Types of Solar Thermal Power Plants

PARABOLIC TROUGHS

The parabolic trough is used in the largest solar power facility in the world located in the Mojave Desert at Kramer Junction, California. This facility has operated since the 1980’s and accounts for the majority of solar electricity produced by the electric power sector today.

A parabolic trough collector has a linear parabolic-shaped reflector that focuses the sun's radiation on a linear receiver located at the focus of the parabola.  The collector tracks the sun along one axis from east to west during the day to ensure that the sun is continuously focused on the receiver.  Because of its parabolic shape, a trough can focus the sun at 30 to 100 times its normal intensity (concentration ratio) on a receiver pipe located along the focal line of the trough, achieving operating temperatures over 400 degrees Celsius.

A collector field consists of a large field of single-axis tracking parabolic trough 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.  A working (heat transfer) fluid is heated as it circulates through the receivers and returns to a series of heat exchangers at a central location where the fluid is used to generate high-pressure superheated steam.  The steam is then fed to a conventional steam turbine/generator to produce electricity.  After the working fluid passes through the heat exchangers, the cooled fluid is recirculated through the solar field.  The plant is usually designed to operate at full rated power using solar energy alone, given sufficient solar energy.   However, all plants are hybrid solar/fossil plants that have a fossil-fired capability that can be used to supplement the solar output during periods of low solar energy. 

SOLAR DISH

A solar dish/engine system utilizes concentrating solar collectors that track the sun on two axes, concentrating the energy at the focal point of the dish because it is always pointed at the sun.  The solar dish's concentration ratio is much higher that the solar trough, typically over 2,000, with a working fluid temperature over 750oC.  The power-generating equipment used with a solar dish can be mounted at the focal point of the dish, making it well suited for remote operations or, as with the solar trough, the energy may be collected from a number of installations and converted to electricity at a central point.  The engine in a solar dish/engine system converts heat to mechanical power by compressing the working fluid when it is cold, heating the compressed working fluid, and then expanding the fluid through a turbine or with a piston to produce work.  The engine is coupled to an electric generator to convert the mechanical power to electric power.

SOLAR POWER TOWER

A solar power tower or central receiver generates electricity from sunlight by focusing concentrated solar energy on a tower-mounted heat exchanger (receiver).  This system uses hundreds to thousands of flat sun-tracking mirrors called heliostats to reflect and concentrate the sun's energy onto a central receiver tower.  The energy can be concentrated as much as 1,500 times that of the energy coming in from the sun.  Energy losses from thermal-energy transport are minimized as solar energy is being directly transferred by reflection from the heliostats to a single receiver, rather than being moved through a transfer medium to one central location, as with parabolic troughs.  Power towers must be large to be economical.  This is a promising technology for large-scale grid-connected power plants.  Though power towers are in the early stages of development compared with parabolic trough technology, a number of test facilities have been constructed around the world.

The U.S. Department of Energy along with a number of electric utilities built and operated a demonstration solar power tower near Barstow, California, during the 1980's and 1990's. Learn more about the history of solar power in the Solar Timeline.

Last Revised: November 2007
Sources: Energy Information Administration, Electric Power Annual, Form EIA-860, Annual Electric Generator Report database, 2006.