CSP plants generate electric power by using mirrors to concentrate (focus) the sun’s energy and convert it into high-temperature heat.

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 500 MW 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. CSP systems can also be much larger, generating up to 1000 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.

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. 

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

A:  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.

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.

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.


Q: Since the sun doesn’t shine 24 hours a day, can we count on solar energy to supply power when we need it?

A: Concentrating solar power (CSP) technologies can include cost-effective thermal storage techniques. These allow a CSP system to set aside the heat energy that accumulates during the day for later conversion to electric power. CSP plants can also be part of a hybrid power system, in which one part runs on fossil fuels. Both options enable CSP plants to generate electricity even when the sun isn’t shining — for example, at night or during cloudy weather.
Q: Since concentrating solar power plants are reliable, why haven’t more been built in the last few years?
A: One reason is the relatively low cost of fossil energy in most areas of the United States. The majority of today’s power plants run on inexpensive coal. And the current utility environment generally favors new natural gas power plants, which have comparatively low initial costs (first costs). With fossil fuel plants, however, customers (ratepayers) must bear the risk of higher fuel costs in the future.

In contrast, the fuel needed to run a concentrating solar power (CSP) plant is sunlight, which is free. A CSP plant uses its field of mirrors to deliver the thermal energy that’s provided by the fossil fuels burned in a conventional (e.g., gas- or coal-powered) plant. So, investing in a CSP plant is the equivalent of buying a lifetime supply of fuel. But the first costs associated with CSP plants can be high. To guarantee that they’ll recover their first costs, most CSP plant operators would probably want to have some long-term power purchase agreements lined up, to minimize the financial risk.

Other factors could also play a role in delayed investment in CSP. These include the perceived risks associated with new technologies and a need for tax equity with conventional technologies. Financial and regulatory incentives, advances in CSP technologies, and cost reductions resulting from economies of scale are just some of the things that could help to increase investments in CSP.

Q: Do concentrating solar power (CSP) plants require a lot of land? How much, exactly?
A: Relatively speaking, no. Consider Hoover Dam, for example. Nevada’s Lake Mead, which is home to the dam, covers nearly 250 square miles. In contrast, a CSP system occupying only 10 to 20 square miles could generate as much power annually as Hoover Dam did in one recent year. And if we take into consideration the amount of land required for mining, CSP plants also require less land than coal-fired power plants do.

It’s hard to say exactly how much land is required for a CSP plant, however, because this depends on its generating capacity and the particular technology used. For example, a 250-kilowatt plant composed of ten 25-kilowatt dish/engine systems requires less than an acre of land. And a parabolic trough system uses about 5 acres for each megawatt of installed capacity. But in any case, the solar resource needed to generate power using CSP systems is quite plentiful. Imagine being able to generate enough electric power for the entire country by covering about 9 percent of Nevada — a plot of land 100 miles on a side — with parabolic trough systems!

Q: Where are the best places to build a concentrating solar plant?
A: In the United States, the Southwest is ideal for concentrating solar power plants. The National Renewable Energy Laboratory has developed highly accurate maps of solar resources for the United States and many other regions; these resource maps allow us to assess potential sites with great accuracy.

Q: What are the environmental impacts of concentrating solar power plants (CSP)?

A: Concentrating solar power plants have few environmental impacts; land use is the primary one. Although a CSP plant’s "footprint," or the amount of land it occupies, is larger than that of a fossil fuel plant, the two actually use about the same amount of land. This is true because fossil fuel plants require a significant amount of land for exploration, mining, and road-building purposes. And CSP plants have the advantage in that they produce no environmental contaminants or greenhouse gases. However, the fossil fuel component of a hybrid power plant does not have the same benefits.

Q: What are the advantages of using concentrating solar power (CSP) rather than other power generation technologies?

A: One key competitive advantage of CSP systems is that they closely resemble most of the nation’s current power plants in some important ways. For example, much of the equipment now used for conventional, centralized power plants running on fossil fuels can also be used for CSP plants. CSP simply substitutes the use of concentrated solar power rather than combustible fossil fuels to produce electricity. This "evolutionary" — in contrast to "revolutionary" or "disruptive" — aspect means CSP can be integrated fairly easily into today’s electric utility grid. It also makes CSP technologies the most cost-effective solar option for large-scale electricity generation.

For example, CSP can make a significant contribution to the increasing need for affordable electricity in California and other "sunshine" states. The nine Solar Energy Generating Station (SEGS) plants in southern California were constructed in less than a year each, and the final two plants each had a capacity of 80 megawatts. The SEGS plants have already demonstrated a production capacity of 200 megawatts per year; this could be reestablished in two years, providing local jobs and a boost to the manufacturing economy. If a few —even four or five —developers began implementing this technology in the southwestern United States, more than 20,000 megawatts could be online by 2020, according to some experts.

Q: What is the benefit of continuing federal support for research and development in concentrating solar power?

A: Concentrating solar power is fast approaching commercial viability, and the U.S. industry is actively seeking commercial projects. To ensure the success of initial power plants and enable large-scale construction of additional ones, the industry requires continuous access to the research base that forms the foundation of CSP plant designs. Eliminating federal support for CSP at this stage could disrupt plans to build critically important, much-needed new commercial plants. Continued funding assures that the benefits of earlier U.S. investments will not be lost, and that future U.S. solar power plant capacity will be provided by a healthy domestic industry.

Q: Are concentrating solar power technologies viable in today’s energy markets? If so, where are the best market opportunities?

A:  Trough systems are commercially available and in use today. However, because of the very low cost of today’s fossil fuels, they cannot yet compete on a cost-of-electricity basis with fossil-based systems. A favorable financing arrangement—one likely to be stimulated by green power markets—could enable parabolic troughs to begin to play a role in the marketplace, however. And as global demand for clean energy sources rises, trough systems will become more financially attractive.

The long-term success of all the concentrating solar power technologies—including dish/engines, which are still in the demonstration phase—depends on continued technological progress. It also depends on an increasing desire for, and commitment to, clean energy. With some of the best direct normal solar resources anywhere on Earth, our nation’s southwestern states are poised to reap large—though as yet largely uncaptured—economic benefits from this important natural resource. Several states are already taking advantage of this opportunity. California, Nevada, Arizona, and New Mexico are all exploring policies that will nurture the development of their solar industries.

In addition to the CSP projects under way in this country, projects are being developed internationally using GEF grants. South Africa, Israel, Iran, and Jordan are also evaluating project opportunities. And independent power producers are beginning to design and develop parabolic trough power projects in Greece and in Spain. If CSP deployment in one or more of these initial markets is successful, many additional project opportunities are expected in these and several other regions.