Improving Output Power of a Torsional-Flutter Harvester in Stochastic Thunderstorms by Duffing—Van Der Pol Restoring Torque1

Abstract

Wind energy harvesters are usually designed to operate in the low wind speed range. They rely on smaller swept areas, as a complement to larger horizontal-axis wind turbines. A torsional-flutter-based apparatus is investigated herein to extract wind energy. A nonlinear hybrid restoring toque mechanism, installed at equally spaced supports, is used to produce energy through limit-cycle vibration. Energy conversion and storage from the wind flow are enabled by eddy currents. The apparatus is used during thunderstorm outflows to explore its efficiency in nonideal wind conditions. The thunderstorm flow model accounts for both nonstationary turbulence and slowly varying mean wind speed, replicating thunderstorm's intensification and decay stages. This paper evolves from a recent study to examine stochastic stability. More specifically, the output power is derived as a random process that is found numerically. Various thunderstorm features and variable apparatus configurations are evaluated. Numerical investigations confirm the detrimental effect of nonideal, thunderstorms on harvester performance with, on average, an adverse increment of operational speed (about +30%). Besides nonlinear damping, the “benign” flutter-prone effect is controlled by the square value of the flapping angle. Since flapping amplitudes are moderate at sustained flutter, activation of the apparatus is delayed and exacerbated by the nonstationary outflow and aeroelastic load features. Finally, efficiency is carefully investigated by quantification of output power and “quality factor.”