Are piezoelectric-electromagnetic hybrid energy harvesting systems beneficial?

Abstract

The primary objective of this work is to investigate the performance of a hybrid energy harvesting system consisting of piezoelectric and electromagnetic transducers. We first show that a single--mechanism generator with negligible electrical losses, referred to as an electrically--lossless harvester, can reach the theoretical power bound regardless of the coupling strength between the mechanical and electrical domains, which renders the use of hybrid systems unnecessary. For a more realistic analysis, the electrically parasitic losses are then taken into account. We introduce effective figures of merit for the piezoelectric and electromagnetic generators that combine transducer coupling and resistive losses. The maximum output power of single--transducer and hybrid systems are determined analytically, expressed as functions of effective figures of merit. We find that there is no benefit to utilizing a hybrid system if one of the two, or both, effective figures of merit exceeds a threshold of $\mathcal{M}^\ast \approx 2.17$. We also derive the narrow conditions under which a resonant hybrid harvester system with multiple transduction mechanisms can outperform its counterpart which uses a single energy conversion. In order to provide a comprehensive assessment of the configurations considered, we analyze the relationships between optimizing system efficiency and maximizing output power. We reveal that the two problems generally yield different solutions. However, for a hybrid structure, these objectives result in a unique solution when the effective figures of merit of the two transductions are equal. This is a distinctive property of a hybrid system compared to a single--mechanism device.