Scavenging energy from ultra-low frequency mechanical excitations through a bi-directional hybrid energy harvester

A bi-directional hybrid energy harvester (HEH) is presented in this paper to scavenge energy from ultra-low frequency mechanical excitations. The proposed HEH consists of two piezoelectric cantilever beams, a suspended magnet, and a set of coil. Specifically, the two piezoelectric beams work as a conventional piezoelectric energy harvester (PEH), whereas the suspended magnet and the coil constitute an electromagnetic energy harvester (EMEH). The two energy-harvesting units (PEH and EMEH), which are sensitive to excitations coming from different directions, are coupled by the suspended magnet, through which the PEH and EMEH are coherently integrated. The suspended magnet not only induces the coil to generate electricity but also actuates the PEH to work, achieving the simultaneous energy extraction from one excitation through two conversion mechanisms. The dynamic model of the HEH is established. Theoretical simulations and experimental measurements under the sinusoidal excitation indicate that the nonlinear interaction between the PEH and EMEH actuates the two energy-harvesting units to oscillate either chaotically or periodically with large amplitudes, which can improve both the PEH and the EMEH power outputs at ultra-low frequencies, not only expanding the HEH working bandwidth but also making the HEH suitable for ultra-low frequency energy harvesting. Moreover, the hand-shaking test shows that the HEH has a better charging performance than an individual energy-harvesting unit for charging a capacitor. Under the hand-shaking induced excitation, the fabricated HEH prototype can also light up tens of light-emitting diodes (LEDs), demonstrating its potential application for powering some portable electronics.