Numerical and experimental study on energy-harvesting piezoelectric flags

Placed in a fluid flow, a cantilevered flexible plate flaps spontaneously above a critical flow velocity. The resulting self-sustained vibrations of such a flag may be used to produce electrical energy and power an output circuit using piezoelectric patches covering the flag that deforms with the flapping motion. Previous work showed only moderate harvesting efficiency with a resistive output circuit, but proposed numerous directions for improvement. We propose a numerical and experimental investigation of the coupled dynamics of such a fluid-solid-electric system, and analyze the influence of the output circuit on the dynamics and harvesting efficiency. In particular, inductive-resistive circuits are considered. Our numerical results show that such resonant circuits lead to a destabilization of the system and a spontaneous flapping at lower fluid velocities. Also they significantly increase the energy harvesting efficiency of the piezoelectric flags as a result of a frequency lock-in between the flag and the electrical circuit. Wind tunnel tests are performed with prototypes of piezoelectric flags and basic resonant circuits. We show that such circuits effectively enhance the energy harvesting performance when they are in resonance with the piezoelectric flag. Meanwhile, the internal resistance of the inductive elements also shows an important influence on the harvested electrical power. In both numerical and experimental studies, the importance of piezoelectric coupling strength is observed. Our results show that promising efficiency enhancements of such flow energy harvesters would be achieved through the optimization of the output circuit as well as development of new piezoelectric materials.