New Accoustic Sterling Engine

Two scientists at Los Alamos National Laboratory have developed a low-tech engine that performs work using simple sound waves and has no moving parts. Scott Backhaus and Gregory Swift believe it has the potential to provide alternative energy, significantly save energy supplies and reduce global pollution.

They said in their study published Thursday in the journal Nature that the revolutionary engine, which is as efficient as a typical car engine, might be used to cool refrigerators or to cryogenically recover a form of natural gas wasted and burned off in oil production.

They already have developed a working collaboration with Denver-based Cryenco Co. to produce a machine for liquefying the natural gas usually burned off or "flared" in oil production.

"We expect our engine to find many additional uses throughout the global power-production environment, ranging from the separation of air into nitrogen and oxygen, to the generation of electricity," they said in their report.

Swift suggested in a statement issued by the lab Wednesday that "small low-cost engines like this could be used in homes for (electrical) cogeneration. That is, they could be used to generate electricity while at the same time to produce heat for warming the home or for hot-water heating."

Their "thermo acoustic Stirling engine" is a long, steel, baseball-bat-shaped resonator with an oval handle on the lower end, which is filled with compressed helium. Electronic devices generate sound waves, which stimulate the gas to alternately expand and contract.

The engine's operation is based on the Stirling principle, named for the Scottish 19th century scientist Robert Stirling, who showed that a confined volume of gas expands at high pressure and contracts at low pressure, creating an alternating cycle that can do work when the gas is heated or cooled through exchangers.

In a separate article published in the same issue of Nature, the research is praised for its ingenuity and environmentally favorable qualities by Steven L. Garrett of the acoustics program at Pennsylvania State University. Garrett expresses admiration for the invention's simplicity, efficiency and lack of environmental intrusiveness.

"The working fluid is pressurized helium, an inert gas, which neither depletes stratospheric ozone nor contributes to global warming," he said.

Garrett said the Los Alamos researchers provide "elegant acoustic solutions" to problems that have vexed acoustic-engine researchers for decades. Researchers, he adds, will soon "exploit this technology for applications that need mechanical or electrical energy."

An obvious drawback of the new thermo acoustic engines is that acoustic amplitudes can reach levels of 190 decibels. That, Garrett said, is "about 10 million times as intense as the front-row levels at a rock concert and 300 times the intensity required to ignite human hair." Fortunately, he said, the sound is produced inside a rigid pressurized vessel, and because the engines only radiate a "single-frequency tone," any escaping sound waves are easily canceled by a complimentary frequency or tone.

Lawrence Spohn writes for The Tribune in Albuquerque, N.M.

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