  This work studies a broadband graded metamaterial, which integrates the
piezoelectric energy harvesting function targeting low-frequency structural
vibrations, lying below 100 Hz. The device combines a graded metamaterial with
beam-like resonators, piezoelectric patches and a self-powered piezoelectric
interface circuit for energy harvesting. Based on the mechanical and electrical
lumped parameters, an integrated model is proposed to investigate the power
performance of the proposed design. Thorough numerical simulations were
conducted to analyse the spatial frequency separation capacity and the
slow-wave phenomenon of the graded metamaterial for broadband and
high-capability piezoelectric energy harvesting. Experiments with realistic
vibration sources show that the harvested power of the proposed design yields a
five-fold increase with respect to conventional harvesting solutions based on
single cantilever harvesters. Our results reveal the significant potential on
exploitation of graded metamaterials for energy-efficient vibration-powered
devices.
