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.     

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.     

The Adaptive Camber Concept—A passive approach for gust load alleviation on wind turbines

A new passive sectional approach to alleviate gust loads on wind turbines—the Adaptive Camber Concept— is introduced. The concept entails fluid‐structure interaction, where flow conditions at the leading edge affect the airfoil shape and vice versa. A two‐dimensional airfoil equipped with the Adaptive Camber Concept is experimentally investigated under steady and unsteady conditions in a wind tunnel. Under steady conditions, the adaptive camber airfoil de‐cambers gradually with increasing angle of attack, yielding a lift curve with declined slope. Unsteady angle of attack fluctuations of various reduced frequencies are generated by means of an active grid. Under unsteady conditions, the adaptive camber airfoil is found to alleviate up to 60% of the fluctuating loads, while generating higher mean lift compared to a rigid reference airfoil.     

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.     

Fluid‐structure interaction of a morphing symmetrical wind turbine blade subjected to variable load

A brief summary of the main challenges of rotor design in wind energy conversion (WEC) systems, most notably the horizontal axis wind turbine (HAWT), are presented. One of the limiting factors in HAWT design is choosing the rated capacity to maximize power output and turbine longevity. One such strategy to accomplish this goal is to widen the operational range of the WEC system by using pitch or torque control, which can be costly and subject to mechanical failure. We present a morphing airfoil concept, which passively controls airfoil pitch through elastic deformation. As a justification of the concept, a two‐dimensional fluid‐structure interaction routine is used to simulate the aeroelastic response of a symmetric NACA 0012 blade subjected to variable loading. The results suggest that the morphing blade can be designed to offer superior average lift to drag ratios over a specified range of attack angles by up to 4.2%, and possibly even higher. This infers that the morphing blade design can increase the power production of WEC systems while conceivably reducing cost because the passive deformation of the morphing turbine does not require active control systems that come at an added upfront and maintenance cost. Copyright © 2011 John Wiley & Sons, Ltd.     

Microtab dynamic modelling for wind turbine blade load rejection

A dynamic model characterising the effect of microtab deployment on the aerodynamics of its base aerofoil is presented. The developed model predicts the transient aerodynamic coefficients consistent with the experimental and computational data reported in the literature. The proposed model is then used to carry out investigation on the effectiveness of microtabs in load alleviation and lifespan increase of wind turbine blades. Simulating a bang–bang controller, different load rejection scenarios are examined and their effect on blade lifespan is investigated. Results indicate that the range of frequencies targeted for rejection can significantly impact the blade fatigue life.Case studies are carried out to compare the predicted load alleviation amount and the blade lifespan using the developed model with those obtained by other researchers using the steady state model. It is shown that the assumption of an instantaneous aerodynamic response as used in the steady state model can lead to inaccurate results.     

A morphing downwind‐aligned rotor concept based on a 13‐MW wind turbine

To alleviate the mass‐scaling issues associated with conventional upwind rotors of extreme‐scale wind turbines (≥10 MW), a morphing downwind‐aligned rotor (MoDaR) concept is proposed herein. The concept employs a downwind rotor with blades whose elements are stiff (no intentional flexibility) but with hub‐joints that can be unlocked to allow for moment‐free downwind alignment. Aligning the combination of gravitational, centrifugal and thrust forces along the blade path reduces downwind cantilever loads, resulting in primarily tensile loading. For control simplicity, the blade curvature can be fixed with a single morphing degree of freedom using a near‐hub joint for coning angle: 22° at rated conditions. The conventional baseline was set as the 13.2‐MW Sandia 100‐m all glass blade in a three‐bladed upwind configuration. To quantify potential mass savings, a downwind load‐aligning, two‐bladed rotor was designed. Because of the reduced number of blades, the MoDaR concept had a favorable 33% mass reduction. The blade reduction and coning led to a reduction in rated power, but morphing increased energy capture at lower speeds such that both the MoDaR and conventional rotors have the same average power: 5.4 MW. A finite element analysis showed that quasi‐steady structural stresses could be reduced, over a range of operating wind speeds and azimuthal angles, despite the increases in loading per blade. However, the concept feasibility requires additional investigation of the mass, cost and complexity of the morphing hinge, the impact of unsteady aeroelastic influence because of turbulence and off‐design conditions, along with system‐level Levelized Cost of Energy analysis. Copyright © 2015 John Wiley & Sons, Ltd.     

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.     

Electric power generation based on variable speed wind turbine under load disturbance

This study was interested in the management of an energy production unit. A variable speed wind turbine (VSWT) was used as a principal source and a supercapacitor (SC) module was used as an energy storage system. Both were connected through a direct current bus. This unit was supplying a three-phase load using an inverter and an inductor and capacitor filter. In order to regulate the direct current bus voltage, the SC storage state was controlled by using a buck-boost converter according to load instructions and wind speed fluctuations. Then, a resonant controller was established to avoid any disturbances and to control the alternating line-to-line voltages of the load which may be unbalanced. This study has shown that the stability of the three-phase voltage source depends on the direct current bus power management and also on the line-to-line voltage control. Simulation results are presented to validate the efficiency of the control strategies used.     

弯扭耦合作用下变频风机叶片断裂分析与治理技术研究

针对某变频运行风机发生的叶片断裂故障,采用无线应变测试技术获取了不同工况下转轴扭矩脉动特性。建立叶轮叶片有限元模型,获取风机各阶模态特性;将实测脉动扭矩作为激励,计算扭矩脉动下叶片所承受的应力特性。结果表明：电机输出谐波可能激发叶片轮盘弯扭耦合共振,导致叶片局部应力增大,进而疲劳断裂;叶片顶部加之字形加强筋可以有效减小叶片振动,预防断裂。     

Full‐scale test of trailing edge flaps on a Vestas V27 wind turbine: active load reduction and system identification

A full‐scale test was performed on a Vestas V27 wind turbine equipped with one active 70 cm long trailing edge flap on one of its 13 m long blades. Active load reduction could be observed in spite of the limited spanwise coverage of the single active trailing edge flap. A frequency‐weighted model predictive control was tested successfully on this demonstrator turbine. An average flapwise blade root load reduction of 14% was achieved during a 38 minute test, and a reduction of 20% of the amplitude of the 1P loads was measured. A system identification test was also performed, and an identified linear model, from trailing edge flap angle to flapwise blade root moment, was derived and compared with the linear analytical model used in the model predictive control design model. Flex5 simulations run with the same model predictive control showed a good correlation between the simulations and the measurements in terms of flapwise blade root moment spectral densities, in spite of significant differences between the identified linear model and the model predictive control design model. Copyright © 2013 John Wiley & Sons, Ltd.