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Posted on November 9, 2009 by  & 

Turning heat into sound, then electricity

Scientists in the US have been developing 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.
"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. Five of Symko's doctoral students have devised methods to improve the efficiency of acoustic heat-engine devices to turn heat into electricity. 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 as an alternative to photovoltaic cells for converting sunlight into electricity. The heat engines also could be used to generate electricity from heat that 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. 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. 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 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. 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. 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.
Source: University of Utah
Top image of Orest Symco source University of Utah
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