Thermoacoustic refrigeration uses sound waves to provide cooling without ozone-depleting refrigerants or moving parts. It was invented in 1992 by Steven Garrett and the first device, called the Space Thermoacoustic Refrigerator, was flown on the Space Shuttle Discovery. The main components are a driver to produce sound waves, a resonator containing a stack of plates through which the sound waves travel, transferring heat. As the waves compress and expand the gas, a temperature difference is created across the stack without vibration. Advantages include reliability, environmental friendliness, and low cost, though efficiency remains lower than conventional refrigerators.

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 Introduction
 Basics of Refrigeration
 Basics of Thermoacoustic Refrigeration
 Thermoacoustics
 Main Parts
 Case Study
 Advantages, Disadvantages and Applications of TAR
 Conclusion

3. INTRODUCTION
 Over the past two decades, physicists and engineers have been
working on a class of heat engines and compression-driven
refrigerators that use no oscillating pistons, oil seals or lubricants.
 Thermo acoustic devices take advantage of sound waves
reverberating within them to convert a temperature differential into
mechanical energy or mechanical energy into a temperature
differential.
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4. STEVEN L. GARRETT
 Leading Researcher
United Technologies Corporation
Professor of Acoustics
The Pennsylvania State University.
 He invented the thermoacoustic
refrigerator in the year 1992 and that
TAR was used in the space shuttle
Discovery(STS-42).
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A refrigerant is a compound used in a heat
cycle that undergoes a phase change from a
gas to a liquid and back.
For example, let us assume that the refrigerant
being used is pure ammonia, which boils at
-27 degrees F. And this is what happens to
keep the refrigerator cool:

6. 1. The compressor compresses the ammonia gas. The
compressed gas heats up as it is pressurized
(orange).
2. The coils on the back of the refrigerator let the hot
ammonia gas dissipate its heat. The ammonia gas
condenses into ammonia liquid (dark blue) at high
pressure.
3. The high-pressure ammonia liquid flows through
the expansion valve. The expansion valve is a small
hole. On one side of it, is high-pressure ammonia
liquid. On the other side of the hole is a low-
pressure area.
4. The liquid ammonia immediately boils and
vaporizes (light blue), its temperature dropping to
-27 F. This makes the inside of the refrigerator cold.
5. The cold ammonia gas is sucked up by the
compressor, and the cycle repeats.
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7. DISADVANTAGES OF
CONVENTIONAL REFRIGERATOR
 Uses harmful refrigerants like ammonia, CFC’s and HFC’s
 Refrigerants if leaked causes the depletion in the ozone layers.
 Refrigerants are costly.
 The moving parts like the compressors require lubrication.
 Leakage of refrigerant may result in adverse human health effects
including cancers, cataracts, immune system deficits, and
respiratory effects, as well as diminish food supplies and promote
increases in vector borne diseases.
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8.  The principle can be imagined as a loud speaker creating high amplitude
sound waves that can compress refrigerant allowing heat absorption
 The researches have exploited the fact that sound waves travel by
compressing and expanding the gas they are generated in.
 Suppose that the above said wave is traveling through a tube.
 Now, a temperature gradient can be generated by putting a stack of
plates in the right place in the tube, in which sound waves are bouncing
around.
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9.  Some plates in the stack will get hotter while the others get colder.
 All it takes to make a refrigerator out of this is to attach heat
exchangers to the end of these stacks.
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10.  Acoustic or sound waves can be utilized to produce cooling.
 The pressure variations in the acoustic wave are accompanied by temperature
variations due to compressions and expansions of the gas.
 For a single medium, the average temperature at a certain location does not
change. When a second medium is present in the form of a solid wall, heat is
exchanged with the wall.
 An expanded gas parcel will take heat from the wall, while a compressed
parcel will reject heat to the wall.
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11.  As expansion and compression in an acoustic wave is
inherently associated with a displacement, a net transport of
heat results.
 To fix the direction of heat flow, a standing wave pattern is
generated in an acoustic resonator.
 The reverse effect also exists: when a large enough
temperature gradient is imposed to the wall, net heat is
absorbed and an acoustic wave is generated, so that heat is
converted to work.
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12.  Thermoacoustics combines the branches of acoustics and thermodynamics
together to move heat by using sound.
 While acoustics is primarily concerned with the macroscopic effects of sound
transfer like coupled pressure and motion oscillations, thermoacoustics focuses on
the microscopic temperature oscillations that accompany these pressure changes.
 Thermoacoustics takes advantage of these pressure oscillations to move heat on a
macroscopic level.
 This results in a large temperature difference between the hot and cold sides of the
device and causes refrigeration.
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13. CARNOT CYCLE
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 The most efficient cycle of
thermodynamics.
 The Carnot cycle uses gas in a
closed chamber to extract work
from the system.

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 The figure traces the basic
thermoacoustic cycle for a packet
of gas, a collection of gas
molecules that act and move
together.
 Starting from point 1, the packet of
gas is compressed and moves to the
left.
 As the packet is compressed, the
sound wave does work on the
packet of gas, providing the power
for the refrigerator. Figure 4.3 THERMOACOUSTIC REFRIGERATION CYCLE (From Reference 2)

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Figure 4.3 THERMOACOUSTIC REFRIGERATION
CYCLE (From Reference 2)
 When the gas packet is at
maximum compression, the gas
ejects the heat back into the stack
since the temperature of the gas is
now higher than the temperature of
the stack.
 This phase is the refrigeration part
of the cycle, moving the heat
farther from the bottom of the tube.
 As the packet is compressed, the sound wave does work on the
packet of gas, providing the power for the refrigerator.

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Figure 4.3 THERMOACOUSTIC REFRIGERATION
CYCLE (From Reference 2)
 In the second phase of the cycle, the gas is returned to the initial
state. As the gas packet moves back towards the right, the sound
wave expands the gas.
 Although some work is expended to
return the gas to the initial state, the
heat released on the top of the stack
is greater than the work expended to
return the gas to the initial state.
 This process results in a net transfer
of heat to the left side of the stack.

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Figure 4.3 THERMOACOUSTIC REFRIGERATION
CYCLE (From Reference 2)
 Finally, in step 4, the packets of
gas reabsorb heat from the cold
reservoir.
 Ant the heat transfer repeats and
hence the thermoacoustic
refrigeration cycle.

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Two main parts are in the TAR
1. Driver
Houses the Loudspeaker
2. Resonator
Houses the gas
The hot and cold heat exchangers
Houses the Stack

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 A loudspeaker (or "speaker") is an electroacoustic transducer
that produces sound in response to an electrical audio signal
input.
 It was invented in the mid 1820’s by the scientist Johann Philipp
Reis.
 It is powered by electricity.
 The magnet or the coil in the speaker vibrates to produce the
waves of required frequency.

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 It is also called as regenerator.
 The most important piece of a thermoacoustic device is the
stack.
 The stack consists of a large number of closely spaced surfaces
that are aligned parallel to the to the resonator tube.
 In a usual resonator tube, heat transfer occurs between the walls
of cylinder and the gas.

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 However, since the vast majority of the molecules are far from the
walls of the chamber, the gas particles cannot exchange heat with
the wall and just oscillate in place, causing no net temperature
difference.
 The purpose of the stack is to provide a medium where the walls
are close enough so that each time a packet of gas moves, the
temperature differential is transferred to the wall of the stack.
 Most stacks consist of honeycombed plastic spacers that do not
conduct heat throughout the stack but rather absorb heat locally.
With this property, the stack can temporarily absorb the heat
transferred by the sound waves.

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 The spacing of these designs is crucial.
 If the holes are too narrow, the stack will be difficult to fabricate,
and the viscous properties of the air will make it difficult to
transmit sound through the stack.
 If the walls are too far apart, then less air will be able to transfer
heat to the walls of the stack, resulting in lower efficiency.
 The different materials used in the Stack are
 Paper
 Alluminium
 Lexan
 Foam

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 Heat exchangers are
devices used to transfer
heat energy from one
fluid to another.

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 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.
 They are widely used in space heating,
refrigeration, air conditioning, power
plants, chemical plants, petrochemical
plants, petroleum refineries, natural gas
processing, and sewage treatment.

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 The Space Thermo acoustic Refrigerator (STAR) was designed
and built by a team at the Naval Postgraduate School led by Steve
Garrett. It has the ability to move about 50 Watts of heat.
 The Space Thermo Acoustic Refrigerator was the first electrically-
driven thermo acoustic chiller designed to operate autonomously
outside a laboratory. It was launched on the Space Shuttle
Discovery (STS-42) on January 22, 1992.
 It was not a very efficient thermoacoustic refrigerator. And hence
did not refrigerate for many years.

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 The refrigerator is driven by a modified compression driver
that is coupled to a quarter-wavelength resonator using a
single-convolution electroformed metal bellow.
 The resonator contains the heat exchangers and the stack.
 The stack is 3.8 cm in diameter and 7.9 cm in length. It was
constructed by rolling up polyester film (Mylar™) using
fishing line as spaces placed every 5 mm.
 The device was filled with a 97.2% Helium and 2.7% Xenon
gas mixture at a pressure of 10 bars.

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 Length of the tube is 35 cm.
 Diameter of the tube is 3.9 cm
 Length of the stack is 7.9 cm
 Diameter of the stack is 3.8 cm
 Gas used is 97.2% Helium and 2.7% Xenon
 Heat pumping capacity is 50 Watts.
 Refrigeration Temperature is 12ᴼC
 Commercial Loudspeaker is used.
 Speaker operates at 135 Hz and 100 W.

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 No moving parts for the process, so very reliable and a long life span.
 Environmentally friendly working medium (air, noble gas).
 The use of air or noble gas as working medium offers a large window of
applications because there are no phase transitions.
 Use of simple materials with no special requirements, which are
commercially available in large quantities and therefore relatively cheap.
 On the same technology base a large variety of applications can be
covered.

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 Out of these, the two distinct advantages of thermo acoustic refrigeration
are that the harmful refrigerant gases are removed. The second advantage
is that the number of moving parts is decreased dramatically by removing
the compressor.
 Also sonic compression or ‘sound wave refrigeration’ uses sound to
compress refrigerants which replace the traditional compressor and need
for lubricants.
 The technology could represent a major breakthrough using a variety of
refrigerants, and save up to 40% in energy.
 Thermo acoustic refrigeration works best with inert gases such as helium
and argon, which are harmless, nonflammable, nontoxic, non-ozone
depleting or global warming and is judged inexpensive to manufacture.

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 Efficiency: Thermo acoustic refrigeration is currently less efficient
than the traditional refrigerators.
 Lack of suppliers producing customized components.
 Lack of interest and funding from the industry due to their
concentration on developing alternative gases to CFCs.
 Talent Bottleneck: There are not enough people who have expertise
on the combination of relevant disciplines such as acoustic, heat
exchanger design etc.

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In order to overcome the drawbacks, some improvements were made.
 In order to improve the efficiency, regenerators are used. The
function of a regenerator is to store thermal energy during part of
the cycle and return it later. This component can increase the
thermodynamic efficiency to impressive levels.
 The extra stress given in using standing waves also paved to be
fruitful. This increased the level of temperature gradient setup
thereby providing more refrigeration effect.

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 Liquefaction of natural gas:
Burning natural gas in a thermo acoustic engine generates
acoustic energy. This acoustic energy is used in a thermo acoustic
heat pump to liquefy natural gas.
 Chip cooling:
In this case a piezoelectric element generates the sound wave. A
thermo acoustic heat pump cools the chip.

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 Electronic equipment cooling on naval ships:
In this application, a speaker generates sound waves. Again a
thermo acoustic pump is used to provide the cooling.
 Electricity from sunlight:
Concentrated thermal solar energy generates an acoustic wave in
a heated thermo acoustic engine. A linear motor generates
electricity from this.
 Upgrading industrial waste heat:
Acoustic energy is created by means of industrial waste heat in a
thermo acoustic engine. In a thermo acoustic heat pump this acoustic
energy is used to upgrade the same waste heat to a useful temperature
level.

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 Thermo acoustic engines and refrigerators were already being considered a
few years ago for specialized applications, where their simplicity, lack of
lubrication and sliding seals, and their use of environmentally harmless
working fluids were adequate compensation for their lower efficiencies.
 In future let us hope these thermo acoustic devices which promise to
improve everyone’s standard of living while helping to protect the planet
might soon take over other costly, less durable and polluting engines and
pumps. The latest achievements of the former are certainly encouraging, but
there are still much left to be done.

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• http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/carnot.html.
• Daniel A. Russell and Pontus Weibull, “Tabletop thermoacoustic
refrigerator for demonstrations,” Am. J. Phys. 70 (12), December 2002.
• G. W. Swift, “Thermoacoustic engines and refrigerators,” Phys. Today
48, 22-28 (1995).
• http://www.howstuffworks.com/stirling-engine.htm.
• http://en.wikipedia.org/wiki/Carnot_cycle.
• Chilling at Ben & Jerry’s: Cleaner, Greener.” Ken Brown. Available:
• http://www.thermoacousticscorp.com/news/index.cfm/ID/4.htm. 17
July 2006.
• S. L. Garrett and S. Backhaus, ‘‘The power of sound,’’ Am. Sci. 88, 516–
• 525 (2000).

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