In a study published online late last month in the scientific journal Proceedings of the National Academy of Sciences, atmospheric science professors Nate Brunsell and David Mechem in KU's Department of Geography evaluated the effects of large wind farms on atmospheric flow and its implications for how much renewable energy the turbines can generate.

The researchers evaluated large-scale limits to wind power generation in a hypothetical scenario of a large wind farm in Kansas using two distinct methods, both of which yielded results that were “energetically” consistent.

“Wind turbines generate electricity by removing energy from the wind, so a larger number of wind turbines should result in a slowdown of the winds in the lower atmosphere," said Mechem, who, with Brunsell, led an international research group which undertook the study.

Wind turbines generate electricity by removing kinetic energy from the atmosphere, and the research shows that the limited replenishment of kinetic energy from aloft limits wind power generation rates at scales “sufficiently large that horizontal fluxes of kinetic energy can be ignored”

While they stressed that no current or planned wind farm approached the size or concentration that would cause the “slowdown effect”, their results indicated that the phenomenon tied to large wind farms would need to be taken into account in future planning of wind energy.

“When just a few wind turbines are installed, each additional turbine results in a similar increase in electricity generated, as you might expect,” Brunsell said.

However, the researchers said that when a substantial number of turbines are installed over a small area, the amount of electricity generated is no longer governed by simple multiplication.

“Instead, because the turbines extract energy from the wind, additional turbines will each generate less and less electricity,” Mechem said.

The team's simulations estimate this “slowdown effect” resulted in a practical upper limit of 1 megawatt per square kilometre that can be generated – far less than previous estimates not accounting for the effect.

While current wind farms are operating well below this generation limit, the researchers found that this slowdown effect needs to be accounted for, particularly when comparing different sources of renewable energy.