Should wind turbines rotate in the opposite direction? A parameter study, changing the rotational direction of a wind turbine under veering inflow

Abstract:

All current-day wind turbine blades rotate in clockwise direction as seen from an upstream perspective. The choice of the rotational direction impacts the wake if the wind profile changes direction with height. Here, we investigate the respective wakes for veering and backing winds in both hemispheres by means of large-eddy simulations. We quantify the sensitivity of the wake to the strength of wind veer, the wind speed, and the rotational frequency of the rotor in the Northern Hemisphere. A veering wind in combination with counterclockwise rotating blades results in a larger streamwise velocity output, a larger spanwise wake width, and a larger wake deflection angle at the same downwind distance in comparison to a clockwise rotating turbine in the Northern Hemisphere (see Figure comparing the streamwise velocity at 7 rotor diameters downwind for a clockwise rotating (a) and counterclockwise rotating (b) turbines). When wind veer is not present (Figure panels (c) and (d)), the wake characteristics are less sensitive to turbine rotational direction. In the Southern Hemisphere, the same wake characteristics occur if the turbine rotates counterclockwise. These downwind differences in the wake result from the amplification or weakening/reversion of the span wise wind component due to the effect of the superimposed vortex of the rotor rotation on the inflow's shear. These effects, on the spanwise wake width and the wake deflection angle between clockwise and counterclockwise rotating actuators, respond to an increase of the directional shear or the rotational frequency of the rotor under veering wind conditions, whereas the wind speed exerts only a subtle impact.