ABSTRACT
Thermoacoustic refrigerator deals with producing
desired refrigeration effect by acoustics/sound waves. Sound waves generated by
ordinary loudspeaker travel through a sealed tube (resonator) and pass over
stack material. Here, gas molecules undergo compression and rare fraction, thus
giving heating and cooling effect respectively, forming the basis of
refrigeration unit.
INTRODUCTION
With
the growing bans on chloro-fluorocarbon (CFC) used in refrigeration technology,
it is of increasing importance to find a suitable alternative for the industry.
Thermo acoustics offers an excellent technique of refrigeration with the help
of sound waves.
Thermo
acoustic is the study of sound & heat. Using sound waves to cool was theory
originally developed in the 1960s. Ford motor company is the only industrial
laboratory, which has published their research in the area. Japan, America,
Australia are also doing research in this area.
NEED
Conventional
refrigerator - our kitchen fridge works by compressing a gas by compressor. The
gas used is Freon, one of the CFCs. Unfortunately, CFCs leak from cooling
system, destroying the atmospheric ozone that protects the earth's surface
ultraviolet radiation.
Thermoacoustic
refrigerators utilize no environmentally hazardous gases. They use inert gases
which are both readily available, inexhaustible & completely
environmentally friendly. So, it is need of hour.
THE
SYSTEM
BASIC COMPONENTS
The main components of
this refrigerator are -
1.
Acoustic Driver:
Acoustic
driver is used to generate sound waves of particular amplitude &
wavelength.
2. Cold & Hot heat exchangers
3. Resonator
tube:
In
this tube the inert gasses will be filled & heat transfer amongst the stack
plates will take place inside this tube with the aid of vibrating gas
molecules.
4. Stack:
These
are nothing but plates of specific material having good thermal conductivity.
These plates are fixed inside the resonator tube one over other in such a
fashion so as to achieve maximum heat transfer rate.
WORKING
Following
is the figure showing main components.
“When
you compress a gas, it gets warmer; expand a gas and it cools. A sound wave is really nothing more than a
periodic compression and expansion of a gas." So a sound
wave heats and cools small parcels of gas along the length of its propagation.
When
a sound wave is sent in the tube with a acoustic driver or a loudspeaker, the
pressure pulsations make the gas inside travel back and forth. This forms
regions where compression and heating take place, plus other areas
characterized by gas expansion and cooling.
A
thermoacoustic refrigerator is a resonator cavity that contains a stack of
thermal storage elements (connected to hot and cold heat exchangers) positioned
so that the back-and-forth gas motion occurs within the stack. When loudspeaker
blasts sound at certain frequency, acoustic wave resonates in the tube &
back-and-forth motion is achieved. The oscillating gas parcels pick up heat
from the stack near the cold heat exchanger and deposit it to the stack near
the hot heat exchanger, forming the basis of refrigeration unit.
DETAILS
OF THE SYSTEM
STACK
Stack
consists of many plates to increase the heat transfer area. Recent
computational & experimental evidences indicate that the flow field in the
stack is dominated by concentrated eddies, whose complex motion significantly
affects the performance of the device. Material of the stack can be metals,
ceramic or plastics etc.
RESONATING TUBE
It
can be made of glass, acrylic, plexi-glass, metals etc. It should be designed
so as to sustain the pressure & temperature of the gas medium.
REFRIGERANT
System
requires gas medium with higher thermal conductivity for good heat transfer.
Air is easily available but has only one sixth the thermal conductivity of
Hydrogen. Being the lightest & cheap, Hydrogen is another option but its
inflammability makes it unsuitable for use. The next best gas is Helium, which
is non-inflammable with high thermal conductivity. Safety factor compensates
it's high cost.
LOUDSPEAKER/ ACOUSTIC DRIVER
Loudspeakers
are available in the market with different wattages. They should be selected
according to the frequency of the sound waves. Higher frequency waves require
rapid heat transfer, increasing the complexity & cost of heat exchangers.
Reducing the frequency involves the enlargement of the tube, increasing gas
cost. So, a balance is required while selecting the frequency.
The
pressure amplitudes within the thermoacoustic resonator are only a small
fraction (5%) of the static internal pressures, which are approximately 20
atmospheres. Given the relatively small acoustic pressure amplitudes, a
pressure vessel which is strong enough to safely contain the static pressure
cannot yield enough under the acoustic pressure variations to radiate much
sound to the environment. If there is any perceptible acoustic radiation at
all, it is usually due to some imbalance in the electroacoustic driver.
HEAT EXCHANGER
Heat
exchangers can be of copper or aluminum tubing. Fins can be added o increase
the contact area with the gas and stack, reducing thermal resistance which in
turns improves heat transfer.
SIZE OF REFRIGERATOR
For
the small power devices built thus far (less than 1,400 Btu/hr = 400 W thermal)
and the larger devices currently under construction (36,000 Btu/hr = 10 kW
thermal), the size and weight are similar to their vapor compression
equivalents. The cooling capacity of
vapor compression units depends upon operating pressure and the amount of
phase-change fluid. The size of a
thermoacoustic device is determined (roughly) by its operating frequency. If small size is important, higher frequency
operation may be required the thermoacoustic device increases in
efficiency, but the vapor compression fridge does not. There are other
advantages to proportional control. You can imagine that it would be nicer if
your home air conditioner would keep the house at a constant cool temperature
rather than cycling between somewhat too hot and somewhat too cold. Similarly,
the performance and lifetime of some types of electronics could be increase by
the steadier temperatures available through proportional control. Proportional
control also eliminates the electronics-damaging "power surges" that
occur throughout the electrical system when the compressor in a conventional
chiller turns on or off.
COST
There
are no intrinsically expensive components in thermoacoustic refrigerators. They
operate at pressures which are similar to vapor compression refrigerators. Thermoacoustic refrigerators do not require
any exotic materials and do not depend upon close tolerances nor do they
require lubrication, since they have no sliding seals
MAINTENANCE
Thermoacoustic
refrigerators will be at least as trouble-free as current home refrigerators.
Thermoacoustic refrigerators and air conditioners use inert gases which will
never be controlled substances and will always be readily available since they
have no sliding seals, they do not require lubrication It appears that proper
design of these can lead to "infinite" lifetimes.
COOLANTS
Like
a conventional refrigeration system, the thermoacoustic system would require
coolants to circulate through pipes. One coolant loop would remove heat from
the cooled space and bring it to the cooled side of the stack while another
would remove heat from the hot side and discard it into the surroundings. The
coolants required would include water & glycol mixtures.
ADVANTAGES
1. Simple, reliable, low cost, eco-friendly.
2. No mechanical moving parts.
3. Avoids use of compressor, lubricants,
eliminating noise.
4. No maintenance
DRAWBACKS
The
only lacuna of the system is that the efficiency is 20-30% lower than the vapor
compression system but it will increase with the advancing research of such a
newly born technique.
APPLICATIONS
1. Household refrigerators
2. Space shuttle cooling
3. Cooling of computers & electronic
instruments.
4. Natural gas liquifier
5. For fishing boats and marine vessels.
FUTURE
TRENDS
EFFICIENCY IMPROVEMENT:
At
the present time, the efficiency of thermoacoustic refrigerators is 20-30%
lower than their vapor compression counterparts. Part of that lower efficiency
is due to the intrinsic irreversibility of the thermoacoustic heat
transport process. This intrinsic irreversibility are also the favorable
aspects of the cycle, since they make for mechanical simplicity, with few or no
moving parts. A greater part of the inefficiency of current thermoacoustic
refrigerators is simply due to technical immaturity. With time, improvements in
heat exchangers and other sub-systems should narrow the gap.
It
is also likely that the efficiency in many applications will improve due only
to the fact that thermoacoustic refrigerators are well suited to proportional
control. One can easily and continuously control the cooling capacity of a
thermoacoustic refrigerator so that its output can be adjusted accurately for
varying load conditions. This could lead to higher efficiencies than
conventional vapor compression chillers which have constant displacement compressors
and are therefore only capable of binary (on/off) control. Proportional control
avoids losses due to start-up surges in conventional compressors and reduces
the inefficiencies in the heat exchangers, since the proportional systems can
operate over smaller temperature gaps between the coolant fluid and the heat
toad.
Researchers
are trying for methodology which uses computer modeling and optimization to
predict the performance of technology for specific application and aid in the
development of prototype designs.
There
is a need for development in loudspeakers for high power, high efficiency and
in the heat exchangers optimized for high frequency oscillatory flow of
compressed gases.
At
the present time, there are no models for the stack/heat exchanger interface.
There are no models for heat transport between the thermo acoustically
oscillating gas and the heat exchanger surfaces which could be used to suggest
what geometries would optimize the useful transfer of heat on and off of the
stack. A stack with a heat exchanger built into it can be a solution to the
problem.
Still,
thermoacoustic refrigerator have real world application due to their low
maintenance and lack of environmentally gases.
CONCLUSION
Thermoacoustic
refrigeration has spurred progress and has undergone a significant evolutionary
period during the past few decades. Further progress in the practical
commercial application of this technology is expected to continue.
In
addition, much is going on to improve the performance of the system at various
levels in most of the developed countries. All of this could lead to more
efficient system replacing our conventional fridge soon.
REFERENCES
Ø
www.theroacs.com
www. thinkeycle.com
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