Chilling out with sound.

Keeping food cold but easily accessible is a resource-intensive undertaking, involving electricity, compressor motors, and insulation, as well as gases such as hydrochlorofluorocarbons and hydrofluorocarbons, which contribute to ozone depletion and the greenhouse effect. Now researchers are developing alternative equipment that uses the environmentally benign gas helium and thermoacoustic engine technology. The research could provide a new freezer that is efficient, environmentally sustainable, and cost-competitive.

The past 20 years have seen considerable progress in making efficient thermoacoustic engines, which convert heat energy into acoustic energy or utilize acoustic energy to pump heat from cold to hot regions. Researchers at the Applied Research Laboratory at The Pennsylvania State University developed thermoacoustic chillers for the U.S. Navy, and they are using that experience to develop a much smaller version for ice cream maker Ben & Jerry's. Ben & Jerry's was acquired in 2000 by Unilever, which owns a staggering 1.2 million freezer cabinets worldwide and thus has considerable stake and a corporate mandate to pursue environmentally friendly technologies.

Essentially, the chiller is a closed pressure vessel that contains a motor producing the acoustic energy, heat exchangers, and a regenerator, or "stack." Inside the stack, the oscillating sound from the motor compresses and expands groups of gas molecules. As they compress and expand, the groups transfer heat from the cooler to the warmer side of the stack, effectively pulling heat away from the interior of the refrigerator. Matt Poese, a Penn State research associate, calls the device "Bellows Bounce" after the metal bellows that, in conjunction with the motor, seals the vessel and generates the acoustic pressure.

The research group completed phase I trials in September 2003 and is in the process of demonstrating they can meet company targets for cooling power, size, and performance, according to Pete Gosselin, director of engineering at Ben & Jerry's. "They've shown that a strong potential exists to take an inert gas and operate it with no performance tradeoff against standard vapor-compression technology," says Gosselin. "The next phase will be taking this engine and integrating it into a cabinet." The goal for this is April 2004.

John Corey, a senior fellow at CFIC-Qdrive, another group developing thermoacoustic chillers, points out that today's home refrigerators are basically commodities driven by very low cost for components, automated production, and well-entrenched manufacturers. Conversely, the market for display coolers and large commercial units is more complex in the sense that a few large companies integrate subsystems such as compressors and heat exchangers from a variety of vendors. Given the more labor-intensive aspect of these largely custom-tailored units, a thermoacoustic unit that is simple and easy to integrate may well succeed.