The Role of Atmospheric Stability on the Induction Zone of a Wind Farm

Abstract:

Wind turbines decelerate the wind upwind as a result of a perturbation in the flow caused by their rotor. This region of slower velocities is called the induction zone, and it can extend up to 2D upstream for a single turbine. Similarly, the presence of an array of wind turbines reduces the inflow wind speed (blockage effect) altering the wind farm energy production. Several studies have shown the existence of an induction zone upstream of large wind turbine clusters using Large-Eddy Simulations (LES), Reynolds-Averaged Navier-Stokes (RANS) simulations, and wind tunnel experiments. LES of a neutral atmospheric boundary layer suggest the spatial extent of the induction region and magnitude of the velocity deficit increase with stronger free-atmosphere stratification (Wu and Porté-Agel 2017). Further, this study also revealed free atmosphere stratification can trigger gravity waves and enhance the wind farm’s blockage effect. Results from RANS simulations (Bleeg et al. 2018) concluded that neglecting the induction zone effect can result in substantial long-term energy prediction bias, undermining the wind resource assessment for a wind farm.

Here, we demonstrate the contrast between the magnitude of upwind blockage as a function of atmospheric stability. Using idealized WRF-LES simulations of a wind farm in flat terrain, we simulate three stability cases (stable, neutral and unstable) of atmospheric boundary layers in a range of wind speed conditions to explore these effects. The turbines are represented using a generalized actuator disk (GAD) which has been previously validated in comparison to scanning lidar measurements of wind turbine wakes. These results may be used to improve energy prediction and reduce bias in operating wind farms.

Bleeg, J., M. Purcell, R. Ruisi, and E. Traiger, 2018: Wind Farm Blockage and the Consequences of Neglecting Its Impact on Energy Production. Energies.

Mirocha, J. D., B. Kosovic, M. L. Aitken, and J. K. Lundquist, 2014: Implementation of a generalized actuator disk wind turbine model into the weather research and forecasting model for large-eddy simulation applications. Journal of Renewable and Sustainable Energy.

Wu, K., and F. Porté-Agel, 2017: Flow Adjustment Inside and Around Large Finite-Size Wind Farms. Energies.