Wind turbines operate in the atmospheric boundary layer and the wind conditions experience a diurnal cycle. The sun heats the ground during daytime, triggering turbulence. This results in a well-mixed layer. Considering a wind turbine, its rotor blades are exposed to the same wind speed and wind direction at each possible blade position. As the ground cools at night, the atmospheric boundary layer becomes stable. The interactions between the Coriolis force and friction at the surface (vegetation, buildings) will cause the wind to rotate and increase with height. In the Northern Hemisphere air masses rotate in clockwise direction with height, while in the Southern Hemisphere they rotate counter-clockwise. At night, the blades of the rotor do not interact with a uniform flow in all heights. Speed and direction of the wind change with height, called a veering wind.
While traditional wind mills turned counter-clockwise (a manufacturing result due to right-handed millers), all modern multi-megawatt wind turbines rotate clockwise when looking downwind at the blades. This has just been the result of market happenstance.
If the atmospheric boundary layer interacts with a rotating turbine, the rotational direction does not matter for a solitary wind turbine or during daytime. However, it does matter for a downwind turbine at night, resulting in a hemispheric dependent situation.
In general, the flow moves the blades of a wind turbine in one direction and is deflected by them in the opposite direction. A common clockwise rotating wind turbine therefore leads to a counter-clockwise rotating flow field directly behind the rotor. At night in the Northern Hemisphere (Fig. 1a), this counter-clockwise rotating flow field conflicts with a veering wind turning in clockwise direction with height, resulting in a deceleration of the spanwise flow up to an inversion of the rotational direction of the flow in the wake.
Considering instead a counter-clockwise rotating turbine at night in the Northern Hemisphere, the deflected flow rotates clockwise. This matches the clockwise rotating atmospheric boundary layer flow field, resulting in the same rotational direction in the whole wake with an amplification of the rotational component of the flow in comparison to the no-veer case (Fig. 1d) (Englberger et al., 2020a).
We investigated some frequently occurring conditions, using large eddy simulations. Figure 1 shows a case, where the wake recovers more rapidly for counter-clockwise rotating blades (Fig. 1c) than for a clockwise rotating rotor (Fig. 1b). This affects the available power in the wind interacting with a downwind turbine (Englberger et al., 2020b).
Englberger, A., Dörnbrack, A., & Lundquist, J. K. (2020a). Does the rotational direction of a wind turbine impact the wake in a stably stratified atmospheric boundary layer? Wind Energy Science, 5(4), 1359-1374.
Englberger, A., Lundquist, J. K. & Dörnbrack, A. (2020b). Changing the rotational direction of a wind turbine under veering inflow: a parameter study. Wind Energy Science, 5(4), 1623-1644.