Thermal solar power as an alternative to coal
Solar thermal energy (STE) is a technology for harnessing solar energy for thermal energy (heat). Solar thermal collectors are defined by the US Energy Information Administration as low-, medium-, or high-temperature collectors. Low temperature collectors are flat plates generally used to heat swimming pools. Medium-temperature collectors are also usually flat plates but are used for creating hot water for residential and commercial use. High temperature collectors concentrate sunlight using mirrors or lenses and are generally used for electric power production. STE is different from photovoltaics, which convert solar energy directly into electricity. While only 600 megawatts of solar thermal power is up and running worldwide in October 2009, according to Dr David Mills of Ausra, another 400 megawatts is under construction and there are 14,000 megawatts of the more serious concentrating solar thermal (CST) projects being developed.
Solar thermal power plants
The most common usage of solar thermal energy is for on site water and space heating. However, with high temperature collectors, electrical energy has been reliably produced by concentrated solar power arrays: mirrors focusing light onto pipes of water or other heat transfer fluid. The hot water is heated to the boiling point and powers a steam turbine to generate electricity, or is preheated for use in fossil fuel based generation. 
Some thermal solar power plants use a highly curved mirror called a parabolic trough to focus the sunlight on a pipe running down a central point above the curve of the mirror. The mirror focuses the sunlight to strike the pipe, and it gets so hot that it can boil water into steam. That steam can then be used to turn a turbine to make electricity. In California's Mojave desert, there are huge rows of solar mirrors arranged in what's called "solar thermal power plants," which makes electricity for more than 350,000 homes. Some solar plants, are a "hybrid" technology: during the daytime they use the sun, and at night and on cloudy days they burn natural gas to boil the water so they can continue to make electricity.
Another form of solar power plants to make electricity is called a solar tower. Sunlight is reflected off mirrors circling a tall tower. The mirrors are called heliostats and move and turn to face the sun all day long. The light is reflected back to the top of the tower in the center of the circle where a fluid is turned very hot by the sun's rays. That fluid can be used to boil water to make steam to turn a turbine and a generator.
Promise of solar thermal for baseload power
Solar power is often seen as intermittent and thus not a reliable source for continuous, baseload power. But new research suggests molten salts can store the sun’s heat during the day and provide power at night. Near Granada, Spain, more than 28,000 metric tons of salt is now in pipes at the Andasol 1 power plant to store sunlight as heat energy for later. Because most salts only melt at high temperatures (table salt, for example, melts at around 1472 degrees Fahrenheit, or 800 degrees Celsius) and do not turn to vapor until they get considerably hotter—they can be used to store a lot of the sun’s energy as heat. Sunlight heats up the salts and, placing the molten salts in proximity to water via a heat exchanger, hot steam can then be made to turn turbines without losing too much of the original absorbed solar energy.
The salts—a mixture of sodium and potassium nitrate, otherwise used as fertilizers—allow enough of the sun’s heat to be stored that the power plant can pump out electricity for up to eight hours after the sun starts to set. The salts can deliver back as much as 93 percent of the energy.
The Andasol 1 power plant, which cost around $380 million (300 million euros) to build, is the first to actually use the technology, so it remains to be seen how it will work in commercial practice. But U.S. government laboratories—U.S. National Renewable Energy Laboratory (NREL) as well as Sandia National Laboratory in Albuquerque, N.M.—have found the technology can work in demonstration projects, like the Solar Two power tower outside Barstow, California. Solar Millennium is planning a twin solar-thermal power plant (Andasol 2), already near completion. Arizona Public Service (APS) has contracted with Abengoa Solar to build a 280-megawatt solar thermal power plant—dubbed Solana or “sunny place”—70 miles (110 kilometers) southwest of Phoenix on nearly 2,000 acres of land.
Thermal energy storage at Andasol 1 or power plants like it costs roughly $50 per kilowatt-hour to install, according to the U.S. NREL. But it doesn’t add much to the cost of the resulting electricity because it allows the turbines to be generating for longer periods, and those costs can be spread out over more hours of electricity production. Electricity from a solar-thermal power plant costs roughly 13 cents a kilowatt-hour, according to Glatzmaier, both with and without molten salt storage systems. That price is still nearly twice as much as electricity from a coal-fired power plant, although solar companies predict costs will come down for solar as its energy efficiency improves, and up for coal as the industry increasingly absorbs the external costs of coal. “The main goal is to find a storage technology that may reduce the actual capital cost” of adding it to a power plant, says Phil Smithers, technical services leader for renewable energy at APS, which is researching those technologies under a U.S. Department of Energy grant.
Siting and Feasibility Studies
The National Renewable Energy Laboratory's "TroughNet" site provides a variety of technical, economic, and market assessment resources on solar thermal technology.
The following studies available from NREL have assessed solar thermal's siting potential:
- Clean and Diversified Energy Initiative: Solar Task Force Report, Report Available from: Western Governors' Association (PDF 3.1 MB), Publication Date: January 2006
- Potential for Renewable Energy in the San Diego Region, Appendix E: Solar Thermal - Concentrated Solar Power, Appendix Available from: San Diego Regional Renewable Energy Study Group (PDF 1.8 MB), Publication Date: August 2005
- Assessment of the World Bank/GEF Strategy for the Market Development of Concentrating Solar Thermal Power, Report Available from: Global Environment Facility (PDF 2 MB), Author: Global Research Alliance for the World Bank, Publication Date: May 2005
- CSP (Concentrating Solar Power) Global Market Initiative, Available from: SolarPaces (PDF 1.7 MB)
- Assessment of Parabolic Trough and Power Tower Solar Technology Cost and Performance Forecasts, NREL Subcontract Report: (PDF 2.4 MB), Author: Sargent & Lundy LLC Consulting Group, Publication Date: October 2003
- Executive Summary: Assessment of Parabolic Trough and Power Tower Solar Technology Cost and Performance Forecasts, NREL Subcontract Report: (PDF 588 KB), Author: Sargent & Lundy LLC Consulting Group, Publication Date: October 2003, Note: Includes additional reference lists
- Fuel from the Sky: Solar Power's Potential for Western Energy Supply, NREL Subcontract Report: (PDF 3 MB), Author: Leitner, A., RDI Consulting, Publication Date: July 2002
- Report to Congress on: Feasibility of 1,000 Megawatts of Solar Power in the Southwest by 2006, Available from: U.S. Department of Energy SunLab (PDF 956 KB), Publication Date: August 2002
- The Commercial Path Forward for Concentrating Solar Power Technologies — A Review of Existing Treatments of Current and Future Markets 2001, Report Available from: SolarPaces (PDF 953 KB), Author: Morse, F., Publication Date: December 2000
- Cost Reduction Study for Solar Thermal Power Plants, Report Available from: SolarPaces (PDF 1 MB), Author: Enermodal Engineering Limited with Marbek Resource Consultants Ltd. for World Bank, Publication Date: May 1999
- Parabolic Trough Solar Power for Competitive U.S. Markets, NREL Conference Paper: (PDF 68 KB), Event: Renewable and Advanced Energy Systems for the 21st Century Conference (RAES '99), Authors: Price, H. W.; Kistner, R., Publication Date: April 1999
- Renewable Energy Technology Characterizations, Reports Available from: U.S. Department of Energy, Author: Electric Power Research Institute, Publication Date: December 1997
- Mining for Solar Resources: U.S. Southwest Provides Vast Potential, Article Available from: Atmospheric Sciences Research Center (PDF 893 KB), Authors: Mehos, M.; Perez, R., Publication Date: 2005
- Assessing the Potential for Renewable Energy on National Forest Service Lands, NREL Booklet: (PDF 5.1 MB), Authors: USDA Forest Service and NREL, Publication Date: January 2005
- Analysis of Siting Opportunities for Concentrating Solar Power Plants in the Southwestern United States, Conference Paper Available from: Ratepayers United of Colorado (PDF 1.1 MB), Event: World Renewable Energy Congress VIII, 29 August - 3 September 2004, Denver, Colorado, Authors: Mehos, M. S.; Owens, B.
- Assessing the Potential for Renewable Energy on Public Lands, U.S. Department of Energy Booklet: (PDF 4.5 MB), Authors: U.S. Department of Energy and U.S. Department of the Interior Bureau of Land Management, Publication Date: February 2003
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- Jon Markman,"It's solar power's time to shine" MSN Money, June 5, 2008.
- Ken Zweibel, James Mason and Vasilis Fthenakis, "A Solar Grand Plan: By 2050 solar power could end U.S. dependence on foreign oil and slash greenhouse gas emissions," Scientific American Magazine, January 2008.
- "The Energy Story: Solar Energy Energy Quest, accessed July 2010.
- Joe Romm, "How to use solar energy at night: Concentrated solar thermal power with storage" Climate Progress, Jan. 16, 2011.
- "A Solar Grand Plan," Ken Zweibel, James Mason and Vasilis Fthenakis, Scientific American, December 2007.
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