Modelling the Impact of Structure Generated Shadowing on Tidal Stream Turbine Performance

Abstract Many tidal stream turbine developers are now opting for fixed yaw devices, in favour of design simplicity and reliability. For every other flood and ebb cycle, these turbines will operate downstream of their floating or fixed foundation, inducing cyclic loading fluctuations impacting power performance and fatigue life. This study investigates the severity and implications of the foundation shadow effect by applying Computational Fluid Dynamics (CFD) modelling techniques to simulate turbine performance in a downstream operating condition. The HydroWing multi-rotor device and the Tocardo 2-bladed Horizontal Axis Tidal Turbine (HATT) are used as case studies for this investigation, and the study is presented so that the results can be applied to all foundation and device types. An Unsteady RANS CFD simulation environment with a sliding mesh is validated against experimental datasets and a turbine in free-stream isolation is simulated as a benchmark case with the modular foundation sequentially introduced to analyse the structure impact. Key findings suggest that operating turbines downstream of the multi-rotor foundation could cause a 10% fluctuation in thrust loading and 20% in power resulting in a mean CP reduction of 5% during operation.