New efficiency measures of energy conversion and their characterization for piezoelectric vibration energy harvesters

To assess the performance of a piezoelectric vibration energy harvester (PVEH) for analysis and design, a well-defined efficiency measure of mechanical-to-electrical energy conversion is desired. Depending on the purpose of use, this work newly derives two efficiency measures of energy conversion, one of which is defined as the ratio of electrical output power to mechanical input power of a PVEH with an optimal impedance–matched load resistor and the other of which is defined as the ratio of electrical output power of a PVEH with an optimal impedance–matched load resistor to mechanical input power of the same PVEH under the short-circuit condition. The proposed efficiency measures are extensively verified by finite element analysis results for four resonance-tuned PVEHs with different piezoelectric materials. New findings on the characteristics of energy conversion and conversion efficiency are also provided for two different impedance matching methods.