Active load alleviation potential of adaptive wind turbine blades using shape memory alloy actuators

Upscaling of wind turbine blades calls for implementation of innovative active load control concepts that will facilitate the flawless operation of the machine and reduce the fatigue and ultimate loads that hinder its service life. Based on aeroelastic simulations that prove the enhanced capabilities of combined individual pitch and individual flap control at global wind turbine scale level, a shape adaptive concept that encompasses an articulated mechanism consisting of two subparts is presented. Shape memory alloy (SMA) actuators are investigated and assessed as means to control the shape adaptive mechanism at airfoil section level in order to alleviate the developed structural loads. The concept is embedded in the trailing edge region of the blade of a 10‐MW horizontal axis wind turbine and acts as a flap mechanism. Numerical simulations are performed considering various wind velocities and morphing target shapes and trajectories for both normal and extreme turbulence conditions. The results prove the potential of the concept, since the SMA controlled actuators can accurately follow the target trajectories. Power requirements are estimated at 0.22% of the AEP of the machine, while fatigue and ultimate load reduction of the flap‐wise bending moment at the blade root is 27.6% and 7.4%, respectively.     

Trailing edge noise reduction of wind turbine blades by active flow control

In the current study, we investigate a route to reduction of the turbulent boundary layer–trailing edge interaction noise. The trailing edge noise is generated by surface pressure fluctuations beneath a turbulent boundary and scattered at the trailing edge of wind turbine blades. Trailing edge noise is considered to be the dominant noise source of modern wind turbines. Therefore, efforts are constantly made to attenuate the noise. Today, noise emission can be reduced by proper airfoil design or passive devices, such as trailing edge serrations. A further improved candidate technology for trailing edge noise attenuation is active flow control in the form of wall‐normal suction. With active flow control, the boundary layer features responsible for trailing edge noise generation can be manipulated, and correspondingly the trailing edge noise can be reduced. Detailed experimental investigations were performed at the Universities of Tel‐Aviv and Stuttgart. The tests showed that steady wall‐normal suction has a positive effect on the trailing edge noise by reducing the boundary layer thickness, and with it the integral length scales of the eddies within the boundary layer. Copyright © 2014 John Wiley & Sons, Ltd.     

A decoupling control strategy for wind turbine blades equipped with active flow controllers

The use of active controls has shown to be of substantial help in supporting the increasing size of wind turbines by reducing peak stresses and fatigue loads. In this respect, this paper proposes the use of intuitive frequency‐based control strategies for reducing loads in wind turbine blades equipped with multi‐input multi‐output (MIMO) active flow controllers. For that purpose, a loop‐shaping approach is considered for analysing the dynamic of actively controlled wind turbine blades. Preliminary aeroelastic simulations are carried out to validate the results. It is shown that the MIMO vibration control problem can effectively be decomposed into a number of decoupled single‐input single‐output control problems because of the strong correlation between the dominant aeroelastic blade dynamics and actuator deployments. As a result, it is demonstrated that classical single‐input single‐output control systems can perform as efficiently as MIMO controllers for damping the aeroelastic dynamics of wind turbine blades. Copyright © 2016 John Wiley & Sons, Ltd.     

Fracture analysis of adhesive joints in wind turbine blades

Modern wind turbine rotor blades are usually made from fibre‐reinforced composite subcomponents. In the final assembly stage, these subcomponents are bonded together by several adhesive joints. One important adhesive joint is situated at the trailing edge, which refers to the downstream edge where the air‐flow rejoins and leaves the blade. Maintenance inspections of wind turbine rotor blades show that among other forms of damage, local debonding of the shells along the trailing edge is a frequent failure type. The cause of trailing edge failure in wind turbine blades is complex, and detailed information is scarce. This paper is concerned with the fracture analysis of adhesive joints in general, with a particular focus on trailing edges. For that, the energy release rates in prescribed cracks present in the bond line of a generic trailing edge joint are investigated. In connection with this examination, the paper elucidates the influence of geometrical non‐linearity in form of local buckling on both the increase of the energy release rate and the change of mode mixity. First, experimental results on adhesively bonded small‐scale subcomponents are presented. Thereafter, a practical approach is presented, which links the experimental results conducted on a small scale to the numerical failure prediction of large‐scale models. The proposed method is based on the virtual crack closure technique and defines the mode mixity at bimaterial interfaces unambiguously. The method is consequently applied to a wind turbine blade submodel in order to predict crack growth in the trailing edge. Thereby, the influence of different crack lengths on crack initiation and propagation is considered. The paper concludes with general thoughts on adhesively bonded trailing edge joints regarding the prevention of local debonding. Copyright © 2014 John Wiley & Sons, Ltd.     

A comprehensive investigation of trailing edge damage in a wind turbine rotor blade

Wind turbine rotor blades are sophisticated, multipart, lightweight structures whose aeroelasticity‐driven geometrical complexity and high strength‐to‐mass utilization lend themselves to the application of glass‐fibre or carbon‐fibre composite materials. Most manufacturing techniques involve separate production of the multi‐material subcomponents of which a blade is comprised and which are commonly joined through adhesives. Adhesive joints are known to represent a weak link in the structural integrity of blades, where particularly, the trailing‐edge joint is notorious for its susceptibility to damage. Empiricism tells that adhesive joints in blades often do not fulfil their expected lifetime, leading to considerable expenses because of repair or blade replacement. Owing to the complicated structural behaviour—in conjunction with the complex loading situation—literature about the root causes for adhesive joint failure in blades is scarce. This paper presents a comprehensive numerical investigation of energy release rates at the tip of a transversely oriented crack in the trailing edge of a 34m long blade for a 1.5MW wind turbine. First, results of a non‐linear finite element analysis of a 3D blade model, compared with experimental data of a blade test conducted at Danmarks Tekniske Universitet (DTU) Wind Energy (Department of Wind Energy, Technical University of Denmark), showed to be in good agreement. Subsequently, the effects of geometrical non‐linear cross‐section deformation and trailing‐edge wave formation on the energy release rates were investigated based on realistic aeroelastic load simulations. The paper concludes with a discussion about critical loading directions that trigger two different non‐linear deformation mechanisms and their potential impact on adhesive trailing‐edge joint failure. Copyright © 2016 John Wiley & Sons, Ltd.     

Aeroelastic design of large wind turbine blades considering damage tolerance

Modern offshore turbine blades can be designed for high fatigue life and damage tolerance to avoid excessive maintenance and therefore significantly reduce the overall cost of offshore wind power. An aeroelastic design strategy for large wind turbine blades is presented and demonstrated for a 100 m blade. High fidelity analysis techniques like 3D finite element modeling are used alongside beam models of wind turbine blades to characterize the resulting designs in terms of their aeroelastic performance as well as their ability to resist damage growth. This study considers a common damage type for wind turbine blades, the bond line failure, and explores the damage tolerance of the designs to gain insight into how to improve bond line failure through aeroelastic design. Flat‐back airfoils are also explored to improve the damage tolerance performance of trailing‐edge bond line failures. Copyright © 2016 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.     

A practical approach to fracture analysis at the trailing edge of wind turbine rotor blades

Wind turbine rotor blades are commonly manufactured from composite materials by a moulding process. Typically, the wind turbine blade is produced in two halves, which are eventually adhesively joined along their edges. Investigations of operating wind turbine blades show that debonding of the trailing edge joint is a common failure type, and information on specific reasons is scarce. This paper is concerned with the estimation of the strain energy release rates (SERRs) in trailing edges of wind turbine blades in order to gain insight into the driving failure mechanisms. A method based on the virtual crack closure technique (VCCT) is proposed, which can be used to identify critical areas in the adhesive joint of a trailing edge. The paper gives an overview of methods applicable for fracture cases comprising non‐parallel crack faces in the realm of linear fracture mechanics. Furthermore, the VCCT is discussed in detail and validated against numerical analyses in 2D and 3D. Finally, the SERR of a typical blade section subjected to various loading conditions is investigated and assessed in order to identify potential design drivers for trailing edge details. Analysis of the blade section model suggests that mode III action is governing and accordingly that flapwise shear and torsion are the most important load cases.Copyright © 2013 John Wiley & Sons, Ltd.     

Aerofoil trailing‐edge noise prediction models for wind turbine applications

This paper proposes a modified TNO model for the prediction of aerofoil trailing‐edge noise for wind turbine applications. The capabilities of the current modified model and four variants of the TNO model are analysed through a comprehensive study which includes 10 aerofoils and involves two different wind tunnels. The Reynolds numbers considered are between 1.13 and 3.41 million, and the effective angles of attack are between ?2.20° and 13.58°. The merit of a model is assessed by comparing two aspects of the numerically predicted and the experimentally measured sound pressure level spectra: the sound pressure level difference between two different aerofoils at similar lift coefficients within a certain frequency range (referred to as the delta noise); and the closeness in terms of spectral magnitude and shape of the predicted and measured sound pressure level spectra. The current modified model is developed by deriving new formulations for the computation of the wall pressure fluctuation spectrum. This is achieved by using the approximate ratio of the normal Reynolds stress components for an anisotropic flow over a flat plate to estimate the vertical Reynolds stress component, and by introducing new stretching factors to take the effects of turbulent flow anisotropy into account. Compared with the four TNO model variants tested, the current modified model has strong delta noise prediction ability, and is able to predict sound pressure level spectra that are more consistent and closer to measurements for the vast majority of aerofoils and flow conditions tested in the two wind tunnels. Copyright © 2017 John Wiley & Sons, Ltd.     

Redefining the design objectives of large offshore wind turbine rotors

Wind turbine rotors are normally designed such that rotor power coefficient is maximized. Much of this methodology has been inherited from the aviation industry. This paper points out that designing machines for maximum rotor aerodynamic efficiency does not necessarily lead to a lower levelized cost of energy. The argument sits on the premise that levelized cost of energy is strongly influenced by machine capital expenditure (CAPEX) and annual energy production (AEP). We therefore assume that the true design objective is to minimize the CAPEX/AEP ratio. The basis of an alternative design path is presented, which centres on the minimization of total volume of structural material in the wind turbine. This is done whilst maintaining a given rated power. This alternative methodology requires the removal of conventional pre‐set design variables and assumptions which relate to the maximization of rotor power coefficient. We examine how changing chord length, axial induction factor and aerofoil lift coefficient affect material volume in the blade. Following this, we use a custom‐made blade element momentum programme to explore the relative CAPEX of machines with varying design axial induction factor and varying lift coefficient. This relative cost is calibrated to the 5 MW National Renewable Energy Laboratory offshore reference turbine. The effects on the rotor, drivetrain and tower are considered. For a 5 MW offshore machine, it is shown that an overall CAPEX/AEP reduction of over 2% can be achieved by using a low‐induction rotor with blades possessing aerofoils operating at non‐peak lift to drag ratios. This economy is delivered notwithstanding a 2.3% drop in design rotor power coefficient. Copyright © 2014 John Wiley & Sons, Ltd.     

Structural health monitoring for delamination detection and location in wind turbine blades employing guided waves

Wind power is becoming one of the most important renewable energies in the world. The reduction in operating and maintenance costs of the wind turbines has been identified as one of the biggest challenges to establish this energy as an alternative to fossil fuels. Predictive maintenance can detect a potential failure at an early stage reducing operating costs. Structural health monitoring together with non‐destructive techniques are an effective method to detect incipient delamination in wind turbine blades. Ultrasonic guided waves offer possibilities to inspect delamination and disunion between layers in composite structures. Delamination results in a concentration of tensions in certain areas near the fault, which can propagate and create the total break of the blade. This paper presents a new approach for disunity detection between layers comparing two real blades, also new in the literature, one of them built with three disbonds introduced in its manufacturing process. The signals are denoised by Daubechies wavelet transform. The threshold for the denoising is obtained by a wavelet coefficients selection rule using the Birgé‐Massart penalization method. The signals were normalized and their envelopes were obtained by Hilbert transform. Finally, a pattern recognition based on correlations was applied.     

System identification and controller design for individual pitch and trailing edge flap control on upscaled wind turbines

Active load reduction strategies such as individual pitch control (IPC) and trailing edge flap (TEF) actuation present ways of reducing the fatigue loads on the blades of wind turbines. This may enable development of lighter blades, improving the performance, cost effectiveness and viability of future multi‐megawatt turbine designs. Previous investigations into the use of IPC and TEFs have been limited to turbines with ratings up to 5 MW and typically investigate the use of these load reduction strategies on a single turbine only. This paper extends the design, implementation and analysis of individual pitch and TEFs to a range of classically scaled turbines between 5 and 20 MW. In order to avoid designing controllers which favour a particular scale, identical scale‐invariant system identification and controller design processes are applied to each of the turbines studied. Gain‐scheduled optimal output feedback controllers are designed using identified models to target blade root load fluctuations at the first and second multiples of the rotational frequency using IPC and TEFs respectively. The use of IPC and TEFs is shown in simulations to provide significant reductions in fatigue loads at the blade root. Fatigue loads on non‐rotating components such as the yaw bearing and tower root (yaw moment) are also reduced with the use of TEFs. Individual pitch performance is seen to be slightly lower on larger turbines, potentially due to a combination of reduced actuator bandwidth and movement of the rotational frequency of larger turbines into a more energetic part of the turbulent spectrum. However, TEF performance is consistent irrespective of scale. Copyright © 2015 John Wiley & Sons, Ltd.     

Structural design of spars for 100-m biplane wind turbine blades

Large wind turbine blades are being developed at lengths of 75–100 m, in order to improve energy capture and reduce the cost of wind energy. Bending loads in the inboard region of the blade make large blade development challenging. The “biplane blade” design was proposed to use a biplane inboard region to improve the design of the inboard region and improve overall performance of large blades. This paper focuses on the design of the internal “biplane spar” structure for 100-m biplane blades. Several spars were designed to approximate the Sandia SNL100-00 blade (“monoplane spar”) and the biplane blade (“biplane spar”). Analytical and computational models are developed to analyze these spars. The analytical model used the method of minimum total potential energy; the computational model used beam finite elements with cross-sectional analysis. Simple load cases were applied to each spar and their deflections, bending moments, axial forces, and stresses were compared. Similar performance trends are identified with both the analytical and computational models. An approximate buckling analysis shows that compressive loads in the inboard biplane region do not exceed buckling loads. A parametric analysis shows biplane spar configurations have 25–35% smaller tip deflections and 75% smaller maximum root bending moments than monoplane spars of the same length and mass per unit span. Root bending moments in the biplane spar are largely relieved by axial forces in the biplane region, which are not significant in the monoplane spar. The benefits for the inboard region could lead to weight reductions in wind turbine blades. Innovations that create lighter blades can make large blades a reality, suggesting that the biplane blade may be an attractive design for large (100-m) blades.     

Effect of rotor–tower interaction,tilt angle,and yaw misalignment on the aeroelasticity of a large horizontal axis wind turbine with composite blades

This paper presents a detailed analysis of the rotor–tower interaction and the effects of the rotor's tilt angle and yaw misalignment on a large horizontal axis wind turbine. A high‐fidelity aeroelastic model is employed, coupling computational fluid dynamics (CFD) and structural mechanics (CSM). The wind velocity stratification induced by the atmospheric boundary layer (ABL) is modeled. On the CSM side, the complex composite structure of each blade is accurately modeled using shell elements. The rotor–tower interaction is analyzed by comparing results of a rotor‐only simulation and a full‐machine simulation, observing a sudden drop in loads, deformations, and power production of each blade, when passing in front of the tower. Subsequently, a tilt angle is introduced on the rotor, and its effect on blade displacements, loads, and performance is studied, representing a novelty with respect to the available literature. The tilt angle leads to a different contribution of gravity to the blade deformations, sensibly affecting the stresses in the composite material. Lastly, a yaw misalignment is introduced with respect to the incoming wind, and the resulting changes in the blade solicitations are analyzed. In particular, a reduction of the blade axial displacement amplitude during each revolution is observed.     

Overall design optimization of dedicated outboard airfoils for horizontal axis wind turbine blades

Designing the primary airfoils for the outboard part of wind turbine blades is a complicated problem of balancing structural, aerodynamic, and acoustic requirements. This paper presents an optimization method for the overall performance of outboard wind turbine airfoils. Based on the complex flow characteristics of the rotor blades and the varying requirements along the span of a blade, the design principles of outboard airfoils were investigated. The requirements for improving the structural performance and reducing the aerodynamic noise were combined with the following aerodynamic design considerations: high efficiency, low extreme loads, stability, and a wide operating region. Thus, this paper proposes a new mathematical model for overall airfoil optimization using the airfoil performance evaluation indicators. Then, an integrated optimization design platform is established for outboard airfoils. Through 2 design cases, new airfoils with desirable aerodynamic characteristics and improved overall performance were obtained. Comparisons between the new airfoils and reference airfoils based on numerical predictions indicate that the proposed method with the newly established mathematical model can effectively balance the complex requirements of the airfoil and improve its overall performance. More notably, the design cases also indicate that the established optimization design method can be used to address special designs of outboard airfoils for different blade requirements.     

On the proof of concept of a ‘Smart’ wind turbine rotor blade for load alleviation

The trend with offshore wind turbines is to increase the rotor diameter as much as possible to decrease the costs per kilowatt‐hour. The increasing dimensions have led to the relative increase of the loads on the wind turbine structure. Because of the increasing rotor size and the spatial load variations along the blade, it is necessary to react to turbulence in a more detailed way; each blade separately and at several separate radial distances. In this paper, a proof of concept study is performed to show the feasibility of the load alleviation abilities of a ‘Smart’ blade, i.e. a blade equipped with a number of control devices that locally change the lift profile on the blade, combined with appropriate sensors and feedback controllers. Theoretical and experimental models are developed of a scaled non‐rotating rotor blade which is equipped with two trailing edge flaps and strain sensors to facilitate feedback control. A pitch actuator is used to induce disturbances with a similar character as a gust or turbulence. A feedback controller based on classical loop shaping is designed that minimizes the root bending moment in the flapping direction. We show that with appropriate control techniques, the loads for periodic disturbances and for turbulence generated disturbances can be reduced up to 90 and 55%, respectively. Copyright © 2008 John Wiley & Sons, Ltd.     

Sizing and control of trailing edge flaps on a smart rotor for maximum power generation in low fatigue wind regimes

An extension of the spectrum of applicability of rotors with active aerodynamic devices is presented in this paper. Besides the classical purpose of load alleviation, a secondary objective is established: optimization of power capture. As a first step, wind speed regions that contribute little to fatigue damage have been identified. In these regions, the turbine energy output can be increased by deflecting the trailing edge (TE) flap in order to track the maximum power coefficient as a function of local, instantaneous speed ratios. For this purpose, the TE flap configuration for maximum power generation has been using blade element momentum theory. As a first step, the operation in non‐uniform wind field conditions was analysed. Firstly, the deterministic fluctuation in local tip speed ratio due to wind shear was evaluated. The second effect is associated with time delays in adapting the rotor speed to inflow fluctuations caused by atmospheric turbulence. The increase in power generation obtained by accounting for wind shear has been demonstrated with an increase in energy production of 1%. Finally, a control logic based on inflow wind speeds has been devised, and the potential of enhanced power generation has been shown by time‐domain simulations. Copyright © 2015 John Wiley & Sons, Ltd.     

Numerical validation of a finite element thin‐walled beam model of a composite wind turbine blade

This paper presents a numerical validation of a thin‐walled beam (TWB) finite element (FE) model of a realistic wind turbine rotor blade. Based on the theory originally developed by Librescu et al. and later extended to suit FE modelling by Phuong, Lee and others, this computationally efficient yet accurate numerical model is capable of capturing most of the features found in large blades including thin‐walled hollow cross section with variable thickness along the section's contour, inner reinforcements, arbitrary material layup and non‐linear anisotropic fibre‐reinforced composites; the present application is, for the time being, restricted to linearity. This one‐dimensional (1D) FE model allows retaining information of different regions of the blade's shell and therefore approximates the behaviour of more complex three‐dimensional (3D) shell or solid FE models more accurately than typical 1D FE beam models. A 9.2 m rotor blade, previously reported in specialized literature, was chosen as a case study to validate the static and dynamic behaviour predicted by a TWB model against an industry‐standard 3D shell model built in a commercial software tool. Given the geometric and material complexities involved, an excellent agreement was found for static deformation curves, as well as a good prediction of the lowest frequency modes in terms of resonance frequencies, mode shapes and frequency response functions; the highest (sixth) frequency mode shows only a fair agreement as expected for an FE model. It is concluded that despite its simplicity, a TWB FE model is sufficiently accurate to serve as a design tool for the recursive analyses required during design and optimization stages of wind turbines using only readily available computational tools. Copyright © 2011 John Wiley & Sons, Ltd.     

Non-destructive and contactless defect detection inside leading edge coatings for wind turbine blades using mid-infrared optical coherence tomography

Leading edge erosion of wind turbine blades is one of the most critical issues in wind energy production, resulting in lower efficiency, as well as increased maintenance costs and downtime. Erosion is initiated by impacts from rain droplets and other atmospheric particles, so to protect the blades, special protective coatings are applied to increase their lifetime without adding significantly to the weight or friction of the blade. These coatings should ideally absorb and distribute the force away from the point of impact; however, microscopic defects, such as bubbles, reduce the mechanical performance of the coating, leading to cracks and eventually erosion. In this work, mid-infrared (MIR) Optical Coherence Tomography (OCT) is investigated for non-destructive, contactless inspection of coated glass-fiber composite samples to identify subsurface coating defects. The samples were tested using rubber projectiles to simulate rain droplet and particle impacts. The samples were subsequently imaged using OCT, optical microscopy, and X-ray tomography. OCT scanning revealed both bubbles and cracks below the surface, which would not have been detected using ultrasonic or similar non-destructive methods. In this way, OCT can complement the existing quality control in turbine blade manufacturing, help improve the blade lifetime, and reduce the environmental impact from erosion.     

Wireless monitoring algorithm for wind turbine blades using Piezo‐electric energy harvesters

Wind turbine blade failure can be catastrophic and lead to unexpected power interruptions. In this paper, a Structural Health Monitoring (SHM) algorithm is presented for wireless monitoring of wind turbine blades. The SHM algorithm utilizes accumulated strain energy data, such as would be acquired by piezoelectric materials. The SHM algorithm compares the accumulated strain energy at the same position on the three blades. This exploits the inherent triple redundancy of the blades and avoids the need for a structural model of the blade. The performance of the algorithm is evaluated using probabilistic metrics such as detection probability (True Positive) and false alarm rate (False Positive). The decision time is chosen to be sufficiently long that a particular damage level can be detected even in the presence of system sensor noise and wind variations. Finally, the proposed algorithm is evaluated with a case study of a utility‐scale turbine. The noise level is based on measurements acquired from strain sensors mounted on the blades of a Clipper Liberty C96 turbine. Strain energy changes associated with damage from matrix cracking and delamination are simulated with a finite element model. The case study demonstrates that the proposed algorithm can detect damage with a high probability based on a decision time period of approximately 50–200 days. Copyright © 2016 John Wiley & Sons, Ltd.