Effects of wind farm on planetary boundary layer development: A WRF-LES study from stable to unstable stratification ranges

With increasing implementation of renewable energy, planetary boundary layer (PBL) processes, including the effects of wind farm (WF), are gaining great attention. This research focuses on the dynamic interaction between WF and the PBL using the Weather Research and Forecasting Model based on a large-eddy simulation (WRF-LES), incorporating WF representation via the actuator disc model. A range of stability conditions, from unstable to stable stratification, are examined along with various wind turbine arrangements.
The results show a significant effect of WF on increasing turbulent kinetic energy (TKE) above the hub-height level under stably stratified to near-neutrally stratified conditions. In contrast, the effect is not profound in the case of unstable stratification.
Three key WF impacts on TKE production can be identified: the effect on mean momentum equation, effect on TKE production due to the WF thrust term, and correction of the length scale. TKE budget analysis reveals that the direct effect of the WF thrust term to TKE production or dissipation is minimal. This implies that the WF effect on TKE production largely depends on the mean-wind profile modulation, increasing shear-production of the TKE. In the case of stably stratified and near-neutrally stratified conditions, the local wind deficits around WT hub height is large. In contrast, the wind speed is entirely decelerated over the boundary layer in case of unstable stratification. This explains why the WF effect is reduced under unstable stratification, where buoyancy-induced mixing dominates over mechanical shear. The length-scale characteristics of the PBL scheme are discussed in terms of how WF modify it.