 A very old engine has found a new future as part of a solar-powered electric generation system.
Robert Stirling, a Scottish minister, invented a simple four-piston reciprocating engine in 1816. Today the Stirling engine designed on the same principles is being coupled to a solar concentrator system to generate electricity. Quiet operation The engine has attracted attention in recent years from the scientific community. It has the The National Renewable Energy Laboratory in Golden, Colo., recently installed a solar concentrator-Stirling engine system for testing. The project is the second phase in a joint venture of NREL, the U.S. Department of Energy, and Science Applications International Corporation to bring the technology to the commercial development stage. SAIC manufactures mirrors for the solar concentrator.
The NREL system produces 25 kilowatts at peak conditions and can produce 200 kW per day - roughly enough to power five to six homes. It is connected to the local utility power grid. Sunlight, focused from the solar concentrator to a receiver on the engine, provides the fuel. Heat from the solar beam expands hydrogen gas in the engine's heater head. The expansion rotates the engine's pistons, which power a crankshaft to operate the generator. The Stirling engine also operates on conventional fuels when sunlight is not available. The power conversion unit includes the solar receiver, engine, generator, cooling system, and an electronic command and control system. The PCU weighs about one ton. Hybrid system The NREL system is a hybrid in the sense that it uses natural gas for fuel in "off-sun" periods. There's no combustion in the engine, so no harmful emissions are produced when operating from solar energy. Low emissions are expected during natural gas operation, which is being verified by NREL testing. Wind loads on the system are also measured and will be used to optimize the design of future collectors. Concave dish The solar concentrator is a concave dish 50 feet in diameter and layered with 16 mirrors, 10.5 feet wide each. The mirrors focus the solar beam to about 10 inches in diameter. Temperatures from the concentrated beam reach 1,400 degrees Fahrenheit on the PCU receiver. The prototype unit costs about $500,000, but mass production could reduce the cost to about $100,000 a unit, according to Kelly Beninga, SAIC assistant vice president. Science Application's long-term goal is to reduce the cost of production to about $50,000 per unit. The technology has widespread application as a power supply for the utility grid and also for areas of the world without any form of electricity. It's estimated that about 2 billion of the world's 5.7 billion people are without electricity. In Mexico alone, there are 200,000 villages without electrical power. The market for power systems to meet this need is estimated at $800 million a year. For irrigation applications, the system can be coupled directly to one or more submersible pump motors. With nearly constant efficiency, the quantity of water pumped is based on insolation available throughout the day. The next phase is a planned commercial phase-in of up to 40 solar-dish Stirling systems in New Mexico. The units will produce between 100 kilowatts and 1 megawatt of power each.
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NREL is collecting hourly data on generation availability and total time "on sun" for six months to a year, according to Mark Mehos, NREL project engineer.
