to the grid in the province of Seville. It has a capacity of 20 MW and uses a thermal molten salt storage system that allows for up to 15 hours of storage at rated power, thus providing electricity from solar energy around the clock during the summer months. In October of 2013, a solar tower plant with a capacity of 420 MW went on the grid in the USA, and another with 120 MW is about to be commissioned.
Dish / Stirling systems
In dish/Stirling systems, a paraboloid dish concentrates the solar radiation onto the heat receiver of a downstream Stirling engine, which then converts the thermal energy into mechanical power or electricity. Efficiencies of over
30 per cent are achieved. There are prototype systems at the Plataforma Solar, for example, in Almeria, Spain. These plants are particularly suitable as stand-alone systems. 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.