• Energy Research

The flow developed on a tidal site can be characterized by combinations of turbulence, shear flows, and waves. Horizontal-axis tidal turbines are therefore subjected to dynamic loadings that may compromise the working life of the rotor and drive train components. To this end, a series of experiments were carried out using a 0.9 m horizontal-axis tidal turbine in a tow tank facility. The experiments included two types of regular waveforms, one of them simulating an extreme wave case, the other simulating a more moderate wave case. The second regular wave was designed to match the peak period and significant wave height of an irregular wave which was also tested. Measurements of torque, thrust, and blade-bending moments were taken during the testing campaign. Speed and torque control strategies were implemented for a range of operational points to investigate the influence that a control mode had in the performance of a tidal stream turbine. The results showed similar average power and thrust values were not affected by the control strategy, nor the influence of either the regular or irregular wave cases. However, it was observed that using torque control resulted in an increase of thrust and blade root bending moment fluctuations per wave period. The increase in fluctuations was in the order of 40% when compared to the speed control cases.

The optimisation of a Numerical Wave Tank is proposed to accurately model the sub surface conditions generated by regular waves superimposed on a uniform current velocity. ANSYS CFX 18.0 was used to develop a homogenous multiphase model with volume fractions to define the different phase regions. By applying CFX Expression Language at the inlet of the model, Stokes 2nd Order Theory was used to define the upstream wave and current characteristics. Horizontal and vertical velocity components, as well as surface elevation of the numerical model were compared against theoretical and experimental wave data for 3 different wave characteristics in 2 different water depths. The comparison highlighted the numerical homogeneity between the theoretical and experimental data. Therefore, this study has shown that the modelling procedure used can accurately replicate experimental testing facility flow conditions, providing a potential substitute to experimental flume or tank testing.