The sound of cool

Published in 2010

For almost a quarter of a century, U.S. physicist Steven Garrett and his collaborators have been quietly labouring to perfect a refrigerator that does not contribute to increased skin cancer or destabilize the Earth’s climate, a refrigerator cunningly cooled by sound.

With the Earth’s ozone layer still at risk and global temperatures climbing, his clean thermoacoustic chiller technology looks very much like an idea whose time has come.

Though half the world’s countries have agreed to phase out ozone-depleting CFCs (chlorofluorocarbons) in refrigerators, 2006 was the worst year yet for atmospheric ozone loss: 40 million tonnes were destroyed, exposing millions of people to searing ultraviolet rays from the sun.

Behind the lack of progress in eliminating CFCs is the fact that developing countries are still making old-style refrigerators in huge numbers, while backyard operators worldwide continue to refurbish old refrigerators and air-conditioners using black market CFCs.

“But even this problem is overshadowed by the fact that the substitute chemicals now being used in modern refrigerators around the world are screamers as global warming gases,” Dr Garrett points out. When it comes to warming the planet, the HCFCs and HFCs used to cool our refrigerators are up to 3,000 times more potent than carbon dioxide as greenhouse gases. “The chemical industry seems to have fixed one problem by creating another,” he reflects.

Steven Garrett (Centre) and Penn State University teammates Robert Smith and Matt
Poese are revolutionizing the technology of refrigeration.

Refrigeration, as we know it today, has been refined over 100 years, and every part of the modern refrigerator is cheap thanks to mass production: to compete, a new cooling technology must not only be exceptionally safe and efficient, but also be inexpensive.

Dr Garrett’s thermoacoustic design relies on components that are uncommon. Recognizing this, he and his team at Pennsylvania State University’s Applied Research Laboratory have focused their efforts during the past 15 years on developing units capable of holding their own in the unforgiving world of commerce.

To do this they teamed up with well-known U.S. ice cream manufacturer Ben & Jerry’s Homemade Inc, which is owned by the Anglo-Dutch firm Unilever. The partnership has yielded an entirely new design, incorporating several brilliant innovations resulting in at least six new U.S. patents and a dozen international patents.

Like a normal refrigerator, the thermoacoustic system uses the compression and expansion of gas to produce cooling and drain off heat. In this case the gas is helium, totally inert and without toxic attributes. Primary cooling is achieved using a moving-magnet loudspeaker-bellows combination, which converts electricity to soundwaves powerful enough to compress and heat the gas. Its efficiency exceeds that of ordinary rotary motors. A heat exchanger conducts away the excess heat, and cooling takes place as the gas moves from high to low pressure.

This all worked quite well, but was not efficient enough to match the performance of conventional refrigerators, so Garrett’s team added some inspired technical innovations that have yielded a refrigeration system that meets commercial specifications. None of them came easily – indeed, Dr Garrett says, it took years to perfect the high-efficiency loudspeaker system alone.

To this they added a “vibro-mechanical multiplier” that operates in phase with the bellows and allows efficient recycling of the acoustic power while orchestrating the acoustical pressure changes and gas flows. As in the traditional Stirling cycle, they introduced a regenerator, a very fine-mesh screen that allows the gas heat to be raised stepwise and conducted away – for another gain in cooling efficiency. Finally they linked the regenerator to a secondary cooling system, containing common motor vehicle antifreeze for heat exhaust and pure grain alcohol to collect the heat at very low temperatures. Alcohol is one of very few materials that remain liquid at ice cream temperatures.

A refrigerator powered by sound might be expected to be noisy and, although the volume of sound created by the loudspeaker is indeed loud, it is confined within the cylinder – and the refrigerator itself is almost silent: gone is the familiar chug and rumble of the domestic cooler. “When we put it on display, the media were rather disappointed because their microphones couldn’t pick up the sound,” says Garrett. “Though based on sound, it’s much quieter than an ordinary refrigerator.”

The Ben & Jerry’s freezer was compelling public proof that sound chills – but Dr Garrett admits that, because of its expensive componentry, it still has some way to go to beat a conventional refrigerator on price.

“If our parts were being turned out in tens of millions like those of current refrigerators, it wouldn’t be a problem,” he explains. “The trouble is there’s no great impetus to make commercial refrigerators less polluting or more efficient and, profit margins being what counts, people still prefer to buy inefficient, dirty, but cheap refrigerators.” However as regulatory pressure grows for greater energy efficiency and tighter control of CO2 production and chemical emissions, he expects it will increasingly favour thermoacoustics.

“Since 2002, our work has been supported commercially, so at least it has reached the level of visibility where people are starting to consider thermoacoustic solutions,” says Garrett. “We are just not good enough yet at getting the efficiency and cost into the right zone to displace the existing technology. But we haven’t quit trying. We’re still moving forward.”

The Ben & Jerry’s prototype ran faultlessly in demonstrations in New York and Brussels. “Our research grant ran out before any flaws showed up,” he adds, with a touch of pride. Since then it has been the familiar story of the innovator struggling to find enough financial support to maintain momentum, while grappling with the technical challenges that invariably emerge in any new technology.

Most recently he and his team at Penn State have been working on a much larger thermoacoustic chiller. “It is working – but not quite as efficiently as we had predicted, so it’s back to the laboratory to figure out why and how to make it do so,” Garrett says.

Another major possibility they are contemplating is developing a secondary cooling system based on thermoacoustics for use in supermarket chillers, to replace part of the cumbersome, leaky, centralized systems of today.

“It’s been more difficult to develop a new kind of refrigerator than I ever imagined it would be when I won the Rolex Award, back in 1993,” the Laureate confesses. “It’s very hard to get support for a new technology that challenges established ones, but the great thing about the Awards is that they give you both exposure and credibility, which can help. These in turn can unlock the resources needed to take the technology forward.

The other reason Steven Garrett keeps going with thermoacoustics through success and setback is summed up in his wry self-assessment: “You’re talking to a professional optimist here.”

Images courtesy of Greg Greico, Penn State University and Rolex Awards.

Julian Cribb

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