22-11-2012, 05:25 PM
Thermoacoustic Refrigeration
[attachment=40715]
ABSTRACT
This project examined the effectiveness
of thermoacoustic refrigeration, which is the
theory of using sound waves as a coolant.
We studied the dynamics of sound,
temperature, and the Carnot cycle, and used
this knowledge to come to an understanding
of thermoacoustics. We then applied this
understanding to the construction of a small
thermoacoustic refrigerator, which was built
from inexpensive and readily available parts.
The experiments showed that while
thermoacoustic cooling was possible, high
efficiency was beyond our reach due to
materials restrictions. However, from these
limitations we devised several proposals for
increasing the efficiency of thermoacoustic
refrigerators. A few suggestions for future
work include using different materials, and
changing the size and shape of the parts
used. While there was no time or resources
for us to conduct further research to test
these suggestions, we hope our findings will
encourage more research in this field.
INTRODUCTION
Refrigerators have become
necessities in modern society. Most
conventional refrigerators operate using a
vapor compression cycle, a process which
involves interaction between vapor and a
refrigerant.[1] While this method of chemical
refrigeration is extremely efficient, the
refrigerants used [once
cholorofluorocarbons (CFCs), now
hydrofluorocarbons (HFCs)] are ozonedepleting
chemicals, which is a major cause
of concern.[2]
Fortunately, an alternative method of
refrigeration, thermoacoustic refrigeration,
has been developed. Unlike conventional
refrigerators, thermoacoustic refrigerators do
not use environmentally-unfriendly
refrigerants to fuel the system. Instead, they
rely on the power of sound, by using sound
waves to generate the work necessary to
compress gases. [
BACKGROUND RESEARCH
Thermoacoustics
The field of thermoacoustics is based
upon the principle that sound waves are
pressure waves. Sound waves are
propagated through the air by the means of
molecular collisions. These collisions create
disturbances in air, resulting in constructive
and destructive interference. Constructive
interference creates a front of high pressure,
compressing molecules, while destructive
interference lowers pressure and in turn
allows molecules in the air to expand. This
property of sound waves is the foundation of
the science behind thermoacoustic
refrigerator. [4]
Thermoacoustic refrigeration uses highamplitude
sounds to pump heat to respective
areas within the device through pressure
oscillations. The wave in the thermoacoustic
engine has a great amount of pressure and
velocity fluctuations through the stack that
the heat is given to the oscillating gas at
high pressure and removed at low pressure.
When it comes to the thermoacoustic
pumps, the process is reversed. Steven
Garrett and Greg Swift contributed a lot of
work in which they found out that linear
thermoacoustic models were developed to
form a basic quantitative understanding. The
most recent attempt at commercializing at
thermoacoustic refrigerating device was by
Ben and Jerry's in 2004 where they
employed the researchers at Penn State to
test and develop a working prototype to be
unveiled at Earth day 2004.
The Carnot Cycle
The most efficient of the various
thermodynamic cycles, the Carnot Cycle, as
shown in Figure 2, is a reversible cycle that
uses two thermal reservoirs. The Carnot
Cycle is the underlying principle of all
refrigeration, so it plays an important role in
this project.
Four processes make up the Carnot
Cycle. Two processes are adiabatic,
meaning they involve the compression or
expansion of gas without heat exchange
outside the system. The remaining two
processes are isothermal, and include a heat
exchange as the gas comes in contact with a
reservoir.
Thermoacoustic Refrigeration
In a thermoacoustic refrigerator,
temperature changes are utilized through the
use of a stack (a set of closely spaced
filaments designed to affect heat transfer)
and a metal heat sink. As a heat pump, the
refrigerator takes energy from input work (in
the form of sound waves) and produces a
cooling effect between two regions in the
tube. This occurs because the stack allows
air molecules that have contracted due to the
increased pressure to expand and absorb
energy. [4]
As shown in Figure 4, molecules are
first propelled by the pressure wave away
from the acoustic driver (the speaker) and
compressed, forcing them to release heat to
the stack. As the pressure wave passes and
the gas molecules are drawn back into a low
pressure zone below the stack towards the
speaker they again expand and attempt to
draw heat back from the stack. However, it
is easier for the gas to transfer heat to the
stack than it to absorb heat from the stack,
so a small temperature gradient is formed.
Since we use frequencies of around 385Hz,
this process happens many times a second,
causing a significant overall temperature
gradient between the two chambers
separated by the stack.
Thermal Penetration Depth
One critical variable in the construction
of the thermoacoustic device was the
spacing between the layers of the stack. This
distance is critical because it directly affects
the efficiency of the device. If the space is
too small, air cannot transfer the sound
waves, which causes the stack to behave as a
stopper. However, if the spacing is too wide,
then too little air gets in contact with the
stack, and not enough heat is transferred.
The distance between these extremes that is
most effective is described by G.W. Swift as
4 thermal penetration depths. A thermal
penetration depth is the distance heat travels
through air in one second. The optimal
distance is four times that distance.
RELATED WORK
Thermoacoustic technology has
progressed rapidly over the last few decades
due to a strong theoretical understanding of
the thermoacoustic process as developed by
N. Rott, J. Wheatly, and G. Swift in the
1960s, 70s, and 80s, respectively.
Improvements need to be made, however, in
designing refrigerators that function at
higher amplitudes and in designing
refrigerator components (heat stacks, etc.).
In addition, thermoacoustic refrigerators
need to be more efficient in order to be
practical for everyday use. [2]
Research has been ongoing in this
promising field of study. A few universities
have carried out large scale research projects
as well. Purdue University developed a
model of a thermoacoustic heat pump. [13]
Studies conducted at Johns Hopkins
University revealed that in thermoacoustic
refrigeration, maximum cooling did not
correlate with maximum efficiency. [11] Out
of all the universities, Penn State has done
the most outstanding research in this field. It
has designed various forms of
thermoacoustic refrigerators, one of its most
exceptional innovations being a
thermoacoustic chiller that was used to cool
shipboard electronics.
CONCLUSION
We set out upon this project with the
simple goal of constructing a cheap,
demonstrative model of a thermoacoustic
refrigerator. To this end we succeeded: this
experiment proved that thermoacoustic
refrigerators indeed work. Additionally, this
experiment did yield some discoveries
regarding the efficiency of thermoacoustic
refrigeration. It was revealed that finding the
optimal frequency is essential for the
maximization of efficiency. This optimal
frequency was found using trial-and-error,
because the equation used to calculate
frequency was ineffective. Another factor
that increased efficiency was the proper
sealing of the apparatus. If the parts are not
properly sealed, heat escapes from the
refrigerator, and it does not function as well.
However, the overall efficiency of such an
apparatus is debatable. The devices used in
the experiment were capable of cooling air,
but only cooled the air a few degrees
Celsius. Also, the refrigerators were only
able to cool the air for a short amount of
time before the cooled air started rising in
temperature.
[attachment=40715]
ABSTRACT
This project examined the effectiveness
of thermoacoustic refrigeration, which is the
theory of using sound waves as a coolant.
We studied the dynamics of sound,
temperature, and the Carnot cycle, and used
this knowledge to come to an understanding
of thermoacoustics. We then applied this
understanding to the construction of a small
thermoacoustic refrigerator, which was built
from inexpensive and readily available parts.
The experiments showed that while
thermoacoustic cooling was possible, high
efficiency was beyond our reach due to
materials restrictions. However, from these
limitations we devised several proposals for
increasing the efficiency of thermoacoustic
refrigerators. A few suggestions for future
work include using different materials, and
changing the size and shape of the parts
used. While there was no time or resources
for us to conduct further research to test
these suggestions, we hope our findings will
encourage more research in this field.
INTRODUCTION
Refrigerators have become
necessities in modern society. Most
conventional refrigerators operate using a
vapor compression cycle, a process which
involves interaction between vapor and a
refrigerant.[1] While this method of chemical
refrigeration is extremely efficient, the
refrigerants used [once
cholorofluorocarbons (CFCs), now
hydrofluorocarbons (HFCs)] are ozonedepleting
chemicals, which is a major cause
of concern.[2]
Fortunately, an alternative method of
refrigeration, thermoacoustic refrigeration,
has been developed. Unlike conventional
refrigerators, thermoacoustic refrigerators do
not use environmentally-unfriendly
refrigerants to fuel the system. Instead, they
rely on the power of sound, by using sound
waves to generate the work necessary to
compress gases. [
BACKGROUND RESEARCH
Thermoacoustics
The field of thermoacoustics is based
upon the principle that sound waves are
pressure waves. Sound waves are
propagated through the air by the means of
molecular collisions. These collisions create
disturbances in air, resulting in constructive
and destructive interference. Constructive
interference creates a front of high pressure,
compressing molecules, while destructive
interference lowers pressure and in turn
allows molecules in the air to expand. This
property of sound waves is the foundation of
the science behind thermoacoustic
refrigerator. [4]
Thermoacoustic refrigeration uses highamplitude
sounds to pump heat to respective
areas within the device through pressure
oscillations. The wave in the thermoacoustic
engine has a great amount of pressure and
velocity fluctuations through the stack that
the heat is given to the oscillating gas at
high pressure and removed at low pressure.
When it comes to the thermoacoustic
pumps, the process is reversed. Steven
Garrett and Greg Swift contributed a lot of
work in which they found out that linear
thermoacoustic models were developed to
form a basic quantitative understanding. The
most recent attempt at commercializing at
thermoacoustic refrigerating device was by
Ben and Jerry's in 2004 where they
employed the researchers at Penn State to
test and develop a working prototype to be
unveiled at Earth day 2004.
The Carnot Cycle
The most efficient of the various
thermodynamic cycles, the Carnot Cycle, as
shown in Figure 2, is a reversible cycle that
uses two thermal reservoirs. The Carnot
Cycle is the underlying principle of all
refrigeration, so it plays an important role in
this project.
Four processes make up the Carnot
Cycle. Two processes are adiabatic,
meaning they involve the compression or
expansion of gas without heat exchange
outside the system. The remaining two
processes are isothermal, and include a heat
exchange as the gas comes in contact with a
reservoir.
Thermoacoustic Refrigeration
In a thermoacoustic refrigerator,
temperature changes are utilized through the
use of a stack (a set of closely spaced
filaments designed to affect heat transfer)
and a metal heat sink. As a heat pump, the
refrigerator takes energy from input work (in
the form of sound waves) and produces a
cooling effect between two regions in the
tube. This occurs because the stack allows
air molecules that have contracted due to the
increased pressure to expand and absorb
energy. [4]
As shown in Figure 4, molecules are
first propelled by the pressure wave away
from the acoustic driver (the speaker) and
compressed, forcing them to release heat to
the stack. As the pressure wave passes and
the gas molecules are drawn back into a low
pressure zone below the stack towards the
speaker they again expand and attempt to
draw heat back from the stack. However, it
is easier for the gas to transfer heat to the
stack than it to absorb heat from the stack,
so a small temperature gradient is formed.
Since we use frequencies of around 385Hz,
this process happens many times a second,
causing a significant overall temperature
gradient between the two chambers
separated by the stack.
Thermal Penetration Depth
One critical variable in the construction
of the thermoacoustic device was the
spacing between the layers of the stack. This
distance is critical because it directly affects
the efficiency of the device. If the space is
too small, air cannot transfer the sound
waves, which causes the stack to behave as a
stopper. However, if the spacing is too wide,
then too little air gets in contact with the
stack, and not enough heat is transferred.
The distance between these extremes that is
most effective is described by G.W. Swift as
4 thermal penetration depths. A thermal
penetration depth is the distance heat travels
through air in one second. The optimal
distance is four times that distance.
RELATED WORK
Thermoacoustic technology has
progressed rapidly over the last few decades
due to a strong theoretical understanding of
the thermoacoustic process as developed by
N. Rott, J. Wheatly, and G. Swift in the
1960s, 70s, and 80s, respectively.
Improvements need to be made, however, in
designing refrigerators that function at
higher amplitudes and in designing
refrigerator components (heat stacks, etc.).
In addition, thermoacoustic refrigerators
need to be more efficient in order to be
practical for everyday use. [2]
Research has been ongoing in this
promising field of study. A few universities
have carried out large scale research projects
as well. Purdue University developed a
model of a thermoacoustic heat pump. [13]
Studies conducted at Johns Hopkins
University revealed that in thermoacoustic
refrigeration, maximum cooling did not
correlate with maximum efficiency. [11] Out
of all the universities, Penn State has done
the most outstanding research in this field. It
has designed various forms of
thermoacoustic refrigerators, one of its most
exceptional innovations being a
thermoacoustic chiller that was used to cool
shipboard electronics.
CONCLUSION
We set out upon this project with the
simple goal of constructing a cheap,
demonstrative model of a thermoacoustic
refrigerator. To this end we succeeded: this
experiment proved that thermoacoustic
refrigerators indeed work. Additionally, this
experiment did yield some discoveries
regarding the efficiency of thermoacoustic
refrigeration. It was revealed that finding the
optimal frequency is essential for the
maximization of efficiency. This optimal
frequency was found using trial-and-error,
because the equation used to calculate
frequency was ineffective. Another factor
that increased efficiency was the proper
sealing of the apparatus. If the parts are not
properly sealed, heat escapes from the
refrigerator, and it does not function as well.
However, the overall efficiency of such an
apparatus is debatable. The devices used in
the experiment were capable of cooling air,
but only cooled the air a few degrees
Celsius. Also, the refrigerators were only
able to cool the air for a short amount of
time before the cooled air started rising in
temperature.