Low-lying stratocumulus clouds can significantly improve the efficiency of offshore wind farms by accelerating the recovery of slow-moving air behind turbines, according to a new study from researchers at the University of Twente in the Netherlands. The findings, published in PRX Energy, suggest that cloud-related processes should be incorporated into wind farm models, particularly in regions like the North Sea where such clouds are common.
For decades, scientists have known that wind farms can be less efficient than isolated turbines because air slows as it passes through the blades, creating a wake that extends for tens of kilometres and can even stretch beyond 100 km. The speed at which this wake recovers depends on how effectively atmospheric mixing draws stronger air from above and from the sides. The new research shows that low-level stratocumulus clouds enhance this mixing through a process called radiative cooling. When air at the top of a cloud radiates heat into space, it becomes cooler and denser, sinking and generating turbulence that intensifies the mixing. This, in turn, helps sustain the cloud by pulling moisture up from the surface.
To model these effects, physicists Davide Selvatici and Richard Stevens used large-eddy simulations to study how low-lying clouds impact the length of a wind farm's wake. They performed high-resolution simulations of stratocumulus-topped boundary layers under three conditions: clear skies, cloudy skies without radiative cooling, and cloudy skies with radiative cooling. The fully three-dimensional simulations solved the Navier-Stokes equations, modified to include heat and moisture effects and the energy extracted by wind turbines.
The results were striking. When cloud-top cooling was present, the mean wind speed within the wake returned to approximately 90% of the incoming wind speed about 17 km beyond the end of the wind farm. Without cloud-top cooling, the wind speed remained below 90% even 32 km beyond the farm. Since wind power production increases with the cube of wind speed, even a modest reduction in wind speed can have a considerable impact on energy output.
«Clouds are inherently coupled to the atmosphere, making it difficult to isolate their effects, but by comparing simulations with and without cloud-top cooling, we identified cloud-top cooling as an important process for wind farm wake recovery,» Selvatici explained. Stevens added that most existing wind-farm models either simplify or exclude cloud-related processes entirely. «Our results demonstrate that these should be taken into account, particularly in regions with frequent low-level clouds as over the North Sea, where many wind farms are found and where the annual cloud coverage is higher than 30%,» he said.
A better understanding of these effects could improve wind farm design and inform decisions about how far apart individual wind farms should be placed. However, Selvatici cautioned that the team is not yet able to offer definite practical advice. «We cannot conclude whether wind farms should indeed be placed closer together in cloudy regions because our simulations represented idealized conditions,» he said. The researchers say further work is needed to translate the findings into real-world guidance for the offshore wind industry.
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