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DOE/GO-102001-1147 FS 128 March 2001 Concentrating Solar : from

Mirror on the wall, what's the The southwestern is focus- greatest energy source of all? The . ing on concentrating because Enough energy from the sun falls on the it's one of the world's best for sun- everyday to power our homes and . The Southwest receives up to twice businesses for almost 30 years. Yet we've the as other regions in the coun- only just begun to tap its potential. You try. This abundance of solar energy makes may have heard about solar concentrating an attrac- to light homes or solar thermal power tive alternative to traditional power plants, used to , but did you know there which burn polluting fossil such as is such a thing as solar thermal-electric oil and . Fossil fuels also must be power? Electric utility companies are continually purchased and refined to use. using mirrors to concentrate heat from the sun to produce Unlike traditional power plants, concen- for , especially in the trating solar power provide an southwestern United States. environmentally benign source of energy, produce virtually no emissions, and con- Photo by Hugh Reilly, Sandia National Laboratories/PIX02186 Photo by Hugh Reilly, This concentrating — known as Solar Two — near Barstow, , is the world’s largest central receiver .

This document was produced for the U.S. Department of Energy (DOE) by the National Laboratory (NREL), a DOE national laboratory. The document was produced by the and Outreach Program at NREL for the DOE Office of Energy Efficiency and Renewable Energy. The Energy Efficiency and Renewable Energy Clearinghouse (EREC) is operated by NCI Information Systems, Inc., for NREL / DOE. The statements contained herein are based on information known to EREC and NREL at the of printing. No recommendation or endorsement of any product or service is implied if mentioned by EREC.

Printed with a renewable-source ink on paper containing at least 50% wastepaper, including 20% postconsumer waste sume no other than sunlight. About cations, ranging from remote power sys- the only impact concentrating solar power tems as small as a few kilowatts (kW) up plants have on the is to grid-connected applications of 200-350 use. Although the amount of land a con- megawatts (MW) or more. A concentrat- centrating solar power plant occupies is ing solar power system that produces 350 larger than that of a plant, both MW of electricity displaces the energy types of plants use about the same amount equivalent of 2.3 million barrels of oil. of land because fossil fuel plants use addi- tional land for and exploration as well as road building to reach the mines.

Other benefits of concentrating solar Concentrator power plants include low operating costs, Receiver Individual trough and the ability to produce power during high-demand energy periods and to help systems currently increase our —our coun- try's independence from foreign oil can generate about imports. Because they store energy, they can operate in cloudy and after 80 MW of electricity. sunset. When combined with fossil fuels as a hybrid system, they can operate around the clock regardless of weather. Concentrating solar power plants also cre- ate two and a half as many skilled jobs as traditional plants. Fig. 1 A

Types of Systems Trough Systems Unlike solar (photovoltaic) cells, which These solar collectors use mirrored para- use light to produce electricity, concentrat- bolic troughs to focus the sun's energy to ing solar power systems generate electric- a fluid-carrying receiver tube located at ity with heat. Concentrating solar the focal point of a parabolically curved collectors use mirrors and to con- trough reflector (see Fig.1 above). The centrate and focus sunlight onto a from the sun sent to the tube receiver, similar to a boiler tube. The oil flowing through the tube, and the heat receiver absorbs energy is then used to generate electricity and converts sun- in a conventional steam generator. light into heat. Many troughs placed in parallel rows are The heat is then called a "collector ." The troughs in transported to a the field are all aligned along a north- steam generator south axis so they can track the sun from or where east to west during the day, ensuring that it is converted the sun is continuously focused on the into electricity. receiver pipes. Individual trough systems There are three currently can generate about 80 MW of main types of con- electricity. Trough designs can incorporate centrating solar thermal storage—setting aside the heat power systems: transfer fluid in its hot —allowing parabolic troughs, for several hours dish/engine sys- into the evening. tems, and central- Currently, all parabolic trough plants are receiver systems. "hybrids," meaning they use fossil fuels to These supplement the solar output during peri- can be used to gen- ods of low solar . Typically, a nat- erate electricity for ural gas-fired heat or a gas steam Photo by Warren Gretz, NREL/PIX00033 Gretz, Photo by Warren a variety of appli- boiler/reheater is used. Troughs also can Nine trough power plants in southern California, with a capacity of 354 MW, meet the energy needs of 350,000 people. 2 be integrated with Dish/engine systems are not commer- existing coal-fired cially available yet, although ongoing plants. demonstrations indicate good potential. Individual dish/engine systems currently can generate about 25 kW of electricity. Dish Systems More capacity is possible by connecting dishes together. These systems can be Dish systems use combined with , and the result- dish-shaped para- ing hybrid provides continuous power bolic mirrors as generation. reflectors to con- centrate and focus the sun's rays onto Central Receiver Systems a receiver, which is mounted above the Central receivers (or power towers) use dish at the dish cen- thousands of individual sun-tracking mir- ter. A dish/engine rors called "" to reflect is a stand- energy onto a receiver located on top of a alone unit com- tall tower. The receiver collects the sun's posed primarily of heat in a heat-transfer fluid () a collector, a that flows through the receiver. The salt's Photo by Warren Gretz, NREL/PIX02342 Gretz, Photo by Warren receiver, and an heat energy is then used to make steam to This concentrating solar power system uses mirrors to generate electricity in a conventional focus highly concentrated sunlight onto a receiver that engine (see Fig.2 converts the sun’s heat into energy. below). It works by steam generator, located at the foot of the collecting and con- tower. The molten salt storage system centrating the sun's energy with a dish- retains heat efficiently, so it can be stored shaped surface onto a receiver that for hours or even days before being used absorbs the energy and transfers it to the to generate electricity. Therefore, a central engine. The engine then converts that receiver system is composed of five main energy to heat. The heat is then converted components: heliostats, receiver, heat to mechanical power, in a manner similar transport and exchange, thermal storage, to conventional , by compressing and controls (see Fig. 3 on page 4). the working fluid when it is cold, heating the compressed working fluid, and then Receiver expanding it through a or with a and Concentrator piston to produce mechanical power. An generator or alternator converts the mechanical power into electrical power. Dish/engine systems use dual-axis collec- tors to track the sun. The ideal concentra- tor shape is parabolic, created either by a single reflective surface or multiple reflec- tors, or facets. Many options exist for Individual receiver and engine type, including Stir- ling cycle, microturbine, and concentrat- dish/engine systems ing photovoltaic modules. Each dish produces 5 to 50 kW of electricity and can Fig. 2 A dish system currently can be used independently or linked together to increase generating capacity. A 250-kW Solar One, Two, “Tres” generate about plant composed of ten 25-kW dish/engine systems requires less than an acre of land. The U.S. Department of Energy (DOE), 25 kW of electricity. and a consortium of U.S. utilities and industry, built this country's first two large-scale, demonstration solar power towers in the near Barstow, Califor- nia. Solar One operated successfully from

3 Solar Two Power Tower

System boundary Receiver

Power tower plants can potentially operate for 65 1,050˚F percent of the year Hot salt Cold salt storage tank storage tank 554˚F without the need for a back-up Steam generator fuel source. Substation

Condenser Steam turbine and electric generator

Fig. 3 Solar Two power tower system

Solar Two—a demonstration power creates electricity. From the steam gen- tower located in the — erator, the salt is returned to the cold can generate about 10 MW of electricity. storage tank, where it stored is and can In this central receiver system, thou- be eventually reheated in the receiver. sands of sun-tracking mirrors called heliostats reflect sunlight onto the By using thermal storage, power tower receiver. Molten salt at 554ºF (290ºC) is plants can potentially operate for 65 pumped from a cold storage tank percent of the year without the need for through the receiver where it is heated a back-up fuel source. Without energy to about 1,050ºF (565ºC). The heated salt storage, solar technologies like this are then moves on to the hot storage tank. limited to annual capacity factors near When power is needed from the plant, 25 percent. The power tower's ability to the hot salt is pumped to a generator operate for extended periods of time on that produces steam. The steam acti- stored solar energy separates it from vates a turbine/generator system that other renewable energy technologies.

4 Concentrating solar power technologies currently offer the lowest-cost solar electricity for large-scale power generation. Photo by Warren Gretz, NREL/PIX 02156 Gretz, Photo by Warren Solar Two near Barstow, California.

1982 to 1988, proving that power towers Solar energy premiums and other incen- efficiently to produce utility-scale tives under review in create an power from sunlight. The Solar One plant attractive market , providing used water/steam as the heat-transfer the economic incentives needed to reduce fluid in the receiver; this presented several the initial high cost and risk of commer- problems in terms of storage and continu- cializing a new . The Spanish ous turbine operation. To address these project, called "Solar Tres" or Solar Three, problems, an upgrade of Solar One was will use all the proven molten-salt technol- planned — Solar Two. Solar Two operated ogy of Solar Two, scaled up by a factor of from 1996 to 1999. Both systems had the three. Although Solar Two was a demon- capacity to produce 10 MW of power. stration project, Solar Tres will be operated by industry as a long-term power produc- Solar Two demonstrated how nitrate salt tion project. This utility-scale solar power (molten salt) could be used as the heat- could be a major source of clean energy transfer fluid in the receiver and as the worldwide, offsetting as much as 4 million heat storage media as well. At Solar Two, metric of carbon equivalent through the molten nitrate salt reached approxi- 2010. mately 1,050˚F (565˚C) in the receiver and then traveled to a storage tank, which had a capacity of 3 hours of storage. Solar Two demonstrated how solar energy can be Future Challenges stored efficiently and economically as heat Solar technology has made huge techno- in tanks of molten salt so that power can logical and cost improvements, but more be produced even when the sun isn't shin- research and development remains to be ing. It also fostered commercial interest in done to make it cost-competitive with power towers. Two of the project's key fossil fuels. Costs can be reduced by industry partners have been pursuing increasing demand for this technology commercial solar power tower plant worldwide, as well as through improved opportunities in Spain. component design and advanced systems.

5 DOE estimates that by 2005, there will be as much as 500 MW of concentrating solar power capacity installed worldwide. Photo by Warren Gretz, NREL/PIX 02334 Gretz, Photo by Warren A technician measures mirror surface quality on a dish concentrator.

Concentrating solar power technologies Future Opportunities currently offer the lowest-cost solar elec- tricity for large-scale power generation (10 Developing countries in Asia, Africa, and MW-electric and above). Current technolo- Latin America—where half the gies cost around $3 per or 12¢ per is currently without electricity and sun- kilowatt-hour (kWh) of solar power. New light is usually abundant—represent the innovative hybrid systems that combine biggest and fastest growing market for large concentrating solar power plants power producing technologies. A number with conventional natural gas combined of projects are being developed in , cycle or coal plants can reduce costs to , , and Mexico. In addition, $1.5 per watt and drive the cost of solar independent power producers are in the power to below 8¢ per kWh. Advance- early stages of design and development ments in the technology and the use of for potential parabolic trough power pro- low-cost thermal storage will allow future jects in (Crete) and Spain. If suc- concentrating solar power plants to oper- cessful, these projects could open the ate for more hours during the day and door for additional project opportunities shift solar power generation to evening in these and other developing countries. hours. Future advances are expected to The southwestern United States can also allow solar power to be generated for benefit from the use of these systems. 4¢–5¢ per kWh in the next few decades. Because the Southwest gets up to twice as Researchers are developing lower cost much sunlight as the rest of the country, solar concentrators, high-efficiency many southwestern states (California, engine/generators, and high-performance Nevada, , and New Mexico) are receivers. The goal is to further develop exploring the use of concentrating solar the technology to increase acceptance of power, especially for use in public utilities. the systems and help the systems pene- trate growing domestic and international energy markets.

6 One key competitive advantage of concen- and 2010. DOE estimates that by 2005, trating solar energy systems is their close there will be as much as 500 MW of con- resemblance to most power plants. centrating solar power capacity installed Concentrating solar power technologies worldwide. By 2020, more than 20 use many of the same technologies and gigawatts of concentrating solar power equipment used by conventional power systems could be installed throughout plants; they simply substitute the concen- the world. trating power of the sun for the combus- tion of fossil fuels to provide the energy for conversion into electricity. DOE analysts predict the opening of specialized niche markets in this country for the solar power industry between 2005

Resources Organizations The following are sources of additional information on American Solar Energy Society (ASES) concentrating solar power technologies. This list is not 2400 Central Avenue, Ste. G-1 exhaustive, nor does the mention of any consti- Boulder, CO 80301 tute a recommendation or endorsement. Phone: (303) 443-3130 Fax: (303) 443-3212 Ask an Energy Expert E-mail: [email protected] DOE’s Energy Efficiency and Renewable Energy Web site: www.ases.org Clearinghouse (EREC) P.O. Box 3048 A national organization dedicated to advancing the use Merrifield, VA 22116 of solar energy for the benefit of U.S. citizens and the 1-800-DOE-EREC (363-3732) global environment. TDD: 1-800-273-2957 DOE’s Concentrating Solar Power Program Fax: (703) 893-0400 E-mail: [email protected] Web site: www.eren.doe.gov/csp/ Online submittal form: a national effort to develop clean, competitive, www.eren.doe.gov/menus/energyex.html and reliable power options using concentrated sunlight. Consumer Energy Information Web site: www.eren.doe.gov/consumerinfo/ DOE’s SunLab Web site: www.eren.doe.gov/sunlab/ Energy experts at EREC provide free general and technical information to the public on many topics Combines the expertise of Sandia National Laboratories and technologies pertaining to energy efficiency and and the National Renewable Energy Laboratory (NREL) renewable energy. to assist industry in developing and commercializing concentrating solar power technologies. DOE’s Energy Efficiency and Renewable Energy Solar Energy and Energy Conversion Laboratory Network (EREN) University of Florida Web site: www.eren.doe.gov Mechanical Your comprehensive online resource for DOE’s energy Box 116300 efficiency and renewable energy information. Gainesville, FL 32611-6300 Phone: (352) 392-0812 Fax: (352) 392-1071 E-mail: [email protected] Web site: www.me.ufl.edu/SOLAR/ Performs fundamental and interdisciplinary engineering applications-oriented research in many areas of solar energy, energy conversion, and power systems. (Continued on page 8)

7 (Continued from page 7) Further Reading Solar Energy Industries Association (SEIA) 1616 H Street, NW 8th Floor Solar Trough Power Plants: Concentrating Power Plants Washington, DC 20006 Have Provided Continuous Generation Since 1984, Phone: (202) 628-7979; (301) 951-3231 produced by NREL for DOE, August 2000. Fax: (202) 628-7779 Available in HTML at Web site: www.seia.org/ www.eren.doe.gov/power/success_stories/ A national trade association of solar energy manufactur- solar_troughs.html ers, dealers, distributors, contractors, and installers. and in PDF at www.eren.doe.gov/power/success_stories/ SolarPACES pdfs/solar_troughs.pdf. International Energy Agency (IEA) 9 rue de la Fédération Power Towers: Proving the Technical Feasibility and 75739 Paris Cedex 15 Cost Potential of Generating Large-Scale Electric Power France from the Sun When It Is Needed, produced by NREL for Phone: (+33) 140 57 65 51 DOE, August 2000. Available in HTML at Fax: (+33) 140 57 65 59 www.eren.doe.gov/power/success-stories/ E-mail: [email protected] power_tower.html Web site: www.solarpaces.org/ and in PDF at IEA Web site: www.iea.org/ www.eren.doe.gov/power/success_stories/pdfs/ Provides a focus for the worldwide development of power_tower.pdf. solar thermal power and energy systems.

Web Sites A Compendium of Solar Dish/Stirling Technology Solstice Web site: solstice.crest.org/renewables/dish-stirling/

Distributed Power Technologies—Concentrating Solar Power DOE’s Distributed Power Program Web site: eren.doe.gov/distributedpower/ pages/tech_csp.html

NREL Photographic Information eXchange (PIX) Web site: www.nrel.gov/data/pix Features a collection of photos on renewable energy and energy efficiency technologies.

Parabolic Troughs: Solar Power Today EREN Web site: www.eren.doe.gov/success_stories/ opt_parabolic.html

TroughNet DOE’s SunLab Web site: www.eren.doe.gov/troughnet/

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