Reconsidering downwind operation by analysis of the National Renewable Energy Laboratory’s Phase VI data

Data are presented comparing upwind versus downwind operation of the National Renewable Energy Laboratory’s Phase VI wind turbine. Power was not reduced as expected for downwind, which may be attributed to inboard three-dimensional effects. Average flap bending loads were reduced with downwind coning and compared well with prediction. Fatigue loads were increased with downwind; however, fatigue was mitigated with a tower shroud. The shroud needs to align with the freestream, demonstrated by an increase in fatigue loads from a 10° shroud misalignment. Pressure data were acquired in the tower wake at the rotor location with and without the shroud. The bare tower wake data compared well with previous work. The shroud wake data at 10° and 20° misalignment showed velocity reduction and turbulence exceeding the bare tower values. Downwind operation, with an aligning tower shroud, should be considered for future designs given the load benefits of downwind coning