Since Robert Stirling invented the Stirling engine in 1816, it has been used in an array of specialized applications. That trend continues today. Its compatibility with clean energy sources is becoming apparent: It is an external combustion engine that can utilize almost any heat source, it encloses a fixed amount of a gaseous working fluid, and it doesn’t require any water — unlike a steam engine.
Power from the Sun
A good example is Phoenix-based Stirling Energy Systems’ (SES) newly designed solar power collection dishes that were unveiled at Sandia National Laboratories this July. Called SunCatchers, these dishes are the next-generation model of SES’s original system. With a high rate of production and cost reduction, they will be used in commercial-scale deployments starting in 2010.
The modular concentrated solar thermal (CSP) SunCatcher uses precision mirrors attached to a parabolic dish to focus the sun’s rays onto a receiver, or heat exchanger, which heats the engine’s working fluid, in this case hydrogen, and rejects heat at ambient conditions. As the gas heats and cools, the working fluid’s pressure rises and falls. This change in pressure drives the piston inside the engine, producing mechanical power, which in turn drives a generator and makes electricity.
The improved design stems from a collaboration between Sandia’s CSP team and SES. The new SunCatcher is about 5,000 pounds lighter than the original, is round instead of rectangular to allow for more efficient use of steel, has improved optics, and consists of 60% fewer engine parts (Figure 2). The revised design also has fewer mirrors — 40 instead of 80 — and the reflective mirrors are formed into a parabolic shape using stamped sheet metal similar to the hood of a car.
2. Hydrogen and Helios. Stirling Energy Systems this July unveiled four newly designed solar power collection dishes at Sandia’s National Thermal Test Facility. The next-generation SunCatcher is about 5,000 pounds lighter than the original, is round instead of rectangular to allow for more efficient use of steel, has improved optics, and consists of 60% fewer engine parts. The engine’s sealed system is filled with hydrogen. This model could be in commercial service by 2010. Courtesy: Randy Montoya
Because the mirrors and engines are made by using automobile manufacturing techniques, the improvements will result in high-volume production, cost reductions, and easier maintenance. But it also minimizes land development and has numerous environmental advantages, said Chuck Andraka, the lead Sandia project engineer. "They have the lowest water use of any thermal electric generating technology, require minimal grading and trenching, require no excavation for foundations, and will not produce greenhouse gas emissions while converting sunlight into electricity."
Tessera Solar, SES’s sister company and the developer and operator of large-scale solar projects using the SunCatcher technology, plans to build a 60-unit, 1.5-MW plant by the end of the year either in Arizona or California. SES said that the proprietary solar dish technology will then be deployed to develop two of the world’s largest solar generating plants in Southern California — with San Diego Gas & Electric in the Imperial Valley and with Southern California Edison in the Mojave Desert — in addition to the recently announced project with CPS Energy in West Texas. SES predicts these projects will produce 1,000 MW by the end of 2012.
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