Clearly, wind energy is favored to its fossil fuel counterparts in terms of its environmental footprint. Zero greenhouse gas emissions. Zero global warming potential. Zero heat islands. Simply, wind energy seems to be a perfect part of the solution to a climate change problem.
At least, this is what we thought.
Until recently, when scientists discovered a surprising link between wind farms and rising land surface temperatures. As it turns out, wind farms may stir air in an atmospheric boundary layer a bit too much – enough to produce a noticeable warming effect after the sundown.
The study, which was published in Nature Climate Change in April, is one of the first ones to consider interactions between wind turbines and the atmospheric boundary layer near the land surface. A team of scientists from State University of New York at Albany and several other institutions presents a long-term observational evidence of the effects of wind farms on surface temperatures. Over the span of nine years, they carefully observed land surface temperatures around four wind farms in west-central Texas. Guided by the satellite data, the team found that wind farms are responsible for “a significant warming trend of up to 0.72°C per decade, particularly at night-time”.
“We attribute this warming primarily to wind farms”, the study confirms.
The researchers suggest that the observed warming effect is created by an enhanced vertical mixing behind the wind turbines. When wind turbines rotate, they create wakes behind them. The turbulent wakes can travel long distances downwind – and stir up air as they go. This is similar to wakes created by motorboats on a lake’s surface.
Yet, during the nighttime, the atmospheric boundary layer is very stable and thermally stratified. Because of the efficient radiative cooling of land surface, air lying closest to the land surface is coldest. From there up, its temperature increases. Therefore, warmer air will typically loom above cooler air. As wind turbines continue to spin during the dark hours, they bring warmer air close to the surface. This creates a warming effect, which is not observed in the absence of wind farms.
Conversely, during the daytime, the atmospheric boundary layer has warm air on the bottom and cool air on the top. As a result, the mixing could produce a cooling effect to some degree, as suggested in the paper. But instead, the results show only negligible effect in daylight temperatures.
Land Surface Temperatures
Satellites used in the study measure Earth’s surface temperatures, which are different from air temperatures we see in weather forecasts. As viewed from the sky, land surface temperatures depend on a number of variables, such as land cover, nature of the surface (for example, roof, road, or forest), reflectance, etc. Plus, land surface temperatures show greater day-to-night variability than air temperatures. The study acknowledges that it is, therefore, possible that the resultant warming effect on air might be slightly less than the calculated effect on the land.
Possible Atmospheric Interactions
Most likely, the role of a wind turbine goes beyond the conversion of kinetic energy into mechanical energy. If the turbine has an active role in vertical air mixing, it changes the moisture content within the atmosphere, too. The amount of moisture present in the atmosphere depends on temperature – warmer air can hold more water vapor than colder air. How this mass and moisture transfer during daytime and nighttime affects regional climate is not yet clear.
Sure, an increase by 0.72°C in a decade doesn’t seem a lot. Yet at the same time, wind power is rapidly expanding all over the map. For example, over the next decade, European Wind Energy Association predicts more than 200% increase in wind energy production in EU countries. Plus, many other countries are joining the race to the top of the global wind industry. With new wind turbines being added each day, the way they interact with environment might be something to think about.
Article by Adela Kuzmiakova