Piezoelectric crystals are one of many small scale energy sources. Whenever piezoelectric crystals are mechanically deformed or subject to vibration they generate a small voltage, commonly know as piezoelectricity. This form of renewable energy is not ideally suited to an industrial situation.
The ability of certain crystals to generate Piezoelectricity in response to applied mechanical stress is reversible in that piezoelectric crystals, when subjected to an externally applied voltage, can change shape by a small amount. This deformation, though only nanometers, has useful applications such as the production and detection of sound.
Probably the best-known use of piezoelectric crystals is in the electric cigarette lighter. Here, pressing the button causes a spring-loaded hammer to hit a piezoelectric crystal, the high voltage produced by this ignites the gas as the current jumps over a small spark gap. This technique also applies to some gas lighters used on gas grills or stoves.
Another common usage of a piezoelectric crystal energy source is that of creating a small motor; such as that used in a reflex camera to operate the auto focus system. These motors operate by vibration. The two surfaces are forced to vibrate at a phase shift of 90 degrees by a sine wave that has been generated at the motors resonant frequency. This forces a frictional force where the two surfaces meet and as one of the surfaces is fixed the other is forced to move.
It has been found that piezoelectric crystals that have been embedded in the sole of a shoe can yield a small amount of energy with each step. This could be applied in a way that the power for instruments such as torches, cell phones or other entertainment devices can be sourced from the movement of the operator.