A recent invention is the The thermoacoustic Stirling heat engine developed by LANL scientists. It works by creating intense acoustic energy which can be used directly in acoustically powered refrigerators or to generate electricity. The power production process is environmentally friendly and up to 30 percent efficient while typical internal combustion engines are 25 to 40 percent efficient.
A, seven chapter, 265 page book on the subject of thermocoustics can be down loaded, in PDF format, from http://www.lanl.gov/projects/thermoacoustics/Book/index.html
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