The concept of thermoacoustic refrigeration – using sound waves to cool something – offers several advantages over conventional cooling systems. Luc Mongeau, assistant professor of mechanical engineering at Purdue University in Lafayette, Indiana, says the simplicity of thermoacoustic devices could lead to refrigerators and cooling systems with low manufacturing and maintenance costs, new kinds of air-conditioning systems, and portable systems for liquefying natural gas.
Mongeau and colleagues James Braun, associate professor of mechanical engineering, and doctoral student Brian Minner have completed a prototype device funded by Johnson Controls.
The device is essentially a hollow metal tube, about 10in. in diameter and two feet long, with a large round cavity on one end, called a Helmholtz resonator. Attached at the opposite end of the tube is an acoustic driver – a vibrating diaphragm similar to a loudspeaker, but sturdier and more powerful. As the diaphragm vibrates, gas atoms inside the enclosure oscillate back and forth, which sets up pressure fluctuations.
Fluctuating pressures inside the cavity are accompanied by a fluctuation in temperature. When gas is compressed it becomes warmer, and when decompressed, it becomes cooler. The gas particles within the device become alternately hot and cold, dynamically, at a typical frequency of 300Hz. The gas atoms transfer their heat to a piece of porous material called a stack, which is located near the acoustic driver. The end result is that heat is pumped towards the acoustic driver, cooling the side of the stack farthest from the driver. The pressure fluctuations propagate as sound waves that are loud, around 180dB.
As in a conventional refrigerator, an appliance cooled by a thermoacoustic device would require coolant to circulate through pipes. One coolant loop removes heat from the space to be cooled and brings it to the cooled side of the stack, while another loop removes heat from the hot side of the stack and discards it to the surroundings. Mongeau and his colleagues have considered water and a combination of water and glycol as coolants for their system. Even though the sound waves are very loud, a thermoacoustic refrigerator should not be any louder than a conventional appliance and may be quieter.
The advantage of this type of cooling is that there are no refrigerants involved, such as chlorofluorocarbons, which are harmful to the environment. All the elements are environmentally benign, including the coolants and the gas inside the device, which is a mixture of inert gases like helium, argon or xenon.
A home appliance based on thermoacoustic refrigeration is still years away, but the research group are examining how to design such devices for optimum efficiency, as well as looking at cost factors. So far, most of the cost resides in the driver, but they believe that this can be reduced.
Thermoacoustic devices are versatile because they not only cool, but also can work in reverse as an engine. If one side of the stack is heated while the other side is kept cool, acoustic power is produced, which could be converted to electricity, or which could power another acoustic driver to produce cooling elsewhere in the system.
According to the researchers, one of the many applications of this type of system is in the gas industry. Natural gas must be cooled to convert it to a liquid for storage and transport. There is a strong interest in portable systems for liquefying natural gas on site.
Using a combination of thermoacoustic devices, electricity would no longer be needed to accomplish the task. Gas could be used as the energy source to heat a thermoacoustic engine, which would power a cooling device for liquefying gas.
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