Swell impacts on an offshore wind farm in stable boundary layer: wake flow and energy budget analysis

Abstract

We investigate the impact of swell on wind turbine wakes and the power output of offshore wind farms under stable atmospheric conditions. We conduct large-eddy simulations, and the influence of swell is modeled by incorporating a wave-induced stress term in the momentum equation. We perform kinetic energy budget analysis, including all relevant source and sink terms. Two typical scenarios in the North Sea area with modest wind speeds and wind-following and wind-opposing fast waves are considered. The results show that swells significantly affect the wind speed profiles and turbulence intensity across the entire operational height of the wind turbines. The impact of the swell predominantly affects the inflow, while its significance progressively diminishes downstream. From kinetic energy budget analysis, we discover that the wave effects are primarily exerted through indirect modification of the advection of energy in the streamwise and vertical dimensions instead of the direct wave-induced energy input/output. The wind shift and yawing adjustment caused by waves play a crucial role in the energy harvesting rate, depending on the specific inflow direction and wind farm layout. The relative changes in total power production reach up to 20.0 %/-27.3% for the wind-following and wind-opposing wave scenarios, respectively.