Performance assessment of a Horizontal Axis Tidal Turbine in a high velocity shear environment

Abstract

The main focus of this thesis was to assess the performance of a full scale Horizontal Axis Tidal Turbine (HATT), using the CFD package, FluentrM, and measured high shear tidal profiles. Two sites are considered: the Anglesey Skerries and a site in the Severn Estuary, both off the Welsh coast. In order to achieve this aim a number of key steps were performed including the use of an existing laboratory scale prototype HATT to establishing the optimum blade pitch angle and provide an experimental data set. Once established the HATT CFD model was used to scale up from the laboratory scale to 30 m diameter. By the use of non-dimensionalised characteristics of power, thrust and torque coefficients, it was shown that the HATT was scaleable and independent of Reynolds number. Using these findings a suitable turbine diameter was determined for site specific analysis. Velocity profiles from the two sites were obtained via vessel mounted Acoustic Doppler Current Profiler (ADCP) surveys. These data were used to define a high velocity shear environment. When non-dimensionalised these data were found to also collapse onto the scaling curves provided a true average for the velocity, across the swept area, is used. In addition, when the HATT was 'positioned' at varying depths down the water column the power extraction was shown to reduce considerably with depth. When positioned close to the seabed, the cyclic torque, power and axial thrust loads were studied with and without a stanchion positioned downstream of the turbine. The presence of a stanchion was also shown to significantly increase the amplitude of the cyclic torque, power and axial thrust during rotation. The findings of this thesis suggest that the complexity of the dynamic torque, power and axial thrust, along with the wake profile, are influenced by the HATT's interaction with the ocean seabed. These complexities are therefore of prime importance when considering a deep water application which encompasses all or part of a high velocity shear regime. The work presented in the thesis shows that it is possible to predict a turbine's performance (for a given geometry) for any scale and velocity profile, from a single diameter. When positioned lower in the water column, the downstream wake also showed a high level of asymmetry which was also shown to influence the upstream flow field