The benefits of wind farms in terms of global climate change are well recognized but according to researchers at the University of Illinois they can also affect local climates as well. The researchers observed that the area immediately surrounding a wind farm is slightly warmer at night and slightly cooler during the day compared to the rest of the region. The discovery could allow for strategies to mitigate those effects in areas where they are undesirable, or take advantage of them in others.

The research was led by University of Illinois professor of atmospheric science Somnath Baidya Roy, who first proposed a model describing the local climate impact of wind farms in a 2004 paper. But until 2009 when he met Neil Kelley, a principal scientist at the National Wind Technology Center who had collected temperature data at a wind farm in San Gorgonio, California, for more than seven weeks in 1989, no field data on temperature were publicly available for researchers to use.

Turbulent discovery

Analysis of Kelley’s data corroborated Roy’s modeling studies and provided the first observation-based evidence of the daytime cooling and nocturnal warming effects of wind turbines. The cause of the temperature change identified by Roy’s models was the mixing of warm and cool air in the atmosphere in the wake of the turbine rotors. As the rotors turn they create turbulence, which pulls upper-level air down towards the surface, while surface air is pushed up, causing the warmer and cooler air to mix.

Roy says the question of whether nocturnal warming or daytime cooling will be the predominant effect depends on the wind farm’s location. In areas where the winds are typically stronger at night, such as the Great Plains region of the U.S., the nocturnal warming effect would be stronger, while regions where daytime winds are stronger will experience a stronger daytime cooling effect.

According to Roy, the nocturnal warming effect could be used in farm areas, such as the Midwestern U.S., to provide some measure of frost protection or even to slightly extend the growing season.

Dealing with the temperature change effect

The researchers also identified two possible strategies to mitigate wind farms’ impact on local climates. The first is to develop low-turbulence rotors that would result in less vertical mixing of the air and would also be more efficient for energy generation. We’ve covered a number of technologies that aim to do just that, such as flexible flaps attached to the trailing edges of a rotor blade or the insect-inspired turbines from Green Wavelength.

The second strategy simply involves locating the turbines in areas that already have a turbulent atmosphere so the consequence of turbulence from the rotors in minimal. Using global data, the researchers identified regions where the temperature effects of large wind farms are likely to be low because of natural mixing in the atmosphere.

“These regions include the Midwest and the Great Plains as well as large parts of Europe and China,” Roy said. “This was a very coarse-scale study, but it would be easy to do a local-scale study to compare possible locations.”

What's next?

Using data from and simulations of commercial rotors and turbines, Roy’s group will next generate models looking at both temperature and moisture transport. They also plan to study the extent of the thermodynamic effects, both in terms of local magnitude and of how far downwind the effects spread.

“The time is right for this kind of research so that, before we take a leap, we make sure it can be done right,” Roy said. “We want to identify the best way to sustain an explosive growth in wind energy over the long term. Wind energy is likely to be a part of the solution to the atmospheric carbon dioxide and the global warming problem. By identifying impacts and potential mitigation strategies, this study will contribute to the long-term sustainability of wind power.”

The team’s paper will appear in online early edition of the Proceedings of the National Academy of Sciences this week.