Researchers at the University of Kansas are looking at how wind turbines affect the wind when a larger number of turbines remove the energy of atmospheric motion.

The Smoky Hills Wind Farm as seen from Interstate 70 in Kansas. Source: KU, Credit: Drenaline via WikiCommonsThe Smoky Hills Wind Farm as seen from Interstate 70 in Kansas. Source: KU, Credit: Drenaline via WikiCommonsThe research group evaluated the effects of large wind farms on atmospheric flow and its implications for how much renewable energy the turbines can generate.

"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," says atmospheric science professor David Mechem of the university’s Department of Geography. The researchers quantified this phenomenon in numerical simulations by applying a model normally used for weather forecasting to one of the windiest regions of the United States.

The team found that a slowdown effect triggered by wind turbines is substantial for large wind farms and results in proportionally less renewable energy generated for each turbine versus the energy that would be generated from an isolated wind turbine.

While the researchers say that no current or planned wind farm approaches the size or concentration that would cause the slowdown, their results suggest the phenomenon tied to large wind farms needs to be accounted for 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," says Nate Brunsell, co-author of the study and a colleague.

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

"Instead, because the turbines extract energy from the wind, additional turbines will each generate less and less electricity," Mechem says.

The team's simulations estimate this slowdown effect results in a practical upper limit of 1 megawatt per square kilometer that can be generated—less than previous estimates not accounting for the effect. Current wind farms operate below this generation limit, but the researchers say that this slowdown effect needs to be accounted for, particularly when comparing different sources of renewable energy. Their research was published in late August in the Proceedings of the National Academy of Sciences.

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