CSP - How it works
In a brief description, CSP plants produce electricity by converting sun's energy into high-temperature heat by concentrating the sunlight. The heat is then converted into mechanical energy to drive a generator which produces electricity.
CSP plants are usually divided in two main sections: the solar field (SF) and the power block (PB). When a plant includes Thermal Energy Storage (TES) systems, it can be included in the power block or be considered as a third main sector.
The solar field is the area where sunlight is concentrated and heat is collected, either in pipes or in a central receiver. The power block is the section where heat is converted into electricity.
Heat from SF to PB is transferred by the Heat Transfer Fluid (HTF). HTF can be water, oil, molten salt or other fluid capable of transferring thermal energy.
CSP can be divided in four main technologies:
In this type of CSP technology, the sun's energy is concentrated by parabolically curved, trough-shaped reflectors onto a receiver pipe running along the inside of the curved surface.
The HTF that flows through the pipes is heated thanks to the concentration of sunlight. At the end of the circuit, HTF is driven to heat exchangers in the power block.
HTF for parabolic trough plants
The currently most used fluid is a synthetic oil, made with a eutectic mixture of Biphenil and Diphenil Oxide, some commercial names are Dowtherm or Therminol.
In this case, the heat collected by the oil is transferred to water through a heat exchanger to produce steam.
It's possible to use water directly in the solar field, this way, we have a plant with Direct Steam Generation, and therefore, it's not necessary a heat exchanger.
Some research projects are using molten salt as HTF. Molten salt is a eutectic mixture of Sodium and Potassium Nitrate. Molten salt can reach higher temperatures than synthetic oil and can be used to store thermal energy.
The PB section of a parabolic trough plant receives the hot HTF. Thanks to the heat exchangers and the steam generation system, the plant produces steam to drive the turbine. Exhaust steam from the turbine is then partially recirculated to improve performance and at the end it's cooled and condensed to be sent to heat exchangers again.
Electricity is generated by the generator coupled with the steam turbine.
To keep HTF above a minimum temperature, plants use to have natural gas boilers. This is a way to improve performance, e.g. at the start-up in the morning, heating HTF quickly or when transient clouds appears.
Layout of a parabolic trough plant
The solar field comprises many troughs in parallel rows aligned on a north-south axis. This configuration enables the single-axis troughs to track the sun from east to west during the day to ensure that the sun is continuously focused on the receiver pipes.
Solar field size is designed as a 'Solar Multiple'. To clarify, a solar multiple of 1 means the size to produce heat enough to drive the turbine at nominal output with the typical DNI value. With higher solar multiple values, we can assure the nominal generation in times of lower DNI or produce enough heat to be stored.
The power block is usually placed in the center of the solar field. The hot HTF (about 393 ºC for synthetic oil) is collected to produce steam and the resulting 'cold' HTF (about 293 ºC) is sent again to the solar field.
Thermal storage in parabolic trough plants
Trough plants can incorporate Thermal Energy Storage (TES) systems, it's usually done by heating molten salts which are stored in a tank. When a plant has storage system, the solar field is 'oversized' to collect enough heat for the normal operation of the plant and to heat the thermal storage medium, usually molten salt.
The TES system is commonly made with two tanks, one for 'hot' salt and other for 'cold' salt. Part of the heat collected in the solar field is transferred to the molten salt which flows from the cold to the hot tank. When it's necessary to use the heat from the TES, e.g. by evening, hot salt then flows to cold salt tank and thanls to a heat exchanger, heat is transferred to the water/steam cycle.
TES capability of CSP plants is commonly measured by hours, what means the total amount of time the stored molten salts can drive the plant with no sunlight. Values range form a couple of hours to up to 9 hours, depending in the availability of solar radiation and the size of the solar field.
Basic diagram of a parabolic plant with TES
Parabolic trough from SENER
Typical aerial view of a CSP plant (La Africana, Spain)
You can download the "Technology Characterization Solar Parabolic Trough (PDF 303KB)" document from SolarPACES website
This is currently the most deployed technology, with about 1.6 GW installed all over the world and about 2.6 GW in construction (january 2012 figures)
These plants are being built with a power output between some megawatts to almost 200 MW. The most common are the 50MW units constructed in Spain (due to an administrative regulation which limit the power to 50 MW in order to access to FiT). The optimum power output seems to be around 125 MW.
Power tower technology, also known as 'central receiver', uses a field of flat -or nearly flat- mirrors which track the sun and focus the sunlight to a receiver at the top of a tower.
The SF of a tower type plant is comprised by 'helisotats', the mirrors that track the sun to reflect the sunlight onto the receiver.
Heliostats can be found in many different sizes, from one or two square meters to the 120 square meters heliostats used by Abengoa.
The receiver, located atop the tower, is also known as a solar boiler. The HTF is pumped up to the tower where concentrated sunlight heats it and is then sent down to the power block.
The receiver is a hey component for the plant, many research projects have been carried out to develop high performance receiver to maximize the efficiency. Currently, two main kind of receiver are being used, the cavity receiver, as those installed at the PS10 and PS20 and cilindrical external receiver, as the ones used in Gemasolar, Ivanpah or CrescentDunes.
In a tower type solar plant, a wide variety of HTF can be used, from air to any fluid that can be heated. Currently, the most common HTF is steam, molten salt or air.
Tower plants can produce steam directly in the receiver and then send it to the turbine, thiis is the case of PS10, PS20 or Ivanpah plants.
Some plants have been designed to use molten salt as HTF, this way, molten salt can be used as HTF and storage medium. This is the case of Gemasolar and Crescent Dunes plants.
Air is also being tested in some small scale plants like the ones developed by AORA. These plants work as hybrid solar-gas plants.
The fist technology to be commercially deployed has been the saturated steam.
ABENGOA's PS-10 power tower plant basic scheme (first commercial power tower plant in the world)
This video by SolarReserve, explains how a molten salts power tower works.
The linear Fresnel technology, is based in the same principles as parabolic trough, thought, in this case, we don't use curved mirrors, but long, narrow flat -or nearly flat- mirrors.
The mirrors are located on a flat platform and the receiver is fixed above them, so that the mirrors rotate on its longitudinal axis to track the sun and reflect the sunlight to the receiver.
This picture shows us a Fresnel demonstration plant at PSA (Solar Platform of Almeria, Spain)
This video by AREVA explains the fresnel technology
In comparison with parabolic trough, it requires less land and less water, but it has a lower performance, this is commonly used as a Direct Steam Generator technology, although some development projects are testing the use of molten salt or
The most common parabolic dish technology is slightly different from the others, it doesn't use a Heat Transfer Fluid (HTF) neither a steam turbine.
This technology uses a Stirling engine, a device which works with heat as fuel. The parabolic dish concentrate the sunlight in a point directly to the Stirling engine, which drives an alternator to generate electricity.
This picture is from the EUROdish prototype at the PSA (Solar Platform of Almeria, Spain)
In this video, Chuck Andraka, a Dish expert from Sandia National Laboratories (US), explains how a parabolic dish with Stirling engine works
The advantages of this technology is the low water needs (just for cleaning the dish), and the simplicity and scalability. This is a perfect technology for off-grid electricity generation.
Parabolic dishes are also being developed to use HTF (e.g. pressurized air) and collect the heat to drive a turbine (gas or steam turbine).
Hybridization is a key point for CSP. Since CSP plants are roughly thermal plants, it's easy to think about mix sources of energy, e.g. a solar field and natural gas.
ISCC that stands for Integrated Solar Combined Cycle uses a solar field to produce steam that is added to the steam turbine of a combined cycle. For now, there are several projects under operation in the US, Moroco, Algeria and Egypt. All of them uses a parabolic trough solar field.
Coal-fired plants can also be boosted with a solar field that adds steam to the system. There are some projects running in Australia.
Biomass suits perfectly with CSP plants and can provide 24/7 renewable energy by mixing a solar field and a biomass boiler. The first project of this type has been built in Spain.