Structural Design And Performance Of Large-scale Sectional Composite Wind Turbine Blades

Continuous fiber reinforced composites have been adopted into mass application in wind energy due to excellent overall performance.Wind energy has already become one of the four major industrial applications of composite materials.With the support of policy and technology promotion,wind energy has developed quickly in the world,and become the third energy in China after fossil and hydroelectric energy.The unit capacity of wind turbines has been larger and larger to capture more wind energy and reduce the levelized cost of energy,and consequently the rotor blades have become longer and longer;more than 60-meter blades have dominated the commercial market,even 80-meter plus blades are being put on the market one after another.Oversized rotor blades bring about lots of troubles in manufacturing,transportation and assembling,simultaneously increase the cost of wind energy and safety risk.Sectional blades were considered an efficient and direct access to tackle the related problems.Though significant and referential works have been done by the researchers,there are several key issues which still need to be further investigated: 1)novel connection configuration with high efficiency and reliability to be suitable for large sectional rotor blades;2)the stiffness and load carrying behaviors of multi-bolted joint structure under in-service load;3)the optimal division location for sectional blades and the systematical analysis methods used;4)the design of bolt allocation and bolt load distribution;5)the impacts due to blade division on the overall performance of the wind turbine and blades.To solve these problems,the author adopted analytical,numerical and experimental methods to perform the comprehensive studies.The building-block study route including unit bolted joint,multi-bolted joint component,full-scale sectional blade and wind turbine was adopted.The contents of studies are summarized as follows.Firstly,the background,development trend and challenges of sectional blade application were reviewed,and the key technological issues urgent to be tackled were summarized.Meanwhile,research methods of tensile bolted joint,which are adopted in sectional blades,were introduced and reviewed.A novel bolted joint configuration combining embedded sleeves and stud bolt was presented as well as the corresponding assembling method.Stiffness of clamped members was deduced on the given configuration,and the cone angle was determined by finite element method to be substituted into analytical equation of members’ stiffness.Based on the proposed bolted joint configuration with real dimensions,finite element model with solid elements was adopted to study the nonlinear behavior of bolt load under external tensile load.Secondly,a sectional composite box-beam connected by 12 tensile bolts was designed and fabricated in consideration of the primary structure and loads of a real blade.Hammer-tapping method was performed out to obtain the natural frequencies,sectional box-beam was then imposed under extreme flap-wise bending load,the strain of spar caps and bolts,and the tip deflection of box-beam were monitored.Meanwhile,solid/shell combined finite element model was built to simulate the nonlinear structural responses even under insufficient preload of bolted joint.The comparative analysis from analytical,numerical and experimental results was conducted to tie the relationship between nonlinear behavior and external loads,and therefore revealed the mechanism of bolt failure of blade connection.Thirdly,a simplified method to determine the optimal division location for sectional blades was presented by evaluating the influence of natural frequencies,connection strength and assembly accessibility.A universally applicable division location was determined to be located at 20% blade length from blade root by analyzing the above effects based on a 38-meter real blade.Meanwhile,the sectional blade was scaled up based on the geometrical similarity rule,some key issues related to application and trend of larger sectional blades were analyzed to offer several instructive suggestions.Fourthly,novel designs on the layout of primary structure,manufacturing process,and assembling of the sectional blade were illustrated.According to IEC regulation,a 38-meter sectional blade was tested under extreme bending loads in four directions.The strains of spar caps,shear webs,bolts and sandwich panels were monitored during the uploading and unloading process.The strength of composite materials,buckling and load allocation of the sectional blade were studied.Fifthly,the wind turbines were modeled by open-source code and commercial software,and the effects on the increments of mass and stiffness,natural frequencies,loads,deflections and aerodynamic performance due to blade division were studied by comparing with the original blade.The extreme edge-wise and flap-wise bending moments at blade root section increase about 10% and 3%,respectively,and the fatigue moments increase 23.2% and 6.0%,respectively,the aerodynamic performance keeps the same as the original blade.Finally,the main contents and conclusions of this dissertation were summarized,and some aspects,which are not carried out but worth further study,were prospected as well.