As wind turbines extract energy from the wind (in order to generate electricity), the wind flow behind the turbine is strongly perturbed, reducing the available energy on the wind. The wind speed decreases and the turbulence increases. Imagine how a water stream changes when we place an object floating on it? The same thing happens to the wind, even though in this case it’s a lot less visible.
This change on the wind flow is what we called the wake effect, and it depends on a lot of factors.
The two most important factors are:
The interaction between the turbine and the wind. In simple words, depending on the way a turbine operates, the more interaction there is between the turbine and the wind, the higher the wake effects. This usually happens when the turbine is operating at maximum thrust, just before it starts feathering (or pitching out) its blades from the wind. As turbines reach their nominal power, they capture less energy from the wind, and therefore the wake effect is less significant.
The wind conditions upwind from the turbine (i.e. how turbulent is the flow before it reaches the wind turbine). If the wind flow is already very turbulent (due to forestry, complex terrain, or atmospheric instability, for example), the wake the turbine will generate is less important than in very stable and low turbulence conditions, like we see on offshore wind farms.
What are the consequences of wake effects?
As the wind flow is perturbed, the turbines located downwind from the first turbine will see lower wind speeds, thus reducing their AEP. This reduction in production is not the only effect: as the wind flow is more turbulent the downwind turbine will be suffering higher loads and more vibrations, which can eventually reduce the lifetime of your wind turbines. These effects will be a lot more important on bigger wind farms, where tens of turbines are grouped together and their different wake effects are combined between themselves.