Cool Sounds

A testing model of the thermoacoustic chiller. Smith says it can be made at half the size.

Since the chiller uses helium as a coolant, it's environmentally friendly. Helium is a harmless gas that doesn't explode, burn or hook up with other chemicals; when it's released into the atmosphere, it drifts into space. Typical refrigerators and air conditioners use harmful chemicals called hydrofluorocarbons, or HFCs, that cool by absorbing heat when the turn from liquid to vapor. They can also damage the atmosphere.

"HFCs are very strong, very potent greenhouse gas pollutants," says Kert Davies, research director of Greenpeace in the U.S. "They are thousands of times stronger per molecule than carbon dioxide, for example. And as a total volume in coming decades, they will be a major percentage of global warming pollution in the world."

Greenpeace has promoted other non-HFC refrigerants for more than a decade. Their "Greenfreeze" movement caught on all over the world, but not in the U.S. "Greenfreeze is unfortunately not available in the United States due to blockage by various manufacturers who still call it unsafe even through it's been proven safe and basically don't want to make the change," says Davies. "Everywhere else in the world you can buy a Greenfreeze refrigerator, from Europe to Japan to Canada."

Most Greenfreeze products use hydrocarbon gases like propane as a coolant. While such gases are flammable, Davies says the risk they pose is minimal, since the amount of gas in a typical refrigerator is the equivalent of what's in two or three cigarette lighters. And while he feels they are safe, he also agrees that a freezer like Smith's might be even better: "This thermoacoustic idea, if it works, only uses helium as a coolant. Obviously helium is safe enough to have at kids' balloons at parties. So that's even better than a flammable gas."

Smith and his collaborators— Steve Garrett and Matt Poese— formed a start-up company with the hope of mass-producing their chiller in two to five years. "We think thermoacoustic refrigeration is a potential alternative," he says. "We think that this is able to use a refrigerant that is inherently safe for the environment. We think it can be something that is quieter than the existing products that are out there, and we think it could be just as reliable or perhaps even more reliable in a typical home refrigerator. In our devices, essentially, there are no parts that are experiencing wear. There are parts that are flexing, very much like your loudspeaker in your home, but there are no parts that are experiencing friction. So we think that provides a path that may make these devices sort of inherently reliable."

The Penn State team will present their prototype with a paper at the at the Gustav Lorentzen Natural Working Fluids Conference in Scotland in August. They received US Patent #6,725,670:"Thermoacoustic Device". Garret published a summary of their research and other thermoacoustic refrigeration research in the January, 2004 issue of American Journal of Physics. The group's work was funded by Unilever, Ben & Jerry's Homemade Inc., and Penn State University.