An Experimental Study on a Wind Turbine Rotor Affected by Pitch Imbalance

An experimental and numerical investigation about the pitch imbalance effect on a wind turbine model is performed. The characterization of the power losses and loads generated on a small-scale model and the validation of an analytical framework for the performance of unbalanced rotors are proposed. Starting from the optimal collective pitch assessment (performed to identify the condition with the maximum power coefficient), the pitch of just one blade was systematically changed: it is seen that the presence of a pitch misalignment is associated with a degradation of the turbine performance, visible both from experiments and from Blade Element Momentum (BEM) calculations (modified to account for the load asymmetry). Up to 30% power losses and a 15% thrust increase are achievable when an imbalanced rotor operates at tip speed ratios around five, clearly highlighting the importance of avoiding this phenomenon when dealing with industrial applications. The numerical model predicts this result within 5% accuracy. Additional numerical simulations showed that, away from the optimal collective pitch, the blade imbalance can provide a power increase or a power decrease with respect to the balanced case, suggesting how an operator can maximise the production of an unbalanced rotor. An analysis of the axial and lateral forces showed a sensitivity of the loads’ standard deviation when imbalance is present. An increase of the lateral loads was observed in all unbalanced cases.