PNNL

Abstract

Asynchronous dual-Doppler lidar scanning techniques were used during the American WAKE ExperimeNt (AWAKEN) to provide height-resolved measurements of wind speed and direction at multiple locations immediately south of lthe leading row turbines in the King Plains wind farm in Oklahoma. These so-called virtual measurements were performed to characterize the inflow into the wind farm and to assess possible upwind blockage effects due to the collective action of the wind farm. The dual-Doppler lidar campaign was conducted from 12 November 2022 to 17 October 2023. During that time 14 unique virtual tower locations were sampled with heights ranging from 240 to 490 m AGL. The wind retrieval algorithm provided estimates of the horizontal winds and their uncertainties with a vertical resolution of about 10m, while also accounting for the tilt of the lidar platform. The virtual tower results are compared to collocated surface station and lidar wind profiling data at the A1 site, which was located roughly 2.4 rotor diameters south of the nearest turbine, and approximately 2 km from each of the two scanning lidars. Comparison of the line-of-sight velocities with a surface meterological station at A1 showed good agreement with a median difference of 11 cm s-1, and a standard deviation of 1.4 m s-1. By contrast, comparisons between the virtual tower and ground-based lidar profilers at A1 showed a significant height-dependence in the wind speed and wind direction differences. Above approximately 240 mAGL the agreement is very good with a median wind speed difference of xx. Below that level the differences increase with decreasing height and exhibit a signicant dependence on wind direction. The largest differences were observed for flow from the south-south-east (SSE), and for waked flow from the north-north-west (NNW). For waked flow the profiler wind speeds were observed to be about 8% lower than the virtual tower wind speeds near hub height. While for flow from the SSE the profiler wind speeds were observed to be ~15% lower than the virtual tower winds near hub height. The large differences observed for SSE flow suggests that the profiler winds are biased due to significant and persistent flow inhomogeneities that are created upwind of the of the King Plains wind farm.