Passive load alleviation on wind turbine blades from aeroelastically driven selectively compliant morphing

Load alleviation control is highly desirable to reduce penalties associated with the added structural mass required to withstand rare load scenarios. This is particularly true for wind turbine designs incorporating long‐span blades. Implementation of compliance‐based morphing structures to modify the lift distribution passively has the potential to mitigate the impact of rare, but integrally threatening, loads on wind turbine blades while limiting the addition of actuation and sensing systems. We present a novel passive load alleviation concept based on a morphing flap exhibiting selective compliance from an embedded bistable element. A multifidelity, aeroelastic tool is used to study the shape adaptability of a morphing flap indicating that passive changes from high lift generation to load alleviation configurations can be achieved by exploiting the energy of the flow. This mechanism offers a method to reduce catastrophic peak loads potentially, thus offering the possibility to lower the overall structural weight of wind turbine blades.     

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.     

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.     

Structural load mitigation control for wind turbines: A new performance measure

Structural loads of wind turbines are becoming critical because of the growing size of wind turbines in combination with the required dynamic output demands. Wind turbine tower and blades are therefore affected by structural loads. To mitigate the loads while maintaining other desired conditions such as the optimization of power generated or the regulation of rotor speed, advanced control schemes have been developed during the last decade. However, conflict and trade‐off between structural load reduction capacity of the controllers and other goals arise; when trying to reduce the structural loads, the power production or regulation performance may be also reduced. Suitable measures are needed when designing controllers to evaluate the control performance with respect to the conflicting control goals. Existing measures for structural loads only consider the loads without referring to the relationship between loads and other control performance aspects. In this contribution, the conflicts are clearly defined and expressed to evaluate the effectiveness of control methods by introducing novel measures. New measures considering structural loads, power production, and regulation to prove the control performance and to formulate criteria for controller design are proposed. The proposed measures allow graphical illustration and numerical criteria describing conflicting control goals and the relationship between goals. Two control approaches for wind turbines, PI and observer‐based state feedback, are defined and used to illustrate and to compare the newly introduced measures. The results are obtained by simulation using Fatigue, Aerodynamics, Structures, and Turbulence (FAST) tool, developed by the National Renewable Energy Laboratory (NREL), USA.     

The application of passive air jet vortex‐generators to stall suppression on wind turbine blades

An experimental study was performed to assess the feasibility of passive air jet vortex‐generators to the performance enhancement of a domestic scale wind turbine. It has been demonstrated that these simple devices, properly designed and implemented, can provide worthwhile performance benefits for domestic wind turbines of the type investigated in this study. In particular, this study shows that they can increase the maximum output power coefficient, reduce the cut‐in wind speed and improve power output at lower wind speeds while reducing the sensitivity to wind speed unsteadiness. A theoretical performance analysis of a 500 kW stall‐regulated wind turbine, based on blade element momentum theory, indicates that passive air jet vortex‐generators would be capable of recovering some of the power loss because of blade stall, thereby allowing attainment of rated power output at slightly lower average wind speeds. Copyright © 2016 John Wiley & Sons, Ltd.     

Vibration suppression for monopile and spar‐buoy offshore wind turbines using the structure‐immittance approach

Offshore wind turbines have the potential to capture the high‐quality wind resource. However, the significant wind and wave excitations may result in excessive vibrations and decreased reliability. To reduce vibrations, passive structural control devices, such as the tuned mass damper (TMD), have been used. To further enhance the vibration suppression capability, inerter‐based absorbers (IBAs) have been studied using the structure‐based approach, that is, proposing specific stiffness‐damping‐inertance elements layouts for investigation. Such an approach has a critical limitation of being only able to cover specific IBA layouts, leaving numerous beneficial configurations not identified. This paper adopts the newly introduced structure‐immittance approach, which is able to cover all network layout possibilities with a predetermined number of elements. Linear monopile and spar‐buoy turbine models are first established for optimisation. Results show that the performance improvements can be up to 6.5% and 7.3% with four and six elements, respectively, compared with the TMD. Moreover, a complete set of beneficial IBA layouts with explicit element types and numbers have been obtained, which is essential for next‐step real‐life applications. In order to verify the effectiveness of the identified absorbers with OpenFAST, an approach has been established to integrate any IBA transfer functions. It has been shown that the performance benefits preserve under both the fatigue limit state (FLS) and the ultimate limit state (ULS). Furthermore, results show that the mass component of the optimum IBAs can be reduced by up to 25.1% (7,486 kg) to achieve the same performance as the TMD.     

Aero‐structural dynamics of a flexible hub connection for load reduction on two‐bladed wind turbines

In this study, an innovative concept for load reduction on the two‐bladed Skywind 3.4 MW prototype is presented. The load reduction system consists of a flexible coupling between the hub mount, carrying the drive train components including the hub assembly, and a nacelle carrier supported by the yaw bearing. This paper intends to assess the impact of introducing a flexible hub connection on the system dynamics and the aero‐elastic response to aerodynamic load imbalances. In order to limit the rotational joint motion, a cardanic spring‐damper element is introduced between the hub mount and the nacelle carrier flange, which affects the system response and the loads. A parameter variation of the stiffness and damping of the connecting spring‐damper element has been performed in the multi‐body simulation solver Simpack. A deterministic, vertically sheared wind field is applied to induce a periodic aerodynamic imbalance on the rotor. The aero‐structural load reduction mechanisms of the coupled system are thereby identified. It is shown that the fatigue loads on the blades and the turbine support structure are reduced significantly. For a very low structural coupling, however, the corresponding rotational deflections of the hub mount exceed the design limit of operation. The analysis of the interaction between the hub mount motion and the blade aerodynamics in a transient inflow environment indicates a reduction of the angle of attack amplitudes and the corresponding fluctuations of the blade loading. Hence, it can be concluded that load reduction is achieved by a combination of reduced structural coupling and a mitigation of aerodynamic load imbalances. Copyright © 2016 John Wiley & Sons, Ltd.