The impact of low-level jets on the power generated by offshore wind turbines

Low-level jets (LLJ) are local maxima in the vertical wind speed profile. They are frequently observed at heights of approximately 50 m to 500 m above sea level in offshore regions. The influence of low-level jets on the power production and loads of wind turbines has not been researched thoroughly. In this paper we investigate the influence of low-level jets on wind turbine performance in an offshore wind farm. We derive vertical wind profiles up to heights of 350 m from lidar plan position indicator scans with different elevation angles at the wind farm Nordergründe in the German Bight, located approximately 15 km from the coast. We detect LLJs with a frequency of occurrence between 2.4 % to 22.6 %, based on different definitions used in literature at the observed location. We analyse their influence on the power production of the turbines using operational wind farm data. We observe a negative influence on power production and increased power fluctuations in low-level jet situations compared to situations with equal wind-veer-corrected rotor equivalent wind speed (REWS) but without LLJs. Further, we conduct aeroelastic simulations for a set of wind profiles with varying veer, shear, turbulence intensity and shape of the LLJ core. Increasing veer and shear both have a negative impact on the simulated power production, while the shape of a low-level jet only slightly alters the energy conversion process at the wind turbine for the same REWS. Thus, we conclude the main driver for the efficiency-lowering effect during the presence of low-level jets to be the combination of positive and negative shear, causing a high absolute shear across the rotor area as well as increased absolute veer.