Description of technology Thermoacoustic refrigeration systems operate by using sound waves and a non-flammable mixture of inert gas (helium, argon, air) or a mixture of gases in a resonator to produce cooling. Thermoacoustic devices are typically characterised as either ‘standing-wave’ or ‘travelling-wave’.
The main components are a closed cylinder, an acoustic driver, a porous component called a "stack, and two heatexchanger systems. Application of acoustic waves through a driver such as a loud speaker, makes the gas resonant. As the gas oscillates back and forth, it creates a temperature difference along the length of the stack. This temperature change comes from compression and expansion of the gas by the sound pressure and the rest is a consequence of heat transfer between the gas and the stack. The temperature difference is used to remove heat from the cold side and reject it at the hot side of the system. As the gas oscillates back and forth because of the standing sound wave, it changes in temperature. Much of the temperature change comes from compression and expansion of the gas by the sound pressure (as always in a sound wave), and the rest is a consequence of heat transfer between the gas and the stack. Loudspeaker Stack Hot heat exchanger Cold heat exchange Resonator Win Figure 1 Sound wave Thermoacoustic engine In the travelling-wave device, the pressure is created with a moving piston and the conversion of acoustic power to heat occurs in a regenerator rather than a stack. The regenerator contains a matrix of channels which are much smaller than those in a stack and relies on good thermal contact between the gas and the matrix. The design is such that the gas moves towards the hot heat exchanger when the pressure is high and towards the cold heat exchanger when the pressure is low, transferring heat between the two sides.
The technology has the potential to offer another refrigeration option but improvements in design are necessary to increase COPs to the level of vapour compression systems. Research effort is currently directed to the development of flow-through designs (open systems) which will reduce or eliminated the use of heat exchangers.
Potential applications in the food coustic refrigerators have the sector Thermoacoustic potential to cover the whole spectrum of refrigeration down to cryogenic temperatures. It is likely that potential market for food applications will be in the low capacity equipment such as domestic and commercial refrigerators, freezers and cabinets.
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