Cool sounds

Environmentally friendly fridges that use sound to cool their contents rather than toxic chemicals are to be launched next year.

Most old refrigerators use chlorofluorocarbons (CFCs) as the cooling agent. These deplete the ozone layer and are now banned under the Montreal protocols. The alternative, hydrofluorocarbons (HFCs), are also listed as greenhouse gases under the subsequent Kyoto accord. A new method of refrigeration has long been a technological goal.

Researchers at the US government’s Los Alamos National Laboratories in California and Penn State University in Pennsylvania have been working on thermoacoustic technology for several years and commercialisation now looks like a reality.

Sponsored by Unilever ice cream firm Ben and Jerry’s, the design is derived from originals developed for the Space Shuttle and a US Navy warship.

The principle is based on the fact that the temperature of a gas of a certain volume increases when it is under pressure, and that sound waves in a gas consist of pressure waves. If a gas carrying a sound wave is confined in a narrow enclosed space between two solid layers, the minute temperature increase due to the pressure can be transferred into the enclosing plates which act as a heat sink.

As the wave bounces back and forth through the enclosed space the gas expands again, cooling down and extracting heat from the opposite end of the heat sink. If a wave bounces backwards and forwards enough times, one end of the sink will heat up and the other cool down. Stacking several plates together enhances the process.

But the technique is not efficient. To solve the problem the developers have used the 19th-century idea of a regenerator. This porous component acts like a valve to help the heat energy flow in one direction through the stack and increases the efficiency enough to make the device feasible, said researcher Steven Garrett.

The prototype is about the size of a beer keg (17 litres) and will be fitted in two retail outlets next year. Garrett confirmed that though it will use two loudspeakers, the sound will be contained within the pressure vessel. It will contain inert gases such as helium or argon, and the temperature be more easily controlled than with the on/off system used in normal fridges.

Garrett felt there were many other possible applications. He has built an air-conditioning prototype for the US Office of Naval Research and expects to enter other markets such as the cooling of electronics and computer chips.

‘It is difficult to penetrate the home appliance industry, since there is such a large investment in the infrastructure for mass-production of home refrigerators and airconditioners and components.’ But, as environmental regulations begin to bite, he hoped more manufacturers would take note.