Cool Sounds

Cool sounds

By Steven Ashley, Associate Editor

Acousticians are working on using the sound-wave-driven phenomenon known as the thermoacoustic effect in simple, reliable, low-cost, environmentally friendly refrigeration units. Although applications such as home refrigerators, electronics chillers, and natural-gas liquefiers have been explored, the efficiencies are not yet sufficiently high to generate the financial investment required for commercialization.

Thermoacoustic refrigerators use standing sound waves to generate low temperatures for cooling.

The thermoacoustic effect is surprisingly simple, according to Steven Garrett, United Technologies Corp. Professor of Acoustics at Pennsylvania State University in State College. "When you compress a gas, it gets warmer; expand a gas and it cools. A sound wave is really nothing more than a periodic compression and expansion of a gas." So a sound wave heats and cools small parcels of gas along the length of its propagation.

When a sound wave is sent down a half-wavelength tube with a vibrating diaphragm or a loudspeaker, the pressure pulsations make the gas inside slosh back and forth. This forms regions where compression and heating take place, plus other areas characterized by gas expansion and cooling.

A thermoacoustic refrigerator, Garrett said, is a resonator cavity that contains a stack of thermal storage elements (connected to hot and cold heat exchangers) positioned so the back-and-forth gas motion occurs within the stack. The oscillating gas parcels pick up heat from the stack and deposit it to the stack at a different location. Garrett said the device "acts like a bucket brigade" to remove heat from the cold heat exchanger and deposit it at the hot heat exchanger, thus forming the basis of a refrigeration unit.

Pioneering work on thermoacoustics began in 1982 at Los Alamos National Laboratory in Los Alamos, N.M., and continues under Greg Swift, who since 1994 has focused on developing a combustion-powered thermoacoustic natural-gas liquefier with no moving parts. Funded jointly by Denver-based Cryenco Inc. and the U.S. Department of Energy, the device burns natural gas to produce heat that is used to form a standing wave inside a helium-filled resonant cavity. The heat-driven cryogenic refrigerator, which has a cooling capacity of approximately 2,000 watts, reaches —150°C.

The Penn State group has developed several thermoacoustic refrigerators over the years. In 1992, the U.S. Air Force launched a thermoacoustic cooler the group built to chill optoelectronic devices (for eventual use on satellites) on the space shuttle Discovery. Two years later, the Shipboard Electric ThermoAcoustic Chiller (SETAC) cooled a radar azimuth converter on a Navy destroyer, the U.S.S. Deyo, for 18 hours. SETAC had the cooling power of a home refrigerator. The team is now fabricating a technically similar yet much larger chiller/air conditioner for the U.S. Navy. The 3-ton unit, called Triton, is being developed with $3 million in funding and is due for completion in April 1999. The 10-thermal-kilowatt unit is expected to cool electronics at the rate of 36,000 Btus per hour.

Other thermoacoustics work is now being conducted at the Naval Postgraduate School in Monterey, Calif.; Purdue University in West Lafayette, Ind.; the University of Utah in Salt Lake City; and the University of Mississippi in University.