How It Works

Concentrated Solar Power (CPS) technologies use large, sun-tracking mirrors to concentrate solar radiation. However, the final steps of generating electricity using CSP systems is similar to conventional electricity generation - the ultimate energy conversion process depends on the use of steam or gas to rotate turbines, or move a piston in a Stirling engine. In a CSP system, however, steam or hot gas is produced by the concentrated solar radiation.

CSP technologies have been constructed in various sizes, from small multi-kW systems, to large power stations of several MW. These power stations have provided the cheapest electricity to be generated using solar power.

Different CSP Technologies

There are three main CSP technologies being promoted internationally. For each of these, there exist various design variations or different configurations. A brief description of the basic options will be given. More details can be obtained from the sites on the LINKS page.

Parabolic Trough plant in California.

  Parabolic Trough Technology

 Parabolic-trough power plants consist of large fields of parabolic trough collectors, a HTF/steam  generation system, a Rankine steam turbine/generator cycle, and optional thermal storage and/or fossil-fired backup systems.
  These systems function as follows:
 The collector field consists of a large field of single-axis tracking parabolic trough solar collectors. The solar field is modular in nature and is composed of many parallel rows of solar collectors aligned on a north-south horizontal axis. Each solar collector has a linear, parabolic-shaped reflector that focuses the sun’s direct beam radiation on a linear receiver located at the focus of the parabola. The collectors track the sun from east to west during the day to ensure that the sun is continuously focused on the linear receiver.

 An heat transfer fluid (HTF) is heated up to 390ºC as it circulates through the receiver and returns to a series of heat exchangers in the power block where the fluid is used to generate high-pressure superheated steam (100 bar, 371ºC). The superheated steam is then fed to a conventional reheat steam turbine/generator to produce electricity. The spent steam from the turbine is condensed in a standard condenser and returned to the heat exchangers via condensate and feedwater pumps to be transformed back into steam. Condenser cooling is provided by mechanical draft wet cooling towers. After passing through the HTF side of the solar heat exchangers, the cooled HTF is recirculated through the solar field.

Example of a Central Receiver plant

  Central Receiver Technology

 Central receiver technology, also known as "power tower technology," is based on the concept of hundreds or thousands of large, two-axis tracking mirrors, called heliostats, that track the sun and reflect the beam radiation to a common focal zone. The focal zone is placed well above the heliostat field to help prevent interference between the reflected radiation and other heliostats. A receiver at the focal zone absorbs the concentrated radiation, converts it to heat, and uses it to heat an HTF to a specified temperature.

 The degree of heating in the receiver depends on several design factors such as heliostat field size, receiver shape and size, HTF limitations, and end-use application. The most common end-use application for central receiver technology is a Rankine power cycle, although the technology can also be used as the heat source for other, more efficient cycles such as Brayton/combined cycle as well as high-temperature process steam.

25kW SES/Boeing Dish/Stirling system

  Parabolic Dish Technology

 Parabolic dish technology has emerged in two particular forms of interest, the modular dish/engine electric-generating system and the solar dish array steam-generating system. In the former, each dish/engine system produces electricity, and a power plant of any size can be built up from individual modules. In the latter, the dish field collects and delivers thermal energy for any given purpose, be it electricity generation, process heat, or other uses.

 The dish/engine system uses a reflective parabolic dish to concentrate the sun’s rays onto a cavity receiver. Thermal energy collected by the receiver is transferred to the engine of a power conversion unit (PCU), which converts the thermal energy into mechanical energy. This mechanical energy can then be converted to electricity or used directly in distributed applications such as water pumping. In a similar manner, the thermal dish array uses reflective parabolic dishes and cavity receivers to collect thermal energy. This heat can then be delivered to a central plant of a solar-only or solar/fossil configuration, uses appropriately sized conventional power generation equipment. For example, a small solar-only system could run a single Brayton cycle, or a large solar field could augment the heat input of a gas-fired power plant. In either the dish/engine or the thermal array case, single or multiple systems can be used to deliver distributed or centralized power, according to the needs of the site.