AERODYNAMIC LOAD CONTROL OF WIND TURBINES

The energy captured by a wind turbine is limited due to the variability of wind speed and direction during typical operating conditions, and also by the efficiency of the turbine (or power coefficient), which represents the amount of power that can be extracted from the available wind power. Utilizing passive and active flow control devices has the potential to improve the energy capture, especially in the pre-stall region. This can be achieved by changing the local aerodynamic characteristics on the aerofoil/blade to increase lift at low wind speeds, potentially allowing the turbine to capture more energy between the cut-in and rated wind speeds. This study explores the feasibility of using circulation control to improve the performance of horizontal axis wind turbines (HAWTs). Firstly, the baseline offshore NREL 5 MW HAWT model was analysed to validate the modelling approach and to determine the most effective region on the blade and its operating conditions, for flow control. Subsequently, a 2D parametric study was performed to generate a high-performance aerofoil with integrated circulation control. Parameters including the jet opening height, its chord-wise location and trailing edge curvature (based on an ellipse) were considered. Results from twenty-eight different combinations of these parameters highlight a robust design solution. This solution is then evaluated over a 5% span-wise region along the NREL 5 MW baseline wind turbine blade, for a range of steady wind speeds and nozzle pressure ratios (NPR). Results show that passive control using centrifugal pumping does not generate a sufficient mass flow rate to be aerodynamically effective. As an alternative, forced pumping using a compressor has the capacity to increase the shaft power at all wind speeds, of up to 11% and 18.4% for NPR ratios of 1.5 and 1.8, respectively. However, the estimated cost to provide this pressurised air exceeds the power gained, and further investigation is required to develop the overall concept.