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PV vs. Solar Thermal

Distributed solar modules are gaining ground on concentrated solar thermal plants.

Fortnightly Magazine - July 2008

2050. 5

Just as with windpower, however, a number of economic questions must be addressed, because solar technologies present challenges and risks that must be factored into the vision in order to be truly attainable.

Solar Thermal Chills Out

Solar thermal technology has dominated the market for utility-scale solar power for decades. Although it still faces technical and economic challenges, interest in this technology remains strong as the industry searches for climate friendly energy sources.

In February 2008, Arizona Public Service (APS) announced plans to build a 280-MW parabolic-trough solar-thermal generating facility near Gila Bend, Ariz., with a levelized cost of 14.4 cents per kilowatt hour over the plant’s estimated 30-year lifetime. APS also plans to issue a second joint solicitation, with other Arizona utilities, to build another 250-MW plant using the same technology. These announcements coincided with the release of several studies of the generation potential for CSP across the southwest United States. 6

The Gila Bend plant will be built by the Spanish firm Abengoa, and it will draw on that firm’s experience with similar technology in Spain. Tracking, concentrating, parabolic-trough collectors will heat synthetic oil, and a two-tank molten salt-storage system will store the heat and allow the plant to generate electricity at near capacity during the long APS summer peak-demand period. The plant will generate electricity using a medium pressure steam turbine with wet cooling towers.

The project’s collector-field technology results from more than 20 years of European and U.S. experiences, most notably the Luz International Solar Electric Generating System (SEGS) projects built in the Mojave dessert in California beginning in 1984. The same cannot be said for the molten salt-storage technology, which never before has been built or operated on this scale. (Smaller similar systems have been tested in the United States and are under construction by Abengoa in Spain.) Given the lack of commercial operating experience, the Electric Power Research Institute (EPRI) declined to include such storage in a recent study of the potential for parabolic-trough electric plants in the southwest United States. 7

Irrespective of molten salt-storage technology, CSP development faces some more mundane challenges.

CSP plants are basically steam turbines that rely on the sun, rather than fossil or nuclear fuel, to make steam. Like most steam-turbine plants, economies of scale dictate that a parabolic-trough CSP plant must be sized in the 100-MW to 300-MW capacity range. The size of the collector field for such a plant, particularly one designed to provide hours of storage is enormous. For example, the Gila Bend plant will require a contiguous 1,900-acre site to build its collector field. (For comparison, a 21-MW thin-film PV plant to be built near Blythe, CA, would require less than 150 contiguous acres.) The size of Gila Bend’s collector field will, in turn, require a detailed environmental impact review, perhaps spanning several jurisdictional boundaries.

The sheer amount of real estate required increases the likelihood of concerns about negative impacts on plant and animal species. Moreover, plants this size likely will require dedicated high-voltage transmission lines that will need to be permitted. These permitting