Machine Design

A sound way to turn heat into electricity

University of Utah physicists have developed small devices that turn heat into sound and then into electricity. Thermoacoustic prime movers hold promise for converting waste heat into electricity, harnessing solar energy, and cooling computers and radar equipment.

A palm-sized, cylinder-shaped resonator contains a stack of material with a large surface area such as metal or plastic plates, or fibers made of glass, cotton or steel wool. The material sits between a cold heat exchanger and a hot heat exchanger. Applying heat to the device moves air and produces sound at a single frequency, similar to air blown into a flute. Longer resonator cylinders produce lower tones, while shorter tubes produce higher-pitched tones. The sound waves then squeeze a piezoelectric element that produces electric current.

Thermoacoustic prime movers lack moving parts, so they will need little maintenance and last a long time. Researchers say the devices won't create noise pollution. Smaller versions under development convert heat to ultrasonic frequencies people cannot hear. Second, sound volume goes down as sound converts to electricity. Finally, sound absorbers placed around devices easily contain the noise.

A 1.5-in. long, 0.5-in. wide cylindrical resonator produces sound with as little as a 90°F temperature difference between hot and cold heat exchangers. Some prototypes produced sound at 135 dB — as loud as a jackhammer. Pressurizing the air in a similar-sized resonator produces more sound, and thus more electricity. Further, raising air pressure lowers the temperature difference needed between heat exchangers to produce sound. This makes the acoustic devices practical for cooling laptop computers and other electronics that emit relatively small amounts of waste heat, researchers say.

For an array to efficiently convert heat to sound and electricity, its individual devices must be "coupled" to produce the same frequency of sound and vibrate in sync.

A resonator made from a 0.25-in.-diameter steel tube bent to form a ring about 1.3 in. across causes sound waves to circle through instead of bounce off the ends as in a cylindrical design. Such ring-shaped resonators are twice as efficient as cylindrical resonators in converting heat into sound and electricity. This is because pressure and speed of air in the ring shape remain synchronized.

Lastly, a cylinder-shaped prototype one-third the size of the other devices (less than half the width of a penny) makes a high-pitched sound at 120 dB, equivalent to the level produced by a siren or a rock concert.

Within a year, researchers plan to use the devices to produce electricity from waste heat at a military radar facility and at the University's hot-water-generating plant.

Thermoacoustic prime movers may also serve as a portable power source on the battlefield to run electronics, as an alternative to photovoltaic cells, or as a way to generate electricity from heat that now escapes from nuclear-powerplant cooling towers.

University of Utah physicist Orest Symko uses a blowtorch to heat a metallic screen inside a plastic tube, which then produces a loud tone, similar to when air is blown into a flute. The sound pressure operates a piezoelectric element that generates an electric current.

TAGS: Energy Archive
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