Study of Solar thermal power plants

Solar thermal power plants

Power plants based on concentrated solar power (CSP) use the sun’s energy to generate electricity on an industrial scale. Solar radiation is optically concentrated, thus generating very high temperatures for the power plant process. This high-temperature heat can be stored, thus allowing electricity to be generated on demand – an important advantage of this technology.

Operating principle of a parabolic trough collector.

Operating principle of a Fresnel collector.

Central receiver operating principle.

Dish / Stirling system operating principle.

DLR, CC-BY 3.0

In a solar tower power plant, solar radiation is concentrated by many ­mirrors onto a receiver at the top of a tower. Temperatures of more than 1,000 degrees Celsius occur at this point and can be used, among other tasks, to produce electricity.

Technology

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 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 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.

Solar thermal power plants are characterised by the fact that the heat generated using this principle 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 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.

Fresnel collectors

• Long, only slightly curved, flat mirrors concentrate the solar radiation onto a fixed absorber tube, thus directly heating and vapourising 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 province of Murcia.

Dish concentrator systems

Central receivers

• In central receiver solar 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 central receiver solar 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 central receiver solar power plant. In mid-2011, the Gemasolar central receiver solar 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.

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. Prototype systems exist, for example, at the Plataforma Solar 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.

Solar thermal power plants – Made in Germany

Germany is the global leader in research into and development of this technology.

As early as 1984 – 1991, German companies supplied the essential components – such as dishes and flexible pipe connections - for parabolic trough plants in California that are still in operation today, and have produced 16,000 GWh of electricity since then. Today, German manufacturers supply the core components for solar fields and power-­station units as well as the necessary measurement and control technology. In other key components, such as p­recision mirrors for parabolic trough plants or solar receivers, German companies have a very high market share worldwide. Besides this, German companies have an internationally acknowledged reputation as technology consultants, certification bodies and experts.

German engineering companies are also involved in the construction and operation of various pioneering projects either in terms of project management or through shareholdings. For example, a central receiver solar power plant with an electrical output of 1.5 MW has been operational at Juelich in Germany since the end of 2008. It is operated as a prototype power plant and research platform in the light of the long-standing experience of German research institutes and companies. Air is used as the heat carrier in a volumetric absorber. It also uses a storage system to compensate for fluctuations in the insolation input. The experience gained from building and operating this prototype power plant are the basis for further optimisation in future projects.