A monostable piezoelectric energy harvester with a square-section bluff body and two bio-inspired pendulum balls for efficient low-speed wind energy harvesting

Efficiently harvesting wind energy under low wind speeds remains challenging due to the limited vibration amplitudes and minimal power output of conventional piezoelectric harvesters. To overcome this, a novel monostable vortex-induced vibration and galloping piezoelectric energy harvester (MVIVGPEH) is proposed. The design comprises a square-section bluff body, three strategically positioned magnets, and two foam pendulum balls suspended from a piezoelectric cantilever beam, inspired by the way spiders hang upside down using silk threads. This configuration facilitates the coexistence and interaction of vortex-induced vibration and galloping responses, significantly enhancing oscillation amplitudes and enabling multi-directional energy harvesting. By adjusting the magnet positions, the harvester achieves a monostable characteristic, eliminating the potential barrier limitation inherent in a bistable design. This monostability greatly enhances beam deflection and reduces the cut-in wind speed required to initiate flow-induced vibrations. Computational fluid dynamics analyses reveal that the pendulum balls effectively amplify aerodynamic excitation, enhancing wake interaction and shear-layer entrainment in the downstream region. Wind tunnel experiments validate that the MVIVGPEH exhibits superior performance compared to its bistable and linear counterparts, achieving a significantly lower cut-in wind speed of 0.8 m/s and higher voltage outputs at low wind speeds. Experimental results demonstrate that the MVIVGPEH consistently generates higher strain amplitudes and power outputs, confirming its effectiveness and reliability for ultra-low wind conditions. This innovative integration of monostable dynamics and bio-inspired aerodynamic structures represents a significant advancement for practical, low-speed wind energy harvesting applications.