Performance analysis of hydrofoil shaped and bi-directional diffusers for cross flow tidal turbines in single and double-rotor configurations

Abstract

With the aim of finding efficient solutions for cross flow turbine (CFT) bi-directional diffusers able to harvest non perfectly rectilinear tidal currents, a 2D CFD analysis of ducted CFTs was carried out with focus on the effects of diffuser shape and yaw angle. The HARVEST hydrofoil shaped diffuser, equipped with a pair of counter-rotating turbines, and a bi-directional symmetrical diffuser were compared in terms of coefficient of power (C P ), torque ripple, overall thrust on diffuser and wake characteristics. Slightly better C P were predicted for the symmetrical diffuser, due to the convergent walls that address the flow towards the blade with a greater attack angle during early and late upwind and to the viscous interactions between the turbine wakes and strong vortices shed by the diffuser. A C P ’s extraordinary improving resulted when yaw increased up to 22.5 ◦ for the hydrofoil shaped and up to 30 ◦ for the symmetrical diffuser. Similar behaviour in yawed flows also occurred in case of a ducted single rotor, demonstrating that it is a characteristic of CFTs. The insertion of a straight throat in the diffuser design proved to be an effective way to mitigate torque ripple, but a C P loss is expected.