Green energy solutions like wind, solar, and hydro have occasionally turned out to be not as green as we'd like. Hydroelectric dams lower fish populations and raise water temperatures. Some types of solar panels require hazardous materials, and utility scale solar requires large parcels of land. Wind turbines trounce birds, are considered an eyesore by some, and drone ceaselessly. Now, research published in PNAS indicates that wind farms can also impact the local air temperature in a significant manner, and this must be taken into account to avoid unintended consequences.

Wind farms change the local weather by changing the air flow patterns around them. The atmosphere just above the ground has a special zone associated with it known as the atmospheric boundary layer, which is generally a few hundred meters or less in height. The air close to the ground can have different humidity, heat, and aerosol content from the air just above the boundary layer, which is why the top of a very tall building may be very cold or windy compared to the ground floor.

The temperature that defines the weather in a region is the surface air temperature below the atmospheric boundary layer. In this paper, the authors used data from wind farms in San Gorgonio, California to show that near-surface air temperatures downwind of the wind farm are higher than the upwind regions during night, while they’re cooler during the day.

Many wind farms are built over agriculture land, so the turbines can actually have beneficial effects, such as the nocturnal warming of ambient air to protect crops from frosts. But as wind farms become larger, the complete environmental cost and possible unintended consequences of these temperature shifts must be considered to ensure the sustainable use of the wind.

In order to build on the available data from the San Gorgonio site and try various alternative wind farm designs, the authors ran simulations. Their working hypothesis is that turbulence generated in the wake of wind turbine rotors mixes air from above and below the atmospheric ground zone; during the night a (typically warm) layer of air from above is mixed down to the surface of the earth, raising temperatures; in the day, the opposite situation occurs.

The authors see two obvious ways to avoid altering the ambient temperatures downwind of wind farms. They performed a “master of the obvious” simulation to show that the increased turbulence caused by the windmill rotors created larger temperatures changes. Choosing to place windmills in geographic regions where the atmosphere is in a state of continuous turbulence anyway ensures that there is no near ground surface layer for a wind farm to mix.

The authors got clever when they took into account the fact that turbulence generated by upstream wind turbines can affect the wind energy harvested by downwind turbines. They found that rotors designed to generate just the right amount of turbulence in their wakes would simultaneously maximize the productivity of the wind farm and minimize temperate variations.

This is a win-win situation because improved placement of wind turbine rotors can simultaneously reduce impacts on surface air temperatures and improve wind farm productivity. If we truly wanted to achieve zero environmental impact with wind turbines we would have to place them in a vacuum, but we should settle for a region without bird migratory routes and with high background atmospheric turbulence.

PNAS, 2010. DOI: 10.1073/pnas.1000493107 (About DOIs).

Listing image by NASA