On the feasibility of a flexible foil with passive heave to extract energy from low wind speeds

Abstract

We explore numerically and theoretically the capability of flexible foils elastically mounted to translational springs and dampers at the leading edge to extract energy from low-speed winds through its passive heave motion. Given the spring and foil stiffnesses, for each damper constant the theory (which is valid for high Reynolds numbers and small foil deflection amplitudes, i.e., in absence of separation) provides analytically a minimum wind velocity for flutter instability, above which energy can be harvested, that depends on the thickness-to-chord-length ratio of the foil. Simple analytical expressions for the flutter frequency are also provided. Minimum wind speeds and corresponding flutter frequencies are characterized for a carbon fiber foil as the spring stiffness and damper constant are varied, finding that energy can be extracted from wind speeds lower than in conventional wind turbines. These theoretical predictions are assessed from full numerical simulations at Reynolds numbers corresponding to these wind velocities and for chord lengths of the order of the meter (i.e. about 106) using appropriate turbulence models, which allow to compute the power extracted from the wind that the flutter stability analysis cannot provide.