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Green Collar Careers - Stirling Engines

The Stirling engine was invented and patented by a Scottish clergyman, Reverend Dr Robert Stirling in 1816.  In contrast to internal combustion engines, Stirling engines have the potential to use renewable heat sources more easily, to be quieter, and to be more reliable with lower maintenance.   Stirling engines can run directly on any available heat source, not just one produced by combustion, so they can run on heat from solar, geothermal, biological, nuclear sources or waste heat from industrial processes.   Since the invention of the Stirling, steam engines took dominance and later internal combustion engines due to the power to weight advantages.  Over the last decade Stirling engines have gained significant interest for solar power applications.

There are two principal types of Stirling Engine, kinematic and free-piston. All Stirling engines have two pistons (functionally speaking), one of which shuttles the working gas between the hot and cold zones and is known as a displacer, whilst the other is subject to the resulting pressure changes and does work to drive the engine. In the kinematic engine, these two pistons are physically connected by a crank mechanism, whereas in the free-piston engine, there is no physical linkage and the displacer oscillates resonantly.

On a perfect New Mexico winter day — with the sky almost 10 percent brighter than usual — Sandia National Laboratories and Stirling Energy Systems (SES) set a new solar-to-grid system conversion efficiency record by achieving a 31.25 percent net efficiency rate.  Compare that to the 12% efficiency of a Solar PV system.

Solar powered Stirling engines have recently been developed to operate in space based on free-piston designs.

Cogeneration
As a mechanical system the Stirling engine also can directly power other mechanical devices such as refrigeration pumps which is spawning a new technology known as cogeneration.  Commercially available cogeneration systems were first available in New Zealand and they are beginning to be distributed in Europe.  Although the technology is relatively new the total economics of the systems are impressive.  Turnkey cogeneration systems that are capable of generating and storing 50 kWh per day (1500kWh/month) are running about $30,000.

Although these systems aren't yet available in the United States there are at least three major US HVAC manufacturers that have development programs with plans for product release by 2012.