Physicists led by Orest Symko, a University of Utah physics professor, have developed small devices that turn heat into sound and then into electricity.
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.
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). 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.
Most of the heat-to-electricity acoustic devices built in Symko’s laboratory are housed in cylinder-shaped ‘resonators.’ Each cylinder, or resonator, contains a “stack” of material with a large surface area – such as metal or plastic plates, or fibres 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. The hot, moving air produces sound at a single frequency, similar to air blown into a flute. Then, the sound waves squeeze the piezoelectric device, producing an electrical voltage.
Longer resonator cylinders produce lower tones, while shorter tubes produce higher-pitched tones.