Concentrating Solar Power: Energy from Mirrors

DOE/GO-102001-1147 FS 128 March 2001 Concentrating Solar Power: Energy from Mirrors Mirror mirror on the wall, what's the The southwestern United States is focus- greatest energy source of all? The sun. ing on concentrating solar energy because Enough energy from the sun falls on the it's one of the world's best areas for sun- Earth everyday to power our homes and light. The Southwest receives up to twice businesses for almost 30 years. Yet we've the sunlight 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 electric power concentrating solar power plants an attrac- to light homes or solar thermal power tive alternative to traditional power plants, used to heat water, but did you know there which burn polluting fossil fuels such as is such a thing as solar thermal-electric oil and coal. 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 environmentally friendly Unlike traditional power plants, concen- electricity for cities, especially in the trating solar power systems 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 solar power tower system — known as Solar Two — near Barstow, California, is the world’s largest central receiver plant. This document was produced for the U.S. Department of Energy (DOE) by the National Renewable Energy Laboratory (NREL), a DOE national laboratory. The document was produced by the Information 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 time 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 fuel 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 environment is land 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 fossil fuel 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 mining 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 energy security—our country's independence from foreign oil can generate about imports. Because they store energy, they can operate in cloudy weather 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 times as many skilled jobs as traditional plants. Fig. 1 A parabolic trough 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 lenses to con- trough reflector (see Fig.1 above). The centrate and focus sunlight onto a thermal energy from the sun sent to the tube heats 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 field." 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 engine 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 phase—allowing parabolic troughs, for electricity generation 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 technologies supplement the solar output during peri- can be used to gen- ods of low solar radiation. 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 natural gas, and the result- dish-shaped para- ing hybrid provides continuous power bolic mirrors as generation. reflectors to concentrate 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 "heliostats" to reflect solar system 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 (molten salt) 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 engines, 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 turbine or with a and Concentrator piston to produce mechanical power. An generator electric generator or alternator converts the mechanical power into electrical power. Dish/engine systems use dual-axis collectors to track the sun. The ideal concentrator shape is parabolic, created either by a single reflective surface or multiple reflectors, 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 desert 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 Heliostat for a back-up Steam generator fuel source. Substation Condenser Steam turbine cooling tower 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 Mojave Desert— 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.

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