Should Wind Turbines Rotate in the Opposite Direction in Stable Stratification in the Northern Hemisphere?

Modern industrial wind turbines usually rotate in a clockwise fashion from the perspective of an observer located upwind and looking downwind at the turbine. For flow without significant vertical wind shear and no vertical wind veer in the height of the rotor, the direction of turbine rotation has a small impact (Vermeer et al., 2003; Shen et al., 2007; Sanderse, 2009; Kumar et al., 2013; Hu et al., 2013; Yuan et al., 2014; Mühle et al., 2017). A stably stratified regime, however, is characterized by vertical wind shear and vertical wind veer. Both reflect the change of the Coriolis force and friction with height, resulting in the corresponding hemispheric dependent Ekman spiral. In the Northern Hemisphere, winds will rotate clockwise with height, while in the Southern Hemisphere, winds will tend to rotate counterclockwise with height. This ambient background veer interacts with the rotational direction of a wind turbine to affect the behavior of the wind turbine wake and therefore the power production and loads of a downwind turbine.

Using large-eddy simulations, we show how the rotational direction of the near wake is determined by the rotational direction of the wind turbine, whereas the rotational direction of the far wake is determined by the Ekman spiral. If a Northern Hemisphere Ekman spiral interacts with clockwise rotating blades, the near wake’s counter-clockwise rotation will slow and become slightly clockwise in the far wake. If the same northern hemispheric Ekman spiral interacts with counterclockwise rotating blades, the near wake consequently rotates in a clockwise direction, which persists for the whole wake, as the rotational direction imposed by the stably stratified regime in the northern hemisphere results also in a clockwise flow rotation of the wake.

This influence of the rotational direction of a wind turbine on the wake with vertical wind veer suggests a preferential rotational direction of a wind turbine in a stably stratified atmospheric boundary layer on each hemisphere could exist, where "preferred" is define by impacts on a downwind turbine's inflow velocity and turbulence loads. For one of our inflow cases (with a geostrophic forcing of 10 m s^-1), a turbine located 7D downwind will experience a wind speed difference of 1.1 m s^-1 depending on if the upwind turbine rotates clockwise or counterclockwise (Figure 1). Given the shape of the wind turbine power curve used here, this difference implies a change of 32% of power production at that wind speed. Modifying wind turbines to rotate in opposite directions would of course imply considerable supply chain challenges, and so the frequency of occurrence of these veered conditions should be evaluated against the possible power gains.

Figure 1: Streamwise wind speeds u as a function of non-dimensional distance (x/D) downwind from a turbine in the northern hemisphere (top panel) or southern hemisphere (bottom panel). Red lines indicate counterclockwise rotation of the upwind turbine’s wake (clockwise rotation of the turbine); blue lines indicate clockwise rotation of the wake (counterclockwise rotation of the turbine). Solid lines are for the top tip of the rotor disk, dashed lines are the bottom tip, and dash-dot lines are averaged over the rotor disk.