Sounds Cool Enough to Chill a Cold One
Thermo-acoustic refrigeration may be the way to keep both food and our environment cool. The trick is getting sound waves in tune with some molecules of helium.

The March 22 issue of The Economist gives a quick primer on keeping cool: since their invention in the 19th century, refrigerators (home and industrial, as well as their air-conditioning kin) have worked on the same principle of getting a cycle of compression and expansion to happen in the right locations. (Allowing a fluid to expand sucks heat away from the surroundings; the necessary opposite, compressing that fluid, dumps heat into the surroundings.)

Commonly used fluids have included ammonia (poisonous!) and CFCs (potent atmosphere destroyers!). And other technologies have been inefficient, requiring more fuel per unit of cool, or limited to specific items e.g using magnets to cool gadolinium (doesn't work on root beer!).

Enter sound. Sounds are just pressure waves, and the ups & downs of pressure create temperature changes. But usually only little ones: even a hot chat raises temperature by only ten-thousandths of a degree. However, raise the volume and the pressure of the gas inside a resonating cavity and you can create some real heat with no moving parts.

Getting real cool is trickier: one needs to resonate at just the right frequency so that cooler and warmer particles of, say, helium, are moved in opposite directions and can be separated. (Remember Maxwell's Demon from high school physics?)

Scientists have been experimenting with thermo-acoustic (although, shouldn't it be "acousti-thermal"?) effects for years, including possible new cooling systems for spacecraft. Now some companies, for example natural gas company Praxair, are starting to commercialize them back on earth.

Read a good explanation of how sound can cool, provided by the Energy Research Center of the Netherlands

Link to the March 22 Economist article (unfortunately, free only to print subscribers)

Link to laboratories at Los Alamos or Penn State University to learn about their research projects (including PSU's "FrankenFridge")

Link to a description of Praxair's applications