| Abstract |
The shortages of global conventional energy and environmental degradation have become
the bottleneck and great challenge of social development and rapid economic growth.
Therefore, the research and utilization of sustainable energy has been paid more and more
attention and favor by the academic and engineering circles. Nearly 70% of the earth surface
is covered by sea water, which has abundant tidal energy reserves and great potential for
development as renewable energy. Tidal current turbine is a key equipment used to obtain
and convert river, current and tidal energy into electric energy. The accurate predictions of
the hydraulic performance of Tidal Current Turbines and the accurate quantification of the
hydrodynamic load quantitative analysis on the rotor have important guiding significance for
the hydraulic design and structural design of the Tidal Current Turbine. The incorrect
quantification of the dynamic load quantitative analysis of the rotor will cause a large
deviation in the design, which will lead to the failure of the blade or rotor during operation
and affect the safe and stable operation of the system.
In this paper, the boundary element model of steady and unsteady state of horizontal axis
tidal turbine is established. The key parameters and its mechanism affecting the performance
of Tidal Current Turbine are studied. The accurate performance prediction method of
horizontal axis tidal current turbine is proposed, and the conditions and parameters selection
principle for obtaining the best prediction results are given. On this basis, the interaction
between surface wave and current and the dynamic load variation characteristics of turbine
caused by the interaction are studied systematically. The influence law of key parameters is
analyzed quantitatively. This work has important theoretical and engineering application
value for guiding the hydraulic and structural design of Tidal Current Turbine and improving
the safety and stability.
Blade element momentum theory (BEMT) is a commonly used method to predict the
performance of tidal current turbines and study the flow field of tidal turbines, especially the
near wake flow field of blades. In this work, the BEMT is improved. By analyzing the
two-dimensional hydrodynamic, lift and drag coefficients, a more accurate performance
prediction method for tidal current turbines is established. The influence mechanism of
hydrodynamic coefficients, Reynolds number and blade span position parameters on the
hydrodynamic forces in the blade element section is clarified. Furthermore, the matching
relationship between hydrodynamic coefficient and Reynolds number, blade span position
parameters and turbine optimal performance are discussed. By using different Reynolds
arrays to calculate the lift and drag coefficients under different Ncrit values, the influence of Ncrit parameters on the lift and drag coefficients is clarified, and the selection principles of
Ncrit parameters and Reynolds number for accurate simulation of turbine performance are
given. The results show that the improved BEMT model is in better agreement with the
experimental data when the lift and drag coefficients are calculated at low Ncrit parameters
and the Reynolds number is calculated at 75% span.
In view of the shortcomings of traditional methods in simulating the wave and current of
tidal turbines, the current dynamic parameters are added to BEM model, and the turbine
model with fixed proportion is placed in the ocean current and wave environment. A
simulation method closer to the actual wave and current environment is proposed, and the
influence of wave and current coupling on turbine performance is studied. The influence
mechanism of actual wave current environment on the dynamic load of turbine is clarified.
In order to predict the dynamic load on the rotor of tidal current turbine, an improved
unsteady boundary element model is proposed. Different wave models such as Dean model,
Kishida model and Fenton model are combined to evaluate the accuracy of wave period, yaw
misalignment, blade bending moment and axial thrust. At last the fifth order Fenton model is
recommended to solve the long steep wave flow field calculation.
In this work, the quantitative analysis method of dynamic load of Tidal Current Turbine is
established. Through the quantitative analysis of dynamic load, the influence mechanism of
turbine design parameters, wave parameters and incoming flow parameters on dynamic load
of Tidal Turbine is explored. It is clearly pointed out that the important factors affecting
dynamic load must be considered in the design stage of Tidal Turbine. The results show that,
compared with the wave period, the velocity and height of the incoming flow have important
effects on the dynamic load. Yaw misalignment will increase the oscillation frequency of
blade bending moment. Excessive increase of turbine speed leads to significant increase of
moment frequency oscillation, which will affect the safe and stable operation of turbine.
Dynamic load changes are caused by the changes of operating conditions, wave parameters
and incoming flow, which must be considered in the design stage of turbine to avoid rotor
failure. In addition, the correct evaluation of the dynamic load can guide the selection of the
best tidal station and avoid the selection in the bad environment.
In order to break through the limitation of the linear superposition method, this paper
combines the modified boundary element model with the linear wave theory and the
nonlinear wave theory, and establishes a model and method for solving the wave current
flow field with further consideration of the wave current interaction. Based on the prediction
and study of the mutual induced characteristics among wave length, wave height, tidal
velocity and tidal depth, the influence mechanism of wave height, wave length, water
velocity and turbine immersion depth on turbine performance is revealed. The comparison of
open test data shows that the wave current interaction model proposed in this paper can significantly reduce the load range of blade root bending moment and the prediction
deviation of dynamic load. |
