Performance optimisation of horizontal axis wind turbine rotors retrofitted with an aerodynamic device

Abstract

Horizontal axis wind turbines (HAWTs) worldwide have a fixed operational performance profile determined by the initial design of the rotor. Over the years, design methods for HAWT rotors have advanced, allowing for customisation based on specific environmental conditions at the installation site. Typically, wind turbines have an average lifespan of 20 years before rotor replacement or decommissioning. Some post-installation methods can enhance a turbine's performance profile during its operational life. This research focused on improving rotor performance by placing an aerodynamic device (a ring vane) near the hub region and upstream of the rotor plane for post installation improvement. The ring vane consists of a ring with aerofoil profile supported by three streamlined struts spaced at 120 degrees and attached to the hub. Various ring vanes with different diameters relative to the rotor diameter were tested. The percentage of ring vane diameter to rotor outer diameter was used to describe the different ring vane sizes. Ring vanes ranged from 15% to 20% of the rotor diameter, with 1% increments. The performance of six retrofitted ring vane rotors was compared to a standard rotor without a ring vane, which served as the baseline. Performance data were obtained through computational fluid dynamics (CFD) simulations and physical testing using a vertical-travel test apparatus that drove the rotors at a constant relative velocity of 0.5 m/s into a body of stationary water. The rotors had a diameter of 280 mm for both physical testing and simulations and used water as the working fluid. Additionally, a case study using simulations was conducted to evaluate the performance of the best-performing ring vane and the standard rotor at a higher relative free stream velocity of 1 m/s. Simulation results at a relative velocity of 0.5 m/s indicated that the 18% ring vane rotor had the best performance improvement, with a peak increase of 2.3% compared to the baseline rotor at 5.3 tip speed ratio. The 18% ring vane rotor outperformed the standard with 4.04% and 3.2% at off design tip speed ratios of 4.7 and 5.9 respectively. Post-simulation processing revealed that the 18% ring vane rotor exhibited enhanced axial and angular induction factors across the blade, indicating aerodynamic benefits. Physical testing at the same relative velocity confirmed that the 18% ring vane rotor had the best performance profile, improving across a range of tip speed ratios from 4.4 to 5, with a peak increase of 4.35% at respective peak tip speed ratio. The 18% ring vane rotor outperformed the standard with 0.24% and 25% at off design tip seed ratios of 4.4 and 5 respectively. The case study at a higher relative velocity of 1 m/s, beyond the capabilities of physical testing. showed that the 18% ring vane rotor continued to improve performance across the entire operational speed range, although with a lower peak increase of 0.8%. The 18% ring vane rotor outperformed the standard with 1.34% and 1.74% at off design tip speed ratios of 4.4 and 6 respectively. The axial and angular induction improvements at this higher velocity confirmed the ring vane's positive aerodynamic impact. Overall, this research demonstrates that retrofitting HAWTs with a ring vane can significantly enhance their operational performance, providing a viable method for improving the efficiency of existing wind turbines.