Thermo Acoustic Refrigeration
1Gorantla Manohar, 2Kummeta Yugandhar
Reddy
1Chaitanya Bharathi Institute of
Technology,Mechanical Engg ¾
2 Chaitanya Bharathi Institute of
Technology,Mechanical Engg 3/4
ABSTRACT
Thermo
acoustic refrigeration is one such phenomenon that uses high intensity sound
waves in a pressurized gas tube to pump heat from one place to other to produce
refrigeration effect. In this type of refrigeration all sorts of conventional
refrigerants are eliminated and sound waves take their place. All we need is a
loud speaker and an acoustically insulated tube. Also this system completely
eliminates the need for lubricants and results in 40% less energy consumption.
By
using this refrigeration system we can completely eliminate the ozone depleting
chlorofluorocarbons(CFCs). Thermo acoustic heat engines have the advantage of
operating with inert gases and with little or no moving parts, making them
highly efficient ideal candidate for environmentally-safe refrigeration with
almost zero maintenance cost.
This
paper proposes the implementation of Thermo acoustic refrigeration over other
commercial refrigeration systems which emits CFCs.
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. These so called 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. Such materials thus can be used, for example,
to generate electricity or to provide refrigeration and air
conditioning. Because thermo acoustic devices perform best with inert gases as
the working fluid, they do not produce the harmful environmental
effects such as global warming or stratospheric ozonedepletion that have been associated with the
engineered refrigerants such as CFCs and HFCs. Recent advances
have boosted efficiencies to levels that rival what can be obtained from internal combustion engines, suggesting
that commercial thermo acoustic devices may soon be a common place
The entire
features mentioned above is possible only because sound waves inthermo acoustic engines and refrigerators
can replace the piston and cranks that are typically built
into any machinery.
These thermo acoustic devices produce or absorb sound power rather than the shaft power characteristic of rotating machinery making it mechanically simple.
BASIC FUNCTIONING
In a nut shell, a thermo acoustic engine
converts heat from a high-temperature source into acoustic power while
rejecting waste heat to a low temperature sink. A thermo acoustic refrigerator does the opposite, using
acoustic power to pump heat from a cool source to a hot sink. These devices
perform best when they employ noble gases as their thermodynamic
working fluids. Unlike the chemicals used in refrigeration over the years, such gases are both nontoxic and environmentally
benign. Another appealing feature of thermo acoustics is that one
can easily flange an engine onto a refrigerator, creating a heat powered
cooler with no moving parts at all .
The principle can be imagined
as a loud speaker creating high amplitude
soundwaves that can compress refrigerant allowing heat abs--orption. 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 travelling 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. 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.
It
is interesting to note that humans feel pain when they hear sound above 120decibels,
while in this system sound may reach amplitudes of 173 decibels. But even
if the fridge is to crack open, the sound
will not be escaping to outside environment, since this intense noise can only
be generated inside the pressurized gas locked inside the cooling system. It is
worth noting that, prototypes of the technology has been built and one has even
flown inside a space shuttle.
THERMO ACOUSTIC EFFECT
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.
As expansion and compression in an acoustic wave are 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.
The principle may find applications in practical refrigerators,
providing cooling ,heat
engines providing heat or power generators providing work. A great
advantage of the technique is that there are
no or only one moving part, in the cold area, which results in high reliability and low vibration levels. Also the use of inert gases make them environmentally safe and hence more in
demand.
FUNCTIONING IN DETAIL
Thermo acoustic refrigerators
now under development use sound waves strong enough to make your
hair catch fire, says inventor Steven L.Garrett but this noise is safely contained in a pressu -rized tube.
if the tube gets shattered ,the noise would instantly dissipate to harmless
levels because it conducts heat, such intense acoustic power is a clean, dependable replacement for cooling systems that use ozone destroyingchlorofluorocarbons (CFCs). Now a scientist Hofler is also developing super coldcryocoolers
capable of temperatures as low as -135˚F (180˚K). he hopes to achieve -243˚F(120˚K) because such cryogenic temperatures would
keep electronic components cool in space or speed the function of new
microprocessors.
The interaction between heat and
sound has been underestimated even by Sir Isaac Newton. This became clear, when Laplace corrected Newton’s
earlier calculation of the speed of sound in air. Newton had assumed
the expansions and compressions of a sound wave in a gas happen without affecting the temperature. Laplace
accounted for slight variations in
temperature that in fact take place, and by doing so he derived the correct speed
of
sound in air, a value that is 18%
faster than Newton’s estimate.
A thermo acoustic refrigerator
functions as follows. First, customized loud speakers are attached to
cylindrical chambers filled with inert, pressurized gases such as xenon and helium.
At the opposite end of the tubes are tightly wound "jelly rolls" made
of plastic film glued to
ordinary fishing line. When the loudspeakers blast sound at
180 decibels, an acoustic
wave resonates in the chambers. As gas molecules begin dancing frantically
in response to the sound, they are compressed and heated, with
temperatures reaching a peak at the
thickest point of the acoustic wave. That's where the super hot gas
molecules crash into the plastic rolls. After transferring their heat to the
stack, the sound wave causes the molecules to expand and cool. "Each one
of these oscillating molecules acts as a member of a 'bucket brigade,'
carrying heat toward the source of the sound," says Garrett. Cold temperatures
can then be tapped for chilling refrigerators, bedrooms, cars, or
electronic components on satellites and
inside computers, according to Garrett. Someday, he says, turning up the
air-conditioner could be accomplished by adjusting a volume-control knob.
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. The design was an extension of the
first thermo acoustic refrigerator built at Los Alamos National Laboratory as
the Ph.D.thesis project of Thomas J. Hofler. Dr. Hofler is currently a member
of the physics faculty at the Naval Postgraduate School in Monterey, CA.
The major parts of Thermo acoustic
refrigeration are loud speaker,
resonator .
Loud speaker
Resonator
In contrast, inside conventional refrigerators and air conditioners, CFC gas iscompressed
and heated by an electrically driven pump, then cooled and condensed by a heat
exchanger in a process known as a "Rankine cycle." When the liquefied
gas is depressurized, it evaporates and drops to a much
cooler temperature. Moving through the freezer coils of a food compartment,
the cold fluid picks up heat, starting the cycle all over again
Before World War II, ammonia and sulfur dioxide were commonly used inrefrigerators,
explains Gregory W. Swift, a thermo acoustics expert at Los Alamos
National Laboratory in New Mexico. But these substances were soon replaced with
CFCs, which are noncorrosive, non flammable, and relatively nontoxic, Swift
says. Unfortunately, he adds, CFCs leak from cooling systems, destroying the
atmospheric ozone that protects the earth's surface from ultraviolet radiation.
Damage to the ozone shield 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.
MERITS OF THE TECHNOLOGY
Although the working principle of
thermo acoustic technology is quite complex, the practical implementation
is relatively simple. This offers great advantages with respect to the economic
feasibility of this technology. Other advantages are
1. No
moving parts for the process, so very reliable and a long life span
2. Environmentally
friendly working medium (air, noble gas)
3. The use of air or noble gas as working medium offers a large window of applications because there are no
phase transitions
4. Use
of simple materials with no special requirements, which are commercially available in large
quantities and therefore relatively cheap.
5. On
the same technology base a large variety of applications can be covered.
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. It also has
fewer moving parts than its competitors, and so is less likely to break
down.
Also sonic compression or ‘sound
wave refrigeration’ uses sound to
compressrefrigerants 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. The system is also an
energy saving drop in current compressors, and projected mass production
cost is very low.
Applications
Speaking of its practical
applicability, prototype of thermo acoustic refrigerators have operated on the
Space Shuttle and abroad a Navy warship. And a powerful thermo acoustic
engine has recently demonstrated its ability to liquefy natural gas on a
commercial scale.
In practice there is a large variety of applications possible for both
thermo acoustic engines and refrigerators and
combination of these. Below, some concrete examples are given of
possible applications:
a.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.
b.Chip cooling:
In this
case a piezoelectric element generates the sound wave. A thermo acoustic heat
pump cools the chip.
c.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
d.Electricity
from sunlight:
Concentrated
thermal solar energy generates an acoustic wave in a heated thermo acoustic
engine. A linear motor generates electricity from this.
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. This latest break through, coupled with
other developments in the design of high power, single frequency loud speakers and reciprocating electric generators suggests
that thermo acoustics may soon
emerge as an environmentally attractive way to power hybrid electric vehicles, capture solar
energy, refrigerate food, air condition buildings, liquefy industrial gases and serve in other capacities
that are yet to be imagined.
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 pump the latest achievements of the former are certainly encouraging, but there are still much left to be done.
References
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