Heat to Sound to Electricity

Posted by jns on 14 June 2007

From a recent Physics News Update comes this half-science, half-technology report about a device that uses heat to make electricity, with sound as an intermediary.

The story is interesting enough by itself, but it is also a useful illustration that sometimes there are new ideas in science and technology that are not as inscrutable as general relativity or string theory, but are nevertheless pretty startling and understandable.

There’s really nothing in this report that requires much in the way of deep technical or scientific understanding, although it might help if I describe the idea of the piezoelectric effect a little. There are some substances, largely ceramics but also some naturally occurring crystals that exhibit this property: applying stress to them (e.g., squeezing them) creates an electrostatic charge, i.e., a voltage across the crystal. Sometimes this property is used in reverse: put a voltage across a piezoelectric substance and it expands by a tiny amount. Piezoelectric devices are often used, therefore, to make precision actuators, devices that move things closer together or further apart depending on an applied voltage.

TURNING HEAT INTO ELECTRICITY THROUGH SOUND has been demonstrated by the University of Utah group of physicist Orest Symko. The group has built devices that can create electricity from the heat that would otherwise be wasted in objects such as computer chips. The devices might potentially make extra electricity from the heat of nuclear power plant towers, or remove extra heat from military electronics.

At last week’s meeting of the Acoustical Society of America in Salt Lake City, five of Symko’s students demonstrated the latest versions of the devices, which they have been developing for a few years. The devices first convert heat into sound, and then sound waves into electricity. Typically, each device is a palm-sized cylinder containing a stack of materials such as plastic or metal or fiberglass. Applying a heat source, such as a blowtorch, to one end of the stack creates a movement of air which then travels down the cylindrical tube. This warm, moving air sets up a sound wave in the tube, similar to the way in which blowing air into a flute creates sound. The pitch, or frequency, of the sound wave depends on the dimensions of the tube; current designs blast audible sound, but smaller devices would create ultrasound. The sound wave then strikes a piezoelectric crystal, a commercially available material that converts sound into electricity when the sound waves put pressure on the crystal.

Symko says a ballpark range of 10-25% of the heat gets converted into sound in typical situations. The piezoelectric crystals then convert about 80-90% of the sound energy into electrical energy. Symko expects the devices to be used in real-world applications within two years, and may provide a better alternative to photovoltaic solar cells in some situations. (Session 5aPA at meeting; also see University of Utah press release at http://www.unews.utah.edu/p/?r=053007-1)

[Phillip F. Schewe and Ben Stein, Physics News Update: The American Institute of Physics Bulletin of Physics News, Number 828, 13 June 2007.]