Aerodynamic characteristics of airfoils used in high tip-speed-ratio tidal rotors for less energetic flows

Is it economically feasible to extract energy from tidal flows with velocities under 2ms−1? Current research and commercial developments have focused on tidal flows above 2ms−1 in an attempt to extract as much energy as possible. However, high velocity flows come with a series of issues associated to the operation, reliability, maintenance and survivability of tidal turbines. All these issues contribute to higher energy costs than other commercial energy sources. Energy extraction from low velocity flows can potentially reduce the impact of said issues, resulting in more attractive energy costs. A proposed 4 m diameter full-scale rotor with blade profile NACA 63-8xx in low velocity waters is analysed in this study. Wind tunnel tests at the University of Strathclyde were conducted using the blade profile at 20% and 70% of the blade length. Flow velocities of 1 and 1.5 m/s were chosen as representative of the low-velocity waters around the Mexican Caribbean and the Philippines. Chord-based Reynolds numbers were matched from water to air. Angles of attack tested range from-30° to 30°. A comparison between the lift and drag coefficients between the experimental campaign, available literature and CFD simulations are presented. For the lift coefficients, the experiments have good agreement for all cases with the values found in the literature and CFD simulations. The experimental drag coefficients present an offset in the order of ten times the literature values and CFD simulations. It is believed to be associated to interactions between the wind and the foil supports.