Efficiency of piezoelectric mechanical vibration energy harvesting

Harvesting efficiency of a piezoelectric vibration energy harvesting system is investigated to provide design guidelines for harvesting devices with optimal performance. Harvesting power efficiency (eta), defined as the ratio of device output power (P-out) to mechanical input power (P-in), is an essential but unexplored metric for comparison of harvesters operating in different power-input environments. Power extracted from piezoelectric harvesters has been of primary interest and proper accounting of mechanical input power and efficiency metrics has not been considered. Here, we present a closed form solution for harvesting efficiency that allows device comparison and furthermore, efficiency-optimized versus power-optimized electrical loading conditions are compared along with a case study. A key finding is that optimal design parameters for efficiency are quite different than optima for output power (e.g., a single optimum versus dual optima at different frequencies), requiring multi-objective design. These new findings provide guidelines on system parameters that can be manipulated for optimized performance in different ambient source conditions.