Finite element analysis of optimized piezoelectric bimorphs for vibrational “energy harvesting”

The concept of “energy harvesting” is to design smart systems to capture the ambient energy and to convert it to usable electrical power to supply small electronics devices and sensors. The goal is to develop autonomous and self-powered devices that do not need any replacement of traditional electrochemical batteries. Piezoelectric devices are commonly used, due to their high conversion efficiency and easy of manufacture. The purpose of this paper is to numerically analyze the electromechanical response of piezoelectric bimorphs subjected to vibrations. The bimorph is made up of two layers of piezoelectric material glued on a stainless steel shim, which form a cantilever beam with a tip mass that has the capability to convert the mechanical bending strain within the piezoelectric layers into electric charges on its external surface The strategy here used to increase the average mechanical strain, and hence the generated power output, is to modify the geometry of rectangular piezoelectric beam, which is traditionally used in applications. Optimized configurations with trapezoidal shapes (direct and reversed), with either constant width or constant volume, have been proposed and numerically analyzed. A detailed 3D finite element model is used to evaluate and to compare the electromechanical response of the proposed optimized bimorphs, in terms of resonant frequency, harmonic transfer function, output voltage and power. The electromechanical vibration response has been studied with a modal analysis and a harmonic coupled simulation with imposed base acceleration. The obtained results confirm an increment in the electric performance of the proposed optimized bimorphs, with a net increase in specific volumetric power compared to the traditional rectangular configuration.