Aero‐structural investigation of biplane wind turbine blades

As wind turbines grow larger, loads increase dramatically, particularly in the inboard region of the blade. A key problem is to design a strong inboard region that supports these loads without sacrificing too much aerodynamic performance. A new design is proposed: a biplane inboard region that transitions into a joint, which connects to a monoplane outboard region. The objective is to develop biplane inboard configurations that improve the aero‐structural performance of blades. To approximately compare a conventional inboard region with a biplane inboard region, cross‐sectional properties of a thick monoplane and a biplane were measured. Numerical simulations were used to explicitly compare the aerodynamic performance of a thick monoplane with a biplane. Then, several model beams were designed to be simple approximations of a conventional blade (‘monoplane beam’) and the biplane blade (‘biplane beam’). Canonical bending loads were applied to each model beam, and their deflections were compared. Numerical simulations show that the lift‐to‐drag ratio is significantly greater for the biplane than the thick monoplane for 0° < α < 15.5°. A parametric analysis of biplane beam configurations shows that their tip deflections are smaller than monoplane beams of the same length. These benefits for the inboard region of (i) improved aerodynamics and (ii) improved strength could lead to weight reductions in wind turbine blades. Innovations that create lighter blades can make large blades a reality. These results suggest that the biplane blade is an attractive design for large blades. Copyright © 2012 John Wiley & Sons, Ltd.     

5 MW风力机叶片模态特性分析

为了研究叶片铺层和主梁形式对大型自适应叶片动态特性的影响,利用Ansys软件建立了5 MW风力机叶片的有限元模型.对不同梁帽铺层角度和主梁形式的叶片进行了模态分析,给出了各模型的前十阶固有频率和振型,分析了叶片梁帽铺层角度、主梁形式和转速对风力机叶片固有频率的影响.结果表明：叶片梁帽铺层角度和叶片主梁形式对固有频率具有重要影响;在叶片的弯扭耦合设计过程中,要考虑设计参数对叶片模态特性的影响,以避免叶片发生振动性能失效.     

Analytic analysis of load alignment for coning extreme‐scale rotors

Extreme‐scale wind turbines (rated powers greater than 10 MW) with large rotor diameters and conventional upwind designs must resist extreme downwind and gravity loads. This can lead to significant structural design challenges and high blade masses that can impede the reduction of levelized cost of wind energy. Herein, the theoretical basis for downwind load alignment is developed. This alignment can be addressed with active downwind coning to reduce/eliminate flapwise bending loads by balancing the transverse components of thrust, centrifugal, and gravitational force. Equations are developed herein that estimates the optimal coning angle that reduces flapwise loads by a specified amount. This analysis is then applied to a 13.2‐MW scale with 100‐m‐level wind turbine blades, where it is found that a load alignment coning schedule can substantially reduce the root flapwise bending moments. This moment reduction in this example can allow the rotor mass to be decreased significantly when compared with a conventional upwind three‐bladed rotor while maintaining structural performance and annual energy output.     

Parked aeroelastic field rotor response for a 20% scaled demonstrator of a 13-MW downwind turbine

Aeroelastic parked testing of a unique downwind two-bladed subscale rotor was completed to characterize the response of an extreme-scale 13-MW turbine in high-wind parked conditions. A 20% geometric scaling was used resulting in scaled 20-m-long blades, whose structural and stiffness properties were designed using aeroelastic scaling to replicate the nondimensional structural aeroelastic deflections and dynamics that would occur for a lightweight, downwind 13-MW rotor. The subscale rotor was mounted and field tested on the two-bladed Controls Advanced Research Turbine (CART2) at the National Renewable Energy Laboratory's Flatiron Campus (NREL FC). The parked testing of these highly flexible blades included both pitch-to-run and pitch-to-feather configurations with the blades in the horizontal braked orientation. The collected experimental data includes the unsteady flapwise root bending moments and tip deflections as a function of inflow wind conditions. The bending moments are based on strain gauges located in the root section, whereas the tip deflections are captured by a video camera on the hub of the turbine pointed toward the tip of the blade. The experimental results are compared against computational predictions generated by FAST, a wind turbine simulation software, for the subscale and full-scale models with consistent unsteady wind fields. FAST reasonably predicted the bending moments and deflections of the experimental data in terms of both the mean and standard deviations. These results demonstrate the efficacy of the first such aeroelastically scaled turbine test and demonstrate that a highly flexible lightweight downwind coned rotor can be designed to withstand extreme loads in parked conditions.     

Design of composite tidal turbine blades

Tidal turbine blades are subjected to significant thrust and torsional loadings due to the high density of the seawater in which they operate. These thrust loadings lead to high bending moments at the blade root, which can prove to be a serious design constraint for these devices and can have implications with respect to cost-effectiveness and scalability. This work presents a combined hydrodynamic-structural design methodology for a commercial scale (1.5 MW) tidal turbine. A hydrodynamic analysis of the blade is carried out to determine force distributions along the blade span under normal and extreme operating conditions. Using output from the hydrodynamic model, a pre-processor for computing blade structural properties is used to determine the strain distribution along the blade spar caps. The strain distributions from this analysis are then compared with a finite element model of the blade which is then used to compare the structural performance of glass fibre reinforced polymer (GFRP) and carbon fibre reinforced polymer (CFRP) as spar cap materials.     

切出风速下风力机变桨故障叶片气动特性及准静态结构响应分析

为探究大型水平轴风力机达到切出风速停机后变桨故障叶片的气动特性及准静态结构响应,基于计算流体力学方法对NREL 5 MW风力机变桨故障/成功叶片气动侧状态进行分析,并利用双向弱流固耦合及曲屈分析对典型方位角下变桨故障叶片展开研究。结果显示：切出风速下变桨故障叶片挥舞力矩平均值为变桨成功叶片的13.8倍,且前者的流场尾迹更为明显。此外,180°方位角变桨故障叶片较之0°方位角变桨故障叶片应力及叶尖位移分别减小29.8%和32.7%,一阶屈曲因子增加20.2%。     

The effect of delaminations on local buckling in wind turbine blades

In this article the effect of delaminations on the load carrying capacity of a large wind turbine blade is studied numerically. For this purpose an 8.65 m long blade section with different initial delaminations in the main spar was subjected to a flapwise dominated bending moment. The model was setup in Abaqus and cohesive elements were chosen for modelling delamination growth.For initial delaminations with a width of 30–50% of the cap width the study showed that delamination close to the surface started to grow in load ranges of normal operation conditions and led to local buckling modes. The local buckling caused high strains and stresses in the surrounding of the delamination, which exceeded the material design properties and therefore should be considered as dangerous.Delaminations placed near the mid-surface of the cap did not have a significant effect on the blade response under normal operation conditions. In the simulations the static load exceeded the design load by more than 40% before delamination growth or cap buckling occurred.It could be concluded that delamination induced near-surface buckling modes have to be considered critical due to an onset of local sublaminate buckling below the design load level.     

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

The 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.     

Combined analytical/FEA-based coupled aero structure simulation of a wind turbine with bend–twist adaptive blades

The simulation of wind turbines with bend–twist adaptive blades is a coupled aero-structure (CAS) procedure. The blade twist due to elastic coupling is a required parameter for wind turbine performance evaluation and can be predicted through a finite element (FE) structural analyser. FEA-based codes are far too slow to be useful in the aerodynamic design/optimisation of a blade. This paper presents a combined analytical/FEA-based method for CAS simulation of wind turbines utilising bend–twist adaptive blades. This method of simulation employs the induced twist distribution and the flap bending at the hub of the blade predicted through a FEA-based CAS simulation at a reference wind turbine run condition to determine the wind turbine performance at other wind turbine run conditions. This reduces the computational time significantly and makes the aerodynamic design/optimisation of bend–twist adaptive blades practical. Comparison of the results of a case study which applies both combined analytical/FEA-based and FEA-based CAS simulation shows that when using the combined method the required computational time for generating a power curve reduces to less than 5%, while the relative difference between the predicted powers by two methods is only about 1%.     

Wind gust detection and load mitigation using artificial neural networks assisted control

The design of a wind turbine implies the simulation of definite conditions as specified in the standards. Among those operational conditions, rare events such as extreme gusts or external faults are included, which may cause high structural loads. Such extreme design load cases usually drive the design of some of the main components of the wind turbine: tower, blades and mainframe. Two different strategies are hence presented to mitigate the loads, deriving from extreme load cases, on the basis of the detection of wind gusts by means of ad hoc synthesized artificial neural networks. This tool is embedded into the main control algorithm and allows it to detect the gust in advance, to anticipate the control reaction, and by doing so reducing extreme loads. One of the strategies performs a controlled stop when wind gust is detected. The other rides through wind gusts without stopping, i.e., without affecting the wind turbine normal operation. Aeroelastic simulations of the Alstom Wind's wind turbines using these techniques have shown significant reductions in the extreme loads for all standard IEC 61400‐1, edition 2 DLC 1.6 cases. In particular, the overall ultimate loads are largely reduced for blade root and tower base bending moments, with a direct impact on the structural design of those components. Copyright © 2013 John Wiley & Sons, Ltd.     

Estimating the angle of attack from blade pressure measurements on the NREL Phase VI rotor using a free wake vortex model: axial conditions

Blade element momentum (BEM) methods are still the most common methods used for predicting the aerodynamic loads during the aeroelastic design of wind turbine blades. However, their accuracy is limited by the availability of reliable aerofoil data. Owing to the 3D nature of the flow over wind turbine blades, the aerofoil characteristics will vary considerably from the 2D aerofoil characteristics, especially at the inboard sections of the blades. Detailed surface pressure measurements on the blade surfaces may be used to derive more realistic aerofoil data. However, in doing so, knowledge of the angle of attack distributions is required. This study presents a method in which a free wake vortex model is used to derive such distributions for the NREL Phase VI wind turbine under different operating conditions. The derived free wake geometry solutions are plotted together with the corresponding wake circulation distribution. These plots provide better insight into how circulation formed at the blades is eventually diffused into the wake. The free wake model is described and its numerical behaviour is examined. Copyright © 2006 John Wiley &Sons, Ltd.     

A review and design study of blade testing systems for utility-scale wind turbines

Since the blades are one of the most critical components of a wind turbine, representative samples must be experimentally tested in order to ensure that the actual performance of the blades is consistent with their specifications. In particular, it must be demonstrated that the blade can withstand both the ultimate loads and the fatigue loads to which the blade is expected to be subjected during its design service life. In general, there are basically two types of blade testing: static testing and fatigue (or dynamic) testing. This paper includes a summary review of different utility-scale wind turbine blade testing methods and the initial design study of a novel concept for tri-axial testing of large wind turbine blades. This new design is based on a blade testing method that excites the blade in flap-wise and edgewise direction simultaneously. The flap motion of the blade is caused by a dual-axis blade resonance excitation system (BREX). Edgewise motion is delivered by the use of two inclined hydraulic actuators and linear guide rail system is used to move the inclined actuators in the flap-wise direction along the blade motion. The hydraulic system and linear guide rail requirements are analyzed and an initial cost estimate of the proposed system is presented. Recommendations for future work on this proposed system are given in the final section of this work.     

Development and validation of a new joint system for sectional blades

The development and validation of a new joint system for sectional blades is presented in this paper. The system is a bolted connection located in the blade spar cap, and its geometry accommodates a higher number of bolts than any other conventional solution requiring inserts. This results in higher transmitted loads per spar cap width. The modular concept of the system is composed of cell units that can be easily integrated into the design and manufacturing of different blade architectures and used in blades of various lengths by selecting the appropriate number of cell units. The design of the system was performed using analytical and finite element analyses. The analyses were validated using risk reduction mechanical tests, which resulted in an optimized joint system. The loads to be transmitted by the joint system were calculated according to IEC61400‐1 using the GH‐Bladed Version 3.80 on a 5 MW onshore class II‐A wind turbine with a tower height of 120 m and a 61.5 m blade with double spar webs. The joint system was successfully tested in a full‐scale mechanical test for its validation under Germanischer Lloyd guidelines and supervision. Copyright © 2014 John Wiley & Sons, Ltd.     

Aeroelastic multidisciplinary design optimization of a swept wind turbine blade

Mitigating loads on a wind turbine rotor can reduce the cost of energy. Sweeping blades produces a structural coupling between flapwise bending and torsion, which can be used for load alleviation purposes. A multidisciplinary design optimization (MDO) problem is formulated including the blade sweep as a design variable. A multifidelity approach is used to confront the crucial effects of structural coupling on the estimation of the loads. During the MDO, ultimate and damage equivalent loads are estimated using steady‐state and frequency‐domain–based models, respectively. The final designs are verified against time‐domain full design load basis aeroelastic simulations to ensure that they comply with the constraints. A 10‐MW wind turbine blade is optimized by minimizing a cost function that includes mass and blade root flapwise fatigue loading. The design space is subjected to constraints that represent all the necessary requirements for standard design of wind turbines. Simultaneous aerodynamic and structural optimization is performed with and without sweep as a design variable. When sweep is included in the MDO process, further minimization of the cost function can be obtained. To show this achievement, a set of optimized straight blade designs is compared to a set of optimized swept blade designs. Relative to the respective optimized straight designs, the blade mass of the swept blades is reduced of an extra 2% to 3% and the blade root flapwise fatigue damage equivalent load by a further 8%.     

Changes in design driving load cases: Operating an upwind turbine with a downwind rotor configuration

This work considers the design driving load cases from a full design load basis analysis on an upwind turbine changed into a downwind configuration. The upwind turbine is a commercial class IIIA 2.1‐MW turbine, manufactured by Suzlon. The downwind turbine shows an increase in the normalized tower clearance by 6%, compared with the upwind concept. Removing the blade prebend increases the normalized minimum tower clearance by 17% in the downwind configuration compared with the upwind configuration. The extreme loads on the longitudinal tower bottom bending moment are seen to generally increase by 17% because of the overhanging gravity moment of the rotor‐nacelle assembly. The extreme blade root bending moments are reduced by 10% flapwise, because of the coning of the rotor in downwind direction. The fatigue loads suffer from the tower shadow, leading to an overall increase of the fatigue loads in the blades with up to 5% in flapwise direction in the downwind configuration. Because of blade deflection and coning direction, the downwind configuration shows a 0.75% lower annual energy production. Removing the prebend increases the annual energy production loss to 1.66%.     

风力机叶片静力测试与分析

利用风力机叶片动力特性实验台测试了风力机叶片在载荷作用下的形变特性,通过改变加载力的大小和加载位置进行多组实验,计算各个截面的弯矩;通过实验和有限元静力分析找出了风力机叶片主要承力部件,对风力机叶片的设计和制造提供了参考依据,对提高风力机的总体性能和优化设计具有重要意义。     

Review of morphing concepts and materials for wind turbine blade applications

With increasing size of wind turbines, new approaches to load control are required to reduce the stresses in blades. Experimental and numerical studies in the fields of helicopter and wind turbine blade research have shown the potential of shape morphing in reducing blade loads. However, because of the large size of modern wind turbine blades, more similarities can be found with wing morphing research than with helicopter blades. Morphing technologies are currently receiving significant interest from the wind turbine community because of their potential high aerodynamic efficiency, simple construction and low weight. However, for actuator forces to be kept low, a compliant structure is needed. This is in apparent contradiction to the requirement for the blade to be load carrying and stiff. This highlights the key challenge for morphing structures in replacing the stiff and strong design of current blades with more compliant structures. Although not comprehensive, this review gives a concise list of the most relevant concepts for morphing structures and materials that achieve compliant shape adaptation for wind turbine blades.Copyright © 2012 John Wiley & Sons, Ltd.     

Dynamic analysis tool development for swept wind turbine blades

Because of their aeroelastic behavior, swept wind turbine blades offer the potential to increase energy capture and lower fatigue loads. This article describes work to develop a dynamic analysis code for swept wind turbine blades. This work was an outgrowth of a U.S. Department of Energy contract on swept blades, where the authors used the Adams? dynamic software (MSC Software Corporation, Santa Ana, CA, USA). The new code is based on the National Renewable Energy Laboratory's FAST code and allows for lower cost analysis and faster computation times for swept blades. The additions to the FAST code include the geometry and mode shapes required for the bending and twisting motion of the swept blade. In addition, a finite element program to determine mode shapes for the swept blade was developed. Comparisons of results obtained with the new code and analytical solutions for a curved cantilever beam show good agreement in local torsional deflections. Comparisons with field data obtained for a 750 kW wind turbine with swept blades were complicated by uncertainties in the test wind speed and turbine controller settings.Copyright © 2012 John Wiley & Sons, Ltd.     

Structural investigation of composite wind turbine blade considering various load cases and fatigue life

This study proposes a structural design for developing a medium scale composite wind turbine blade made of E-glass/epoxy for a 750 kW class horizontal axis wind turbine system. The design loads were determined from various load cases specified at the IEC61400-1 international specification and GL regulations for the wind energy conversion system. A specific composite structure configuration, which can effectively endure various loads such as aerodynamic loads and loads due to accumulation of ice, hygro-thermal and mechanical loads, was proposed. To evaluate the proposed composite wind turbine blade, structural analysis was performed by using the finite element method. Parametric studies were carried out to determine an acceptable blade structural design, and the most dominant design parameters were confirmed. In this study, the proposed blade structure was confirmed to be safe and stable under various load conditions, including the extreme load conditions. Moreover, the blade adapted a new blade root joint with insert bolts, and its safety was verified at design loads including fatigue loads. The fatigue life of a blade that has to endure for more than 20 years was estimated by using the well-known S–N linear damage theory, the service load spectrum, and the Spera's empirical equations. With the results obtained from all the structural design and analysis, prototype composite blades were manufactured. A specific construction process including the lay-up molding method was applied to manufacturing blades. Full-scale static structural test was performed with the simulated aerodynamic loads. From the experimental results, it was found that the designed blade had structural integrity. In addition, the measured results of deflections, strains, mass, and radial center of gravity agreed well with the analytical results. The prototype blade was successfully certified by an international certification institute, GL (Germanisher Lloyd) in Germany.     

A comparative study on dynamic responses of spar‐type floating horizontal and vertical axis wind turbines

Interest in the exploitation of offshore wind resources using floating wind turbines has increased. Commercial development of floating horizontal axis wind turbines (FHAWTs) is emerging because of their commercial success in onshore and near‐shore areas. Floating vertical axis wind turbines (FVAWTs) are also promising because of their low installation and maintenance costs. Therefore, a comparative study on the dynamic responses of FHAWTs and FVAWTs is of great interest. In the present study, a FHAWT employing the 5MW wind turbine developed by the National Renewable Energy Laboratory (NREL) and a FVAWT employing a Darrieus rotor, both mounted on the OC3 spar buoy, were considered. An improved control strategy was introduced for FVAWTs to achieve an approximately constant mean generator power for the above rated wind speeds. Fully coupled time domain simulations were carried out using identical, directional aligned and correlated wind and wave conditions. Because of different aerodynamic load characteristics and control strategies, the FVAWT results in larger mean tower base bending moments and mooring line tensions above the rated wind speed. Because significant two‐per‐revolution aerodynamic loads act on the FVAWT, the generator power, tower base bending moments and delta line tensions show prominent two‐per‐revolution variation. Consequently, the FVAWT suffers from severe fatigue damage at the tower bottom. However, the dynamic performance of the FVAWT could be improved by increasing the number of blades, using helical blades or employing a more advanced control strategy, which requires additional research. Copyright © 2016 John Wiley & Sons, Ltd.