Flow misalignment and tidal stream turbines

Extensive Research and Development (R&D) within the tidal energy industry is pushing this sector towards commercial viability, with full scale prototypes starting to meet the challenges of the marine environment. This paper combines velocity data collected from Ramsey Sound (Pembrokeshire, Wales), with Computational Fluid Dynamics (CFD) to assess the impact of non-rectilinear flows on turbine rotor performance. This requires both the geometry of the turbine and the surrounding free stream velocity to be studied. From the site data, the majority of the velocities tend to fall within a ±20° misalignment to the principle flow direction for velocities greater than the economic viable threshold of 2 ms -1 . From the CFD it was found that the non-dimensional performance parameters reduced with increasing angles of misalignment between the axis of rotation and free stream velocity. The resultant magnitude of the bending moments about the head of the driveshaft for the misaligned turbines were found to be up to nine times greater, than the aligned turbine. The paper shows that the tolerance to axial flow misalignment between the free stream velocity and axis of rotation of a turbine requires defining, in order to avoid the detrimental effects it has on performance and