Cool Sounds

Most modern refrigerators use chemicals called Hydro-Chloroflurocarbons — or HCFCs.

If these chemicals leak into the air, they act as greenhouse gases. They’re thousands of times better at holding in heat than carbon dioxide. That’s why scientists at Penn State University are developing more environmentally friendly refrigerators. Instead of using HCFC’s, the new fridges use helium and sound waves. Here’s acoustics researcher Matt Poese:

Matt Poese: We use a motor, a big souped-up loudspeaker, to create an acoustic wave and then that acoustic wave makes things colder.

When sound travels through a gas, the gas itself expands and compresses. When it expands, it gives off a little heat. Scientists have learned how to use just the right sound waves to move heat out of the gas and into metal plates that conduct heat out of a chamber. This so-called “thermoacoustic fridge” uses helium gas, which has no impact on global warming.

Poese and his colleagues know the technology works in the laboratory — but new funding, from Ben and Jerry’s Ice Cream, will let them to build a refrigerator that’s efficient and inexpensive enough to use in the home or in industry.

Hydro-Chloroflurocarbons — or HCFCs

Every time you pop a pint of ice cream in the freezer or a jug of juice in the fridge, you make a small contribution to global warming.

Most modern refrigerators use chemicals called Hydro-Chloroflurocarbons — or HCFCs. In the 1990s, these chemicals began to replace chemicals called CFCs that were linked to the destruction of Earth’s protective ozone layer. But the new replacement chemicals have their own environmental drawbacks. When they leak into the air, HCFC’s act as greenhouse gases. They’re as much as 2,000 to 3,000 times more potent than carbon dioxide.

People recognized the interaction between sound and heat as much as 200 years ago. In the late nineteenth century, the British Lord Rayleigh discovered that Isaac Newton had made a 20 percent error in calculating the speed of sound because he hadn’t realized that the transmission of sound in the atmosphere is insulated, rather than conducted.

One of the biggest barriers to thermoacoustic refrigeration is that conventional, or vapor compression, refrigerators already work extremely well, so there isn’t much incentive to improve them. And while sound and helium are more environmentally friendly than chemical refrigerants, thermoacoustic refrigerators still use a good deal of electricity to operate. So, sound-powered refrigerators will have to become much more efficient and cheaper to make to be feasible for mass production.

At 175 decibels, sound levels inside a thermoacoustic refrigerator’s resonator are intense enough to blow out your eardrums. But the machine contains all of that sound, so it is actually quieter than a conventional fridge and just as safe.

Still, thermoacoustic refrigerators might some day transform the way we cool all sorts of things, Poese says. “My wildest dreams for success of this technology,” he says, “are to see it slowly infiltrate into niche applications like ice cream freezers for Ben and Jerry’s, like soda vending machines, like electronic cooling for computer servers.” Likewise, air conditioners in cars, trains, and planes could eventually run on sound.

Other applications for thermoacoustics are also in the works. Researchers at the Los Alamos National Laboratory are working on a natural gas liquefier, which would use sound waves to cool natural gas into a liquid. The liquid could then be put on barges and shipped from remote sites, providing more energy sources and reducing our dependence on foreign oil.

The newest development in thermoacoustics is using sound to separate mixtures of gases, says Los Alamos physicist Greg Swift. “If you have a sound wave in a gas mixture in the right geometry, one component can be made to go with the sound wave while the other swims upstream,” he says. “If the engine refrigeration stuff blows your mind, this really ought to amaze you.” The technique might some day have applications for drug delivery and other medical uses.

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