A hybrid piezoelectric-triboelectric generator for low-frequency and broad-bandwidth energy harvesting

In this paper, we report a hybrid generator to harness energy from low-frequency ambient vibrations. The generator is designed with a piezoelectric energy harvester (PEH) patch, a triboelectric nanogenerator (TENG) patch, a spring-mass system and an amplitude limiter. The spring-mass system receives energy from excitations and applies forces to the piezoelectric element and the triboelectric layers. The unique amplitude limiter is deliberately introduced into the system, not only achieving the desired frequency up-conversion effect, but also boosting the voltage responses significantly. The limiter also causes hardening nonlinearity and dynamic bifurcation triggering superharmonic resonance so that the generator resonates at a frequency of about 3 Hz. Furthermore, the proposed PEH adopts the strong compressive operation mode, and employs a truss mechanism to effectively amplify the impact forces. In experiments, open-circuit voltages are 58.4 V from PEH and 60 V from TENG under an excitation of 1.0 g at resonance. The hybridized generator is capable of achieving a maximum power of 19.6 mW from the two sources with matched impedances. The working bandwidths of the PEH and the TENG reach up to 5.39 Hz and 7.25 Hz, respectively, out of our targeted frequency domain [2.5 Hz, 10 Hz] due to the multiple resonances and nonlinearity. In the applications to charge capacitors, the high saturation voltage and relatively short charging time validate the effectiveness of our power management technique. Furthermore, the generator proves to be able to effectively scavenge energy from human body motions and charge a capacitor of 4.7 mu F to 7.6 V in around 50 s, which indicates a great potential of practical applications in wearable devices.