THERMOACOUSTIC REFRIGERATION
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Transcript of THERMOACOUSTIC REFRIGERATION
THERMOACOUSTIC REFRIGERATION
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. While this method of chemical refrigeration
is extremely efficient, the refrigerants used [once chlorofluorocarbons (CFCs), now hydro
fluorocarbons (HFCs)] are ozone depleting chemicals, which is a major cause of concern.
From creating comfortable home environments to manufacturing fast and
efficient electronic devices, air conditioning and refrigeration remain expensive, yet
essential, services for both homes and industries. However, in an age of impending
energy and environmental crises, current cooling technologies continue to generate
greenhouse gases with high energy costs.
Thermoacoustic refrigeration is an innovative alternative for cooling that is
both clean and inexpensive. Through the construction of a functional model, we will
demonstrate the effectiveness of thermoacoustics for modern cooling. Refrigeration relies
on two major thermodynamic principles. First, a fluid’s temperature rises when
compressed and falls when expanded. Second, when two substances are placed in direct
contact, heat will flow from the hotter substance to the cooler one. While conventional
refrigerators use pumps to transfer heat on a macroscopic scale, thermoacoustic
refrigerators rely on sound to generate waves of pressure that alternately compress and
relax the gas particles within the tube. The model constructed for this research project
employed inexpensive, household materials. Although the model did not achieve the
original goal of refrigeration, the experiment suggests that thermoacoustic refrigerators
could one day be viable replacements for conventional refrigerators.
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CHAPTER 1: BASICS
The first and second laws of thermodynamics place an upper bound on the
efficiency of heat engines. If TH and TC are the hot and cold thermal reservoirs,
respectively, and QH and QC the associated heat flows with W the work flows as shown
in Figure 1.1, in the usual case of cyclic engines operation, QH and QC and W are time
averaged powers. The operation is assumed steady-state, so that the time-averaged state
of the engine itself does not change. The first law of thermodynamics states that
The second law states that the entropy generated by the system must be positive
or zero. Since the engine is in (time averaged) steady state, the net entropy increase in the
reservoirs is
For the prime mover, the efficiency of interest is H Q W = η . Combining
Equations (1) and (2) to eliminate Qc,
The temperature ratio in Equation (4) is called the Carnot efficiency,
c η . It is the highest efficiency that a prime mover can achieve. Meanwhile for a heat
pump, the efficiency is called the coefficient of performance, W Q COP C = , reflecting
the fact that QC is the desired output of the refrigerator. Combining Equation (1) and (3)
to eliminate QH,
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Thermoacoustic systems operate in a similar manner with the heat engine
generating acoustic power and the heat pump requiring acoustic power. The efficiency
and COP, however, are not derived similarly.
.
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CHAPTER 2: THERMOACOUSTIC THEORY
Thermoacoustic effects had been observed for a long time, with the two most
famous devices, the Sondhauss tube and Rijke tube being described in 1850 and 1859
respectively (Wheatley et.al., 1985). However, a theoretical explanation to the
thermoacoustical effects observed in these devices is only available through Lord
Rayleigh whose discussion is mostly qualitative. According to Rayleigh, heating and
cooling could create acoustic power “if heat be given at the moment of greatest
condensation, or be taken from it at the moment of greatest rarefaction” and the heating
and cooling could be created by an acoustic wave (Backhaus and Swift, 2002). A
quantitative theoretical explanation is available only by 1970s through the works of
Nikolaus Rott. These theories are later used in the development of a thermoacoustic heat
engine.
Thermoacoustic heat engines are able to function as a prime mover or a heat
pump owing to the nature of the thermoacoustical phenomena where acoustic power is
generated if oscillatory thermal expansion and contraction is created and oscillatory
thermal expansion and contraction could be caused by a temperature gradient. The
difference of the function of the heat engine is therefore dependant on whether thermal or
acoustic power is given. Acoustic power is provided through an acoustic driver while
thermal power or heat is provided through the heat exchangers.
Thermoacoustic heat engines are further divided into two categories, standing
wave engines and traveling-wave engines. The traveling wave engine is better known as a
Stirling engine (Backhaus and Swift, 2002), while thermoacoustic heat engines normally
refers to the standing wave heat engine. In standing-wave engines, a standing wave is
generated within the resonator and a stack with moderately spaced plates is introduced in
the resonator to ensure a poor but nonzero thermal contact. Fluid in traveling-wave
engine oscillates in a traveling wave and the plates in the stack are closely spaced to
ensure a perfect thermal contact between fluid and stack (Swift, 1988)
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CHAPTER 3: SOUND WAVES AND PRESSURE
Thermoacoustics is based on the principle that sound waves are pressure
waves. These sound waves propagate through the air via molecular collisions. The
molecular collisions cause a disturbance in the air, which in turn creates constructive
and destructive interference. The constructive interference makes the molecules
compress, and the destructive interference makes the molecules expand. This principle
is the basis behind the thermoacoustic refrigerator.
One method to control these pressure disturbances is with standing
waves. Standing waves are natural phenomena exhibited by any wave, such as light,
sound, or water waves. In a closed tube, columns of air demonstrate these patterns
as sound waves reflect back on themselves after colliding with the end of the tube. When
the incident and reflected waves overlap, they interfere constructively, producing a
single waveform. This wave appears to cause the medium to vibrate in isolated
sections as the traveling waves are masked by the interference. Therefore, these “standing
waves” seem to vibrate in constant position and orientation around stationary
nodes. These nodes are located where the two component sound waves
interfere to create areas of zero net displacement. The areas of maximum
displacement are located halfway between two nodes and are called antinodes. The
maximum compression of the air also occurs at the antinodes. Due to these node and
antinode properties, standing waves are useful because only a small input of power is
needed to create a large amplitude wave. This large amplitude wave then has
enough energy to cause visible thermoacoustic effects.
All sound waves oscillate a specific amount of times per second, called the
wave’s frequency, and is measured in Hertz. For our thermoacoustic refrigerator
we had to calculate the optimal resonant frequency in order to get the maximum heat
transfer rate. The equation for the frequency of a wave traveling through a
closed tube is given by:
where f is frequency, v is velocity of the wave, and L is the length of the tube.
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The surroundings now do work on the system, adiabatically
compressing the gas and allowing the piston to fall back to its rest position.
However, because it is easier to compress the cooler gas than to add heat to the warm
gas, net work is done on the surroundings. To determine the efficiency of the cycle, the
total useful work done is compared to the total heat transferred. In Figure 3,
the total heat transferred equals the red area plus the white area. The work extracted
from the system is represented by the white area. Even the Carnot cycle, the ideal
thermodynamic process where each step is reversible and involves no change in entropy,
transfers more heat than it does work. However, the Carnot cycle has the best work
output with the given temperature difference and entropy difference, so it is
defined to be 100% efficient.
CHAPTER 4: EXPERIMENTAL DESIGN
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Device Construction In the construction of our thermoacoustic devices, we
followed the methods of Russell et al. Our materials included a boxed loudspeaker, a
Plexiglas tube, an aluminum stopper, film, and 15lb nylon fishing line.
A diagram of the thermoacoustic device
1. Stack A lot of time was spent making the most important feature of the device, the
stack. It was created by gluing fishing line at evenly spaced intervals along the roll of
film. To do this, we wound fishing line around a 1 meter long cardboard loom with slits
cut every 5mm along the edges. After the line was wound, a meter of photographic film
was secured to a stable surface and then sprayed with adhesive. The loom and line were
then pressed onto the film, weighted, and allowed to dry overnight. Once dry, the
cardboard and excess fishing line was removed. The film was rolled compactly and
placed inside a Plexiglass tube with a diameter of ¾ cm and a length of 23 cm. The stack
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was placed approximately 5 cm from the open end of the tube. Small holes were then
drilled above and below the stack to serve as entry points for the thermocouples.
2. Thermocouples
To construct the thermocouples, a high power small scale welder was to flash-
melt the chromel and alumel wires together on one end, while other ends were connected
to a K tap connector. The welded ends were then inserted into the previously drilled
holes.
3. Adhesives and Sealant
Another Plexiglas plate was cut so that it would cover the speaker entirely. A
hole was drilled in the center of this plate in order to allow the placement of the tube. To
secure an airtight seal between the tube and the plate, an epoxy was used, while a silicone
caulk was used on all the other areas which had potential for leakage (connection of plate
to loudspeaker, thermocouple holes).
4. Loudspeaker
These are selected as per requirement of frequency for wave generation (generally
400 Hz).
Fig- The final modified thermoacoustic device with heat sink.
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CHAPTER 5: THERMOACOUSTIC REFRIGERATION
The thermoacoustic effect occurs in the stack region and requires the presence
of two thermodynamics media i) Stack ii) working fluid (gases). This region also calls as
thermoacoustic core.
Figure: Thermoacoustic Refrigeration
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. 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. The purpose of the stack is
to provide a medium for heat transfer as the sound wave oscillates through the resonator
tube.
A functional cross section of the stack we used is shown in figure b. In typical
standing wave devices, the temperature differences occur over too small of an area to be
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noticeable. In a usual resonator tube, heat transfer occurs between the walls of cylinder
and the gas. 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. In a typical column, 99% of the air
molecules are not near enough to the wall for the temperature effects to be noticeable.
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. 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.
Working:-
Fig: Thermoacoustic Cycle
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Figure: Transport of heat along a stack plate
The cycle consists of two adiabatic steps (1 & 3) and tow isobaric steps (2 &
4). The acoustics standing wave moves the gas parcel forward, the gas parcel is
adiabatically compressed causing its temperature to rise, let’s say by tow arbitrary units
to reach the temperature T++, as indicated in figure 1.3, step (1). At this stage the gas
parcel is warmer than the stack plate and irreversible heat transfer from the working fluid
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towards the stack plate takes place. In step (2), the gas parcel cools down by one arbitrary
unit to the temperature T+. In the process of adiabatic expansion, the gas parcel moves
back to its initial location and cools down by two arbitrary units to reach the temperature
T-, as indicated in step (3). At this stage the gas parcel is colder than the stack plate and
irreversible heat transfer from the stack plate towards the gas parcel takes place in the
fourth step. During the described cycle, the gas parcel has returned to its initial position
and initial temperature T and therefore the cycle can start again. Since there are many
gas parcels moving along the stack plate, and heat that is dropped by one gas parcel, is
transported further by the adjacent parcel, a temperature gradient develops along the
stack plates.
Fig. Temperature variation above (Thot) and below (Tcold) the stack as afunction of time.
Figure shows typical results for the temperatures above the stack (Thot) and
below the stack (Tcold) as a function of time. The starting temperatures were normalized
to zero, so the plot shows the changes in temperature as measured by each thermocouple.
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To produce this plot the thermocouple leads were connected to a two-channel digital
oscilloscope with an 8 minute capture time. The plot shows that the temperature below
the stack (Tcold) begins decreasing immediately after the sound is turned on, dropping 4
°C in the first 15 seconds, with the rate of temperature change decreasing with time. After
4 minutes of operation the temperature below the stack has dropped by 10.5 °C and is
still decreasing. The temperature above the stack (Thot) increases, also more rapidly at
first, as the heat is being pumped through the stack. After approximately 2 minutes the
temperature above the stack has increased by 5 °C. After that it stops increasing as the
rate at which heat is moved through the stack equals the rate at which heat is conducted
through the aluminum cap into the surrounding room. After 4 minutes of operation, the
temperature difference between the top and bottom of the stack is about 15.5 °C, a
difference large enough to be detected by touching a finger along the outside of the
acrylic tube. The trends in Fig. are similar to those found in the literature.
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CHAPTER 6: POSSIBLE MODIFICATIONS
One possible way to dissipate more heat is to increase the surface
area of the cap by cutting grooves into each end of the aluminum plug. The
increased surface area gives air particles a larger area to collide into the aluminum plug
and transfer heat, allowing for there to be more collisions at a single time, thus
increasing the rate of heat conduction of the aluminum plug from the top end of the tube
into the surrounding air. The grooved aluminum plug will decrease the temperature in the
top end of the tube by dissipating heat faster than the flat aluminum plug could. This
will decrease the temperature difference between the top end and the bottom end,
allowing the bottom end to become colder than with the flat plug before the temperature
difference reaches the point that it exceeds the temperature gradient created by the
sound waves and heat can no longer be transferred.
Another possible method of dissipating the heat from the refrigerator would
involve heat absorption by water. Thin pipes could be run across the top end of the
stack. Liquid could flow through the stack, effectively transferring the excess
heat from the system. Water, with a relatively high heat capacity, would absorb the
heat quickly. The hot water could then be used for other applications, such as spinning a
turbine in a generator or an engine. This would be using the device as a heat pump to
power a device.
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Figure: The unmodified model data. The top red bar shows the readings of the warm
thermocouple. The bottom blue bar shows the readings for the cooler thermocouple.
Figure 10: The modified model data. The top red bar shows the readings of the warm
thermocouple. The bottom blue bar shows the readings for the cooler thermocouple. As
shown in the diagram, the actual temperature difference was slightly greater in this
design, but not significantly different.
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CHAPTER 7: APPLICATIONS
1. In Telecommunications:
Thermal management has always been a concern for computer systems
and other electronics. Computational speeds will always be limited by the amount of
noise produced by computer chips. Since most noise is generated by waster heat,
computer components and other semiconductor devices operate faster and more
efficiently at lower temperatures. If thermoacoustic cooling devices could be scaled for
computer applications, the electronic industry would realize longer lifetimes for
microchips, increased speed and capacity for telecommunications, as well
as reduced energy costs.
2. In Freezers:
Ben and Jerry’s Ice Cream, in collaboration with Professor Garrett’s
research team, has begun production of thermoacoustic freezers to keep its ice cream
cold. Investing over $600,000 in Garrett’s program, Ben and Jerry’s has
already placed the freezers in many of its New York stores. The ice cream
company’s experiment has successfully demonstrated the viability of
thermoacoustic refrigeration.
3. In Automobiles:
Figure : Example arrangement of an ideal thermoacoustic heat engine driving an ideal
thermoacoustic heat pump in an automobile.
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Compared to current automotive refrigeration systems, thermoacoustic systems
are quite simple and inexpensive to construct, using steel, aluminium or even plastics
manufactured to low tolerances. These devices are expected to weigh no more than
equivalent vapour compression systems, and operate at lower pressures (usually less than
half the 2,000kPa of typical compressors). Although arguably only in development for
the last 25 years, thermoacoustic systems are highly capable devices with wide ranging
applications: from electricity generation to liquefaction of natural gas, and from cooling
of electronics racks in US Naval warships to onboard the Space Shuttle Discovery.
Figure: The recently completed thermoacoustic refrigerator (TAR).
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CONCLUSION
This device worked as a proof of concept device showing that a
thermoacoustic device is possible and is able to cool air, abet for only a short period of
time. If they were able to build the device with better materials, such has a more
insulating tube, we might have been able to get better results. In order to create a
working refrigerator we probably would have to attach a heat sink to the top of the
device, thus, allowing the excess heat to dissipate to the surroundings. However,
our device did demonstrate that thermoacoustic device have the ability to create and
maintain a large temperature gradient, more than 20 degrees Centigrade, which would be
useful as a heat pump.
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REFERENCES
1. “Standing Waves.” Rod Nave, Georgia State University.
Available: http://hyperphysics.phyastr.gsu.edu/hbase/waves/standw.html. 17 July 2006.
2. http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/carnot.html
3. http://www.howstuffworks.com/stirling-engine.htm
4. http://en.wikipedia.org/wiki/Carnot_cycle
5. Daniel A. Russell and Pontus Weibull, “Tabletop thermoacoustic refrigerator for
demonstrations,” Am. J. Phys. 70 (12), December 2002.
6. G. W. Swift, “Thermoacoustic engines and refrigerators,” Phys. Today 48, 22-28
(1995)
7. http://www.rolexawards.com/laureates/laureate-36-lurie_garrett.html
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Refrigerators.” H.H. Jung and S.W.K Yuan.
Available: http://www.yutopian.net/Yuan/papers/Intel.PDF. 17 July 2006.
9. “Thermoacoustic Refrigeration for Electronic Devices: Project Outline.” Stephen Tse,
2006 Governor’s School of Engineering and Technology.
10. “Chilling at Ben & Jerry’s: Cleaner, Greener.” Ken Brown.
Available: http://www.thermoacousticscorp.com/news/index.cfm/ID/4.htm. 17 July 2006.
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