Abstract
A comparison of a tidal turbine's performance and structural loads is conducted using lab-scale numerical models and experimental testing under multiple current-only and wave-current conditions at the IFREMER wave-current flume. Experimental testing, used to validate CFD models, was accomplished using a 0.9 m diameter, 3-bladed tidal turbine and had a blockage ratio of 8% while the turbine was submerged. Initial investigations analysed the performance and loads on the turbine under uniform and profiled current-only conditions. The presence of a profiled velocity gradient was found to have a negligible effect on the average performance characteristics; however, transient thrust, torque and out of plane bending moment loads experienced much greater variations. These load fluctuations were further increased with increasing levels of shear in the velocity profile, while peaks in the turbine loads coincided with its rotational frequency. The addition of regular, Stokes 2nd Order Theory waves added to the complexity of the flow conditions experienced by the turbine. The effect on the average performance characteristics were negligible while the total turbine thrust and torque fluctuations increased by 35 times that of the current-only cases. Peaks in the loads aligned with the wave surface elevation, indicating the importance of transient analyses of dynamic loads.
Highlights
World energy consumption is predicted to increase by 28% from 2015 to 2040 (U.S Energy Information Administration, 2017)
This study presents the turbine perfor mance under uniform and profiled current-only conditions, as well as under wave-current conditions using Stokes 2nd Order Theory (S2OT) regular waves which are superimposed upon the profiled current ve locities
Regular waves travelling in the same direction as the current flow will have a wave period (Tr), angular frequency and wave celerity (Cr) in a frame of reference that is moving at the same velocity as the average current (W)
Summary
World energy consumption is predicted to increase by 28% from 2015 to 2040 (U.S Energy Information Administration, 2017). Oscillatory motions induced by waves presented significant cyclic variations, responsible for fluctuations in the thrust and torque loadings in excess of 35% of the mean rotor load (Galloway et al, 2010; Ordonez-Sanchez et al, 2019) This can lead to extreme loadings on the drivetrain and accelerated fatigue of the individual turbine components. A study by Tatum et al (2016) analysed the individual turbine component loadings induced by a uniform current velocity and surface waves, finding that wave action had a significant effect on the fluctuation of the thrust and power, as well as the shaft bending moment. The focus of this study is to examine the impact that complex flow conditions, such as profiled velocity gradients or surface waves, can have on the individual turbine component loadings and performance of a HATT.
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