Cool Sounds

A conventional refrigerator—your kitchen fridge—works by compressing a gas, then letting it expand, all within a closed system. Expansion cools: it's basic thermodynamics. Once the gas has chilled your beer and your bologna, it is routed back to the compressor, where the cycle starts again.

Unfortunately, the gas employed is freon; well-suited to the purpose, but when those pipes get old and leaky, it plays havoc in the atmosphere.

But listen, says Penn State physics professor Jay Maynard. A sound wave causes compression and expansion too.

In this case the gas is air. Chao Zhang, a Ph.D. student in Maynard's lab, suggests thinking of a discrete parcel of air, a blob, riding those waves of sound in an enclosed space. The waves are small, so the displacement for any given blob is minute—a few hundred microns, or even less, depending on the wave's frequency. But each squeeze and release is accompanied by a corresponding shift in temperature.

Maynard and Zhang are working on gathering up these little changes and putting them to good use. They are building a thermoacoustic refrigerator: an icebox powered by sound.

There are a number of challenges involved in fleshing out the concept. "The first problem," Maynard says, "is that there's no heat flow in air. The thermal conductivity is too low.

"So what you do is get yourself a boundary, a solid surface, and propagate sound waves across that." The surface, he explains, ferries the heat — but not too fast. Thermoacoustic cooling depends on establishing and maintaining a temperature gradient.

It works like this: Take a thin plate of stainless steel or other suitable material. To both ends of it fix a heat exchanger—a thin hollow coil with water or another liquid running through it to carry heat away, like a car radiator. Enclose this assembly in a well-sealed tube. Then apply sound.

cut away of silver device with ball on top

Design for a refrigerator that cools by sound. An acoustic driver (bottom) , like a stereo speaker, pushes sound waves through a stack of heat-absorbing plates (center) attached to a resonator (top). Air is cooled by compression and expansion.

For demonstration purposes, a standard stereo speaker funneled to one end of the tube will do nicely. Hit the right frequency and the tube becomes a powerful resonator. Maynard sketches the configuration on a blackboard.

"You've got a tremendous amount of sound in there, 190 decibels. Above the pain threshhold," he says. "But it all stays inside that rigid tube, reflecting back and forth, building up." The steady pulse sets the air in the resonator quivering.

"There's a pressure differential created," Zhang explains. "The pressure in the tube is highest at the ends, and lower in the middle. So when a blob of air moves toward the end of the tube, the pressure on it is increased. The blob is compressed, and its temperature rises. Then it moves back toward the middle, where the pressure is less. It expands and cools off."

Here's where the steel plate comes in. At each end of its little back-and- forth waggle, the blob must pause to change direction. That instant, if the blob is in contact with the steel surface, is time for a transfer of heat.

Toward the middle of the tube, the relaxed blob—being cooler than the plate—takes on heat. At the other end of its swing, all scrunched and warmed up, it gives the heat back. Then it goes back and repeats the cycle, over and over. So, like a tiny bucket brigade, hundreds of blobs transfer heat from one end of the plate to the other—and eventually from one heat exchanger to the other. If you keep your starting heat exchanger at room temperature, say by circulating tap water through it, Zhang says, your ending heat exchanger will soon cool off nicely.

It's important to use the right material for your plate, Zhang adds. "You want it to hold a lot of heat, but not conduct it very well, so you don't ruin the temperature gradient you're trying to create." Ceramic would be ideal for the purpose, he suggests, except that ceramics cannot easily be made thin enough to function efficiently. "Most designs use plastic or stainless steel." To create enough cold, you also need lots of surface area, which is why thermoacoustic devices typically employ a stack of a dozen or more plates.

One persistent design problem, Maynard says, has to do with improving the interface between the stack and the heat exchanger. "It's what we call a link-scale problem," he explains. The typical heat exchanger is made of copper tubing and fins, with a gauge of three millimeters. Copper tubing can't be made much smaller. The plates in a stack, on the other hand, might be only a tenth of a millimeter apart—a distance fixed by the laws of thermal absorption. So the ratio is 30 to one. This size disparity means a lot of heat is lost in transfer.

To get around this problem, Zhang has created a composite design—a stack with a heat exchanger built into it. He pulls out an early prototype, one he painstakingly made of Pyrex. It fits in the hand, embedded with a score of tiny plates and a maze of tubing not much thicker than pencil lead. "This was very difficult to construct," he smiles. "It took me over a year." After he built it, however, Zhang had a better idea—a design improvement he can't discuss, because he and Maynard have applied for a patent on it.

Theoretically, Maynard says, once the bugs are worked out and efficiency is improved enough to make them commercially feasible, thermoacoustic refrigerators will be cheaper to make and more reliable than today's ozone eaters. "The promise is there. There are no moving parts, for one thing."

But, he adds, "There's a lot that has yet to be worked out." It will likely be some time, then, before household refrigerators are humming a different tune.

Chao Zhang is a Ph.D. student in physics in the Eberly College of Science, 104 Davey Laboratory, University Park, PA 16802; 814-865-7841; zhang@phys.psu.edu. His adviser, Julian D. Maynard, Ph.D., is distinguished professor of physics, 104 Davey; 865-6353; maynard@rayleigh.phy.psu. edu. The research reported here is funded by the Office of Naval Research. For more information on the Maynard lab, visit http://www.phys.psu.edu/MAYNARD/maynard.html. To learn about another thermoacoustics group at Penn State, headed by Steven Garrett, United Technologies Corporation professor of acoustics in the College of Engineering, visit http://www.acs.psu.edu/users/sinclair/thermoacoustics.html. Design for a refrigerator that cools by sound. An acoustic driver (bottom), like a stereo speaker, pushes sound waves through a stack of heat-absorbing plates (center) attached to a resonator (top). Air is cooled by compression and expansion.

Last Updated January 01, 1998