Widening the bandwidth of vibration energy harvesters using a liquid-based non-uniform load distribution

Vibration energy harvesters typically have a narrow bandwidth, which allows them to generate high amounts of power, but only at a specific frequency. This paper presents a novel method of increasing the bandwidth of a cantilever beam by creating a non-uniform load. The concept uses a liquid filled mass, which causes the structures overall centre of gravity to shift as the beam bends. The overall centre of gravity shifts due to the mass change caused by the dynamic behaviour of the fluid. This paper validates the concept both numerically and experimentally by using a custom manufactured fluid filled mass on a piezoelectric cantilever. A water filled mass demonstrated a 2.8x increase in bandwidth for low acceleration (<1 g) and low frequency cantilever (27 Hz) devices. The effects due to liquid density and liquid viscosity are also experimentally measured. The numerical estimations match well with the experimental results for low accelerations (<0.5 g). Above 0.5 g acceleration the liquid water used in the cavity became chaotic, which caused liquid droplets to separate from the bulk liquid, thus reducing the overall mass for given point in time. This non-linear liquid dynamic behaviour further increases the bandwidth by creating a larger variation in the resonant frequency. In addition, as the fluid viscosity increases, the acceleration from the vibration source required to cause movement in the fluid also increases. The measured open circuit peak to peak voltage demonstrated an increase in bandwidth without significant loss in voltage.