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Solar FAQs — Concentrating Solar Power — The Basics

Q: What is concentrating solar power?

A: The real powerhouse in CSP plants is focused sunlight. CSP plants generate electric power by using mirrors to concentrate (focus) the sun's energy and convert it into high-temperature heat. That heat is then channeled through a conventional generator. The plants consist of two parts: one that collects solar energy and converts it to heat, and another that converts the heat energy to electricity. Within the United States, over 350MW of CSP capacity exists and these plants have been operating reliably for more than 15 years.

CSP systems can be small enough (Stirling systems as small as 10 kilowatts are under development) to help meet a small village's power needs. (For comparison, a typical U.S. home might require a system generating about 5 to 15 kilowatts to meet most of its power needs, according to some renewable energy experts.) CSP systems can also be much larger, generating up to 100 megawatts of power for use in utility-grid-connected applications. Some CSP systems include thermal storage to provide power at night or when it's cloudy. Others are combined with natural gas systems in hybrid power plants that provide power on demand.

The amount of power generated by a concentrating solar power plant depends on the amount of direct sunlight at the site. CSP technologies make use of only direct-beam (rather than diffuse) sunlight.

The amount of power generated by a concentrating solar power plant depends on the amount of direct sunlight at the site. CSP technologies make use of only direct-beam (rather than diffuse) sunlight.

Today's CSP systems can convert solar energy to electricity more efficiently than ever before. Utility-scale trough plants are the lowest cost solar energy available today and further cost reductions are anticipated to make CSP competitive with conventional power plants within a decade. So, CSP is a very good renewable energy technology to use in the southwestern United States as well as in other sunny regions around the world.

Other Resources: 

Q: How does concentrating solar power (CSP) work?

A:  Some of the following documents are available as Adobe Acrobat PDFs. Download Acrobat Reader.

Basically, CSP systems collect and concentrate (focus) the solar energy in sunlight to generate electricity. The three kinds of concentrating solar power systems — parabolic troughs, power towers, and dish/engines — are classified according to how they collect solar energy.

Parabolic Troughs: In parabolic trough systems, curved, trough-like collectors reflect and concentrate sunlight onto a receiver, a pipe running along the inside of the curved surface of the trough. The concentrated solar energy heats a heat transfer fluid (usually oil) flowing through the pipe; this heated fluid is then used to run a conventional steam generator for electricity production.

If we install numerous troughs in parallel rows, we have what's known as a collector field. The field is typically aligned on a north-south axis, which allows the troughs to track the sun from east to west during the day. This ensures that the sunlight is continuously focused on the receiver pipes and that electrical output is highest in the summer months when it is needed most. Trough systems with thermal storage capabilities can also store thermal energy for electricity generation later in the evening. The largest trough systems operating today generate about 80 megawatts of electricity (for comparison, a 5- to 15-kilowatt system can provide most of the power needs of an average U.S. home), however, it may be possible to build plants as large as 400 megawatts which can greatly reduce the cost of delivered energy.

Currently, all parabolic trough plants are hybrids. This means that they include a fossil fuel system to supplement the solar energy at night or when it's cloudy. The fossil fuel part of a hybrid system runs on natural gas.

Power towers: A power tower system is made up of many large, sun-tracking mirrors (heliostats) that focus sunlight on a receiver at the top of a tower. The sunlight heats up a heat transfer fluid in the receiver, which then is used to generate steam. The steam, in turn, is used in a turbine-generator to produce electricity.

In early power towers (such as the Solar One plant), steam was the heat transfer fluid. Current designs (including Solar Two, pictured) made use of molten nitrate salt because of its superior heat transfer and energy storage capabilities. Individual commercial plants can be small or large enough to produce anywhere from 50 to 200 megawatts of electricity.

Dish-engine systems: A solar dish-engine system is an electric generator that "burns" sunlight instead of gas or coal to produce electricity. The major parts of the system are the solar concentrator and the power conversion unit.

The dish, or solar concentrator, is the primary solar component. It collects the sun's direct-beam energy and concentrates it on a receiver located at the focal point of the dish. The reflective surface of the concentrator is made of glass mirrors, which reflect approximately 92% of the sunlight that strikes them.

The power conversion unit includes the thermal receiver and the engine/generator. The thermal receiver the interface between the dish and the engine/generator absorbs the concentrated solar beam, converts it to heat, and transfers the heat to the engine/generator. A thermal receiver can be a bank of tubes with a gas, usually hydrogen or helium, which is the heat transfer medium. Thermal receivers can also be heat pipes in which an intermediate fluid boils and condenses to transfer heat to the engine. The engine/generator uses heat from the thermal receiver to produce electricity. The most common type of heat engine in dish-engine systems is the Stirling engine, which uses heat from an external source (like the sun) to create mechanical power that in turn drives a generator to produce electricity. The Solar Energy Technology Program is investigating concentrating PV receivers that use high-efficiency PV cells to generate electricity—the advantage being the elimination of moving parts and potential for very high efficiencies and low cost.

Other Resources:  For more information about trough systems, see the following documents: Technology Characterization: Solar Parabolic Trough (PDF 303 KB)
Solar Trough Power Plants - HTML (PDF 230 KB)

Parabolic Trough Roadmap (PDF 1053 KB)

For more information about the most recent analysis work done on parabolic troughs and power towers, please see "Assessment of Parabolic Trough and Power Tower Solar Technology Cost and Performance Forecasts"

For more information about power tower systems, see the following documents:
Technology Characterization: Solar Power Towers (PDF 303 KB)

Solar Two Demonstrates Clean Power for the Future (PDF 557 KB)

For more information about dish-engine systems, see the following documents: Technology Characterization: Solar Dish Engine (PDF 888 KB)

Solar Dish/Engine Systems (PDF 200 KB)

Q: What's the difference between concentrating solar power (CSP) and other solar technologies?

A: They all make use of the abundant energy of sunlight. But they differ in the ways that they capture and use solar energy to produce heat or electricity. Most solar water- and space-heating technologies, for example, use sunlight directly to produce heat rather than using the sun's heat to produce steam that drives a generator to produce electricity, the way CSP does.

Electricity can also be generated by photovoltaic (PV) systems. These technologies convert sunlight directly to electricity using the semiconductor materials in solar panels.

CSP technologies first concentrate the sun's energy using reflective devices such as troughs or mirror panels. The resulting concentrated heat energy is used to power a conventional turbine and produce electricity. In the future, CSP technologies will be used to power concentrating PV technologies.

Other Resources: 

U.S. Department of Energy
1000 Independence Avenue, SW
Washington, DC 20585
www.eere.energy.gov www.energy.gov