Dynamic Loads on Tidal Turbine Blades

Abstract

Tidal turbines convert kinetic energy from tidal currents into electricity in a similar way as wind turbines extract energy from wind. The wind energy industry being more developped, tidal turbine designers use therefore the same methods as their wind turbine counterparts to develop their power plants. The standard engineering method in use is the Blade Element Momentum (BEM) theory, which combines the division of the blade in blade elements characterised by their hydrodynamic properties (lift and drag coefficient), their twist angle and their cord length, and the momentum theory, based on the assumptions of actuator disc, irrotational and uniform inflow, among others. However, the differences in the inflow and in the rotor inertia between wind and tidal turbines questions the validity of this method. For instance, contrary to wind turbulence, ocean waves have a frequency close to the rotor frequency, leading to significant variations in the inflow within one rotor revolution. To model loads on tidal turbine blades, one can also call on two other methods, namely model testing and Computational Fluid Dynamics (CFD) simulations. These methods are appropriate for design optimisation and/or validation but require respectively expensive test facilities and time consuming simulations on high performance computers. Therefore, these methods are less flexible than the quasi-static BEM method, for which results can be obtained relatively instantaneously for various flow conditions. To get an idea of the accuracy of the BEM method, a two-bladed tidal turbine rotor was designed and tested in a towing tank with a regular wave field and at runaway, i.e. when the rotor is suddenly uncoupled from the grid and rotates freely. These experimental setups were also reproduced in the BEM code, as well as in CFD simulations, using ANSYS CFX. The thrust force and shaft torque obtained by these three approaches could then be compared to point out the need for modeling dynamic effects in the quasi-static BEM method. This was done by analysing the time series in time domain and comparing statistically the mean values, extreme values and standard deviations of the wave tests. A spectral analysis was also used to check if all frequency content appearing in the experimental data was also captured by the BEM, and to what extent. As a result, the mean values of the wave tests match the BEM mean values, as long a there is no stall on the blade under wave troughs. However, the spectral analysis showed that the dynamic (expertimental or CFD) thrust and torque signals may be obtained by high-pass filtering the quasi-static (BEM) values (low-pass filtering the induction factors), observed through a phase delay and an amplification of the peaks at high frequencies. This can be explained by the dynamic behaviour of the wake. Dynamic wake models developped for wind turbines may be used to correct empirically the BEM codes, with coefficients determined using vortex-based numerical methods. However, vortex codes are not valid for the turbulent wake state, which is often observed passed tidal rotors in waves. Therefore, a correction of the BEM method, as simple as those used for wind turbines, are a priori not adapted for tidal turbines in wave fields. In addition, the actuator disc simplification, necessary in the BEM method, decreases the influence of blade position on rotor loading under regular waves (showing a non-uniform inflow due to the decreasing wave velocity profile with depth). Therefore, the spectral peaks observed at sum frequency (wave+rotor frequency) in the experimental results are attenuated or even not captured by the BEM. As a conclusion, the smaller, but not unsignificant, load cycles at sum frequency are not taken into account in a fatigue analysis based on BEM predictions and would lead to a wrong life time estimation. Concerning the runaway case, the tests highlighted a critical peak in thrust force, rising extremely fast after the release of the rotor. This peak does not appear on the BEM results because it arises from the dynamic properties of the wake. This corroborates with the response of a highpass filter to a step, as previously observed for the wave cyclic loads. The highly loaded rotor makes the wake reach the vortex ring state, whose dynamics cannot be captured with available engineering models. This shows the necessity of an efficient control system braking the rotor in case of runaway. Many questions were raised during this PhD project and further research in this field is necessary to include corrections to the BEM method, so that it takes into account the wake dynamics. Those corrections should be calibrated using CFD simulations and validated by comparing with experimental results.