Modeling wind farms’ influence on weather

The amount of electricity generated worldwide from wind has been increasing by roughly an order of magnitude per decade; in 2012 wind power generated 520 terawatt-hours, according to the US Energy Information Administration. The fast growth is prompting researchers to study not just how airflows affect the extraction of wind energy by wind farms (see, for example, the Quick Study by John Dabiri, Physics Today, October 2014, page 66 ) but also how wind farms affect the atmosphere. That influence extends through the atmospheric boundary layer, a region of turbulent, well-mixed air that strongly couples to Earth’s surface and whose height ranges from tens of meters to a few kilometers. Understanding the interactions between wind farms and the boundary layer is important for modeling weather and other large-scale atmospheric processes. Flows around individual wind turbines can’t be spatially resolved in weather models, so they must instead be parameterized. One common approach is to treat wind farms as sinks of momentum and sources of turbulence at finite, realistic elevations. Mahdi Abkar and Fernando Porté-Agel of the École Polytechnique Fédérale de Lausanne now put that approach on an analytical footing that can take into account wind-farm densities, farm layouts, and wind direction. In particular, the researchers show the importance of various factors affecting the wind velocity inside wind farms. Incorporating those considerations into the parameterization produced good agreement with large-eddy simulations of the boundary layer for the vertical profiles of both the drag forces and the turbulent energy induced by wind farms in different configurations. (M. Abkar, F. Porté-Agel, J. Renewable Sustainable Energy 7, 013121, 2015, doi:10.1063/1.4907600 .)