Thermo Acoustic Air conditioning

THERMOACOUSTIC REFRIGERATION

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Department of Mechanical EngineeringDepartment of Mechanical EngineeringChandubhai S. Patel Institute of Technology, Changa Chandubhai S. Patel Institute of Technology, Changa

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Introduction

Basics of Refrigeration

Basics of Thermoacoustic Refrigeration

Thermoacoustics

Main Parts

Case Study

Advantages, Disadvantages and Applications of TAR

Conclusion

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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:

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 Steven L. Garrett Leading ResearcherLeading Researcher

United Technologies Corporation Professor of United Technologies Corporation Professor of Acoustics The Pennsylvania State University.Acoustics The Pennsylvania State University.

He invented the thermoacoustic refrigerator in the He invented the thermoacoustic refrigerator in the year 1992 and that TAR was used in the space year 1992 and that TAR was used in the space shuttle Discovery (STS-42).shuttle Discovery (STS-42).

• 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.

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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.

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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.9

• 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.

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CARNOT CYCLE

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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.

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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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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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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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Two main parts are in the TAR

1. Acoustic Driver

Houses the Loudspeaker

2. Resonator

Houses the gas

The hot and cold heat

exchangers

Houses the Stack

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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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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 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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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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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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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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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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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.

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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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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.

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