Wind-wave interaction effects on offshore wind energy

University dissertation from Department of Energy Sciences, Lund University

Abstract: This thesis is devoted to the investigation of the impacts of fast moving ocean surface waves on the aerodynamics of offshore wind turbines. The impacts of non-locally generated waves (swell) on the Marine Atmospheric BoundaryLayer (MABL) and thereby on offshore wind turbine aerodynamics are studied numerically by using Large Eddy Simulations and the Actuator Line Method.The MABL is often interacting with ocean surface waves causing mass, heat and momentum exchange between the air and the underlying waves’ surface. Due to this coupling between the MABL and the surface waves, the MABL differs from boundary layer over land. The effects of ocean waves on the MABL are believed to be small and usually taken into account as a roughness height when offshore wind farms are designed. This roughness height is commonlytreated either as a constant or as a function of the friction velocity without regard to its dependency on the sea state (i.e. the waves’ height, slope and velocity). However, recent field observations and numerical simulations have shown that the impact of the waves, in particularly swell, on the MABL might be stronger than previously assumed. Wave statistics show that the earth’s oceans are strongly dominated by swell waves almost all the time. Hence, abetter understanding of swell effects on the MABL would provide a valuable information that can lead to improve: the offshore wind turbine design, the layout of offshore wind farms and the accuracy of wind farm power extractionrate estimations.The results presented in this thesis show that the swell impacts on the MABL are significant. By comparing the MABL over moving waves to that over flat surface (calm sea), the effects of swell are isolated from the effects of atmospheric turbulence. The wave-induced stress reduces the total wind stress resulting in higher wind velocity, less wind shear and lower turbulence intensity level. These effects increase by increasing the wave age and/or wave steepness. These modifications in the MABL in the presence of fast moving swells propagating in the direction of the local wind invalidate the use of the Monin–Obukhov Similarity Theory widely used in wind energy applications and indicate that the extrapolation of a wind speed measured at a certain height to another height assuming a logarithmic wind speed profile is questionable in the presence of swell. Moreover, the results show that fast moving waves have pronounced effects on wind turbine aerodynamics. Longer wind turbine wake regions and weaker velocity deficits downstream a stand-alone wind turbine with higher power extraction rates are obtained in the presence of swell. More remarkably, higher overall power extraction rates are obtained from a 2 by 2 wind farm in the presence of swell for the same hub-height wind velocity.