Future Tech: Backhaus Sound Engine

This tube of supercharged sound could make the biggest bang since internal combustion.

By Brad LemleyMay 1, 2000 5:00 AM


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A thrum, low and insistent, emanated from the room next to engineer Greg Swift's office at Los Alamos National Laboratory in the late evening of May 3, 1998. "It was very weird," he recalls. "Those guys had never made any noise before." Swift hurried next door and found postdoc Scott Backhaus in a state of disbelief. Backhaus's experimental sound-powered engine, wrapped in heater tape and suspended from a framework of steel pipes, had suddenly fired up on its own.

"I was heating it up to test expansion and contraction," Backhaus says. "I didn't expect it to start up at such a low temperature." He was both surprised and relieved: "My major reaction was, "Thank God, it works.' "

BACKHAUS'S SOUND ENGINE is driven by a hot source and a cold sink (at left) that amplify acoustic waves until they can do the work of steel--but with no moving parts. Courtesy Presely Salaz/Los Alamos National Laboratory

Backhaus's Thermoacoustic Stirling Hybrid Engine, which he affectionately refers to as TASHE, performs the same basic job as an ordinary car engine or gas-fired turbine: It converts heat into motion. But the similarity ends there. TASHE operates entirely on pressure waves, using high-intensity sound to do the work of steel. As a result, it has no moving parts, can be constructed from cheap, basic materials, and yet it is just as efficient as a typical modern internal combustion engine. Ultimately, sound engines could take dozens of forms, from big ones that liquefy plumes of natural gas to little ones laboring in the cellar, that would provide supplemental home electricity. "What sound allows us to do is build invisible machinery. It's the next level of mechanical engineering," says Tim Lucas, president and CEO of Macrosonix Corp., a research and development company in Richmond, Virginia.

Although the idea of using sound to drive an engine is new, TASHE relies on a mechanical blueprint that dates back to the era of steam power. In 1816, Robert Stirling, a multitalented minister of the Church of Scotland, patented a simple design for an external combustion engine; unfortunately, it proved too costly to mass produce. Stirling's engine consists of a sealed chamber filled with gas that shuttles back and forth between a "cold" end, often at room temperature, and a "hot" end, which can be heated by any energy source. A displacer piston within the chamber moves the gas between the two ends, while a power piston oscillates in response to the movement of the gas as it expands when heated and cools when chilled. The power piston can be attached to a crankshaft to do the work.

Time and the internal combustion engine passed the Stirling engine by, but it continued to intrigue scientists and engineers. Then, in 1979, Peter Ceperley, a physics professor at George Mason University in Fairfax, Virginia, published a paper showing that the work done by heat in a Stirling engine could also be carried out by a sound wave. After all, sound is nothing but motion--we hear because pressure waves traveling through the air vibrate our eardrums at varying frequencies. Those waves, Ceperley realized, could bat a slug of gas back and forth in a Stirling-like cycle, just as heat moves a piston back and forth.

"Lots of people tried to put flesh on that idea, with limited success," says Swift, a fellow at Los Alamos. "About three years ago, we said, ÔLet's put a postdoc on it full time.' " That postdoc was Scott Backhaus. To test Ceperley's ideas, he built his own test engine, starting with a baseball-bat-shaped resonator made from inexpensive steel pipe. The resonator determines the operational frequency of the engine, in the same way that the length of an organ's pipe determines its pitch. At the "handle" end of the bat, Backhaus bolted on a doughnut-shaped metal chamber to hold the hot (about 1,300 degrees Fahrenheit, or 700 degrees Celsius) and cold (70 degrees Fahrenheit, or 20 degrees Celsius) heat exchangers. Then he filled the device with compressed helium.

The heat exchangers in TASHE act like a huge stereo speaker--creating sound, sending it down the resonator, and amplifying the feedback repeatedly until it becomes inconceivably powerful. "If you were in that wave, permanent hearing loss would be the least of your problems. It's loud enough to set your hair on fire," says Swift. The operating engine is remarkably muted, however, quieter than an idling car. Quarter-inch-thick steel walls, needed to contain the highly compressed helium, maintain the silence. "The cavity walls are extremely stiff. They don't flex, so the sound wave hardly escapes," Swift says.

He and Backhaus are just beginning to sort out what their sound contraption can do. Soon it may provide a better way to recover natural gas. In the course of drilling, offshore oil rigs can liberate natural gas, which is often just burned as a waste product. The sound engine could provide a cost-effective way to capture and ship the gas to the mainland. In cooperation with Cryenco Inc., a Denver gas-transport firm, the Los Alamos engineers are building a huge model of the engine--40 feet tall and four feet in diameter--that can cool and liquefy 500 gallons of natural gas per day. The heat needed to run it will come from burning a little of the cast-off fuel. "We're both conserving a resource and cutting the pollution caused by flaring off that gas," Swift says.

Sound engines could perform a similar conservation coup in the home. Gas-fired hot-water heaters dump unused warmth into millions of basements around the country. The sound engine could tap that thermal waste and use it to move a spring-mounted piston driven by acoustic waves. The piston, in turn, could run a household generator. Swift has teamed up with the Clever Fellows Innovation Consortium in Troy, New York, to develop such a hybrid device. "You burn natural gas, and instead of putting the heat directly into the water, you'll use that heat to run an acoustic engine to make electricity," says Swift.

SUPER-RESONATOR draws in air when pressure piles up at the left end of the chamber (top). When the pressure wave rebounds (bottom), it could be released as part of a cooling system for people or computers. Courtesy: Macrosonix

And there's more than one way to tap into the power of sound. At Los Alamos in the late 1980s, Tim Lucas worked on a resonator capable of creating far more intense sound waves than those generated by devices like TASHE. By vibrating the resonator with an electric motor, he generated sound waves having energy densities thousands of times greater than had ever before been achieved. After forming Macrosonix in 1990, Lucas began exploring ways to use extreme sound to perform a variety of jobs that normally require complex machinery, such as manufacturing pharmaceuticals, grinding up materials, mixing chemicals rapidly, compressing gas, turbocharging engines, and recycling plastics. "It's a factory in a bottle. There's a level of control there that has never existed before," he says.

Chemicals can be heated and cooled 600 times per second over temperature swings as large as hundreds of degrees Celsius, or turbulently mixed 1,200 times per second. "We can create a wide range of physical effects that were simply impossible to attain before," Lucas says. He calls his sound-generating process resonant macrosonic synthesis, and he thinks it will someday find applications as diverse as the laser. But first the technology will show up in more conventional applications, such as acoustic compressors, which can be used in refrigerators, air conditioners, and cooling systems for microprocessors.

Given the way the this work is going, Backhaus's little experiment at Los Alamos could soon become the thrum heard 'round the world.

To find out more about TASHE, see the Los Alamos National Laboratory's Web site:

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