Energy capture from ambient flows using piezoelectric flutter harvesters
Creators: name: McCarthy, Jesse
Publisher: name: RMIT University
Year: 2024

description: A wind-energy extraction method is examined, which exploits flutter of piezoelectric harvesters immersed in a wind flow. The harvesters nominally consisted of compliant piezoelectric patches coupled via a revolute hinge to a polymeric, triangular “leaf”, which might be incorporated en-masse into a “tree”. Several questions remained unanswered about the potential of such a device due to the complexities of the aero-structural-electrical physics coupling involved, aerodynamic proximity effects, and the lack of studies performed with these harvesters in turbulent (outdoor) flows.<br><br>A parametric study was conducted in smooth, parallel flow at representative wind speeds, to examine the effect of changing the leaf geometry on the output-power characteristics of a single harvester. Increasing the leaf area past a certain size caused chaotic flutter of the harvester and inconsistent power output, because of the combined effects of increasing system length and width, tending the system to instability. Changing the leaf aspect ratio did not affect the flutter regime of the harvester for the wind speed testing range, showing that leaf area distribution does not affect flutter regime in a significant way for average wind speeds. Larger aspect ratios caused delayed flutter cut-in due to the increased stability of the system through the lower width – demonstrating that width of the immersed harvester had a greater effect on the stability rather than length. Delayed flutter cut-in for larger aspect ratios was also due to the larger inertia of the leaf about the hinge, requiring greater start-up energy from the flow.<br><br>The performance of a single harvester was also evaluated over a range of different wind directions and flow speeds, and also in aspects of replicated ABL turbulence. It was found that the Atmospheric Boundary Layer (ABL) turbulence generally reduced harvester power output compared to smooth-flow conditions for parallel flow. Off-axis flow was generally detrimental to power output in smooth and turbulent flow, due to harvester flutter deviating from Limit-Cycle Oscillations (LCOs), but turbulent flow provided a marginal advantage over smooth flow due to the energy of fluctuations in the inertial subrange exciting higher-order modes of the harvester.<br><br>Aerodynamic proximity effects were also examined for two harvesters in tandem using dual-camera smoke-flow visualisation, synchronised with voltage data acquisition from the harvesters. This permitted observation of detached flow structures from the leading harvester, which acted to amplify the output power of the trailing harvester at certain tandem spacings. There were two salient flow structures identified: a horseshoe cone vortex that tilted above and beneath the smoke-generation plane, and a Leading-Edge Vortex (LEV) that was present on the suction side of the harvester flutter cycle. The horseshoe vortices increased the trailing harvester power output by elevating the PVDF patch tip velocity by inducing a pressure gradient towards the vortex core, increasing output voltage. The LEVs did not influence the trailing harvester in a significant way.<br>