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Sustainable Energy

An Engine that Harnesses Sound Waves

The device uses thermoacoustics for greater efficiency.

A startup company has developed a new type of engine that could generate electricity with the efficiency of a fuel cell, but which costs only about as much as an internal combustion engine.

Sound barrier: This prototype engine uses thermacoustics to turn heat into electricity.

Etalim, based in Vancouver, Canada, says its engine, roughly the size of a basketball, could improve the economics of electricity production for the cogeneration of power and heat in homes, and as a way to harness the heat produced at concentrating solar collectors. The company has created a prototype, but has yet to achieve the kind of efficiencies—in excess of 40 percent—that its computer models indicate are within reach.

The device shares some principles of a Stirling engine, in which an external heat source is used to expand a fixed amount of working gas (usually helium), which then contracts when it is pushed into a cooler space. This expansion-contraction cycle repeats itself, turning heat into mechanical work by driving a piston.

Etalim’s CEO Ron Klopfer says a fundamental problem with Stirling engines is that they need to run at very high temperatures and pressures to be efficient, making it difficult to keep the gas sealed inside the cylinder that encases the piston. “At these temperatures, you can’t use traditional methods of sealing,” he says. “You can’t use rubber, lubricants. It must be dry metal on metal, and those are very expensive, high-precision parts that lead to high costs.”

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Etalim’s founder and chief scientist, Thomas Steiner, saw an opportunity to eliminate all the rubbing parts and seals that are prone to wear and leakage by using a design based on thermoacoustics—which employs heat to control the intensity of sound waves within a sealed cavity.

Encased within the core of Etalim’s engine is a plate of metal that replaces the function of a piston in a conventional Stirling engine. When pressurized helium on the top side of the metal plate is heated, sound waves traveling through the gas are amplified, causing the plate to vibrate, and a metal diaphragm below (separated by a cooler layer of helium) to push down on a shaft. All mechanical friction is eliminated. The shaft is attached to an alternator that produces electricity.

The vibration of the plate only moves the shaft two-tenths of a millimeter per cycle, so not much helium is being moved with each cycle. But the engine achieves a rapid 500 cycles per second. “If you go to high-frequency, you can get more power out of it,” says Greg Swift, a thermoacoustics expert at Los Alamos National Laboratories who saw an early version of Etalim’s engine. “Steiner has really done a good job of taking a different [design] direction and not making any mistakes.”

The company has ambitious goals. A first prototype, completed last year, demonstrated that the concept works, but relatively low heat was used, so its efficiency was only 10 percent. A second prototype that aims for 20 to 30 percent efficiency at 500 °C is expected this spring.

A commercial product with 40 percent efficiency running at 700 °C is targeted for 2012. It will initially be sold as a cheaper and longer-lasting competitor to fuel cells used for residential cogeneration. The company believes it can manufacture the engine for less than $1 per watt, and has a long-term target of 15 cents per watt, which would make it less expensive than a comparable internal combustion engine.

“Everything to get us to 40 percent efficiency is right in line with what we’ve seen from our prototype so far,” says Klopfer, adding that 50 percent is the longer-term target. “To get from 40 percent to 50 percent, we need to raise the temperature to 1,000 °C, and that requires some use of ceramics.”

Mike Hayden, a professor of physics at Simon Fraser University, says Etalim’s design is promising, but a lot of engineering works lies ahead to prove that the device can handle high temperatures and achieve the kind of efficiencies that would make it stand out. “But there’s no doubt these guys have something interesting,” he says.

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