Scientists Use Sound, Not Chemicals, in Cooling Units

MONTEREY, Calif. —  Scientists at the Naval Postgraduate School here are pioneering a new approach to building refrigeration units that would use sound waves to produce cooling instead of the chemicals that are destroying the Earth’s ozone layer.

While it is still too early to tell whether the radical new technology will lead to a new generation of air conditioners and refrigerators, the scientists believe their work holds some promise of providing a solution to the ozone problem that has eluded experts around the world.

Cooling units now in use rely on chemicals called chlorofluorocarbons (CFCs) that are eating away at the layer of ozone that protects the Earth from ultraviolet radiation from space. Nearly all the research that is now under way elsewhere is aimed at replacing those chemicals with others that would work in conventional refrigerators, but so far no satisfactory substitute has been found.

The new approach, according to a team of acoustical physicists here, would use sound waves to cause a harmless gas to oscillate in a way that can be used to cool--or heat--adjacent areas. The scientists are adding the finishing touches to a small refrigerator that is to be flown aboard the space shuttle sometime in the coming months for the Air Force Space Test Program in El Segundo.

“We feel confident that we can do a commercial design” of a cooling system that would use their technology instead of chlorofluorocarbons, said Steven L. Garrett, professor of physics at the postgraduate school.

Garrett concedes that far more research is needed. But he is so convinced of the technology’s promise that he said he wishes he had spent more time working on a home cooling system than the space unit, which could be used to control temperatures in satellites.

“We should have gone right to house air conditioning,” he said. “We would be rich by now.”

“We could make a thermal acoustics (cooler) comparable to the ones you buy now,” added a fellow researcher, physicist Thomas J. Hofler.

The experiment has its doubters, but there are few experts who can comment on the claim at all, since there are only a handful of scientists working in the field. While the researchers here are confident, they caution that they are not promising an instantaneous solution to a problem that has frustrated literally thousands of other scientists.

“We don’t want this to turn into another cold fusion,” Garrett said, referring to the now-infamous claim by chemists at the University of Utah that they had achieved fusion in a jar, thus promising to end the world’s energy woes.

“This is based on proven experiments, and our results have been published” in professional journals, Garrett added.

The biggest problem, he said, is that “it’s a brand new technology” and the technology that led to today’s ozone crisis is “entrenched.”

The change from chemical to acoustical cooling units would mean switching from a widely understood technology serviced by thousands of technicians to a new industry that would need to retrain people for a radically different system.

“Only five or six people know how to design one of these,” Garrett said.

Garrett, Hofler and a graduate student, Jay Adeff, built upon pioneering work by Gregory W. Swift, a physicist with the Los Alamos National Laboratory in New Mexico, who demonstrated a decade ago that sound waves could be used to alter temperatures and, conversely, that changes in temperature could be used to produce sound waves. They concluded that sound waves could be used to control temperatures in satellites and would be more reliable than the liquefied gas coolers used today.

They put together a proposal and the Defense Department decided to fund an experiment, which led to the creation of the small cooling device that is to be flown aboard the space shuttle.

Ironically, Swift is one of those who has doubts about the project. He said a refrigeration unit that runs on acoustics would produce heat as a byproduct, and removing that may prove to be an insurmountable problem.

“It has yet to be proven that the efficiency would be high enough to satisfy federal requirements for appliances,” Swift said.

But even Swift believes the technology may be extremely adaptive to space vehicles because the current generation of coolers are not always reliable and have a limited life span.

“The requirements in space are different” from those on Earth, Swift said. The primary need there is for “reliability instead of efficiency, because it’s hard to send up a repairman whenever anything goes wrong.”

An acoustical system, with very few moving parts, should be very reliable, the Navy researchers said.

The models built here use some parts from off-the-shelf sound equipment. The heart of the system is a thin metal diaphragm from a conventional loudspeaker. Electronic impulses cause the diaphragm to vibrate, just as it would in a speaker, sending out acoustical waves that would be deafening if they could be heard.

However, the system is enclosed in a non-vibrating plastic tube, so it is very quiet. The sound waves generated by the diaphragm travel through a tube that contains an inert gas, such as helium, which, coincidentally, would not interact with the ozone even if released. As the sound waves travel through the gas, the molecules move back and forth, compressing and expanding as the waves pass through.

When gas molecules compress, they heat up; when they expand, they cool. This in turn sets up a thermal gradient in the system. By tapping into cold areas with some form of heat exchanger, the cooler temperature can be extracted and used to refrigerate some other area.

Swift’s concern, however, is that gas moving back and forth also causes friction, which causes heat, thus lowering the efficiency of the system.

Garrett doesn’t fully disagree with that, but he maintains that no system in the future that relies on anything but the highly efficient chlorofluorocarbons will work with the same efficiency.

CFCs have been considered the ideal chemical for refrigeration because the bonding of the molecules is so strong that the gas does not break down. Once released in the air, a single molecule can last for a century, slowly rising in the Earth’s atmosphere. At high altitudes, however, it is exposed to ultraviolet radiation, which causes it to break down, releasing free chlorine, the agent that destroys ozone.

None of the other chemicals now being studied as substitutes promises to be as effective as chlorofluorocarbons, which must be phased out under international agreements by the end of the century. All substitutes now being tested have severe drawbacks, including an incompatibility with lubricants. Some are highly corrosive. And all would be far less efficient.

Because this inefficiency would increase the amount of electricity needed to run a cooling system, the demand for electricity in the United States, using any substitute chemical, would rise at least 3%, according to a study by the Oak Ridge National Laboratory.

Garrett maintains that this scenario will dictate that federal energy standards for appliances will have to be relaxed, no matter which solution is picked. Thus, he said, even if the acoustical refrigerator takes a little more juice, it should not be ruled out.

And he argues that there are other components in the system that could be made more efficient--an acoustical unit has far fewer moving parts than a typical compressor, for example--so energy savings could be made elsewhere.

Some applications would be harder to achieve than others, he added. It might be easier to build a home air conditioner than a refrigerator because cooling loops containing water could be used in an air-conditioning unit but would not be effective for the colder temperatures needed for a refrigerator.

Garrett believes his main hurdle will be to get others to take his program seriously.

“This option came out of the blue,” he conceded. “It came from nowhere.”