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Cheaper Solar Thermal Power

A simpler design could reduce the cost of solar power generated by concentrating sunlight on Stirling engines.
July 28, 2009

Stirling Energy Systems (SES), based in Phoenix, has decreased the complexity and cost of its technology for converting the heat in sunlight into electricity, allowing for high-volume production. It will begin building very large solar-power plants using its equipment as soon as next year.

Sun catchers: This is the latest design of a system for focusing sunlight on a Stirling engine to generate electricity.

The company is currently building a 1.5-megawatt, 60-unit demonstration plant that will use the company’s latest design. Stirling expects to finish that project by the end of the year. It also has contracts with two California utilities to supply a total of 800 megawatts of solar power in Southern California. The first of the plants that will supply this power could be built starting the middle of next year, pending government permits and loan guarantees from the U.S. Department of Energy (DOE).

The projects are part of a resurgence in what’s known as solar thermal power. Various solar thermal technologies were developed starting in the 1970s, but a breakdown in government funding and incentives caused them to stall before they reached a scale of production large enough to drive down costs and allow them to compete with conventional sources of electricity. “It was a classic problem with solar. The market support to bring solar to high volume wasn’t there,” says Ian Simington, the chairman of SES and chief executive of the solar division of NTR, a company based in Dublin, Ireland, that bought a controlling share of SES last year.

Recent state mandates and incentives for renewable energy have led to a new push to commercialize the technology. There are over six gigawatts of concentrated solar power under contract in the southwestern United States right now, says Thomas Mancini, program manager for concentrated-solar-power technology at Sandia National Laboratory in Albuquerque, NM. That’s equivalent to about six nuclear-power plants. BrightSource Energy has contracts to provide 2.6 gigawatts of solar power with concentrated solar power (a previous version of this story cited only one of two 1.3 gigawatt contracts), and Solar Millenium has announced a project that would generate nearly one gigawatt of power.

Stirling Energy Systems technology uses 12-meter-wide mirrors in the shape of a parabolic dish to concentrate sunlight onto a Stirling engine. The difference in temperature between the hot and cool sides of the engine is used to drive pistons and generate 25,000 watts of electricity. The first phase of the company’s large-scale projects will use 12,000 of these dishes to generate 300 megawatts of power. Simington expects electricity from the systems to cost between 12 and 15 cents per kilowatt hour, higher than the cheapest sources of electricity–such as coal-fired power plants–but competitive in many markets, especially in the afternoon, when prices are highest.

Earlier this month the company unveiled its production design. Compared to several prototypes that have been tested for several years at Sandia National Laboratory, the new design cuts about two metric tons from the weight of each dish and reduces the number of mirrors in each from 80 to 40. The simplified design can be built in large quantities using equipment in existing factories for automobiles.

The company’s design has certain advantages over other approaches to concentrated solar power. In other systems, heat is collected over a large area and used to drive turbines in a central facility. These turbines require large amounts of water for cooling, Mancini says, whereas the SES system uses a closed-radiator system that doesn’t consume water. Water use is an important consideration for solar thermal technologies, Mancini adds, since they work best in areas with a lot of direct sunlight–that is, in deserts. (These concentrated-solar-power systems are quite different from solar water heaters used in homes.)

Another advantage of the SES system is its modularity. With other approaches, the entire solar collection and generation system has to be in place to start generating electricity. With the Stirling engine system, power can come online as the dishes are installed, and more generating capacity can easily be added by building more dishes, without any need to enlarge a central generating plant.

But the system also has a significant disadvantage. Other solar thermal power plants collect heat in a central place where it can easily be stored, making it possible to generate electricity when the sun isn’t shining. “There’s no obvious way to do this with the dishes,” Mancini says.

Although there has been a resurgence in contracts for solar thermal power, obstacles to the plants being built still remain. The new projects could be stalled by slow action from the government. Permits originally thought to be ready by the end of this year are now expected no sooner than next May. What’s more, the current economy has made financing hard to come by, says Sean Gallagher, SES’s vice president for market strategy and regulatory affairs. That has forced his company and others to rely on Department of Energy loan guarantees. But, Gallager says, although the DOE has promised to speed up its process for issuing these, it has yet to issue even the rules for applying for the guarantees included in February’s stimulus package.

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