EGU 2012: Potential ecosystem effects of offshore wind farms

By Rachel Berkowitz

The European Geosciences Union (EGU) general assembly in Vienna dedicated several sessions to climate change and the need to reduce society's dependence on fossil fuels. Offshore wind turbines are one solution, but as Göran Broström of the Norwegian Meteorological Institute puts it, the oceanographic and atmospheric community needs to “sit down and have a serious discussion on what the influence may be before [large scale] wind farms are built.”

In 2008 Broström made a case that large wind farms significantly affect nearby wind speeds. A wind speed of 5–10 m/s may generate ocean upwelling and downwelling velocities of more than 1 m/day near the turbines. Upwelling draws nutrient-rich deep water toward the surface and changes the temperature structure and availability of nutrients in the vicinity of the wind farm. That may lead to changes in the local ecosystem.

“The biggest wind farms as of today are close to the coast so the impact on the ocean is difficult to see,” says Broström. And currently operating wind farms are small enough that ocean circulation effects might not be detectable.

A wind farm generates strong horizontal shear in the wind stress via atmospheric convection; that shear leads to surface divergence and convergence in the upper ocean. Thus circulation and an associated upwelling pattern are engendered by the wind farm.

Ekman transport explains these changes. Ekman currents are the net motion of fluid perpendicular to the wind stress due to balance between Coriolis force and drags generated by the wind and water. Variable winds mean that Ekman transports are not uniform and that they lead to convergence and divergence of surface currents. Water uplifted from below balances surface divergence at the turbines.

Modeling the dynamics

Ludewig used the Hamburg-Shelf-Ocean-Model, which simulates oceanic, coastal, and sea-shelf dynamics, in a sensitivity study on the effects of a proposed wind farm. According to her presentation at EGU, Germany has plans to harvest 8700 MW worth of wind energy in the North and Baltic seas by building the equivalent of 20 offshore wind farms with a total of 80 turbines.

“The width [of the farm] is more important than the density of the wind mills,” explains Broström. He suggests that upwelling and local current changes should be noticed at 3- to 4-km2 wind farms.

The Hamburg team showed that a small wind farm of 12 turbines with rotor diameter 80 m rapidly leads to upwelling and downwelling zones in North Sea conditions. The circulation change affects an area 160 times as big as the wind farm itself, with key results being a change of a few millimeters in sea level and a tilt of the thermocline.

“We're still checking the dynamics,” says Ludewig. The next simulations of the North Sea will include investigating local climate: “does it become warmer, or fresher, or what?” Those results will help to analyze ecosystem effects and reef development within the wind farms.

Permission to build wind farms is granted by national authorities, but proposed mechanisms for ocean circulation and ecosystem change, though based on solid theoretical ground, have not been verified by measurements. Ludewig believes that turbines will continue to be built, but hopes that “if we have results they'll think about how many . . . to build.”

Broström adds that “different wind farms may interact, giving rise to a combined influence that is greater than their own influence.” Oceanographers may provide valuable input for understanding the ecological effects, and fisheries will see changes.

While large-scale offshore wind farms have great potential for easing environmental impact by replacing fossil fuel use with renewable energy, we shouldn't underestimate their potential for other effects on the environment.