Observing and Simulating Wind Turbine Wakes During the Evening Transition

Wind turbine wake evolution during the evening transition (ET) introduces variability to wind farm power production at a time of day typically characterized by high electricity demands. During the ET, the atmosphere evolves from an unstable condition to a stable regime, and vertical stratification of the wind profile begins to develop as the residual planetary boundary layer decouples from the surface layer. We examined the evolution of wind turbine wakes during the ET from two perspectives: wake observations from single turbines and simulations of multiple turbine wakes using the mesoscale Weather Research and Forecasting (WRF) model. Throughout the ET, the wake's wind speed deficit and turbulence enhancement became confined within the rotor layer when the atmospheric stability changed from unstable to stable. The height variations of maximum upstream-downstream differences of wind speed and turbulence intensity gradually decreased during the ET. After verifying the WRF-simulated upwind wind speed, wind direction and turbulent kinetic energy profiles with observations, we investigated the wind-farm-scale ET wake evolution using the WRF Wind Farm Parameterization (WFP). As the evening progressed, the modeled hub-height wind speed deficit monotonically increased. The figure below presents the one-hour average horizontal wind speed difference, subtracting wind speed of the control WRF run from wind speed of the 100-turbine WFP run, interpolated at hub height at a wind farm in central Iowa over 4 hours. The black vectors represent the one-hour average wind direction of the control run interpolated at hub height, and the vector lengths are proportional to the wind speed magnitude. The turbine locations are labelled as dots in cyan. Moreover, the downwind surface sensible heat flux reduction slowed down in the evening, while the magnitude of downstream turbulence generation fluctuated with time. During the ET, modeled power output declined due to the drop in wind speed when the atmospheric stability changed. Overall, wind turbine wakes respond to the evolving ET atmospheric boundary layer, thereby affecting total wind farm power production.