Everyone’s familiar with the ‘shading effect’ that occurs on solar energy schemes. If a tree or high rise building casts a shadow on your photovoltaic cell’s glass face, that means your system isn’t generating as much energy as it could, because it’s not catching as much sunlight as it could.

The Horns Rev wake effect, in the North Sea. Picture courtesy renewableenergyworld.com and NOAA.

But did you know that wind turbine farms are hindered by a similar phenomenon? It’s known as a wake effect, and oftentimes, it’s caused by the wind turbines themselves. A now famous picture of the so termed ‘Horns Rev wake effect’ was taken off the coast of Europe in the North Sea of an offshore wind farm.

Imagine you could see the wind. It’s flowing along peacefully across the ocean. Then someone puts an obstacle in its way. And not just any obstacle, it’s a spinning vortex of metal. It tends to cause some turbulence. In the instance captured by this picture, the turbulent wind streams are flowing right into the path of the turbines behind the front line. This causes the same loss of efficiency as shading does in photovoltaics, because the wind turbines are no longer receiving a steady stream of wind.

The way scientists have tried to reduce this effect is through optimizing the layout of wind farms; that is, placing each turbine where it would be least affected by shadowing. Many times this means minimizing a wind turbine’s overall contact with turbulent flow by placing it far enough away from other turbines that the turbulence can’t be ‘felt.’ Sometimes, however, site managers can capitalize on the eddy created by a turbine by placing the subsequent turbine just at the end of the wake’s boundary layer. Think about drafting behind someone in the Tour de France. If you’re right behind the next biker, you get no drag. But lag behind just a little bit, the wake of air created by your fellow cyclist is way higher than it would be if you were biking by yourself! That’s the flow I’m talking about.

Small wind farm off the coast of Denmark, attempting to minimize wind turbine ‘shading.’ Image courtesy NREL.

The process for precisely determining what a turbine’s wake effect looks like is expensive and time-consuming, because it relies heavily on computer modelling of the complicated field of turbulent fluid dynamics. Most companies nevertheless find that the monetary savings brought in by an increase in efficiency are worth it. So next time you drive by one of those huge wind farms, see if the wake effect is visible!