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FEATURE ARTICLE

The Power of Sound

Sound waves in "thermoacoustic" engines and refrigerators can replace the pistons and cranks that are typically built into such machinery

Steven Garrett, Scott Backhaus

A Stereo Refrigerator

The simplicity of the hardware involved in thermoacoustic machines is best appreciated by examining a concrete example. In the mid-1990s, one of us (Garrett) and his colleagues at the Naval Postgraduate School in Monterey, California developed two thermoacoustic refrigerators for the Space Shuttle. The first was designed to cool electronic components, and the second was intended to replace the refrigerator-freezer unit used to preserve blood and urine samples from astronauts engaged in biomedical experiments.

Figure 5. Gas pressure within a cylinderClick to Enlarge Image

This "thermoacoustic life sciences refrigerator," as we called it, produced good results in the laboratory, yet NASA sponsorship ended abruptly, ostensibly for lack of funds. Because the project was progressing so well by that time, we were quite puzzled. But six months later we discovered that the managers of our program at the NASA Life Sciences Division were enmeshed in a controversial FBI investigation of kickbacks and bribery at the Johnson Space Flight Center in Houston. Clearly, they were preoccupied with something other than evaluating our technical progress. Fortunately, the U.S. Navy was in need of a similar chiller and took over support of our efforts.

Because we had originally designed this refrigerator to operate in the rather demanding environment of space, we chose a "stereo" configuration to provide redundancy in case one of the loudspeakers failed. The two loudspeakers are similar to those used for sound reproduction, but they are much more powerful and operate over a limited range of frequencies. They also differ from normal speakers in that the cones are inverted, having their large diameter at the voice coil and small diameter where the sound is radiated. The moving parts of these speakers are joined to a stationary U-shaped resonant cavity by small metal bellows.

Click to Enlarge Image

The U-tube contains two separate stacks, each with two water-filled heat exchangers, which resemble small car radiators, attached at the ends. Two of these heat exchangers exhaust waste heat, and two provide cooling. In this incarnation, chilled water from our "life sciences refrigerator" circulated through racks of radar electronics on the USS Deyo, a Navy destroyer. The maximum cooling capacity we achieved in our sea trials proved to be in excess of 400 watts, using just over 200 watts of acoustic power. At the lowest temperature of operation we could comfortably attain without risking the water freezing and blocking the pipes (about 4 degrees C), the refrigerator performed at 17 percent of the efficiency that could, in principle, be coaxed from a perfect refrigerator operating over the same temperature span—a fundamental limit imposed by the Second Law of Thermodynamics. The refrigerator itself reached 26 percent of the maximum, but inefficiencies of the heat exchangers reduced the useful cooling to the 17-percent value. That level is little better than half of what conventional chillers of similar size and cooling capacity can boast.

Although we could have improved the performance substantially with some modest changes, thermoacoustic refrigerators of this type will always have an intrinsic limit to their efficiency, which is imposed by the way heat flows between the gas and the stack. But recently, one of us (Backhaus) and his colleagues at Los Alamos National Laboratory demonstrated a technique that has enabled thermoacoustic engines to break this seemingly insurmountable barrier by, strangely enough, borrowing a technique that a Scottish minister patented in 1816—the very year Laplace first correctly calculated the speed of sound.




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