Profiling wind LiDAR measurements to quantify blockage for onshore wind turbines

Flow modifications induced by wind turbine rotors on the incoming atmospheric boundary layer (ABL), such as blockage and speedups, can be important factors affecting the power performance and annual energy production (AEP) of a wind farm. Further, these rotor-induced effects on the incoming ABL can vary significantly with the characteristics of the incoming wind, such as wind shear, veer, and turbulence intensity, and turbine operative conditions. To better characterize the complex flow physics underpinning the interaction between turbine rotors and the ABL, a field campaign was performed by deploying profiling wind LiDARs both before and after the construction of an onshore wind turbine array. Considering that the magnitude of these rotor-induced flow modifications represents a small percentage of the incoming wind speed (approximate to 3%$$ \approx 3\% $$), high accuracy needs to be achieved for the analysis of the experimental data and generation of flow predictions. Further, flow distortions induced by the site topography and effects of the local climatology need to be quantified and differentiated from those induced by wind turbine rotors. To this aim, a suite of statistical and machine learning models, such as k-means cluster analysis coupled with random forest predictions, are used to quantify and predict flow modifications for different wind and atmospheric conditions. The experimental results show that wind velocity reductions of up to 3% can be observed at an upstream distance of 1.5 rotor diameter from the leading wind turbine rotor, with more significant effects occurring for larger positive wind shear. For more complex wind conditions, such as negative shear and low-level jet, the rotor induction becomes highly complex entailing either velocity reductions (down to 9%) below hub height and velocity increases (up to 3%) above hub height. The effects of the rotor induction on the incoming wind velocity field seem to be already roughly negligible at an upstream distance of three rotor diameters. The results from this field experiment will inform models to simulate wind-turbine and wind-farm operations with improved accuracy for flow predictions in the proximity of the rotor area, which will be instrumental for more accurate quantification of wind farm blockage and relative effects on AEP.

Automated summary

Flow modifications induced by wind turbine rotors on the incoming atmospheric boundary layer (ABL) can significantly affect the power performance and annual energy production (AEP) of a wind farm. A field campaign was conducted to better understand the complex flow physics between turbine rotors and the ABL. Statistical and machine learning models were used to quantify and predict flow modifications for different wind and atmospheric conditions. The experimental results showed that rotor-induced effects on wind velocity were significant and varied with different wind and atmospheric conditions, but became negligible at a certain distance from the rotor area.