Evaluating global blockage engineering parametrizations with LES
Under the term global blockage, the cumulative induction of wind turbines in a wind farm has been recently suspected to be responsible for observed overestimations of the energy yield in large-size wind farms. In this paper, the practice of modeling this effect after linear superposition of single turbine inductions, calculated with three of the most recent analytic models, is compared to Large-Eddy-Simulations of idealized wind farms. We compare the models across two different farms, composed of 9 and 49 turbines, with two different heights of the atmospheric boundary layer, 300 and 500 m. The results show that the differences between the analytical models are negligible while they substantially differ from the LES results. The linear superposition of induction consistently underestimates the upstream velocity deficit with an error that increases as the wind farm size grows and the ABL height decreases. Also when calculating the power output at the turbines of the farm, the analytical models considered do not agree well with the LES. These comparisons reveal that the farm interactions with the atmospheric boundary layer may highly outclass the turbine induction in determining the extent of the global blockage effect. Therefore, we present a first dimensional approach to the problem based on LES, aimed at simplifying its characterization.