Solar thermal power uses solar energy instead of combustion. Solar thermal power plants use the sun’s rays to heat a fluid to high temperatures. The fluid is then circulated through pipes so that it can transfer its heat to water and produce steam. The steam is converted into mechanical energy in a turbine, which powers a generator to produce electricity.
Solar thermal power generation works essentially the same as power generation using fossil fuels, but instead of using steam produced from the combustion of fossil fuels, the steam is produced by heat collected from sunlight. Solar thermal technologies use concentrator systems to achieve the high temperatures needed to produce steam.
Types of solar thermal power plants
There are three main types of solar thermal power systems:
- Parabolic trough
- Solar dish
- Solar power tower
Parabolic troughs are used in the longest operating solar thermal power facility in the world, which is located in the Mojave Desert in California. The Solar Energy Generating System (SEGS) has nine separate plants. The first plant, SEGS 1, has operated since 1984, and the last SEGS plant that was built, SEGS IX, began operation in 1990. The SEGS facility is one of the largest solar thermal electric power plants in the world.
A parabolic trough collector has a long parabolic-shaped reflector that focuses the sun’s rays on a receiver pipe located at the focus of the parabola. The collector tilts with the sun as the sun moves 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 from 30 times to 100 times its normal intensity (concentration ratio) on the receiver pipe located along the focal line of the trough, achieving operating temperatures higher than 750°F.
The solar field has many parallel rows of solar parabolic trough collectors aligned on a north-south horizontal axis. A working (heat transfer) fluid is heated as it circulates through the receiver pipes and returns to a series of heat exchangers at a central location. Here, the fluid circulates through pipes so it can transfer its heat to water to generate high-pressure, superheated steam. The steam is then fed to a conventional steam turbine and generator to produce electricity. When the hot fluid passes through the heat exchangers, it cools down, and is then recirculated through the solar field to heat up again.
The power plant is usually designed to operate at full power using solar energy alone, given sufficient solar energy. However, all parabolic trough power plants can use fossil fuel combustion to supplement the solar output during periods of low solar energy.
Solar dish/engine systems use concentrated solar collectors that track the sun, so they always point straight at the sun and concentrate the solar energy at the focal point of the dish. A solar dish’s concentration ratio is much higher than a solar trough’s concentration ratio, and it has a working fluid temperature higher than 1,380°F. 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 into 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 because 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. 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 1980s and 1990s. Learn more about the history of solar power in the Solar Timeline.
There are two operating solar power tower projects in the United States:
- A 5-Megawatt, two-tower project, located in the Mojave Desert in southern California
- A 392-Megawatt project located in Ivanpah Dry Lake, California
A 110-megawatt project located in Nevada is undergoing testing.