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The Honolulu Advertiser

Posted on: Wednesday, December 11, 2002

Sound waves make noise as Earth-friendly refrigerant

By Dan Vergano
USA Today

A fridge chilled by sound waves may be headed for the ice-cream parlor and your kitchen, researchers report.

Today's coolers and refrigerators use refrigerants linked to ozone depletion and global warming, raising interest in "green" alternatives like the "thermoacoustic" fridge. Unveiled recently at an acoustics meeting in Mexico, the sound-chilled prototype uses air as a refrigerant.

Led by Steven Garrett of Pennsylvania State University in State College, Pa., the researchers see thermoacoustic technology leading to environmentally benign refrigerators, air conditioners and home electricity generators that run off natural-gas lines.

Partly financed by ice-cream maker Ben & Jerry's, the team hopes to turn its prototype into a line of ice-cream cabinets the same size as present ones, says Garrett.

Thermoacoustic refrigeration takes advantage of the natural heating a gas undergoes when compressed and the cooling it undergoes when it expands, like all refrigerators.

Within the 15-inch-tall, nine-inch-wide prototype, pressurized air is blasted with a high-pitched 173-decibel sound wave. (By comparison, the noise level standing 80 feet from a jet taking off is 140 decibels.) The sound waves alternately squeeze and expand the gas, removing heat from the refrigerator side of a heat exchanger and dumping it into the hot side. No sound can be heard outside the sealed heat exchanger, and temperatures on the cold side drop as low as eight degrees below zero.

"We're not at the finish line yet, but this is exciting stuff," says acoustic scientist Greg Swift of the Los Alamos (N.M.) National Laboratory. Advances in thermoacoustic devices to power small-scale refrigerators, air conditioners and heat pumps have been growing for the past two decades, he says.

In terms of energy efficiency, the technology still lags conventional devices, but improvements have come only in the past two years.

The technology has limits. For instance, very cold "cryogenic" temperatures near absolute zero, about 459 degrees Fahrenheit, are too difficult to achieve for this technology.

However, it also offers unique advantages, says Garrett. Simply upping the frequency of the thermoacoustic sound waves can lead to smaller refrigerators. And sound-driven air conditioners can be made to turn their output up or down gradually, unlike conventional ones that operate at maximum power for a time and then turn off until the heat bumps up again.

"This can lead to much greater efficiencies," he says.