Tapered Two-Layer Broadband Vibration Energy Harvesters

Two-layer piezoelectric vibration energy harvesters using convergent and divergent tapered structures have been developed for broadband power output. The harvesters consist of a base cantilevered beam, which is attached to an upper beam by a spacer to develop a two-layer configuration. Two masses are attached to each layer to tune the resonance frequencies of each harvester and one of these masses also serves as the spacer. By varying the positions of the masses, the convergent and divergent tapered harvesters can generate close resonance frequencies and considerable power output in the first two modes. A broadband harvester design strategy is introduced based on a modal approach, which determines the modal performance using mass ratio and modal electromechanical coupling coefficient (EMCC). The required modal parameters are derived using the finite element method. Mass ratio represents the influence of the modal mechanical behavior on the power density directly. Since the dominant mode causes the remaining modes to have smaller mass ratios, smaller EMCC, and poor performance, the design strategy involves the selection of the harvester configurations with close resonances and favorable values of mass ratio initially, and deriving the EMCC and power density of those selected configurations.