Simple Sound Can Be Really Cool

June 30, 1997

WEST LAFAYETTE, Ind. -- It's a chilling thought -- using a loudspeaker to power your refrigerator, without the use of refrigerants that can harm the environment.

The concept is called thermoacoustic refrigeration -- using sound waves to cool something. The technology offers several advantages over conventional cooling systems, says a Purdue University engineer who is developing a prototype device based on the idea.

Luc Mongeau, assistant professor of mechanical engineering at Purdue, says the simplicity of thermoacoustic devices could lead to refrigerators and cooling systems with low manufacturing and maintenance costs; new kinds of air-conditioning systems; and portable systems for liquefying natural gas, to name a few.

"Another advantage of this type of system is capacity control," he says. "When you don't need a lot of cooling, you just turn the volume down, so to speak."

Mongeau and colleagues James Braun, associate professor of mechanical engineering, and doctoral student Brian Minner, from Idaho Falls, Idaho, will complete their prototype device by the end of July. It looks something like a large doorknob with a long shaft, and here's how it works:

The device is essentially a hollow metal tube, which varies in diameter along its length and is capped on one end. Attached at the opposite end of the tube is an acoustic driver - a vibrating diaphragm similar to a loudspeaker, but sturdier and more powerful. As the diaphragm vibrates, gas atoms pressurized to 20 atmospheres inside the enclosure oscillate back and forth, which sets up pressure fluctuations.

"Fluctuating pressures inside the cavity are accompanied by a fluctuation in temperature," Mongeau explains. "When you compress a gas quickly it becomes warmer, and when you decompress it quickly, it becomes cooler. The gas particles within the device are becoming alternately hot and cold, dynamically, at a typical frequency of 200 Hertz, or 200 oscillations per second."

The gas atoms transfer their heat to and from a piece of porous material called a stack, which is located near the acoustic driver. The end result is that heat is pumped toward the driver, cooling the side of the stack farthest away from the driver.

"The pressure fluctuations propagate as sound waves that are very loud, around 180 decibels," Mongeau says. "If you were inside, it would be unbearable to listen to."

But Mongeau says a thermoacoustic refrigerator shouldn't be any louder than a conventional appliance, and may be even quieter.

"My specialty is acoustics and noise control, so I think we can design a quiet system," he says.

As in a conventional refrigerator, an appliance cooled by a thermoacoustic device would require coolant to circulate through pipes. One coolant loop removes heat from the space to be cooled and brings it to the cooled side of the stack -- another loop removes heat from the hot side of the stack and discards it to the surroundings.

Mongeau and his colleagues have considered water and a combination of water and glycol as coolants for their system.

""The main advantage to this type of cooling system is that there are no phase-change refrigerants involved, which may be harmful to the environment," Mongeau says. "With thermoacoustic refrigeration, all the elements are environmentally benign, including the coolants and the gas inside the device, which is a mixture of inert gases like helium, argon or xenon that present no hazard to the atmosphere.

"Also, because there are no mechanical compressors and lubricants as in conventional refrigerators, you don't have to worry about that kind of maintenance," he says.

A home appliance based on thermoacoustic refrigeration is still years away, Mongeau says, but he and his research group are working on several aspects of the technology.

"We're examining how to design these devices for optimum efficiency, as well as looking at cost factors," he says. "So far, most of the cost resides in the driver, but we think there are ways of reducing that cost. In order to have maximum efficiency, you can't just hook up any stereo speaker to this device. You have to design a driver specifically for the system."

In addition to home refrigerators and air-conditioning, the Purdue team is evaluating the feasibility of thermoacoustics for several other niche applications, where it will most likely first be used, Mongeau says.

Researchers at other institutions have considered thermoacoustic technology for cooling computer chips and other electronic equipment, and one researcher has a patent on a thermoacoustic device to cool seismic instruments in the earth's crust.

Thermoacoustic devices are versatile because they not only can cool, but also can work in reverse as an engine, Mongeau says.

"If you heat one side of the stack while keeping the other side cool, you produce acoustic power, which could be converted to electricity or which could power another acoustic driver to produce cooling elsewhere in the system," he says. Mongeau demonstrates this principle in his lab by dipping the open end of a device into liquid nitrogen, where it becomes super-cold. "When we take it out, it warms up and a self-sustained oscillation is created inside. It starts singing."

One of the applications of this type of system could benefit the natural gas industry. Natural gas must be cooled to convert it to a liquid for storage and transport.

"There is a strong interest in portable systems for liquefying natural gas on-site," Mongeau says. "Using a combination of thermoacoustic devices, you wouldn't need electricity to do this. You could use gas as the energy source to heat a thermoacoustic engine, which in turn powers a cooling device for liquefying gas."

Using sound waves to cool is not a new idea, says Mongeau, noting that the theory was developed in the 1960s and a few researchers in Europe and the United States began working on the idea in the 1980s. Since that time, scientists have gained a better understanding of these systems and have built several useful devices.

Mongeau, who with his colleagues has published and presented research at scientific meetings, says the Purdue team will continue its optimization work and will develop prototypes in order to evaluate the feasibility of the technology. He and colleagues are among a handful of groups conducting research in this area. Others are at Johns Hopkins and Penn State universities, the University of Mississippi, the University of Texas, Los Alamos National Laboratory, Ford Motor Co., and the U.S. Naval Postgraduate School.

Purdue University

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