26 July 2012
The Stirling Engine has been around for almost two hundred years, but Sam Weaver, Chief Executive Officer of Cool Energy, has given new life to this device, allowing it to produce electricity from diesel generator waste heat and solar thermal systems.
A hot gas in the engine produces more power in expansion than the same gas requires to re-compress when it is colder, and the Stirling engine uses this difference in compressibility to convert heat energy into mechanical energy, which can then be converted into electricity. The amount of electricity extracted from the heat is related to the difference in temperature between the hot side of the engine and the cold side.
Previously, the use of kilowatt-scale Stirling engines has been relegated to really hot things (with temperature differences in the range of many hundreds of degrees). At Cool Energy, they are able to extract electricity from a difference as low as 100 C. Because a smaller temperature difference is required, they are able to extract thermal energy that would not have been previously usable.
Cool Energy began with the idea that the Stirling engine could be coupled to a small solar thermal device for off-grid electricity generation. They were funded by the NSF and the DOE to develop their first device, which currently operates in Cool Energy’s Boulder laboratory. Cool Energy chose to use evacuated tube solar thermal collectors which allow the mineral oil to reach higher temperatures than traditional solar thermal panels. Because the Stirling engine’s output is directly tied to how big a temperature difference they can generate, the hotter the heat-conducting liquid, the more electricity they can pump out. The mineral oil is pumped through copper tubes in the panels, then pumped down to a 3 kW Stirling engine which extracts the heat and produces electricity. The engine is also hooked up to an electrical heater, allowing them to run the engine when the sun isn’t shining.
Since the development of the first solar thermal prototype, Cool Energy has primarily grown their business in the direction of waste heat recovery from off-grid diesel generators, from which, testing has shown, they can improve fuel economy by up to 20%. Despite this, they remain excited about the future of the solar thermal-Stirling engine, especially for remote cell towers, small off-grid communities and other projects with small load requirements where electricity is unavailable or very expensive and maintenance is difficult to do.
Written by Sydney Kaufman, Contributing Editor. Solar Novus Today