Optimal design of two-layer vibration energy harvesters using a modal approach

Piezoelectric vibration energy harvesters (VEHs) with two-layer structures are developed. The attached masses are used to tune the frequencies and as spacers between the two layers. By changing the dimensions of the layers and masses and relocating the positions of the masses, the VEHs can generate close resonance frequencies and considerable power output. The modal approach is introduced to determine the modal performance using the mass ratio and the modal electromechanical coupling coefficient, where the mass ratio represents the influence of the modal mechanical behaviour on the power density directly, and the modal parameters required are derived using the finite element method. The findings indicate that a mode with too large mass ratio will cause the remaining modes to have small mass ratios and poor performance. Then, a screening process for the identification of the configurations of VEHs with optimal or near-optimal performance is developed using the modal approach. This procedure facilitates the selection of VEH configurations with close resonances and favourable values of mass ratio initially before carrying out full analysis. Furthermore, the approach can be used to develop VEHs of different sizes ranging from a few millimeters to hundreds of millimeters with the power ranging from microwatts to milliwatts. © 2014 IOP Publishing Ltd.