Hot sounds

Led by physicist Orest Symko, a University of Utah group has turned heat into electricity through sound.

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.

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 fibreglass. 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.