Spar Cap Research Articles

Efficient structural optimisation of a 20 MW wind turbine blade

This paper presents, through the structural design of a 20 MW wind turbine blade, a selection of novel analysis and optimisation methods for wind turbines. These methods are integrated in the software—Aeroelastic Turbine Optimisation Methods (ATOM). A key feature is the novel, computationally-efficient piecewise linear model for running rapid design load case simulations (up to 16 times speed-up over conventional methods). Further, a comprehensive set of realistic design constraints is also proposed to ensure structural feasibility and aeroelastic stability. To demonstrate these methods, a sequential gradient-based optimisation process is employed, relying on the globally convergent method of moving asymptotes (GCMMA). The process begins with an aerodynamic optimisation to generate twist distributions, followed by an iterative loop during which load envelope updates and ‘frozen-load’ blade structural optimisations are performed independently. Aeroelastic loads are therefore considered, but are not directly optimised for. The present study investigates the structural design of a 20 MW wind turbine blade with hybrid carbon-glass spar caps. The optimised 122 m blade is found to have a mass of 83,622 kg, which decreases to 81,396 kg (-2.66%) with the addition of sweep. The GCMMA is found to converge successfully at each structural optimisation step. By contrast, the iterative loop is observed to oscillate, albeit within small bounds. Finally, results suggest that convergence of multi-step optimisation methods for aeroelastic blade design may not be guaranteed if design variables inducing aeroelastic couplings are considered.

Adhesive strength analysis and real-scale simulation for smart curing in a large turbine blade with carbon fiber-reinforced plastic spar cap

Various approaches for the smart curing processes in the fabrication of large hybrid composite blades have been employed to determine whether smart curing, which has multiple heating and cooling processes during the total curing time, is applicable to the adhesive process between carbon fiber-reinforced plastic (CFRP) and glass fiber-reinforced plastic (GFRP). Therefore, investigating the adhesive strength and residual thermal stress between CFRP and GFRP is very important. In an experimental approach, the adhesive strengths of CFRP/GFRP single-lap joint specimens were compared according to surface treatment techniques and curing conditions. The proper roughness of the CFRP surface and the application of smart curing are very effective for the manufacture of CFRP spar cap wind turbines. The simulation results from the analytical approach demonstrated that when the final curing temperature was reduced by the smart curing process, the residual thermal stress and thermal shear stress in the blade were also reduced, resulting in an increased adhesive strength in the adhesive layers between the CFRP spar caps and GFRP skin. It was determined that large hybrid wind blades with CFRP spar caps could be efficiently manufactured using a smart curing process.

Adhesive curing cycle time optimization in wind turbine blade manufacturing

The curing of adhesives in wind turbine blades is a cost and time-intensive manufacturing step. Bondlines are critical to the structural integrity of the blade, but substantial variation in bondline thickness can result in different thermal histories and even degradation of the adhesive due to its exothermic reaction. Models to predict the thermal behavior of adhesive bondlines during the curing step, taking manufacturing conditions and variation in thickness into account, are needed to minimize cure times while avoiding excess temperature. In this study, a finite element model capable of tracing the thermal and conversion histories of the adhesive was developed. Standard heat transfer equations were coupled with cure kinetics in the model through user subroutines, and the cure kinetics were characterized and validated experimentally. A 2D cross-section model of a wind turbine blade was created to simulate the adhesive curing cycle in a manufacturing setting. This model was used to highlight critical aspects of the curing cycle and test methods to shorten the cycle time while maintaining the integrity of the bond. Several improvement methods and their potential cycle time savings were investigated, such as implementing individual heating and cooling zones for each adhesive area and additional spar cap heating/cooling.

Preliminary design procedure for large wind turbine blades based on the classical lamination theory

Designing the structure of a wind turbine blade is a nontrivial task. Nowadays, finite element software is widely applied for its design and computation. However, huge computational resources and time are required for modeling and evaluation of large composite turbine blades. Additionally, a significant amount of design parameters is required to be handled; thereby, it is difficult to perform a systematic parametric study. This paper presents a preliminary procedure for the initial structural design of a large wind turbine blade based on the classical lamination theory (CLT). By analyzing the existing blade, the optimized numbers of the spar cap layers in the blades are estimated. Furthermore, a parametric study using the carbon fiber-reinforced plastics (GFRP) and glass fiber-reinforced plastic (CFRP) at multiple fiber orientation angles for the spar cap design is attempted with respect to the axial and twist coupling stiffness coefficients for each material design. Based on the constitutive equation, an analytical design approach of a laminated composite blade is developed and compared with a three-dimensional finite element blade model. The results show good agreement; thus, the design procedure provides a foundation for the initial structural design of a wind turbine blade.

Mathematical programming approach to productivity improvement in wind turbine-blade manufacturing through a case study

Wind energy has attracted remarkable attention in recent years as one of the most important renewable energy sources. However, wind energy production is still a costly process and one of the reasons is production processes which are usually not optimized. The purpose of this paper is to improve the efficiency of spar cap production process as an important phase of a wind turbine blade production in a manufacturing system. For this purpose, the present paper introduces a new optimization problem for optimal assignment of fiberglass rolls along the spar cap. In order to solve the roll allocation problem (RAP) optimally, two alternative mathematical programming models based on mixed-integer programming (MIP) and constraint programming (CP) are developed. Subsequently, two cases form a real manufacturing setting are presented and solved by using both MIP and CP models. Although proposed models are capable to solve problems optimally, it is observed that CP outperforms MIP. Solving RAP optimally reduces walking time/distance of workers considerably and provides significant improvements in comparison with current manufacturing practices. Besides, the total operation time will also decrease correlatively, and this will lead to an increase in productivity of turbine blade production in the studied system.

A Feature-Based Cost Estimation Model for Wind Turbine Blade Spar Caps

A problem for wind turbine operators is decreasing prices for wind-generated electricity. Many turbines are approaching their rated 20-year lives. A more economically viable and sustainable solution that reduces Levelized Cost of Energy (LCOE) and avoids expensive turbine replacement is retrofitting new spar caps blades. A new cost model assesses the feasibility of retrofitting 35 to 75 m turbines with GFRP (glass fiber reinforced polymer composite) and longer length CFRP (carbon fiber reinforced composite) spar caps. Spar cap cost scales with features such as mass, volume fraction and complexity. Organizational learning is a cost factor. Material and direct labor increase as proportions of total cost while tooling, capital, utilities, and indirect labor decrease. There is good agreement between a manufacturer and the model. Twenty-year turbines were compared with retrofitted spar caps over 25 years for LCOE. Same length GFRP and longer length CFRP spar cap retrofits decrease LCOE. Longer length CFRP spar caps decrease LCOE compared with GFRP retrofits over 25 years. CFRP material cost impacts CFRP retrofit feasibility. Retrofitted turbines must meet engineering, operational performance, and planning requirements criteria. Software algorithms may improve human learning and enable automatic updates from varying design and cost inputs, thereby increasing cost prediction accuracy.

Experimental Study of Stepped-Lap Scarf Joint Repair for Spar Cap Damage of Wind Turbine Blade in Service

The objective of this paper was to design configuration parameters for a stepped-lap scarf joint repair, which can be used for spar cap damage of a wind turbine blade in service and to realize the post-repair monitoring. Two experimental studies were included. First, tensile test for the unidirectional tape specimens with a large aspect ratio repaired using a multiple stepped-lap scarf joint method was carried out. The results showed that the reinforcement layer could effectively improve the load-carrying capacity of the repaired zone. The stepped-lap joint surface was identified as the weak part of the spar cap repair, which should be monitored. Second, by embedding carbon nanotube buckypaper sensors on the stepped-lap joint surface of the repaired specimens, quasi-static tensile tests and fatigue tests were carried out. According to the resistance response of the sensors, the quasi-static tensile test confirmed the failure processes, namely the stiffness turning point, damage evolution, crack propagation, and fracture. The fatigue test could accurately identify the progressive failure, namely the initial damage, damage accumulation, initial cracking, and crack propagation to structural failure. The above tests provided an important configuration parameter basis for evaluating the spar cap repair scheme and presented a promising method for the health monitoring of a spar cap after repair.

New investigation of mechanical properties of a horizontal axis wind turbine blade based on a hybrid composites with kenaf fibers

The present work aims to assess the benefit of introducing simultaneously kenaf and carbon fibers in a composite wind turbine blade made initially of epoxy glass fiber. After an in-depth analysis of the optimal design of the blade using the Blade Element Momentum theory, an algorithm is generated in order to, firstly, establish the foundation of the blade geometry and secondly, to make a conservative static loading estimation representing a 50-years extreme gust. After performing the blade geometry on the 3D-modeler CATIA® V5, it was imported to PATRAN® software in order to create the mesh of the turbine blade. By modifying the way of stacking layups at each zone of the blade and by taking into account the plies properties, seven models were created. A further comparison was made between these models with a view of finding the best combination of fibers to satisfy the allowable thresholds. After several attempts of fibers combinations, it was finally revealed that the best way to combine the three fibers consists of replacing all unidirectional glass fibers of the blade skin with kenaf fibers and substituting those of the spar cap with carbon fibers while maintaining intact the remaining blade layups.

Study on the Effect of Different Delamination Defects on Buckling Behavior of Spar Cap in Wind Turbine Blade

Delamination is detrimental to the composite materials, and it may occur in the manufacturing process of the unidirectional laminate of the spar cap in wind turbine blades. This paper studies the effect of different delamination defects on the strength of the unidirectional laminate. The finite element model of laminate with different delamination areas and delamination heights is established using solid elements. The eigenvalues of laminates have different parameters calculated based on the finite element method. The final coupon test is used to verify the conclusions of simulation results. The finite element method presented in this study shows excellent capabilities to predict the buckling behavior of the laminate. The buckling eigenvalue of tested laminate is negatively correlated with the delamination area and positively correlated with the delamination height under the edgewise load. The S11, which is too high at the boundary of the delamination region, plays a significant role in buckling failure. It has a particular reference value for testing the laminate of blade both in theory and practice.

Fatigue life evaluation of composite wing spar cap materials

AbstractAn effective method for evaluating the fatigue strength of thick unidirectional composite laminates of wing spar caps has been presented here. A typical width-tapered bending specimen has been developed for a four-point loading set up. Static and fatigue loading was performed at a load ratio R = ‒1. The complex stress state has been investigated numerically by finite element analysis. Finally, the concept is proven experimentally on specimens made of pultruded fibre rods. The fatigue behavior of the continuously tapered width by water jet cutting has been compared to discrete tapering via rod-drop. The new method enables a screening of material fatigue behavior caused by normal and shear loading.

Optimal structural design of biplane wind turbine blades

Biplane wind turbine blades have been shown to have improved structural performance, aerodynamic performance, and reduced aerodynamic loads compared to conventional blade designs. Here, the impact of these factors on blade mass is quantified for the first time. The objectives of this work are to quantify the mass of biplane wind turbine blades which have been designed for realistic loads, and to understand the mass-driving constraints for such blades. A numerical optimization approach is used to design the internal structure of biplane wind turbine blades, minimizing blade mass subject to a number of design requirements which are imposed as constraints. The mass reductions are significant, showing that the optimal biplane blades are more than 45% lighter than a similarly-optimized monoplane blade. This is primarily due to the improved resistance to flapwise deflection when compared to the monoplane blades, which allows for considerably less spar cap material to be used in the biplanes. Biplane blades are also shown to have improved resistance to edgewise fatigue damage, requiring less trailing edge reinforcement. Given such large mass reductions, some criticality is required, and the limitations of the present approach are discussed. The results of the optimization present strong evidence that biplane wind turbine blades may be an enabling concept for the next generation of lighter, larger, and more cost-effective wind turbine blades.

Structural collapse characteristics of a 48.8 m wind turbine blade under ultimate bending loading

To further study the structural failure mechanism and response characteristics of large wind turbine blades under extreme conditions, a full-scale 48.8 m blade collapse test was carried out. The strong coupling action of different damage modes are descripted by the gradual collapse process recorded by cameras. The damage behavior of suction side and pressure side, and interior and skin behavior of test blade are analysed. The test shows that the buckling of the suction side spar cap at about 21 m and cracks of the root shear web are the essential factor leading to the catastrophic failure of the entire blade, the test result is consistent with the finite element analysis. The load-deformation curve of the suction side spar cap, the deflection curves at 24 m, 36 m and tip, and the strains curves along the blade span are plotted to perform the data analysis. The critical load of 78% and deflection behavior in collapse process are determined by data analysis. The research results have great significance for improving the ultimate strength of entire blade, and provide a theoretical basis for the structural improvement of the weak points of blade.

Evaluating Structural Failure of Load-Carrying Composite Box Beams with Different Geometries and Load Conditions

The load-carrying component of wind turbine blades is a composite box beam that often consists of two spar caps and two shear webs. Local buckling of such beams usually leads to failure of spar caps and/or shear webs. As the failure modes change with cross-sectional geometries and load conditions, it is of interest to develop a method for efficient structural failure evaluation. As a correlation exists between linear buckling response and the potential structural failure, this study presents comprehensive numerical studies on the box beams with different external shapes and layer thicknesses to establish buckling mode maps that can be useful for the preliminary structural design. The results show that the changes of cross-sectional aspect ratio affect the buckling strength more than the change of the weight or the material usage when the spar cap buckling dominates the failure. Larger curvature of spar caps can significantly improve the buckling strength due to better resistance to the cross-sectional flattening. The evaluation method only uses the modeling techniques readily available in common commercial FE software, and no in-house user subroutines is needed, thus allowing the failure evaluation to be performed efficiently in the early design of the load-carrying box beams in wind turbine blades.

Structural failure test of a 52.5 m wind turbine blade under combined loading

Blade is one of the basic and key components in wind turbines. To get closer to the actual loading condition of blade under complex and extreme wind conditions to more accurately analyse the structural failure characteristics of large wind turbine blades. The structural collapse test of a 52.5 m wind turbine blade under combined loading was carried out. The entire process of damage under ultimate load was recorded and analysed in detail. The joint analysis of the failure region after the blade collapse and the failure mode during the test process were performed. The research shows that the geometrical nonlinear buckling of spar cap and shear web, the delamination result from non-uniform stress distribution and further fracture of Aft panel cause catastrophic failure of the blade. The torsional moment cause oblique cracks and oblique bulges, aggravates the expansion of the internal and external composite cracks of blade, affects the final failure mode after blade collapses (critical failure mode), and make the spar cap at suction side to clockwise twist from blade tip to root. The research results lay a theoretical foundation for subsequent model establishment and simulation analysis of large wind turbine blades under combined loading.

Study on composite bend-twist coupled wind turbine blade for passive load mitigation

The wind turbine size is nowadays becoming increasingly larger as an effective approach to reduce cost of energy. Passive load control technique is introduced to alleviate the increasing loads on an upscaling wind turbine blade to improve its fatigue life. In this paper, the composite bend-twist coupled blade is investigated and utilized to mitigate the cyclic fluctuating loads in shear wind. The NREL 5-MW blade is firstly inversely redesigned of its composite layup configuration. The fiber on the spar cap is rotated away from the blade axis to implement the bend-twist coupling (BTC). An advanced aeroelastic model of the wind turbine blade is developed to conduct the study. The geometrically exact beam theory is used to account for the large deformation effects, while the tool VABS is utilized to generate the cross-section stiffness matrices. Meanwhile, the free wake lifting surface model is adopted to calculate the aerodynamic loads, which is physically more realistic than the traditionally used BEM method. Based the established model, the influences of BTC on the rotor aerodynamic performance are investigated. Load mitigation effects of the blades with various spar cap fiber angles are evaluated. The influences of coupling region and different wind speeds are also discussed.

Manufacturing and Flexural Characterization of Infusion-Reacted Thermoplastic Wind Turbine Blade Subcomponents

Reactive thermoplastics are advantageous for wind turbine blades because they are recyclable at end of life, have reduced manufacturing costs, and enable thermal joining and shaping. However, there are challenges with manufacturing wind components from these new materials. This work outlines the development of manufacturing processes for a thick glass fiber–reinforced acrylic thermoplastic resin wind turbine blade spar cap, with consideration given to effects of the exothermic curing reaction on thick composite parts. Comparative elastic properties of these infusible thermoplastic materials with epoxy thermoset materials, as well as thermoplastic coupon components, are also included. Based on the results of this study it is concluded that the thermoplastic resin system is a viable candidate for the manufacturing of wind turbine blades using vacuum-assisted resin transfer molding. Significant gains in energy savings are realized avoiding heated molds, ability for recycling, and providing an opportunity for utilizing thermal welding.

The Influence of Uneven Temperature Distributions on Local Shear and Interlaminar Fracture Toughness Properties of a Thick Composite Laminate

Abstract Because wind turbine blade length has increased over the years, so have the root and spar cap thicknesses. Temperature gradients and residual strains related to the manufacturing process of thick laminates are factors that determine the final laminate mechanical properties. The aim of this study is to investigate the manufacturing process influence on shear and fracture toughness properties for an epoxy glass fiber-reinforced polymer. Epoxy-infused laminates of 84 layers (55–60 mm) thick were manufactured. The laminate was manufactured using vacuum infusion on a single-sided mold. Thermocouples and strain gauges were embedded at different thickness positions to monitor temperature and residual strain during the curing process. The laminate was manufactured using the sublaminate technique: with the help of peel plies at different thickness positions, sublaminates were extracted and separately tested as standard coupons. Shear coupons were tested according to ASTM D7078-12, Standard Test Method for Shear Properties of Composite Materials by V-Notched Rail Shear Method, and fracture toughness double-cantilever beam coupons were tested in Mode I according to ASTM D5528-13, Standard Test Method for Mode I Interlaminar Fracture Toughness of Unidirectional Fiber-Reinforced Polymer Matrix Composites. Based on the extracted sublaminates, the relations between local curing cycles, residual strains, and the influence on the shear and fracture toughness were studied. This work reports temperature profiles and residual strain measurements that were monitored during thick laminate manufacturing. In addition, experimental data from shear and fracture toughness tests (static and fatigue) are correlated with the local residual strains and temperature measurements.

A parametric study of flutter behavior of a composite wind turbine blade with bend-twist coupling

Now a days wind turbine blades are generally designed to have length as high as 60 m or more to maximize power production. Aeroelastic instabilities such as flutter are major concerns for these long, flexible and slender blades. Stiffness coupling between bending and twisting modes can be used to improve the aeroelastic performance of such blades. In composite blades bend twist coupling can be achieved by imparting unbalance in the lamination sequence. In the present work, a parametric study has been conducted to study the effect of unbalances in different parts of a wind turbine blade on flutter instability. An eigenvalue-based approach has been used for flutter analysis. It has been observed that flap-torsional stiffness has high impact on critical flutter speed. The critical flutter speed is increased by 40% with flap-torsional stiffness due to the unbalance in the entire section of the blade with symmetric skin, while for asymmetric skin; the achievable increment is 100%. The unbalance in the spar cap of the blade has less critical flutter speed as compared to the unbalance in the entire section of the blade. Unbalance in the entire section of the blade with asymmetric skin can lead to highest flutter speed.

Integration of multiple passive load mitigation technologies by automated design optimization—The case study of a medium‐size onshore wind turbine

AbstractThe present work considers the application to a medium‐size onshore wind turbine of passive load mitigation technologies, first individually and then integrated together. The study is conducted with the help of a comprehensive automated design optimization procedure, which eases the generation and comparison of consistent solutions, each satisfying the same overall requirements. Passive load mitigation is here obtained by inducing bend‐twist coupling to the blades. The coupling is generated by rotating the fibers of anisotropic laminates, by the aerodynamic sweeping of the blade and by offsetting the spar caps in opposite directions on the pressure and suction sides. The first two solutions yield significant benefits, while the third, for this particular wind turbine, is ineffective. In addition, the typical power losses associated with bend‐twist coupled blades are reduced by a novel regulation strategy that varies the fine pitch setting in the partial load region. After having considered each load mitigation technology by itself, fiber rotation and sweeping are combined together and used to design a rotor with a larger swept area. The final design generates cost of energy savings thanks to a large‐diameter, highly coned, soft‐in‐bending rotor that results in lower turbine costs and a higher energy capture compared with the baseline design.

Design and nonlinear structural responses of multi-bolted joint composite box-beam for sectional wind turbine blades

This paper deals with the nonlinear structural responses of multi-bolted joint box-beam under inadequate preload, the composite box-beam was designed to refer to a real blade. The simplified analytical, solid-shell finite element and experimental methods were jointly conducted to investigate additional bolt loads, clamped loads, and deflections of the sectional box-beam under flap-wise bending loads. The validation of the finite element model is confirmed by the good prediction of the natural frequencies, strains of composite spar caps and the additional bolt loads of the sectional box-beam. The numerical model shows superior precision in describing the structural responses of the sectional box-beam even after separation. The simplified analytical method offers a conservative solution to calculate bolt loads. The global movement at the divided section will not appear ideally due to continuous increase of the total clamped load. The slight jump of clamped load in PS implies the probability of residual deflection of the sectional box-beam. Meanwhile, the relative movement tends to brings about the bolt preload reduction in the case of periodic loading, and then fatigue failure rapidly under on-going reduction of preloads. This study provides a deep insight into the fatigue strength of bolted joint for sectional rotor blades.