SOME really cool sounds are wafting round the corridors of certain
research departments at Purdue University in Indiana.
But the scientists involved are not starting a garage band鈥攖hey are
using sound to make a fridge. Luc Mongeau and colleagues in the university鈥檚
school of mechanical engineering admit that sound-driven refrigerators have been
developed before, but none is as cheap as theirs. Cost is crucial in persuading
people to forego ozone-depleting refrigeration systems, particularly in
countries such as China, where demand for fridges is escalating as citizens
become wealthier.
At the heart of the fridge is a hollow, cylindrical tube, 60 centimetres in
length, which is filled with a helium and xenon gas mixture and fitted at one
end with what looks like a giant doorknob. The 鈥渄oorknob鈥� is a Helmholtz
resonator that reflects sound waves blasted into the cylinder from a loudspeaker
at the other end.
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The sound cools the fridge indirectly. 鈥淚t鈥檚 the shaking of gas by sound
waves in the device that鈥檚 important,鈥� says Mongeau.
As waves of sound bounce from one end of the cylinder to the other at a
frequency of 300 hertz, they repeatedly compress then decompress the gas mixture
filling the device. The gas heats up when compressed and cools when
decompressed.
Mongeau and his colleagues siphon off this heat into a solid, honeycomb-like
interceptor device called a stack, which sits near the loudspeaker. Each time
the gas is pressurised by the sound waves, it diffuses into pores within the
honeycomb, and dumps some of its heat into the stack walls. Decompressed gas
soaks heat back out.
The stack鈥檚 finely tuned structure means the heating and cooling do
not cancel each other out. Instead, a temperature differential builds up across
the stack, with the hot end closest to the loudspeaker reaching around 37 掳C
and the cooler end near the Helmholtz resonator dropping to 鈥�13
掳颁.
Heat exchangers linked to the stack translate the
temperature difference into the energy for refrigeration. This process regulates
the temperature of the coolant in coils in the body of the fridge that suck heat
out of the interior, maintaining a temperature between 3 掳C and 7 掳颁.
The coils contain a mixture of water vapour and ethylene instead of
ozone-damaging refrigerants.
The most expensive part, says Mongeau, is the loudspeaker,
although the aluminium diaphragm that 鈥渞attles鈥� the gas is the cheapest yet
tried in a sound-driven refrigerator system, he says.
Thick walls and tight sealing make the fridge no noisier than existing
models, even though the sound levels inside reach 180 decibels.
