Flow-energy harvesting using a fully passive flapping foil : a guideline on design and operation

Abstract

Following our previous work [Int. J. Mech. Sci. (2020), vol. 177, 105587], we searched the best power extraction performance of a novel flow-energy harvester, which utilizes a fully passive flapping foil to extract energy from air/water flows. A series of water-tunnel experiments were conducted on the same test model at the Reynolds number around 105. Through investigating the effects of two unexplored key parameters, a higher overall maximum power conversion efficiency of 42.7% was obtained at water speed 0.71 m/s and foil pitching amplitude 60∘, corresponding to a larger mean power output of about 1.51 W. A quasi-steady theoretical model was also developed to fast predict the system performance in a larger parameter space. It was found that, in addition to typical dynamics, i.e., a flapping cycle includes two pure-heaving phases and two stroke-reversal phases, the foil system can also continuously operate in a “no-pure-heaving” zone where a flapping cycle only includes two successive stroke-reversal phases. Through these experimental and theoretical studies, a useful guideline was proposed on the design and operation of the foil system.