“The United States currently produces about 7% of its electricity from wind energy,” said Sara Pryor, professor in the Department of Earth and Atmospheric Sciences. “This research shows that a quadrupling of the installed capacity of wind turbines from 2014 levels will allow us to attain the goal of 20% of electricity from the wind, without requiring additional land, or negative impacts on systemwide efficiency or local climates.”

Pryor worked with Rebecca Barthelmie, professor in the Sibley School of Mechanical and Aerospace Engineering, and postdoctoral researcher Tristan Shepherd to develop scenarios for how wind energy can expand from current levels to one-fifth of the entire U.S. electricity supply by 2030, as outlined by the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) in 2008.

Called the “20% Wind Scenario,” the NREL report noted that generating 20% of U.S. electricity from wind could eliminate approximately 825 million metric tons of carbon dioxide emissions in the electrical energy sector in 2030.

But, the researchers asked, does quadrupling the number of wind turbines reduce the efficiency of turbine fleets that gather energy? And can that generation negatively affect the local climate?

In high-density arrays of large wind turbines, the researchers examined possible declines in systemwide efficiency associated with a phenomenon called “wind turbine wakes,” where the wind speed is slowed by extraction of momentum by wind turbines. This wake is eroded by mixing with undisturbed air in the atmosphere, but can reduce the wind speed that impinges on downstream wind turbines.

“The ‘theft’ of wind by upstream wind turbines reduces the overall power produced by the total ensemble of wind turbines and the enhanced mixing (turbulence) can alter local climate conditions close to wind turbines,” said Barthelmie.

The researchers offered scenarios — such as repowering turbines with improved technology — for expanding the installed capacity of wind turbines without using additional land. The researchers demonstrated that expansion of the installed capacity has a tiny influence on systemwide efficiency and very small impacts on local climate that are reduced by deploying large, state-of-the-art turbines.