Thermoacoustic hot air engines (sonic heat pump and refrigeration or thermoacoustic heat pump and refrigeration) of which nearly all are thermoacoustic stirling engines is a technology that uses high-amplitude sound waves in a pressurized gas to pump heat from one place to another; or uses a heat temperature difference to induce sound, which can be converted to electricity with high efficiency, with a piezoelectric sensor.
This type of heat pump or refrigerator has no ozone-depleting or toxic coolant and few moving parts. A device consisting of a series of small parallel channels, referred to as a ‘stack’, is fixed in place at a set location inside a tube. In a standing wave thermoacoustic engine, the pressure and velocity fluctuations through the stack are such that heat is given to the oscillating gas at high pressure and removed at low pressure; "If heat be given to the air at the moment of greatest condensation, or be taken from it at the moment of greatest rarefaction, the vibration is encouraged. On the other hand, if heat be given at the moment of greatest rarefaction, or abstracted at the moment of greatest condensation, the vibration is discouraged, for self-sustained oscillation and by this process heat is converted into acoustic power. For thermoacoustic pumps, the process is reversed. By using thermal delays in the stack, this process approximates the highly efficient Stirling Cycle, but without the cranks, sliding seals or excess weight found in Stirling engines.
Modern research and development of thermoacoustic systems is largely based upon the work of Rott (1980) and later Steven Garrett, and Greg Swift (1988), in which linear thermoacoustic models were developed to form a basic quantitative understanding, while commercial interest has resulted in niche applications such as small to medium scale cryogenic applications. The technology is also suitable for air-conditioning for homes, commercial buildings, vehicles and other cooling and heating applications.
The most efficient thermoacoustic devices built to date have an efficiency approaching 40% of the Carnot limit, or about 20% to 30% overall (depending on the heat engine temperatures). The efficiency of high-end TA engine is comparable with an average internal combustion engine, or with low-end domestic vapor compression systems (a high-end compressor by itself will yield efficiencies of up to 65% for the compression process alone, however the overall cycle efficiency will be much less, due to the Carnot limit).
Higher hot-end temperatures may be possible with TA devices because there are no moving parts, thus allowing the Carnot efficiency to be higher. This may partially offset their lower efficiency, compared to conventional heat engines, as a percentage of Carnot, thus yielding overall efficiencies similar to conventional heat engines.
"...the engine's 30% [absolute] efficiency and high reliability may make medium-sized natural-gas liquefaction plants (with a capacity of up to a million gallons per day) and residential cogeneration economically feasible..."
The history of thermoacoustic hot air engines start about 1887, where Lord Rayleigh discusses the possibility of pumping heat with sound. Little further research occurred until Rott's work in 1969.
Cool Sound Industries, Inc. is engaged in developing thermoacoustics for air-conditioning, heating and refrigeration.
Orest Symko began a research project in 2005 called Thermal Acoustic Piezo Energy Conversion (TAPEC). The research group has built several prototypes, including a ring-shaped model designed by student Ivan Rodriguez that currently has the highest efficiency.
The development of a combined electrical generator, refrigerator based on two coupled thermoacoustic Stirling engines, has recently been disclosed. The name is SCORE (Stove for Cooking, Refrigeration and Electricity). Score was awarded £2M in March 2007 to research a cooking Stove that will produce electricity and cooling using the Thermo-acoustic effect for use in developing countries.