Wind energy harvesting inspired by Palm leaf flutter: Observation, mechanism and experiment

Abstract

Environmental adaptation of wind energy harvester (WEH) would be improved by mimicking the leaf flutter in Mother Nature. This study presents aeroelastic modelling and laboratory testing of Palm leaves to understand the mechanism of leaf flutter, furtherly designs an artificial leaf WEH and tests it in wind tunnel. Three modes of investigation are fulfilled: (1) Observation: Palm leaf flutter is observed in urban wind. The material properties of Palm leaves are measured by microscopic images or tensile tests. (2) Modelling: The aeroelastic model is established using finite element method and doublet-lattice aerodynamics to determine the flutter boundary of Palm leaves. The mechanism of mode coalescence between bending and twisting is found, and the model is further applied in designing the artificial leaf. (3) Experiments: Palm leaf oscillations under several flow speeds are measured with ultra-high-speed laser displacement sensor in wind tunnel. The critical speed and frequency of Palm leaf flutter are then obtained. Furthermore, the artificial leaf is mounted on the piezoelectric cantilever and evaluated in wind tunnel. At preliminary trial, the leaf harvester starts working at as low wind speed as 2 m/s, with stable frequency of 3.56 Hz. It reaches output power density of 1.238 µW/cm3 in our laboratory. The study highlights the bio-inspired design of wind energy harvester with low cut-in wind speed and stable output frequency.