Environmental Engineering Reference
In-Depth Information
Often flat-plate collectors are located on roofs of buildings or on the ground, tilted toward the
prevailing direction of the sun and fixed in place. Some may be mechanized to follow the path
of the sun through the sky.
In concentrating collectors, the area intercepting solar radiation is sometimes hundreds of times
greater than the absorber area (USEIA 2011f). Solar thermal power plants use the sun's rays to
heat a fluid to very high temperatures. The fluid is then circulated through pipes so it can transfer
heat to water and produce steam. The steam, in turn, is converted into mechanical energy in a
turbine and into electricity by a conventional generator coupled to the turbine.
Solar thermal power generation works essentially the same way as generation from fossil fuels
except that instead of steam being produced from combustion of fossil fuels, steam is produced by
the heat collected from sunlight. Solar thermal technologies use concentrator systems to achieve
high temperatures needed to heat the fluid. The three main types of solar thermal concentrating
power systems are parabolic trough, solar dish, and solar power tower.
Parabolic Trough
In the most common type of solar power plant, 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 it moves from east to west during the day to ensure it is continu-
ously focused on the receiver. Because of its parabolic shape, a trough can focus the sun at thirty
to 100 times its normal intensity (concentration ratio) on the receiver pipe located along the focal
line of the trough, achieving operating temperatures over 750°F (USEIA 2011d).
Many parallel rows of solar parabolic trough collectors are aligned on a north-south horizontal
axis in a solar field. A 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 heat to water and 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 heat exchangers, it cools down and is then recirculated through the solar field to heat it
up again. Parabolic troughs are 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 1980s and
accounts for most solar electricity produced by the electric utility industry in the United States
today (USEIA 2011d).
Solar Dish
A solar dish/engine system uses concentrating solar collectors that track the sun and concentrate
solar energy at the focal point of a dish. A solar dish's concentration ratio is much higher than
a solar trough's, typically over 2,000, with a working fluid temperature over 1,380°F (USEIA
2011f). Power-generating equipment used with a solar dish can be mounted at the focal point of
the dish or, as with the solar trough, 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 mechanical power to electric power (USEIA 2011f).
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