Non-dimensional scaling of tidal stream turbines

The impact of local depth-wise velocity profiles on tidal turbine performance is important. Although the use of standard power laws for predicting velocity profiles is common, these laws may underestimate the magnitude of the depth-wise velocity shear and power attenuation. Predicting the performance of a tidal turbine in a high velocity shear is crucial in terms of power extraction. This paper discusses the dimensional scaling of a turbine using CFD and experimental data. Key performance characteristics (power, torque and thrust coefficients) were studies with increasing diameters and velocities, by generating. a series of non-dimensional curves. This provides a first order approximation for matching turbine performance characteristics to site conditions. The paper also shows that the use of a volume-averaged velocity derived from the upstream velocity profile can be used to determine these key performance characteristics. These are within 2% of those determined assuming a uniform flow. The paper also shows that even changes in the blade pitch angle results in new turbine characteristics under uniform velocity conditions and it is expected that these can be used for profiled flow.

► Scaling of tidal stream turbines, using non-dimensional groups, is valid for Reynolds numbers >3 × 105.
► The scaling criteria work for flows with velocity profiles provided the volumetric averaged velocity is used.
► Changing the blade pitch angle alters the turbine's characteristics.
► Turbine performance charts can be constructed for a given turbine geometry.