Research On Aerodynamic And Structural Designs Of Blades For Large Offshore Wind Turbines

Environment-friendly development is the common mission for people around the globe and it is universally acknowledged that the renewable energy rate should be increased due to the climate change and the energy crisis.Chinese government has made an agreement that non-fossil energy will account for 20% of the total energy consumption in 2030 and China is expected to be at the first position on the utilization of the offshore wind energy in 2020.The fundamental part of wind turbine is the blade which determines the key parameters like power coefficient,design load,etc.However,with the wind turbine developing towards large-scale and offshore,the size of the blade is becoming extremely large which poses challenges to the precise aerodynamic calculations,the aerodynamic and structural designs.This research is supported by the grant from the National Natural Science Foundation of China(No.51175526),the National High Technology Research and Development program of China(No.2012AA051301,863 Program)and the China Scholarship Council.This research was also helped by the Danish Council for Strategic Research under the project name OffWindChina(Sagsnr.0603-00506B).In this paper,a research topic,research on aerodynamic and structural designs of blades for large offshore wind turbines,has been proposed and studied.Based on the characteristics of large offshore wind turbines,new airfoils are designed to be utilized on the main power production zone of 2 MW and 5 MW wind turbine blades,respectively.Then,a new theoretical model of high accuracy for the wind turbine aerodynamic computations are developed,and then compared with the MEXICO wind tunnel experiment.Based on the new theoretical model,blade shape optimization of a 5 MW wind turbine is performed.Based on the new blade aerodynamic shape and one coastal wind condition in China,blade structural optimization of the 5 MW wind turbine is finished.The main study and achievements are as follows:(1)First,based on the Reynolds number requirement of a classcial 2 MW wind turbine blade and the smooth airfoil condition in the offshore wind farm,airfoil with relative thickness of 18% is optimized using integral expression theory and CFD tools.ICEM and FLUENT are coupled to generate airfoils,reconstruct mesh,set boundary conditions and calculate aerodynamic performances.The results show that the new airfoil has a higher lift coefficient and lift-to-drag ratio in on-design and off-design operation conditions and in the main range of attack angles.Whant is more,based on the Reynolds number requirement of 5 MW wind turbine blades,the composite materials layup condition and the requirement of roughness sensitivity in the offshore wind farm,an integrated aerodynamic and structural design for wind turbine airfoils has been carried out with the help of the classical plate theory.Besides to a higer lift coefficient and lift to darg ratio,the optimization goal also considers the smooth decrease of lift after stall which is important to the blade fatigue life and is an intrinsic property of the commonly used commercial airfoils.The new airfoil has a higher lift coefficient and lift to drag ratio,has good sturctural performances and slight decrease of lift after stall,which provide a soild foundation for the design of large offshore wind turbine blades with good aerodynamic and structural performances.(2)The blade element momentum(BEM)theory is widely used in aerodynamic performance calculations and optimization applications for wind turbines.The fixed point iterative method is the most commonly utilized technique to solve the BEM equations.However,this method sometimes does not converge to the physical solution,especially for the locations near the blade tip and root where the failure rate of the iterative method is high.The stability and accuracy of aerodynamic calculations and optimizations are greatly reduced due to this problem,and these outcomes will be much sever especially for the large scale wind turbine.The intrinsic mechanisms leading to convergence problems are addressed through both theoretical analysis and numerical tests.When the initial inflow angle is set larger than the critical inflow angle and the relaxation methodology is adopted,the convergence ability of the iterative method will be greatly enhanced.Numerical tests have been performed under different combinations of local tip speed ratio,local solidity,local twist and airfoil aerodynamic data.Results show that the simple iterative methods have a good convergence ability which will improve the aerodynamic or structural design of the large wind turbines.(3)However,the classic BEM method is not quite accurate which often tends to under-predict the aerodynamic forces near root and over-predict its performance near tip.The reliability of the aerodynamic calculations and design optimizations is greatly reduced due to this problem.These shortcomes of the classical BEM theory will be amplified on the long blade of the large wind turbine,which will bring large errors for the balde design.To improve the momentum theory,in this paper the influence of pressure drop due to wake rotation and the effect of radial velocity at the rotor disc in the momentum theory are considered.Thus the axial induction factor in far downstream is not simply twice of the induction factor at disc.To calculate the performance of wind turbine rotors,the improved BEM theory is considered together with both Glauert's tip correction and Shen's tip correction.Numerical tests have been performed for the MEXICO rotor.Results show that the improved BEM theory gives a better prediction than the classic BEM method,especially in the blade tip region,when comparing to the MEXICO measurements.This new model provides solid foundation for the accurate aerodynamic calculations of the large wind turbine.(4)Based on the newly developed BEM model,the improved partical swarm optimization method is applied to optimize the blade twist for the 5 MW wind turbine.It is certified that the power coefficient of the optimized rotor is greatly improved and results also validate the excellent aerodynamic performance of the newly designed airfoil.After the optimization,the maximum twist of the balde and the rate of twist variation along the blade span is decreased,which is beneficial to the blade manufacture process.In short,this optimization has provided good blade shape for the subsequent structural design.(5)Based on the optimized blade shape,the parametric finite element model for the composite material blade is established.The extreme loads of the large 5 MW wind turbine blade is calculated under the wind condition of an offshore wind farm located on the south-east cost of China.With the aim of minimize the blade mass,the structural optimization design has been carried out.The goal of decreasing the blade mass after optimization is achieved on the prerequisite of good strength and rigidity,which provides references for the balde design of offshore wind turbines with 5 MW or a higher power level.