Basic Principle

The principle can be imagined as a loud speaker creating high amplitude sound waves that can compress refrigerant allowing heat absorption. Researchers have exploited the fact that sound waves travel by compressing and expanding the gas they are generated in.

Considering the above said wave is traveling through a tube, 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.

Thermoaccoustic 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 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.

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.

Thermoaccoustic Cycle

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.

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.

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.

Finally, in step 4, the packets of gas reabsorb heat from the cold reservoir.

And the heat transfer repeats and hence the thermoacoustic refrigeration cycle.

Thermoaccoustic Refrigerator (TAR)

Two main parts are in the TAR

1. Driver - Houses the Loudspeaker

2. Resonator:

• Houses the gas
• The hot and cold heat exchangers
• Houses the Stack

STACK

It is also called as regenerator. and is the most important piece of a thermoacoustic device. 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.

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.

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 Aluminium, Lexan and Foam.