Concentrated solar power (CSP), is a solar-powered system in which high-temperature fluids are used with steam turbines to produce electricity. Early small-scale applications of CSP mainly involved its use for the pumping of water, however, since the mid-1980s several large-scale power systems with a power output of up to 80 MW have been built. CSP technology is especially useful in desert regions where almost all the solar radiation is incident as direct radiation.
The systems usually consist of a collector, where solar energy is absorbed, a storage system (usually water or phase change storage), a boiler that acts as a heat exchanger between the operational fluids of the collector and the heat engine, and the heat engine itself, which converts thermal energy into mechanical energy. In a large CSP plant, the heat engine is usually a steam turbine, where the energy stored in hot steam is converted to rotational energy as the steam drives the turbine. This rotational energy can be further used to drive an electric generator. The collectors are built as concentrator systems such that they are able to reach the high temperatures required to operate the heat engine.
CSP systems generate solar power by using mirrors or lenses to concentrate a large area of sunlight onto a small area. This concentrated heat eventually spins the turbine that generates electricity. The main difference between CSP systems and PV systems is that CSP systems depend on moving parts for electricity generation, whereas PV systems don’t. Different types of concentrators in the system produce different peak temperatures and hence different thermodynamic efficiencies, due to different ways of tracking the sun and focusing light. Innovations in this field are constantly leading to more energy-efficient and cost-effective CSP systems.
At the moment, there are four concentrator models used in CSP systems, three of these rely on steam turbine power to drive a turbine.
The most common one is the Parabolic Trough type that forms about 96% of all installed CSP installations. It is also called a Linear Concentrator and uses curved mirrors to focus the sun’s energy onto a receiver tube running in the trough’s centre. Inside the receiver tube, runs a high-temperature heat transfer fluid (like synthetic oil) that absorbs the sun’s thermal energy, to temperatures of 150℃ to 350℃. The steam drives a conventional steam turbine power system to generate electricity. The solar collector field contains hundreds of parallel rows of troughs connected as a series of loops, which are placed on a north to south axis so that the troughs can track the sun from east to west. Individual collector models are typically 15-20 feet tall and 300-450 feet long. This technology is highly developed and well-known and some examples include the Nevada Solar One, USA, and the Plataforma Solar de Almeria (PSA), Spain, power plants.
The second concentrator type is that of Compact Linear Fresnel Reflectors (CFLR) or Fresnel concentrators, which use the principles of curved-mirror trough systems but with long parallel rows of lower-cost flat mirrors. The modular reflectors focus the sun’s heat energy onto elevated receivers, which consist of a system of tubes through which water flows. The generation of high-pressure steam by the boiling water is used directly for power generation.
In the Dish Stirling or Dish Engine system, mirrors are distributed over a parabolic dish surface in order to concentrate sunlight on a receiver fixed at a focal point. In contrast to the other CSP technologies that use steam to create electricity using a turbine, a dish engine system uses a working fluid such as hydrogen, which is heated up to about 500℃ in the receiver to drive a Stirling engine (a heat engine that operates on the cyclic compression of a working fluid such that the heat energy is converted to mechanical work). Each dish rotates along two axes to track the sun. With about 31.25% efficiency demonstrated at Sandia National Laboratories, this design currently has the highest demonstrated efficiency.
The last concentrator type is that of Solar Power Tower Plants. They use a central receiver system (the tower), which allows for higher operating temperatures and thus greater efficiencies. Computer-controlled flat mirrors, called heliostats, track the sun along two axes and focus solar energy on a high tower. The focused heat energy is used to heat a transfer fluid to 500℃ or even 1000℃, which then produces steam to run a central power generator. Examples of solar power towers are Solar One and Solar Two in the USA and the Eureka project in Spain.
Other advanced CSP technologies use molten nitrate salt because of its superior heat-transfer and energy storage capabilities. It is this energy storage capability that allows the system to continually produce electricity during cloudy weather or even at night. In 2018, CSP had a world’s installed capacity of 5,500 MW, up from 354 MW in 2006. Of which, Spain accounts for almost half the world’s capacity at 2,300 MW, making it the world leader at the end of 2018. The USA follows Spain with 1,740 MW of CSP.
One problem that CSP systems face is that the efficiency of the collector diminishes as its operating temperature rises, while the efficiency of the engine increases with temperature. Hence, a compromise between the two has to be found when choosing the operating temperature. Current systems have efficiencies of up to 30%.