Stephen Evanczuk writes:
Micro-harvesting, or energy scavenging, relies on extracting power from minute but pervasive sources of ambient energy such as light, heat, RF, or vibration. With piezoelectric devices, energy from vibration can supply low-power applications, such as wireless sensors that are difficult to reach and maintain, for equipment or structural monitoring. By following a few key design considerations, engineers can build applications powered by piezoelectric transducers from Parallax, Measurement Specialties/Schaevitz, and Mide Technology and power management devices from Linear Technology.
Compared to other micro-harvesting energy sources, vibration and motion are relatively robust sources of ambient power (Figure 1). Placed on motors, for example, vibration-powered sensors can harvest power precisely when it is needed during motor operation. In a practical application, the piezoelectric transducer would likely be used to charge a high efficiency storage device rather than provide power directly to application circuits.
Energy Scavenging with Piezoelectric Transducers - [Link]