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Thermoacoustic refrigeration

Thermoacoustic refrigeration

Overview
Thermoacoustic engines (sometimes called "TA engines") are thermoacoustic
Thermoacoustics
Thermoacoustics is about the interaction between thermodynamic and acoustic phenomena. Thermoacoustics is a relatively new field of science and engineering. Few devices based on this principle have been made thus far...

 devices which use high-amplitude sound waves to pump heat
Heat pump
A heat pump is a machine or device that effectively "moves" thermal energy from one location called the "source," which is at a lower temperature, to another location called the "sink" or "heat sink", which is at a higher temperature. An air conditioner is a particular type of heat pump, but the...

 from one place to another, or conversely use a heat difference to induce high-amplitude sound waves. In general, thermoacoustic engines can be divided into standing wave
Standing wave
In physics, a standing wave – also known as a stationary wave – is a wave that remains in a constant position.This phenomenon can occur because the medium is moving in the opposite direction to the wave, or it can arise in a stationary medium as a result of interference between two waves traveling...

 and travelling wave
Wave
In physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...

 devices. These two types of thermoacoustics devices can again be divided into two thermodynamic
Thermodynamics
Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...

 classes, a prime mover (or simply heat engine
Heat engine
In thermodynamics, a heat engine is a system that performs the conversion of heat or thermal energy to mechanical work. It does this by bringing a working substance from a high temperature state to a lower temperature state. A heat "source" generates thermal energy that brings the working substance...

), and a heat pump
Heat pump
A heat pump is a machine or device that effectively "moves" thermal energy from one location called the "source," which is at a lower temperature, to another location called the "sink" or "heat sink", which is at a higher temperature. An air conditioner is a particular type of heat pump, but the...

.
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Encyclopedia
Thermoacoustic engines (sometimes called "TA engines") are thermoacoustic
Thermoacoustics
Thermoacoustics is about the interaction between thermodynamic and acoustic phenomena. Thermoacoustics is a relatively new field of science and engineering. Few devices based on this principle have been made thus far...

 devices which use high-amplitude sound waves to pump heat
Heat pump
A heat pump is a machine or device that effectively "moves" thermal energy from one location called the "source," which is at a lower temperature, to another location called the "sink" or "heat sink", which is at a higher temperature. An air conditioner is a particular type of heat pump, but the...

 from one place to another, or conversely use a heat difference to induce high-amplitude sound waves. In general, thermoacoustic engines can be divided into standing wave
Standing wave
In physics, a standing wave – also known as a stationary wave – is a wave that remains in a constant position.This phenomenon can occur because the medium is moving in the opposite direction to the wave, or it can arise in a stationary medium as a result of interference between two waves traveling...

 and travelling wave
Wave
In physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...

 devices. These two types of thermoacoustics devices can again be divided into two thermodynamic
Thermodynamics
Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...

 classes, a prime mover (or simply heat engine
Heat engine
In thermodynamics, a heat engine is a system that performs the conversion of heat or thermal energy to mechanical work. It does this by bringing a working substance from a high temperature state to a lower temperature state. A heat "source" generates thermal energy that brings the working substance...

), and a heat pump
Heat pump
A heat pump is a machine or device that effectively "moves" thermal energy from one location called the "source," which is at a lower temperature, to another location called the "sink" or "heat sink", which is at a higher temperature. An air conditioner is a particular type of heat pump, but the...

. The prime mover creates work using heat, whereas a heat pump creates or moves heat using work.
Compared to vapor refrigerators
Vapor-compression refrigeration
Vapor-compression refrigeration is one of the many refrigeration cycles available for use. It has been and is the most widely used method for air-conditioning of large public buildings, offices, private residences, hotels, hospitals, theaters, restaurants and automobiles...

, thermoacoustic refrigerators have no ozone
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...

-depleting or toxic coolant and few or no moving parts therefore require no dynamic sealing or lubrication.

Overview of device


A thermoacoustic device basically consists of heat exchanger
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...

s, a resonator
Resonator
A resonator is a device or system that exhibits resonance or resonant behavior, that is, it naturally oscillates at some frequencies, called its resonant frequencies, with greater amplitude than at others. The oscillations in a resonator can be either electromagnetic or mechanical...

, and a stack (on standing wave devices) or regenerator
Regenerative heat exchanger
A regenerative heat exchanger, or more commonly a regenerator, is a type of heat exchanger where the flow through the heat exchanger is cyclical and periodically changes direction. It is similar to a countercurrent heat exchanger. However, a regenerator mixes the two fluid flows while a...

 (on travelling wave devices). Depending on the type of engine a driver
Speaker driver
A speaker driver is an individual transducer that converts electrical energy to sound waves, typically as part of a loudspeaker, television, or other electronics device. Sometimes the transducer is itself referred to as a speaker, particularly when a single one is mounted in an enclosure or as...

 or loudspeaker
Loudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...

 might be used as well to generate sound waves.

Consider a tube closed at both ends. Interference can occur between two waves traveling in opposite directions at certain frequencies. The interference causes resonance
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...

 creating a standing wave. Resonance only occurs at certain frequencies called resonance frequencies, and these are mainly determined by the length of the resonator.

The stack is a part consisting of small parallel channels. When the stack is placed at a certain location in the resonator, while having a standing wave in the resonator, a temperature difference can be measured across the stack. By placing heat exchangers at each side of the stack, heat can be moved. The opposite is possible as well, by creating a temperature difference across the stack, a sound wave can be induced. The first example is a simple heat pump, while the second is a prime mover.

Heat pumping


To be able to create or move heat, work must be done, and the acoustic power provides this work. When a stack is placed inside a resonator a pressure drop occurs. Interference between the incoming and reflected wave is now imperfect since there is a difference in amplitude causing the standing wave to travel little, giving the wave acoustic power.

In the acoustic wave, parcels of gas adiabatically compress and expand. Pressure and temperature change simultaneously; when pressure reaches a maximum or minimum, so does the temperature. Heat pumping along a stack in a standing wave device can now be described using the Brayton cycle
Brayton cycle
The Brayton cycle is a thermodynamic cycle that describes the workings of the gas turbine engine, basis of the airbreathing jet engine and others. It is named after George Brayton , the American engineer who developed it, although it was originally proposed and patented by Englishman John Barber...

.

Below is the counter-clockwise Brayton cycle consisting of four processes for a refrigerator
Refrigerator
A refrigerator is a common household appliance that consists of a thermally insulated compartment and a heat pump that transfers heat from the inside of the fridge to its external environment so that the inside of the fridge is cooled to a temperature below the ambient temperature of the room...

 when a parcel of gas is followed between two plates of a stack.
  1. Adiabatic compression of the gas. When a parcel of gas is displaced from its rightmost position to its leftmost position, the parcel is adiabatic compressed and thus the temperature increases. At the leftmost position the parcel now has a higher temperature than the warm plate.
  2. Isobaric heat transfer. The parcel's temperature is higher than that of the plate causing it to transfer heat to the plate at constant pressure losing temperature.
  3. Adiabatic expansion of the gas. The gas is displaced back from the leftmost position to the rightmost position and due to adiabatic expansion the gas is cooled to a temperature lower than that of the cold plate.
  4. Isobaric heat transfer. The parcel's temperature is now lower than that of the plate causing heat to be transferred from the cold plate to the gas at a constant pressure, increasing the parcel's temperature back to its original value.


Travelling wave devices can be described using the Stirling cycle
Stirling cycle
The Stirling cycle is a thermodynamic cycle that describes the general class of Stirling devices. This includes the original Stirling engine that was invented, developed and patented in 1816 by Reverend Dr...

.

Temperature gradient


An engine and heat pump both typically use a stack 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.


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


The critical temperature gradient is a value depending on certain characteristics of the device like 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 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


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

Derivations


Using the Navier-Stokes equations for fluids, Rott was able to derive equations specific for thermoacoustics. Swift continued with these equations, deriving expressions for the acoustic power in thermoacoustic devices.

Efficiency in practice


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
Heat engine
In thermodynamics, a heat engine is a system that performs the conversion of heat or thermal energy to mechanical work. It does this by bringing a working substance from a high temperature state to a lower temperature state. A heat "source" generates thermal energy that brings the working substance...

 temperatures).

Higher hot-end temperatures may be possible with thermoacoustic devices because there are no moving parts
Moving parts
The moving parts of a machine are those parts of it that move. Machines comprise both moving and fixed parts. The moving parts have controlled and constrained motions....

, 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.

Research in thermoacoustics


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, and numeric models for computation. Commercial interest has resulted in niche applications such as small to medium scale cryogenic applications.

History


The history of thermoacoustic hot air engines started about 1887, when Lord Rayleigh
John Strutt, 3rd Baron Rayleigh
John William Strutt, 3rd Baron Rayleigh, OM was an English physicist who, with William Ramsay, discovered the element argon, an achievement for which he earned the Nobel Prize for Physics in 1904...

 discussed the possibility of pumping heat with sound. Little further research occurred until Rott's work in 1969.

A very simple thermoacoustic hot air engine is the Rijke tube
Rijke tube
Rijke's tube turns heat into sound, by creating a self-amplifying standing wave. It is an entertaining phenomenon in acoustics and is an excellent example of resonance.-Discovery:...

 that converts heat into acoustic energy. This device however uses natural convection.

Current research


Orest Symko at University of Utah
University of Utah
The University of Utah, also known as the U or the U of U, is a public, coeducational research university in Salt Lake City, Utah, United States. The university was established in 1850 as the University of Deseret by the General Assembly of the provisional State of Deseret, making it Utah's oldest...

 began a research project in 2005 called Thermal Acoustic Piezo Energy Conversion (TAPEC).

Score Ltd. was awarded £2M in March 2007 to research a cooking stove that will also deliver electricity and cooling using the Thermo-acoustic effect for use in developing countries.

Cool Sound Industries, Inc. is developing an air-conditioning system that uses thermoacoustic techology, with a focus on HVAC
HVAC
HVAC refers to technology of indoor or automotive environmental comfort. HVAC system design is a major subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer...

  applications. The system is claimed to have high efficiency and low costs compared to competing refrigeration technologies, and uses no HFC, no HCFC, and no mechanical compressor.

Q-Drive, Inc. is also engaged in developing thermoacoustic devices for refrigeration, with a focus on cryogenic applications.

See also

  • SASER
    Sound Amplification by Stimulated Emission of Radiation
    A SASER is the acoustic analogue of the laser. It is capable of producing highly coherent, concentrated beams of ultrasound, using methods very similar to those employed in the laser. First experimentally demonstrated in the Gigahertz range in 2009, the SASER is being developed at the University...

    , Sound Amplification by Stimulated Emission of Radiation

Further reading


  • Frank Wighard "Double Acting Pulse Tube Electroacoustic System" US Patent 5,813,234


External links