Impact of tidal turbine support structures on realizable turbine farm power

This thesis discusses the importance of tidal turbine support structures through analytical and computational modelling. A head-driven analytical channel model was first developed to determine the sensitivity of the flow to the presence and type of support structures. It showed that there was a significant potential reduction in farm power output even when only considering approximate force coefficients for rotor and support structure. To confirm these findings, computational simulations were performed on a full-scale turbine to obtain more accurate force coefficients considering full rotor-support structure interactions. The flow interaction effects between the rotor and its support structure were studied using Computational Fluid Dynamics (CFD) for different support structure shapes for a range of tidal velocities including the power-capping zone. The integrated rotor force coefficients were higher in the presence of the cylindrical support structure than the elliptical support due to the higher opposing thrust from the cylinder in the channel redirecting the flow and increasing the flow velocity over the top half of the rotor. The presence of rotor caused a drop in the stream-wise forces on the support structure. The amplitude of the stream-wise sectional forces along the support structure height was lower in the case of an elliptical than a circular cylinder due to more streamlined shape of the ellipse. At device scale, the computational model was used to study the turbine performance in the power-capping zone by pitching the blades to feather. The influence of pitch-to- feather power-capping strategy was examined by studying the forces and angle of attack on the turbine blades, and the wake at three different blade pitch angles. Increasing blade pitch angle resulted in a significant drop in the average load on the blade. Also since the tidal channel flow has a shear in its velocity profile, the influence of shear on turbine performance was studied by comparing it to the same turbine in a uniform flow. The analytical channel flow model was used to investigate the characteristics of tidal stream energy extraction for large tidal farms deployed in tidal channels with specific focus on the limitations to realizable farm power due to turbine support structure drag and constraints on volume flow rate reduction. The force coefficients dataset from computational modelling was used to obtain a better estimate of the farm power output. Support structures were seen to contribute significantly to the overall resistive force in the channel and thus reduce the overall flow rates in the channel, leading to losses in realizable power. Over a wide range of channel characteristics, realistic levels of support structure drag lead to up to a 10% reduction in realizable power, and an associated reduction in the number of turbines that can be economically installed.