thumb|upright=1.3|A schematic representation of a thermoacoustic hot-air engine. The hot side of the heat exchanger is connected to hot heat reservoir – and the cold side to cold heat reservoir. The electro-acoustic [[transducer, e.g. a loudspeaker, is not shown.]]

Thermoacoustic engines (sometimes called "TA engines") are thermoacoustic devices which use high-amplitude sound waves to pump heat from one place to another (this requires work, which is provided by the loudspeaker) or use a heat difference to produce work in the form of sound waves (these waves can then be converted into electrical current the same way as a microphone does).

These devices can be designed to use either a standing wave or a travelling wave.

Compared to vapor refrigerators, thermoacoustic refrigerators have no coolant and few moving parts (only the loudspeaker), therefore require no dynamic sealing or lubrication.

History

The ability of heat to produce sound was noted by glassblowers centuries ago. In the 1850s experiments showed that a temperature differential drove the phenomenon, and that acoustic volume and intensity vary with tube length and bulb size.

Rijke demonstrated that adding a heated wire screen a quarter of the way up the tube greatly magnified the sound, supplying energy to the air in the tube at its point of greatest pressure. Further experiments showed that cooling the air at its points of minimal pressure produced a similar amplifying effect. using natural convection.

In about 1887, Lord Rayleigh discussed the possibility of pumping heat with sound.

In 1969, Rott reopened the topic. Using the Navier-Stokes equations for fluids, he derived equations specific for thermoacoustics.

Linear thermoacoustic models were developed to form a basic quantitative understanding, and numeric models for computation. Swift continued with these equations, deriving expressions for the acoustic power in thermoacoustic devices.

In 1992 a similar thermoacoustic refrigeration device was used on Space Shuttle Discovery.

Niche applications such as small to medium scale cryogenic applications. Score Ltd. was awarded £2M in March 2007 to research a cooking stove that also delivers electricity and cooling for use in developing countries.

A radioisotope-heated thermoacoustic system was proposed and prototyped for deep space exploration missions by Airbus. The system has slight theoretical advantages over other generator systems like existing thermocouple based systems, or a proposed Stirling engine used in ASRG prototype.

SoundEnergy developed the THEAC system that turns heat, typically waste heat or solar heat into cooling with no other power source. The device uses argon gas. The device amplifies sound created by the waste heat, converts the resulting pressure back into another heat differential and uses a Stirling cycle to produce the cooling effect.

Higher hot-end temperatures may be possible with thermoacoustic devices because they have 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.

The ideal Stirling cycle, approximated by traveling wave devices, is inherently more efficient than the ideal Brayton cycle, approximated by standing wave devices. However, the narrower pores required to give good thermal contact in a travelling wave device, as compared to a standing wave stack which requires deliberately imperfect thermal contact, also gives rise to greater frictional losses, reducing practical efficiency. The toroidal geometry often used in traveling wave devices, but not required for standing wave devices, can also boost losses due to Gedeon streaming around the loop.

See also

  • Sound amplification by stimulated emission of radiation (SASER)

References

Further reading

  • Semipopular introduction to thermoacoustic effects and devices.
  • Frank Wighard "Double Acting Pulse Tube Electroacoustic System" US Patent 5,813,234
  • Los Alamos National Laboratory, New Mexico, USA
  • Thermoacoustics at the University of Adelaide, Australia, web archive backup: Discussion Forum
  • Adelaide University
  • Hear That? The Fridge Is Chilling, Wired Magazine article