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CONTENTS Introduction
Basics of Refrigeration
Basics of Thermoacoustic Refrigeration
Thermoacoustics
Main Parts
Case Study
Advantages, Disadvantages and Applications of TAR
Conclusion
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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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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:
BASICS OF REFRIGERATION
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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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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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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.
BASICS OF THERMOACOUSTIC REFRIGERATION
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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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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.
THERMO ACOUSTIC EFFECT
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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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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.
THERMOACOUSTICS
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CARNOT CYCLE
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)
THERMOACOUSTIC CYCLE
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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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THERMOACOUSTIC REFRIGERATOR
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MAIN PARTS
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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LOUDSPEAKERS
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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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STACK
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 EXCHANGER
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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CASE STUDY
SPACETHERMOACOUS
TICREFRIGERATOR
(STAR)
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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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SPECIFICATIONS 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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ADVANTAGES OF TAR
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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DISADVANTAGES OF TAR
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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IMPROVEMENTS MADE
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.
35
APPLICATIONS
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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CONCLUSION
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.
38
REFERENCES
• 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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THANK YOU
PRESENTED BY
NIMALAN I