Concentrating solar power (CSP) plants use mirrors or lenses to concentrate sunlight, creating temperatures high enough to drive traditional steam turbines or engines that in turn create electricity. The most cost-effective CSP plants are hundreds of megawatts (MW) in size, making them attractive as wholesale energy suppliers to utilities. Today, over 520 MW of CSP plants operate in the United States and there are over 1,950 MW of CSP projects with signed PPAs.
(Photo courtesy of SkyFuel Inc.)
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 (e.g., synthetic oil) absorbs the sun’s energy, reaching temperatures of around 700° F, 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
(Photo courtesy of AREVA Solar)
To reduce some of the up-front capital costs of plant construction, Compact Linear Fresnel Reflector (CLFR) developers rely on the principles of curved-mirror trough systems, but use 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.
(Photo courtesy of Sandia National Laboratories)
Seeking higher operating temperatures for greater efficiencies, other plant designers opt for a central receiver system. Computer-controlled flat 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 (800° F to 1,000° F) to produce steam and run a central power generator.
(Photo courtesy of Sandia National Laboratories/Randy Montoya)
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 such as the Stirling engine. Each dish rotates along two axes to track the sun
Key Requirements for Concentrating Solar Power Plants
Nine CSP plants, totaling over 350 MW, have been in daily operation near Kramer Junction, Calif., for over 20 years. In the last four years, new plants have come online in Arizona, Nevada, and California. The plants’ locations reflect the important conditions required for this type of project. CSP plants need:
Financing – The primary barrier to utility-scale solar power is project financing. The 2008 economic crisis severely restricted the private sector capital that is typically used to finance renewable energy projects. Commercial banks today simply do not have enough appetite for long-term, low interest debt to finance construction of every project in the queue.
Areas of high direct normal solar radiation – In order to concentrate the sun’s energy, it must not be too diffuse. This feature is captured by measuring 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.
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.
Access to water resources – Like other thermal power plants, such as natural gas, coal and nuclear, some 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.
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.