Should wind turbines rotate in the opposite direction?

Abstract. Wind turbine blades rotate in clockwise direction seeing from an upstream position. This rotational direction impacts the wake in a stably stratified atmospheric boundary layer, in which the wind profile is characterised by a veering or a backing wind. Here, we challenge the arbitrary choice of the rotational direction of the blades by investigating the interaction of the rotational direction with veering and backing winds in both hemispheres by means of large-eddy simulations. Likewise we quantify the sensitivity of the wake to the strength of stratification, the strength and type of wind veer, and the wind speed in the Northern Hemisphere. A veering wind in combination with counterclockwise rotating blades would result in a power output increase of 11.5 % for a downwind turbine in comparison to a clockwise rotating upwind turbine in the Northern Hemisphere. In the Southern Hemisphere, the power output of a downwind turbine would decrease by the same value if the upwind turbine rotates counterclockwise. These wake differences result from the interaction of a veering or a backing wind with the rotational direction of the near wake. In the common case of a clockwise rotating rotor and a veering wind in the Northern Hemisphere, or similarly a backing wind in the Southern Hemisphere, the rotational direction differs in the far wake compared to the near wake. In contrast, if a counterclockwise rotating rotor interacts with a veering wind in the Northern Hemisphere or a backing wind in the Southern Hemisphere, the rotational direction of the near wake persists throughout the entire wake. Under veering wind conditions in the Northern Hemisphere, enhancing the thermal stability or increasing the strength of the veering wind further enlarges the power output difference up to 23 %. The positive impact on the potential power production can be explained by an intensified entrainment of the ambient air and the more rapid wake recovery under shared wind conditions and counterclockwise rotating blades.