Low-level jets' influence on the power conversion efficiency of offshore wind turbines

Abstract

Low-level jets (LLJs) are local maxima in the vertical wind speed profile. They are frequently observed at heights of approximately 50–500 ma.s.l. in offshore regions. The influence of LLJs on the power conversion of the energy flux through the rotor-swept area and loads of wind turbines has not yet been thoroughly investigated. In this paper, we study the influence of LLJs on wind turbines in an offshore wind farm located approximately 15 km from the coast. We derive vertical wind profiles up to heights of 350 m from lidar plan position indicator scans with different elevation angles. We detect LLJs with occurrence rates of between 2.4 % and 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 the power conversion efficiency and increased power fluctuations in LLJ 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. Comparing situations with the same REWS revealed that increasing veer and shear both have a negative impact on the simulated power conversion efficiency, while the shape of an LLJ only slightly alters the energy conversion process. Thus, we conclude the main driver for the lower efficiency during the presence of LLJs to be the combination of positive and negative shear, causing a high absolute shear across the rotor area as well as increased absolute veer.