Offshore wind farm global blockage measured with scanning lidar

Abstract

Abstract. The objective of this paper was the experimental investigation of the accumulated induction effect of a large offshore wind farm as a whole, i.e. the global-blockage effect, in relation to atmospheric-stability estimates and wind farm operational states. We measured the inflow of a 400 MW offshore wind farm in the German North Sea with a scanning long-range Doppler wind lidar. A methodology to reduce the statistical variability of different lidar scans at comparable measurement conditions was introduced, and an extensive uncertainty assessment of the averaged wind fields was performed to be able to identify the global-blockage effect, which is small compared to e.g. wind turbine wake effects and ambient variations in the inflow. Our results showed a 4 % decrease in wind speed (accuracy range of 2 % to 6 %) at transition piece height (24.6 m) upwind of the wind farm with the turbines operating at high thrust coefficients above 0.8 in a stably stratified atmosphere, which we interpreted as global blockage. In contrast, at unstable stratification and similar operating conditions and for situations with low thrust coefficients (i.e. approx. 0 for not operating turbines and ≤ 0.3 for turbines operating far above rated wind speed) we identified no wind speed deficit. We discussed the significance of our measurements and possible sources of error in long-range scanning lidar campaigns and give recommendations on how to measure small flow effects like global blockage with scanning Doppler lidar. In conclusion, we provide strong evidence for the existence of global blockage in large offshore wind farms in stable stratification and the turbines operating at a high thrust coefficient by planar lidar wind field measurements. We further conclude that global blockage is dependent on atmospheric stratification.

A case study of wind farm effects using two wake parameterizations in the Weather Research and Forecasting (WRF) model (V3.7.1) in the presence of low-level jets

“…The wind farm wakes lead to a reduced wind speed up to the jet nose, a higher jet nose and a higher wind shear beneath the jet nose. This upward shift of the jet nose in the presence of a wind farm was also modeled in different LES studies (Sharma et al, 2017;Abkar et al, 2016).…”

Quantifying wind plant blockage under stable atmospheric conditions

“…On one hand, several simulations found wind decelerations larger than 10% at a distance of 2D upstream of the first row of turbines (Allaerts and Meyers, 2018;Wu and Porté-Agel, 2017). On the other hand, experimental results and most numerical simulations suggest wind speed slowdowns that are one order of magnitude smaller at a distance of 2D upstream of the first row of turbines (Bleeg et al, 2018;Wu and Porté-Agel, 2017;Segalini and Dahlberg, 2019;Schneemann et al, 2021). The large spread in the results primarily comes from simulations where the wind plant triggers gravity waves in the domain.…”

“…Numerical and experimental studies show the extent and magnitude of the induction zone can vary significantly depending on the size and layout of the wind plant, atmospheric conditions, wind turbine characteristics, and wind speed Meyers, 2017, 2018;Bleeg et al, 2018;Wu and Porté-Agel, 2017;Segalini and Dahlberg, 2019;Schneemann et al, 2021). Whereas some studies show slowdowns up to ∼30D upstream (Wu and Porté-Agel, 2017;Segalini and Dahlberg, 2019;Schneemann et al, 2021;Bleeg et al, 2018), others show slowdowns that extend up to ∼80D upstream of first row of the wind plant (Allaerts and Meyers, 2018;Wu and Porté-Agel, 2017). The spread in the wind speed deficit observed just upstream of the plant is even larger.…”

“…Schneemann et al (2021), on the other hand, were able to quantify the magnitude of upstream blockage in an operational offshore wind plant using scanning lidar measurements. Schneemann et al (2021) estimate a freestream velocity for each scan using the mean wind speed across the whole sampling area, which includes the induction zone region. Still, there is not an accepted methodology for defining the unperturbed conditions upstream of wind plants, and hence their blockage effect.…”

“…Just as wind speed and atmospheric stability affect turbine power production (Wharton and Lundquist, 2012;Vanderwende and Lundquist, 2012;St. Martin et al, 2016) and wake structure (Bodini et al, 2017;Rhodes and Lundquist, 2013), the induction zone of a wind plant is modified with atmospheric static stability (Allaerts and Meyers, 2018;Schneemann et al, 2021;Wu and Porté-Agel, 2017). Scanning lidar observations at a 400 MW offshore wind plant show upstream blockage occurring during stable conditions and winds between cut-in and rated speed, where the turbines are operating at high thrust coefficients (Schneemann et al, 2021).…”