Concentrating Solar Power (CSP)
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|Summer School on Global Sustainability|
Most people are familiar with photovoltaic modules, which convert direct and diffuse sunlight directly into electricity. But there is another solar electric technology that is more akin to a typical power plant. Concentrating solar power (CSP) technology focuses direct sunlight to achieve the high temperatures needed to efficiently run a thermodynamic heat engine. Resource studies have shown that even after practical limitations are taken into account, CSP located under clear skies in the Southwest could provide six times the current US national electricity capacity. The most established CSP technology, parabolic troughs, has been successfully providing over 350 megawatts of electricity in the Mojave Desert for over 20 years. This technology focuses sunlight onto a fluid-carrying pipe, which transfers heat to a boiler and runs a conventional steam cycle. Power towers, or central receivers, use a field of tracking mirrors (or heliostats) that focus sunlight onto a tower, again providing heat to a steam cycle. Parabolic dishes focus sunlight onto a small Stirling-cycle engine that generates electricity at each dish.
Concentrating solar power technology offer a means to deploy very large solar plants. Utilities are generally comfortable with trough and tower technology because they rely on the use of conventional steam cycle power plants. In addition, they lend themselves to the use of relatively low-cost thermal storage. By over-sizing a collector field, a hot tank of molten salt can be charged during the day. The molten salt in the tank can be used to produce steam after the sun sets, thus providing electricity during a utility's peak evening load period. A parabolic trough plant with 7 hours of thermal storage is now operating in Spain, and Arizona Public Service has ordered a 280 MW plant with six hours of storage to provide electricity to Phoenix. There are now contracts for over 7,000 MW of CSP plants around the world with over 4,000 MW of this in the U.S.
CSP technology benefits from a 30% federal investment tax credit, which was extended through the year 2016. With that tax credit, CSP electricity is about 3 cents per kWh more expensive than electricity from a new combined-cycle natural gas plant. But that cost premium provides protection against future increases in natural gas prices. The cost of CSP technology is expected to drop considerably as a result of R&D, economies of scale, and learning curves.