In the corner of Steve Garrett's lab, behind the cluttered workbenches and banks of monitors, sits a small steel cylinder that could change the cold-air business for good. Garrett, a Penn State professor of acoustics, along with research associate Matthew Poese and engineer Robert Smith, has developed a refrigerator that is powered by sound.
"This isn't a complicated technology," says Poese. "There are no crazy materials it took years to develop." Nor is the theory difficult. Remember high-school physics? As a gas expands it gets cooler; squeezed, it heats up.
According to Garrett, physicist John Wheatley of Los Alamos National Laboratory was the first to exploit the implications of sound's thermal properties. In the early 1980s, Garrett recounts, "Wheatley was working on heat engines, a generic term for a device that uses heat to either create mechanical energy or move heat against a temperature gradient, which is what a refrigerator does." Assisted by Greg Swift, another Los Alamos physicist, Wheatley discovered that sound could be used to cool.
During the late '70s Garrett and Swift had studied low-temperature physics together as graduate students at the University of California at Berkeley. When Swift moved on to Los Alamos, Garrett took a faculty position at the Naval Postgraduate School in Monterey, California. When, a few years later, Wheatley and Swift were working on their first prototype refrigerator, they turned to Garrett to recommend a loudspeaker powerful enough to drive it. Fascinated by thermoacoustics, Garrett had no trouble getting the Navy to fund his own subsequent research in the field. "It makes sense that the Navy took so much interest," explains Poese. "From food to computer chips, a lot of their operational capability depends on efficient cooling."
During the early '90s, Garrett developed two sound-based refrigerator systems: The first was installed on the space shuttle Discovery in 1992, and in 1995 the second went on board the warship USS Deyo. During May of that year, Garrett moved to Penn State, where the Navy has continued to fund his work. Last year, he picked up a new sponsor: the ice cream makers Ben and Jerry's.
At a foot tall and eight inches in diameter, the steel cylinder that holds the cooling apparatus for Garrett's latest prototype would easily fit on the back of a standard refrigerator. It stands on its end atop a small flat motor. From the other end protrude several looping plastic tubes circulating a clear liquid. The motor is a 200-watt loudspeaker lodged in one end of the cylinder, Garrett explains. "Inside the cylinder we compress helium, chosen for its excellent thermal properties, to almost 10 times the atmosphere's pressure." The speaker blasts a long unchanging note, equivalent to a low G on the piano, further compressing the helium to produce a volume of 196 decibels—16 decibels higher than the sound of a shuttle launch at ground zero. Fortunately, the sound is contained almost entirely within the sealed container. "The only noise is a low hum, quieter than your refrigerator at home," says Poese.
When the "music" starts, pressure changes caused by the sound waves bouncing in the sealed space alternately heat and cool the enclosed gas. Starting at the speaker end of the cylinder, the heated helium passes through a series of finemesh stainless-steel screens installed to siphon off its heat. "The gas is forced through tiny pores in the screen," Garrett explains, "and the helium transfers heat to the cool steel."
As the sound passes through the screens, the wave's falling pressure allows the helium to expand and cool further before it reaches several metal fins which extend into a reservoir of ethyl alcohol on the outside of the pressurized cylinder. The cold helium draws heat from the alcohol (in this prototype, vodka), and the frigid liquid is then pumped through the walls of the ice cream sales cabinet. After cooling the ice cream in the box, the alcohol cycles back to the reservoir, where the helium re-chills it.
"The gas functions like a sponge, soaking up heat, squeezing it out, and coming back for more," says Poese. On its way back to the loudspeaker end, the warm gas passes over a bed of tiny, water-filled aluminum tubes that protrude through the walls of the cylinder. This water collects the helium's heat and exhausts it outside the cylinder.
Because the system contains none of the global-warming or ozone-destroying gases that standard refrigeration depends on, Poese notes, the thermoacoustic refrigerator is environmentally friendly. "It also has fewer moving parts than its competitors, and so is less likely to break down," says Smith. In addition to being useful on shipboard, this technology could be adapted for softdrink machines, medicine storage, computer chips, and food-transport companies.
Whether it can compete economically with your kitchen Frigidaire is another matter. "We are up against a very cheap, efficient, and reliable machine," says Poese. In the event of a further tightening of international environmental legislation similar to the 1996 Montreal protocol that banned CFCs, however, the thermo-acoustic refrigerator could become a viable mass-market alternative.
For now, the team has already achieved standard ice cream temperature: ñ4 degrees F. "We have started to close in on some heat leaks that are reducing cooling capacity," says Poese. Small problems like these, he is confident, will be ironed out quickly. Once they are, the sound-based cooler may be chilling pints of Cherry Garcia down at your local Ben and Jerry's.
Steven Garrett, Ph.D., is United Technologies Corporation professor of acoustics in the College of Engineering,220 Applied Science Bldg, State College, PA 16804; 814-863-6373; sxg185@psu.edu. Matthew Poese, M.S., is a doctoral candidate and research associate in acoustics, 105 Applied Science Bldg; 863-1899; poese@psu.edu. Robert Smith, M.S., is a research associate engineer, 204M Applied Science Bldg, 865-5883; rws100@psu.edu.