A sound way to turn heat into electricity

From Science Daily:

University of Utah physicists developed small devices that turn heat into sound and then into electricity. The technology holds promise for changing waste heat into electricity, harnessing solar energy and cooling computers and radars.

University of Utah physicist Orest Symko demonstrates how heat can be converted into sound by using 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. Symko and his students are developing much smaller devices that not only convert heat to sound, but then use the sound to generate electricity. The devices may be used to cool electronics, harness solar energy in a new way, and conserve energy by changing waste heat into electric power.

"We are converting waste heat to electricity in an efficient, simple way by using sound," says Orest Symko, a University of Utah physics professor who leads the effort. "It is a new source of renewable energy from waste heat." Five of Symko's doctoral students recently devised methods to improve the efficiency of acoustic heat-engine devices to turn heat into electricity. They will present their findings on Friday, June 8 during the annual meeting of the Acoustical Society of America at the Hilton Salt Lake City Center hotel.

Symko plans to test the devices within a year to produce electricity from waste heat at a military radar facility and at the university's hot-water-generating plant.
The research is funded by the U.S. Army, which is interested in "taking care of waste heat from radar, and also producing a portable source of electrical energy which you can use in the battlefield to run electronics" he says. Symko expects the devices could be used within two years as an alternative to photovoltaic cells for converting sunlight into electricity. The heat engines also could be used to cool laptop and other computers that generate more heat as their electronics grow more complex. And Symko foresees using the devices to generate electricity from heat that now is released from nuclear power plant cooling towers.

Symko's work on converting heat into electricity via sound stems from his ongoing research to develop tiny thermoacoustic refrigerators for cooling electronics.
In 2005, he began a five-year heat-sound-electricity conversion research project named Thermal Acoustic Piezo Energy Conversion (TAPEC). Symko works with collaborators at Washington State University and the University of Mississippi. The project has received $2 million in funding during the past two years, and Symko hopes it will grow as small heat-sound-electricity devices shrink further so they can be incorporated in micromachines (known as microelectromechanical systems, or MEMS) for use in cooling computers and other electronic devices such as amplifiers.

Using sound to convert heat into electricity has two key steps. Symko and colleagues developed various new heat engines (technically called "thermoacoustic prime movers") to accomplish the first step: convert heat into sound. Then they convert the sound into electricity using existing technology: "piezoelectric" devices that are squeezed in response to pressure, including sound waves, and change that pressure into electrical current. "Piezo" means pressure or squeezing. Most of the heat-to-electricity acoustic devices built in Symko's laboratory are housed in cylinder-shaped "resonators" that fit in the palm of your hand. Each cylinder, or 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 -- placed between a cold heat exchanger and a hot heat exchanger.

When heat is applied -- with matches, a blowtorch or a heating element -- the heat builds to a threshold. Then the hot, moving air produces sound at a single frequency, similar to air blown into a flute. "You have heat, which is so disorderly and chaotic, and all of a sudden you have sound coming out at one frequency," Symko says.
Then the sound waves squeeze the piezoelectric device, producing an electrical voltage. Symko says it's similar to what happens if you hit a nerve in your elbow, producing a painful electrical nerve impulse.

Longer resonator cylinders produce lower tones, while shorter tubes produce higher-pitched tones. Devices that convert heat to sound and then to electricity lack moving parts, so such devices will require little maintenance and last a long time. They do not need to be built as precisely as, say, pistons in an engine, which loses efficiency as the pistons wear.

Symko says the devices won't create noise pollution. First, as smaller devices are developed, they will convert heat to ultrasonic frequencies people cannot hear. Second, sound volume goes down as it is converted to electricity. Finally, "it's easy to contain the noise by putting a sound absorber around the device," he says.

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