Concentrating solar power (CSP) plants use mirrors to concentrate the sun’s energy to drive traditional steam turbines or engines that create electricity. The thermal energy concentrated in a CSP plant can be stored and used to produce electricity when it is needed, day or night. Today, roughly 1,815 megawatts (MWac) of CSP plants are in operation in the United States.
Parabolic Trough
Parabolic trough systems use curved mirrors to focus the sun’s energy onto a receiver tube that runs down the center of a trough. In the receiver tube, a high-temperature heat transfer fluid (such as a synthetic oil) absorbs the sun’s energy, reaching temperatures of 750°F or higher, and passes through a heat exchanger to heat water and produce steam. The steam drives a conventional steam turbine power system to generate electricity. A typical solar collector field contains hundreds of parallel rows of troughs connected as a series of loops, which are placed on a north-south axis so the troughs can track the sun from east to west. Individual collector modules are typically 15-20 feet tall and 300-450 feet long.
Compact Linear Fresnel Reflector
CLFR uses the principles of curved-mirror trough systems but with long parallel rows of lower-cost flat mirrors. These modular reflectors focus the sun’s energy onto elevated receivers, which consist of a system of tubes through which water flows. The concentrated sunlight boils the water, generating high-pressure steam for direct use in power generation and industrial steam applications.
Power Tower
Power tower systems use a central receiver system, which allows for higher operating temperatures and thus greater efficiencies. Computer-controlled mirrors (called heliostats) track the sun along two axes and focus solar energy on a receiver at the top of a high tower. The focused energy is used to heat a transfer fluid (over 1,000° F) to produce steam and run a central power generator. Energy storage can be easily and efficiently incorporated into these projects, allowing for 24-hour power generation.
Dish-Engine
Mirrors are distributed over a parabolic dish surface to concentrate sunlight on a receiver fixed at the focal point. In contrast to other CSP technologies that employ steam to create electricity via a turbine, a dish-engine system uses a working fluid such as hydrogen that is heated up to 1,200° F in the receiver to drive an engine. Each dish rotates along two axes to track the sun.
Key Requirements for Concentrating Solar Power Plants
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Financing – The primary challenge for any utility-scale energy-generating facility, including CSP, is project financing.
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Areas of high solar radiation – In order to concentrate the sun’s energy, it must not be too diffuse. This is measured by the direct normal intensity (DNI) of the sun’s energy. Production potential in the U.S. Southwest stands apart from the rest of the U.S.
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Contiguous parcels of land with limited cloud cover – A CSP plant operates most efficiently, and thus most cost-effectively, when built in sizes of 100 MW and higher. While land needs will vary by technology, a typical CSP plant requires 5 to 10 acres of land per MW of capacity. The larger acreage accommodates thermal energy storage.
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Access to water resources – Like other thermal power plants, such as natural gas, coal and nuclear, most CSP systems require access to water for cooling. All require small amounts of water to wash collection and mirror surfaces. CSP plants can utilize wet, dry, and hybrid cooling techniques to maximize efficiency in electricity generation and water conservation.
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Available and proximate transmission access – CSP plants must be sited on land suitable for power generation with adequate access to an increasingly stressed and outdated transmission grid. Access to high-voltage transmission lines is key for the development of utility-scale solar power projects to move electricity from the solar plant to end users. Much of the existing transmission infrastructure in the Southwest is at full capacity and new transmission is urgently needed.
