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 Summer 2011

How it Works

Concentrating Solar Power

When you think of solar energy, photovoltaic panels on rooftops probably come to mind. But if you have ever used a magnifying glass to ignite a piece of paper, you have dabbled in a primitive form of concentrating solar power (CSP). CSP (or solar thermal) plants apply this principle on a much larger scale, creating intense heat to generate clean, carbon-free electricity.

Parabolic troughs produce steam by concentrating sunlight on fluid-filled pipes.


How to Catch Rays

There are several different types of CSP technology. The most popular is the parabolic trough, which uses curved mirrors to concentrate sunlight onto fluid-filled pipes above each mirror. A similar technology, linear Fresnel reflectors, uses flat mirrors (rather than curved) to concentrate sunlight onto pipes. In both systems, the sun heats fluid in the pipes that then boils water, which creates steam that drives a turbine and generates electricity. The mirrors also track the sun’s movement throughout the day, optimizing energy generation. There are 17 parabolic trough plants in operation around the world today; the first series of plants was built in California’s Mojave Desert in the 1980s and, at 354 megawatts (MW) of generating capacity, remains the largest. In contrast, only three linear Fresnel plants are in operation so far.

Power towers use a large array of multiple flat mirrors to direct sunlight toward a single liquid-filled boiler atop a central tower. So far, there are five power towers operating in the world, and one under construction in the Mojave Desert—the Ivanpah project—will be the largest CSP plant in the world, with an expected generation capacity of 392 MW.

Power towers focus sunlight on a centralized boiler.

The least-used CSP technology to date is dish/engine systems, which use satellite dish-shaped mirrors to concentrate sunlight onto a Stirling engine. The sun’s heat expands air or a gas, usually helium or hydrogen, in the engine and the resulting change in pressure powers the engine, generating electricity. Since this technology does not involve steam, much less water is needed—an advantage for the sunny, arid climates best suited for CSP. And since each unit generates its own electricity, it can be built on a smaller scale appropriate for targeted, local power needs. Arizona is home to the only dish/engine CSP plant in operation today, a demonstration unit with a generating capacity of 1.5 MW.

The choice of CSP technology is often driven by location-specific factors, but steam-driven systems have benefited from the fact that they use the same turbines as conventional gas- and coal-fired power plants, allowing them to be deployed more quickly and at a lower cost than technologies that require fully customized parts. In addition, unlike some other renewable energy technologies, CSP systems that use trough or tower technology can store the heat they collect and use it to generate electricity when the sun is not shining. The heat is either stored as a hot liquid or transferred to another substance like molten salt or graphite. For example, a 50 MW parabolic trough facility in Granada, Spain, can store seven hours’ worth of electricity-generating heat. This thermal storage capability makes CSP more competitive with large coal and nuclear plants in terms of both output and reliability.

On the Horizon

Slaking CSP’s Thirst

As water becomes more scarce in a warming world, CSP plants must minimize their consumption.

CSP plants traditionally require significant amounts of water, primarily to cool the steam that drives the generating turbine. Water use estimates for such “wet-cooled” CSP facilities range from 700 to 1,000 gallons per megawatt-hour; in comparison, wet-cooled nuclear and fossil fuel plants use roughly 400 to 1,000 gallons per megawatt-hour.

Fortunately, a far less resource-intensive cooling technology exists: “dry cooling,” which uses large fans instead of water to cool the steam. This can reduce water use at CSP plants by about 90 percent (water is still needed to wash the mirrors and compensate for any leaks in the steam pipes). However, dry-cooled CSP plants are less efficient—and therefore more expensive—than wet-cooled plants for two reasons: air is not as good as water at cooling steam, and a significant amount of electricity is needed to power the cooling fans, which reduces the plant’s electricity output between 5 and 25 percent.

All the CSP plants operating today use wet cooling, but with increasing demands on freshwater supplies and conflicts over its use, plant owners will feel more and more pressure to conserve water. Some of the CSP plants recently approved for development in California, including the Ivanpah plant already under construction, will use dry cooling.

The 26 CSP plants operating in the world today have the capacity to generate more than 1,200 MW of power (more than 430 MW in the United States). Although this is a minuscule total compared with the current capacity of fossil-fuel-fired power plants (a typical coal-fired plant has a capacity of 600 MW), the CSP industry has grown rapidly since the early 2000s and will continue to expand in market share. There are approximately 60 CSP plants under development around the world, 33 of which are planned for the United States (mostly in the desert Southwest).

One of the biggest obstacles to the growth of CSP will likely be economics; generation costs for CSP (per kilowatt of capacity) are declining, but not as rapidly as those for other renewable technologies, especially solar photovoltaics. The other important consideration, as with any proposed generation facility, is environmental impact. Most CSP facilities require large swaths of intensely sunny, relatively level land—as much as 5 to 10 acres per megawatt of capacity. The most suitable locations are usually desert ecosystems, so developers must take care to minimize or avoid disruption to natural habitats and protected species. CSP projects can also strain water supplies in areas where water is a scarce resource.

These hurdles are leading project developers to invest in low-water technologies (see the sidebar), arrange their mirrors more efficiently, and choose sites that have already been “disturbed” by previous activities, minimizing the project’s impact on the land and its species. With help from strong state-level policies and federal tax incentives, combined with responsible permitting practices, CSP can play an increasingly important role in ending our national dependence on fossil fuels, combating the threat of global warming, and securing a future based on clean, safe, and reliable energy.

 Laura Wisland is an analyst in the UCS Climate and Energy Program.

Learn more about CSP and other renewable electricity technologies in “Clean Energy 101.”