Thickness-variable composite beams for vibration energy harvesting

Researchers have recently focused on improving the efficiency of piezoelectric energy harvesters by redesigning their substrate beams. Two approaches have been explored, including changing the shape of the cross-section area, i.e., width variation, and changing the thickness of cantilever beams. The width changing approach suffers from the deduction of output power from the piezoelectric elements of the trimmed-off regions, while performance deterioration does not occur in the thickness-variable approach. The benefit of thickness-variable beams on energy harvesting has been analyzed theoretically, but its detailed experimental study is still absent. In this study, we for the first time fabricate a thickness-variable harvester based on the Garolite FR-4 epoxy laminate, and validate the superiority of the variable thickness beam on the harvester performance enhancement. A theoretical model is developed based on the Euler-Bernoulli beam theory and numerically solved via the Rayleigh-Ritz method. We compare the thickness-variable composite beam configuration with a conventional uniform-thickness counterpart and unveil the beneficial effect of evenly-distributed strain on performance enhancement. Experiments indicate that the thickness-variable method increases on the energy harvester efficiency by 78%.