In the searing heat and weighty humidity of a Midwestern summer, when it pays to think cool thoughts, James Braun and his Purdue University colleagues are using more than their imaginations. They're building their own refrigerator. But this cooler is not your ordinary Fridgidaire. For starters, it's powered by sound.
The thermoacoustic system uses a loudspeaker to summon the energy needed to generate cool air from a mixture of liquid helium and argon, replacing the need for the traditional Freon-filled compressor found in refrigerators and air-conditioning systems. And this is just what Braun and his colleagues hope to do - all in the name of clearing the air.
"It's fairly quiet to a person, but it's high volume inside," said Braun, associate professor of mechanical engineering at Purdue's Ray W. Herrick Laboratory, an institution teaming with researchers specializing in either thermal or acoustical sciences.
The sound-powered refrigeration unit is not a marriage between the stereo and the ice box. Braun - who used to play the trumpet - said no musical talent nor sense of rhythm was necessary in building the cooling contraption. A medium-pitched, monotonal sound is focused on a sealed aluminum tube that holds the helium/argon concoction.
Sound waves from the tone cause the liquid mixture to oscillate inside the tube to create a temperature gradient that creates an airflow. Fluid at one end takes in heat energy in the form of cool air and travels to the other end, where it gives off the heat energy in the form of hot air.
This entire system is enclosed in an aluminum shell which, Braun says, absorbs much of the sound waves which operate at a frequency of 50 to 100 Hz, depending upon the resonant frequency of the device.
Currently, Braun and his fellow researchers are putting the finishing touches on a prototype that they want completed by early July. Braun said the technology would likely be used in small-scale cooling systems such as those used on scientific instruments. But commercial applications could follow, including air-conditioning systems and refrigerators.
"It's environmentally friendly, simple technology that has the potential to be part of low-cost systems," he said.
