Monday, June 11, 2007

Thermoacoustic Energy


A few weeks ago I reported here on a thermoacoustic stove that also provides cooling and electric power. The device that makes the thermoacoutsti stove work is a closed pipe that when heated creates high volume sound waves. Now a Prof. at the University of Utah has taken that idea on a slightly different tangent. Prof. Orest Symko uses the thermoacoustic effect to create pressure waves in a tube that vibrates a piezoelectric material that converts the sound energy directly into electricity.

“We are converting waste heat to electricity in an efficient, simple way by using sound,” said Symko in a Science Daily interview, “It is a new source of renewable energy from waste heat.”

Piezo electric devices create electricity when subject to mechanical stress. Most piexo electric materials are crystals.

Here are summaries of the studies by Symko’s doctoral students:

– Student Bonnie McLaughlin showed it was possible to double the efficiency of converting heat into sound by optimizing the geometry and insulation of the acoustic resonator and by injecting heat directly into the hot heat exchanger.

She built cylindrical devices 1.5 inches long and a half-inch wide, and worked to improve how much heat was converted to sound rather than escaping. As little as a 90-degree Fahrenheit temperature difference between hot and cold heat exchangers produced sound. Some devices produced sound at 135 decibels — as loud as a jackhammer.

– Student Nick Webb showed that by pressurizing the air in a similar-sized resonator, it was able to produce more sound, and thus more electricity.

He also showed that by increasing air pressure, a smaller temperature difference between heat exchangers is needed for heat to begin converting into sound. That makes it practical to use the acoustic devices to cool laptop computers and other electronics that emit relatively small amounts of waste heat, Symko says.

Student Brenna Gillman learned how to get the devices — mounted together to form an array — to work together.

Gillman used various metals to build supports to hold five of the devices at once. She found the devices could be synchronized if a support was made of a less dense metal such as aluminum.

– Student Ivan Rodriguez used a different approach in building an acoustic device to convert heat to electricity. Instead of a cylinder, he built a resonator from a quarter-inch-diameter hollow steel tube bent to form a ring about 1.3 inches across.

In cylinder-shaped resonators, sound waves bounce against the ends of the cylinder. But when heat is applied to Rodriguez’s ring-shaped resonator, sound waves keep circling through the device with nothing to reflect them.

– Student Myra Flitcroft designed a cylinder-shaped heat engine one-third the size of the other devices. It is less than half as wide as a penny, producing a much higher pitch than the other resonators. When heated, the device generated sound at 120 decibels — the level produced by a siren or a rock concert.

“It’s an extremely small thermoacoustic device — one of the smallest built — and it opens the way for producing them in an array,” Symko says.

Note: This story has been adapted from a news release issued by University of Utah.

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