View source for Thermoacoustic heat engine

{{Short description|Heat pump powered by sound}}
[[File:Thermo-acoustic_cooling_machine.png|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 [[Thermoacoustics|thermoacoustic]] devices which use high-amplitude sound waves to [[heat pump|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 [[Wave|travelling wave]]. 

Compared to [[Vapor-compression refrigeration|vapor refrigerators]], thermoacoustic refrigerators have no coolant and few moving parts (only the loudspeaker), therefore require no dynamic sealing or lubrication.<ref>{{cite journal | author=Ceperley, P. | title=A pistonless Stirling engine – the travelling wave heat engine | journal=J. Acoust. Soc. Am. | year=1979 | volume=66 | issue=5 | pages=1508–1513 | doi=10.1121/1.383505|bibcode = 1979ASAJ...66.1508C }}</ref>

== History ==
The ability of heat to produce sound was noted by glassblowers centuries ago.<ref name=":0">{{Cite web|url=https://newatlas.com/soundenergy-thermoacoustic-cooling/58169/|title=Electricity-free air con: Thermoacoustic device turns waste heat into cold using no additional power|website=newatlas.com|date=25 January 2019 |language=en|access-date=2019-01-26}}</ref> 

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. 

[[Pieter Rijke|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.<ref name=":0" /> A [[Rijke tube]] converts heat into [[acoustic energy]],<ref>[[Pieter Rijke|P. L. Rijke]] (1859) Philosophical Magazine, '''17''', 419–422.</ref> using natural convection.

In about 1887, [[John Strutt, 3rd Baron Rayleigh|Lord Rayleigh]] discussed the possibility of pumping heat with sound. 

In 1969, Rott reopened the topic.<ref>{{Cite web|url=http://uw.physics.wisc.edu/~timbie/P325/Fahey_thermoacoustic_oscillations.pdf|title=Thermoacoustic Oscillations, Donald Fahey, Wave Motion & Optics, Spring 2006, Prof. Peter Timbie|access-date=2007-10-16|archive-date=2008-03-09|archive-url=https://web.archive.org/web/20080309163233/http://uw.physics.wisc.edu/~timbie/P325/Fahey_thermoacoustic_oscillations.pdf|url-status=dead}}</ref> Using the [[Navier–Stokes equations|Navier-Stokes equations]] for fluids, he derived equations specific for [[thermoacoustics]].<ref>{{cite journal|author=Rott, N.|year=1980|title=Thermoacoustics|journal=Adv. Appl. Mech.|series=Advances in Applied Mechanics|volume=20|issue=135|pages=135–175|doi=10.1016/S0065-2156(08)70233-3|isbn=9780120020201}}</ref> 

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.<ref name="Swift">{{cite journal|last1=Swift|first1=Gregory W.|date=1988|title=Thermoacoustic engines|url=http://scitation.aip.org/content/asa/journal/jasa/84/4/10.1121/1.396617|journal=The Journal of the Acoustical Society of America|volume=84|issue=4|page=1145|bibcode=1988ASAJ...84.1145S|doi=10.1121/1.396617|access-date=9 October 2015}}</ref>

In 1992 a similar thermoacoustic refrigeration device was used on [[Space Shuttle Discovery]].<ref name=":0" />

Orest Symko at [[University of Utah]] began a research project in 2005 called ''Thermal Acoustic Piezo Energy Conversion'' (TAPEC).<ref>[http://www.physorg.com/pdf100141616.pdf physorg.com: A sound way to turn heat into electricity (pdf)] Quote: "...Symko says the devices won’t create [[noise pollution]]...Symko says the ring-shaped device is twice as efficient as cylindrical devices in converting heat into sound and electricity. That is because the pressure and speed of air in the ring-shaped device are always in sync, unlike in cylinder-shaped devices..."</ref>

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.<ref>{{Cite web|url=https://arstechnica.com/uncategorized/2007/05/new-stove-generator-refrigerator-combo-aimed-at-developing-nations/|title=Cooking with sound: new stove/generator/refrigerator combo aimed at developing nations|last=Lee|first=Chris|date=May 28, 2007|website=Ars Technica}}</ref><ref>[http://www.score.uk.com/ SCORE (Stove for Cooking, Refrigeration and Electricity)], [http://www.score.uk.com/index.php/research-summary/ illustration]</ref>

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 [[Advanced Stirling radioisotope generator|ASRG]] prototype.<ref name="Airbus">{{Cite web|url=http://pamir.sal.lv/2014/cd/container/B.7.02=DidierA_Pamir14.pdf|title=Thermo-Acoustic Generators for space missions}}</ref>

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.<ref name=":0" />

== Operation ==
A thermoacoustic device takes advantages of the fact that in a sound wave parcels of gas [[Adiabatic process |adiabatically]] alternatively compress and expand, and pressure and temperature change simultaneously; when pressure reaches a maximum or minimum, so does the temperature. It basically consists of [[heat exchanger]]s, a [[resonator]] and a stack (on standing wave devices) or [[regenerative heat exchanger|regenerator]] (on travelling wave devices). Depending on the type of engine a [[Speaker driver|driver]] or [[loudspeaker]] might be used to generate sound waves.

In a tube closed at both ends, interference can occur between two waves traveling in opposite directions at certain frequencies. The interference causes [[resonance]] and creates a standing wave. The stack consists of small parallel channels. When the stack is placed at a certain location in the resonator having a standing wave, a temperature differential develops across the stack. By placing heat exchangers at each side of the stack, heat can be moved. The opposite is possible as well: a temperature difference across the stack produces a sound wave. The first example is a heat pump, while the second is a prime mover.

=== Heat pump ===
Creating or moving heat from a cold to a warm reservoir requires work. Acoustic power provides this work. The stack creates a pressure drop. Interference between the incoming and reflected acoustic waves is now imperfect. The difference in amplitude causes the standing wave to travel, giving the wave acoustic power.

Heat pumping along a stack in a standing wave device follows the [[Brayton cycle]].

A counter-clockwise Brayton cycle for a [[refrigerator]] consists of four processes that affect a parcel of gas between two plates of a stack.

#''Adiabatic compression of the gas.'' When a parcel of gas is displaced from its rightmost position to its leftmost position, the parcel is adiabatically compressed, increasing its temperature. At the leftmost position the parcel now has a higher temperature than the warm plate.
#''[[Isobaric process | Isobaric]] heat transfer.'' The parcel's higher temperature causes it to transfer heat to the plate at constant pressure, cooling the gas.
#''Adiabatic expansion of the gas.'' The gas is displaced back from the leftmost position to the rightmost position. Due to adiabatic expansion the gas cools to a temperature lower than that of the cold plate.
#''Isobaric heat transfer.'' The parcel's lower temperature causes heat to be transferred from the cold plate to the gas at a constant pressure, returning the parcel's temperature to its original value.

Travelling wave devices can be described using the [[Stirling cycle]].

=== Temperature gradient ===
Engines and heat pumps both typically use stacks and heat exchangers. The boundary between a prime mover and heat pump is given by the temperature gradient operator, which is the mean temperature gradient divided by the critical temperature gradient.

:<math>\Iota = \frac{\nabla T_{m}}{\nabla T_{crit}} </math>

The mean temperature gradient is the temperature difference across the stack divided by the length of the stack.

:<math>\nabla T_{m} = \frac{\Delta T_{m}}{\Delta x_{stack}}</math>

The critical temperature gradient is a value that depends on characteristics of the device such as frequency, cross-sectional area and gas properties.

If the temperature gradient operator exceeds one, the mean temperature gradient is larger than the critical temperature gradient and the stack operates as a prime mover. If the temperature gradient operator is less than one, the mean temperature gradient is smaller than the critical gradient and the stack operates as a heat pump.

=== Theoretical efficiency ===
In thermodynamics the highest achievable efficiency is the [[Carnot's theorem (thermodynamics)|Carnot]] efficiency. The efficiency of thermoacoustic engines can be compared to Carnot efficiency using the temperature gradient operator.

The efficiency of a thermoacoustic engine is given by

:<math>\eta = \frac{\eta_{c}}{\Iota}</math>

The [[coefficient of performance]] of a thermoacoustic heat pump is given by

:<math>COP = \Iota \cdot COP_{c}</math>

== Practical efficiency  ==
The most efficient thermoacoustic devices have an efficiency approaching 40% of the [[Carnot heat engine#Efficiency of real heat engines|Carnot]] limit, or about 20% to 30% overall (depending on the [[heat engine]] temperatures).<ref>web archive backup: [https://web.archive.org/web/20080801212651/http://www.lanl.gov/mst/engine/ lanl.gov: More Efficient than Other No-Moving-Parts Heat Engines]</ref>

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 [[toroid]]al 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.{{elucidate|date=July 2015}}

== See also ==
*[[Sound amplification by stimulated emission of radiation]] (SASER)

== References ==
{{Reflist|30em}}

== Further reading ==
{{Refbegin}}
* {{cite journal | last1=Gardner |first1=D. |last2=Swift |first2=G. | title=A cascade thermoacoustic engine | journal=J. Acoust. Soc. Am. | year=2003 | volume=114 | issue=4 | pages=1905–1919 | doi=10.1121/1.1612483 | pmid=14587591|bibcode = 2003ASAJ..114.1905G }}
* {{cite journal |last1=Garrett |first1=Steven |last2=Backaus |first2=Scott |title=The Power of Sound |journal=American Scientist |volume=88 |page=561 |date=November 2000 |issue=6 |doi=10.1511/2000.6.516 |url=http://www.americanscientist.org/issues/pub/the-power-of-sound/1}} Semipopular introduction to thermoacoustic effects and devices.
* Frank Wighard "Double Acting Pulse Tube Electroacoustic System" US Patent 5,813,234
* {{cite conference |last1=de Blok |first1=Kees |date=February 2013 |url=http://www.aster-thermoacoustics.com/wp-content/uploads/2013/02/Multi-stage-traveling-wave-feedback-thermoacoustics-in-practice-Kees-de-Blok.pdf |title=Multi-stage traveling wave thermoacoustics in practice |book-title=19th International Congress on Sound and Vibration 2012 |conference=ICSV 19 |volume=2 |place=[[Red Hook, New York]] |publisher=Curran Associates |pages=1573–1580 |citeseerx=10.1.1.454.1398 |isbn=978-1-62276-465-5 |access-date=2021-12-08}}
{{Refend}}

== External links ==
{{Commons category|Thermoacoustic heat engines}}
{{wikibooks|1=Engineering Acoustics|2=Thermoacoustics}}
* [http://www.lanl.gov/thermoacoustics/ Los Alamos National Laboratory, New Mexico, USA]
* [http://www.mecheng.adelaide.edu.au/avc/thermoacoustics/ Thermoacoustics at the University of Adelaide, Australia], web archive backup: [https://web.archive.org/web/20080723204600/http://www.mecheng.adelaide.edu.au/anvc/thermoacoustics/forum/ Discussion Forum]
*[http://www.mecheng.adelaide.edu.au/avc/publications/public_papers/2005/preprint_bamman_aas2005.pdf  Adelaide University]{{Dead link|date=August 2023 |bot=InternetArchiveBot |fix-attempted=yes }}
* [https://www.wired.com/2003/01/hear-that-the-fridge-is-chilling/ Hear That? The Fridge Is Chilling], [[Wired Magazine]] article

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{{DEFAULTSORT:Thermoacoustic Hot Air Engine}}
[[Category:Hot air engines| ]]
[[Category:Cooling technology]]
[[Category:Heat pumps]]
[[Category:Acoustics]]