"In our proof-of-concept test system, there is no 'test freezer,' we simply cool an electrically-heated piece of window screen. The coldest temperature we have achieved with this test rig is eight degrees below zero – well below the freezing point of water," Garrett says. However, although the test rig doesn't look anything like the freezer display case where you usually pick up your pint of Cherry Garcia, Garrett says it is a big step in the "green" ice cream sales cabinet direction.
"We have achieved proof-of-concept for making a compact chiller that has a volume which is substantially smaller than earlier thermoacoustic chillers," he explains. "And we did it with a system that was carefully and redundantly instrumented for both accurate performance measurement and performance diagnostics."
The team's progress will be detailed in a paper, "Performance of a Small Low-Lift Regenerator-based Thermoacoustic Refrigerator," December 4 , 2002 at the First Pan-American/Iberian Acoustics Meeting in Cancun, Mexico. Matthew Poese, doctoral candidate in acoustics, is first author of the paper. The work is part of his doctoral thesis. Garrett explains that his group's thermoacoustic chiller uses a souped-up loudspeaker to generate high amplitude sound energy in an environmentally safe gas – currently the air we breathe – that is converted directly into useful cooling. The high amplitude sound levels are hundreds of thousands of times beyond even rock concert levels.
The loudspeakers used in thermoacoustics do not need to produce a range of frequencies or tones like a radio. So, Garrett's group improves their efficiency by operating them at resonance or at the tones they produce by the natural free oscillation of the system. The Penn State group has developed loudspeakers that not only operate near their natural resonance frequencies but also use metal bellows that replace loudspeaker cones to compress the environmentally safe gas -- air in the test case -- used for chilling.
"We have been operating loudspeakers at resonance and using bellows in thermoacoustic devices for 20 years," Garrett adds. "Now, by putting the entire refrigeration core inside the bellows, we've substantially reduced the size."
Robert Smith, the third member of the Penn State Applied Research Laboratory team working on the Ben & Jerry's project, made important contributions to the loudspeaker in his master's thesis. Garrett notes, "What began as basic research on the fundamental connections between sound waves and heat transport, funded by the Office of Naval Research, is getting closer to providing an environmentally benign substitute for traditional engine and refrigeration technology."
Gary Epright, Ben & Jerry's lead process engineer, said "I am proud that Ben & Jerry's has taken the initiative to explore this beneficial technology with Steve Garrett's team at Penn State. It is a tremendous opportunity to participate in an innovative technology that could revolutionize the way we understand and use refrigeration. With refrigeration based on sound, using environmentally safe gases, we could go a long way toward restoring atmospheric balances."