Concentrated solar power (CSP) is a method of electric generation fueled by the heat of the sun, an endless source of clean, free energy. Commercially viable and quickly expanding, this type of solar technology requires strong, direct solar radiation and is primarily used as a large, centralized source of power for utilities. In contrast, photovoltaic cells are effective in a wider range of regions and applications. CSP plants generate power best during the late afternoon â€“ during peak demand â€“ and can displace the use of fossil fuel plants that emit the greenhouse gases that cause climate change. As energy storage technology continues to advance, more CSP plants will be able to provide baseload power throughout the night.
CSP, also called solar thermal power, uses mirrors to focus sunlight onto a heatâ€transfer medium. The steam produced from the heatâ€transfer medium powers a turbine or engine that generates electricity. Depending on the type, CSP plants can supply up to 100 megawatts (MW) with a potential to produce up to 300 MW, on par with other utilityâ€scale power plants. Effective CSP requires solar radiation of at least 5.5 kWh/m2/day â€“ California averages 6.75â€8.25 kWh/m2/day1 â€“ and functions best in arid, flat locations. The U.S. Southwest, Sahara Desert, and Australia have the highest potential capacity for CSP in the world.
TYPES OF CSP SYSTEMS
Parabolic Trough: Long, curved mirrors pivot to concentrate sunlight onto tubes filled with a heat transfer fluid, generally oil or water, whose steam moves a powerâ€generating turbine. These systems are the most developed CSP technology and have operated in the United States since the 1980s.
Optimal capacity size is 150â€250 MW â€“ enough to power 44,000 homes â€“ although 80 MW is the largest plant size today. Alternately, one U.S. company, Sopogy Inc., has created a â€œMicro CSPâ€ system that uses a scaledâ€down parabolic trough system for distributed generation on rooftops.Linear Fresnal Reflectors (LFR): Still in the demonstration stage, LFR systems function like parabolic trough systems but use flat mirror strips instead of curved mirrors. Although less efficient than other CSP systems, the cheaper expense of flat mirrors lowers initial investment cost.
California Parabolic Trough Plants Productive for 25 Years Californiaâ€™s Solar Electric Generating Systems Iâ€IX have generated 12 million MW and earned more than $2 billion since 1984.
Environmental and Energy Study Institute
Dish/Engine: Mirrored dishes (resembling those for satellite television) track the sun and concentrate its heat onto a powerâ€generating unit that has an engine powered by a heatâ€responsive fluid. Stirling engines, the most common type of engine for this system, do not require the extensive water cooling system needed for steam engines because its engine is powered by the expansionâ€contraction of hydrogen gas as it is heated and cooled. The newest systems have a 31.5 percent sunâ€toâ€grid energy conversion efficiency, the highest among CSP plants.4 However, they have a smaller production capacity of 0.003â€0.025 MW.5 The first commercial deployment of a dish/Stirling system array is planned for 2010.6
Power tower: Fields of flat mirrors focus sunlight onto a central receiver filled with a heatâ€transfer fluid, most often molten salt, which can trap thermal energy for long periods. These systems concentrate heat at higher temperatures than other CSP systems, improving their conversion efficiency. A 20 MW power tower system came online in April 2009 outside Seville, Spain,7 and the early Solar Two demonstration plant, a 10 MW facility that operated from 1996â€1999 in Barstow, California, had a storage tank which provided three hours of electricity when the sun was not shining.
CSP generates power during daylight hours when demand for electricity is greatest. The heat transfer process ensures stable generation for 15â€30 minutes, enough time to endure passing clouds, but during the night or extended cloud cover, power generation requires one of two options: supplemental fuels or thermal storage.
The majority of CSP today is supplemented with natural gas so a plant can provide baseload power at all times. Like most CSP systems, many natural gas plants use steam engines to generate power, so the two systems can be hybridized easily. Alternatively, thermal storage technology can allow CSP plants to meet baseload demand without the use of backup fuels. CSP systems with storage can operate by sunlight alone for 70 percent of the year, as opposed to 15â€30 percent without storage. 9 One branch of the U.S.
Department of Energy (DOE)â€™s Solar Energy Technologies Program is conducting research on advanced heatâ€transfer fluids and storagetechniques.
WATER AND LAND IMPACTS
Because CSP functions best in sunny desert climates, water scarcity is often an issue. CSP plants with a steam engine require a cooling system to recirculate the water used. Wet cooling systems can use 758â€957 gallons of water per MWh, a level comparable with coal plants.10 Dry cooling systems, such as the Heller system, use air instead of water and can reduce water usage up to 97 percent. However, they are more expensive and can reduce energy efficiency by 5 percent.
Land requirements vary from a single rooftop for Micro CSP to 500 acres for a power tower system. The largest nonhybrid CSP system, Nevada Solar One, is a 64 MW parabolic trough plant on 400 acres. The acreage needed to generate power from coal or hydroelectric dams is higher when the mining or reservoir sites are considered.12 In 2009, the Department of the Interior designated 24 areas in six Western states â€” totaling 670,000 acres of federal land â€” as Solar Energy Study Areas, where environmental impact statements and solar resource surveys will be conducted by a new task force in the Bureau of Land Management.13 This advance work will accelerate the permitting process for future projects. Andasol I Powers through the Night This 50 MW parabolic trough plant located in Spain has a twoâ€tank molten salt heat storage system that can generate electricity for an additional 7.5 hours, even after the sun goes down. It began operation in December 2008, and two sister plants nearby will be finished in 2011. German parent company Solar Millennium also plans to build a similar plant in Nevada by 2011.
Environmental and Energy Study Institute
All solar power, including photovoltaics, generated only 0.09 percent of U.S. energy supply in 2008, but capacity is growing.15 Currently, the United States is the world leader in installed CSP capacity, with 429 MW operating in three states (see chart below). Approximately 7,000 MW from CSP is in development in the United States alone,16 and of that, 3000 MW from CSP is expected to be operational by 2011.17 The DOE projects that 2 million homes could be powered by CSP in the United States in 2020.18
CSP Plants Operating in the United States
Name and Location Built Principals Capacity Type SEGS (Solar Electric Generating Systems) Iâ€II Daggett, CA 1985â€1986 Cogentrix (owner/operator), Luz (developer), Southern California Edison (utility) 44 MW Troughâ€natural gas hybrid; SEGS Ihad a thermal storage system with a threeâ€hour capacity that was damaged by a fire in 1999.
Kramer Junction, CA 1987â€ 1989 NextEra (owner/operator), Luz (developer), Southern California Edison (utility) 5 x 30MW each
Troughâ€natural gas hybrid
SEGS VIIIâ€IX -Harper Lake, CA 1990â€1991 NextEra (owner/operator), Luz (developer), Southern California Edison (utility) 2 x 80 MW each Troughâ€natural gas hybrid
APS Saguaro Tucscon, AZ 2006 Arizona Public Service (owner/operator/utility), Solargenix Energy (developer/solar provider) 1 MW Trough
Nevada Solar One Boulder City, NV 2007 Acciona/Solargenix Energy(developer/operator), Nevada Power (utility) 64 MW Trough
Kimberlina Solar Thermal Energy Project20 Bakersfield, CA
2008 Demonstration project by Ausra USA -5 MW Linear Fresnal Reflector Keahole Solar Project Kailuaâ€Kona, HI
2008 Demonstration project by Sopogy 0.5 MW Micro Trough Sierra SunTower22 Lancaster, CA
2009 eSolar (developer), Southern California Edison (utility) 5 MW Power Tower
Selected CSP Projects in Development
â€¢34 projects planned with 9,183 MW potential capacity, including:
â€¢1600 MW from Stirling engine systems
â€¢747 MW from power tower systems
â€¢177 MW from LFR systems
â€¢8 projects, totaling 4228 MW, are currently in the review process.
â€¢1 project with 553 MW potential capacity planned for construction midâ€2009â€2011
â€¢25 year power purchase agreement made in 2007 between Solel and Pacific Gas & Electric
â€¢Currently the world’s largest solar plant in development
â€¢A hybrid of gas and parabolic trough systems with 75 MW potential capacity of solar power
â€¢Projected to supply 11,000 homes in Martin
County by 2011
â€¢Power purchase agreement between Lauren Engineers & Constructors and Florida Power & Light
â€¢500 MW photovoltaicâ€ CSP project for the Army training center at Fort Irwin, California
â€¢Commission won by private company partnership between Clark Energy Group and Acciona
â€¢Possible expansion to 1000 MW
â€¢The project value could be worth $2 billion