MW Reference Wind Turbine Research Articles

NAUTILUS-DTU10 MW Floating Offshore Wind Turbine at Gulf of Maine: Public numerical models of an actively ballasted semisubmersible

This study presents two numerical multiphysics models of the NAUTILUS-10 floating support structure mounting the DTU10 MW Reference Wind Turbine at Gulf of Maine site, and analyses its dynamics. With the site conditions and the FAST model of the onshore turbine as the starting point, the floating support structure: tower, floating substructure with its corresponding active ballast system and station keeping system, was designed by NAUTILUS. The numerical models were developed and the onshore DTU wind energy controller was tuned to avoid the resonance of the operating FOWT by TECNALIA, in the framework of H2020 LIFES50+ project. This concept and its subsystems are fully characterised throughout this paper and implemented in opensource code, FAST v8.16. Here, the mooring dynamics are solved using MoorDyn, and the hydrodynamic properties are computed using HydroDyn. Viscous effects, not captured by radiation-diffraction theory, are modelled using two different approaches: (1) through linear and quadratic additional hydrodynamic damping matrices and (2) by means of Morison elements. A set of simulations (such as, decay, wind only and broadband irregular waves tests) were carried out with system identification purposes and to analyse the differences between the two models presented. Then, a set of simulations in stochastic wind and waves were carried out to characterise the global response of the FOWT.

Down-regulation and individual blade control as lifetime extension enablers

As more and more wind turbines are coming close to the end of their design lifetime, evaluation of end of life strategies is becoming highly relevant. Moreover, as turbine technology matures and wind farms grow larger, lifetime extension becomes a financially attractive option compared to re-powering and decommissioning. Present work suggests control strategies, namely down-regulation and individual blade control, as lifetime extension enablers. The concept of using them as retrofit control implementations is explained. Their individual and combined potential in fatigue load reduction is evaluated, along with their effect on other performance and pitch system metrics. Finally, the possible period of extension, beyond the nominal 20 years, is evaluated in an example case where the retrofit control strategy is applied after 15 years of baseline operation. The aeroelastic simulations are performed with a 10 MW reference wind turbine, according to load certification standards. Results show that the two methods complement each other in load alleviation. The pitch actuator demands are also significantly decreased when the two methods are combined.

Numerical investigation of modeling frameworks and geometric approximations on NREL 5 MW wind turbine

Abstract The key to the better design of an industrial scale wind turbine is to understand the influence of blade geometry and its dynamics on the complicated flow-structures. An industrial-scale wind turbine can be numerically represented using various approaches (from simpler 2 D steady flow to complex 3 D with moving mesh, with and without the inclusion of hubs) that can alter the results substantially. At the same time, to standardize and test methodologies, the reference industrial-scale NREL 5 MW wind turbine is gaining popularity. The wind energy community will learn through an improved understanding of the aerodynamics of this NREL 5 MW reference mega-watt size wind turbine by taking into consideration the various levels of geometric approximations. Hence, in this work the NREL 5 MW reference wind turbine is used for both: (a) understanding the associated flow-complexities due to various geometric approximation, and (b) comparison and validation of the performance of different numerical approaches for flow-simulation. The various geometric approximations considered here are related to simulating the influence of the shape of turbines structure (blade, an inclusion of hub, tower) and motions of blades. It involves (a) influence of 2D, 2.5D and 3D turbine blade geometry to highlight the impact of section bluntness, (b) influence of steady frozen rotor and rotating rotor simulations and (c) influence of inclusion of hub through a full-scale geometry of the complete reference turbine. Furthermore, the key features of the flow dynamics of a rotor and full machine are identified and parametric studies are conducted to evaluate overall performance prediction under variable Tip Speed Ratios (TSR = 6, 6.5, 7, 7.5, 8, 8.5, 9) and variable level of geometric and flow modeling approximations.

Fatigue damage assessment of wind turbine composite blades using corrected blade element momentum theory

This paper presents a method for estimating the fatigue life of composite blades of horizontal axis wind turbines, and a real blade example of the NREL 5 MW reference wind turbine is employed to verify this proposed method. First, the distributions of aerodynamic loads are analysed by new corrected blade element momentum theory, and then the aerodynamic performance of the blade under various wind speeds is assessed. The time-history of the wind speed is generated by using the Weibull distribution based on the offshore wind parameters available. The maximum stress caused by the aerodynamic loads due to wind is then calculated through the finite element analyses. The fatigue cycle during the certain time period is estimated by fast Fourier transform for the load spectrum, which converts the time-stress spectrum to stress cycles. The fatigue damage and remaining service life of wind turbine composite blades are estimated by the Goodman diagram and S-N curve for stress cycles during the service time. Finally, the remaining service life is predicted by utilising the Miner's law for linear fatigue damage accumulation. The results of the real wind turbine composite blade show that the proposed approach can provide an effective tool for evaluating the fatigue damage and assessing the structural performance of the wind turbine composite blades during the service life.

Experimental investigation of the unsteady aerodynamics of FOWT through PIV and hot-wire wake measurements

This work presents a selected set of results from an extensive experimental campaign aimed at characterizing the wake aerodynamics of a floating offshore wind turbines (FOWT) in dynamic conditions. The campaign was carried out in the Politecnico di Milano 14m x 4m atmospheric boundary layer test section, under UNAFLOW project (UNsteady Aerodynamics of FLoating Wind turbines). Wake measurements were carried out either through hot-wire anemometers with a traversing system as well as through Particle Image Velocimetry (PIV) technique. A 1/75 scale model of the DTU 10 MW reference wind turbine was adopted, which was mounted on a hydraulically actuated mechanism imposing sinusoidal surge motion at different frequencies and amplitudes, and for different tip speed ratios λ. Selected results are reported with the aim of giving a first interpretation of the novel set of measurements, also completing previous measurements conducted by the authors, to put the basis of a more comprehensive understanding of unsteady aerodynamics of FOWT for control and wind farm purposes.

On the effect of blade deformations on the aerodynamic performance of wind turbine rotors subjected to yawed inflow

This work is aimed to investigate the effects of elastic blade deformations on the aerodynamics of large wind turbine rotors subjected to yawed inflow. Due to the increasing rotor size and advanced light weight blade designs, significant blade deflections can be observed on a regular basis for modern wind turbines. However, especially for complex flow situations like yawed inflow, the role of blade deformations is still not completely understood. In this paper, numerical simulations are conducted on the DTU 10 MW reference wind turbine to gain a better understanding of the involved phenomena. Results are obtained by two numerical methods of different fidelity. First, by the aero-elastic simulation tool FAST, which is based on the low fidelity Blade Element Momentum Theory (BEM) and makes use of common skewed wake correction models. Secondly, by a high-fidelity framework which couples the open-source Computational Fluid Dynamics (CFD) toolbox OpenFOAM with the in-house geometrically non-linear beam solver BeamFOAM. The evaluation of the results is based on the analysis of azimuthal variations of the sectional forces along the blade span and reveals a generally good agreement between the used numerical approach in terms of force amplitudes and variation phases. However, especially for cases of larger yaw angles, the BEM models clearly over-predict the variation amplitudes of the forces up to 40% in the outer blade part.

Fluid-structure coupled computations of the NREL 5 MW wind turbine by means of CFD

Abstract This paper presents a fluid-structure coupled simulation tool for high-fidelity simulations of wind turbine rotors. Coupling the open source Computational Fluid Dynamics (CFD) code OpenFOAM and the inhouse structural solver BeamFOAM, the developed tool allows the analysis of flexible wind turbines blades by means of CFD without a significant increase in computational costs. To demonstrate the capabilities of the coupled solver, the aero-elastic response of the NREL 5 MW reference wind turbine is computed for various conditions and specific results are compared to findings of other authors. The solver framework is then used to investigate the effect of blade deformations on aerodynamic key parameters such as power, thrust and sectional forces. It is shown, that the structural deformations have a clear influence on the aerodynamic rotor performance. Especially for the case of yawed inflow, significant implications can be observed in terms of loads and local induction factors. Compared to the fluid-structure coupled framework, the rigid CFD solver underpredicts the forces acting on the blades for most of the cases. Consequently, the presented results are expected to contribute to improve the correction models used in aerodynamic models of lower fidelity like those based on the Blade Element Momentum theory.

Aero-elastic wind turbine design with active flaps for AEP maximization

Abstract. In optimal wind turbine design, there is a compromise between maximizing the energy producing forces and minimizing the absolute peak loads carried by the structures. Active flaps are an attractive strategy because they give engineers greater freedom to vary the aerodynamic forces under any condition. Flaps can be used in a variety of different ways (i.e. reducing fatigue, peak loads), but this article focuses on how quasi-static actuation as a function of mean wind speed can be used for annual energy production (AEP) maximization. Numerical design optimization of the DTU 10 MW reference wind turbine (RWT), with the HAWTOpt2 framework, was used to both find the optimal flap control strategy and the optimal turbine designs. The research shows that active flaps can provide a 1 % gain in AEP for aero-structurally optimized blades in both add-on (i.e. the flap is added after the blade is designed) and integrated (i.e. the blade design and flap angle is optimized together) solutions. The results show that flaps are complementary to passive load alleviation because they provide high-order alleviation, where passive strategies only provide linear alleviation with respect to average wind speed. However, the changing loading from the flaps further complicates the design of torsionally active blades; thus, integrated design methods are needed to design these systems.

Generalized analytical displacement model for wind turbine towers under aerodynamic loading

The goal of this work is two-fold: 1) to determine the angular deflection and displacement of the NREL 5 MW reference wind turbine tower under different atmospheric thermal stratifications, and under overlapping wind turbine wake effects using a comprehensive numerical analysis; 2) to develop and verify a generalized analytical model allowing to efficiently estimate wind turbine tower displacements under a variety of flow conditions. Large-eddy simulations are used to generate atmospheric flows similar to those of a characteristic diurnal cycle. One hour periods are selected and inputted into an aero-elastic simulation code which resolves the rotor-disk aerodynamic loading components, which are then used as inputs to a 3-D finite element model of the tower to compute the angular deflection and displacement. To improve efficiency in tower displacement predictions, a generalized analytical model is developed based on a simplified 2-D cantilever beam and 1-D momentum theory. Results are compared with those obtained from the numerical simulations. Results indicate that the tower deflection standard deviation increases with increasing atmospheric turbulence and wake superposition. Also, differentiated tower displacements are experienced under different atmospheric stratifications. Interestingly, lone standing wind turbines undergo larger tower deflections compared to those located within very large wind farms. The wind turbine rotor thrust and the aerodynamic thrust on the tower are found to have the greatest and least influence, respectively, on tower deflection. Displacement results from the finite element analysis are used to verify the accuracy of the new generalized analytical model showing a very good agreement. This new model has the advantage of estimating the tower deflection by just knowing the inflow velocity for any wind turbine, independent of the atmospheric stratification and wind farm arrangement.

Fully integrated load analysis included in the structural reliability assessment of a monopile supported offshore wind turbine

The present work investigates where cost reductions in the support structure are possible while keeping a sound and safe design. A probabilistic design tool is presented, which involves a coupling between reliability analysis and wind turbine simulation tools. A proof of principle of the proposed tool is given in a case study considering a 4 MW reference wind turbine and a limited lumped load set. The assumptions made in this case study are presented and the results show the reliability index is higher than recommended by DNV. The main contributors to the variance of the fatigue limit state function are the Miner’s rule uncertainty and log(C2) parameter in the SN-curve.

Comparison of numerical response predictions for a bottom-fixed offshore wind turbine

Numerical simulations are widely used for response calculations on offshore wind turbines. Code-to-code comparisons are frequently used for verification of the codes, as full-scale measurements can be difficult to obtain. However, most code comparisons performed focus on documenting the responses predicted by the different codes, or on the effect of specific differences between the codes. Little insight is provided to how these differences would affect design calculations, such as the fatigue utilization. In this paper, the response predictions of the programs SIMA, vpOne and FAST are compared using the DTU 10 MW reference wind turbine on a monopile foundation. While differences in the models are first highlighted through a number of simplified load cases, a lifetime fatigue evaluation of the model is then performed for the monopile at mudline. In the deterministic load cases the response of all models are quite similar, while some differences become apparent in the stochastic analysis. For the fatigue calculations, a difference of 14 % is found in the damage equivalent bending moment at mudline. This demonstrates how sensitive the fatigue utilization is to small differences is code capabilities and modelling.

The Triple Spar campaign: Model tests of a 10MW floating wind turbine with waves, wind and pitch control

Results of a test campaign for a floating wind turbine in simultaneous wind and wave forcing at scale 1:60 are presented. The floater is the Triple Spar floater, a hybrid between a spar buoy and a semi submersible tri-floater, tested here for the first time. The turbine is a model scale version of the DTU 10 MW reference wind turbine, which, also for the first time, is tested with active blade pitch control. The tests focus on the effects of aerodynamic damping and interaction effects between the wind forcing, wave forcing and the blade pitch control algorithm. Special focus is devoted to the instability of the platform pitch natural mode, that can occur if a standard land-based controller is applied.Results of three control strategies are reported: Fixed blade pitch, a standard land-based controller, and a tuned controller designed to eliminate the instability. Special rotor ID tests were carried out to characterize the rotor and identify the control parameters. Results for regular waves, focused wave groups, a wind-only test and irregular waves are presented in terms of time series, power spectra and exceedance probability plots. Among the key results are the wind-induced steady offset of floater pitch and surge and the aerodynamic damping of the floater pitch motion. Further, a clear example of the onset of unstable motion at the platform natural pitch frequency is provided along with a detailed analysis of the dynamic response for irregular wave conditions with the three control strategies. The paper ends with the first results of numerical re-modelling of the response for a waves-only test. A good match is found, thus encouraging further work in this direction.

Design and fatigue analysis of monopile foundations to support the DTU 10 MW offshore wind turbine

This study aims to investigate the challenges and feasibility of extending monopile technology for larger wind turbines and deeper water.Preliminary monopile designs to support the DTU 10 MW reference wind turbine were established for water depths 20 m, 30 m, 40 m and 50 m. To properly account for the pile-soil interaction and the rigid pile behavior of large-diameter piles, lateral soil stiffness was derived using the finite element software Plaxis 3D.To verify the preliminary design, FLS analyses were done using the SIMO-RIFLEX computational tool. As the water depth increases, the contribution of hydrodynamic loads to fatigue damage is found to be larger. A method for predicting total fatigue damage using fewer representative sea states was introduced: a fatigue Damage Parameter (FDP) was established to correlate fatigue damage to environmental condition by means of thrust, significant wave height (Hs) and wave peak period (Tp), and to select representative conditions. Using at least 30% of the total number of conditions resulted in at least 90% accuracy in damage prediction.

Wind energy harnessing of the NREL 5 MW reference wind turbine in icing conditions under different operational strategies

Icing is a strong limitation for the performance of wind turbines in cold climates and the prediction of the performance loss due to ice accretion is essential for designing effective ice mitigation measures. This paper presents a numerical approach capable of simulating the ice accretion transient phenomenon and its effects on wind turbine performance. This approach is applied to the NREL 5 MW reference wind turbine to predict (i) its performance during and after an icing event lasting for 8 h and (ii) the potential improvement in energy harnessing due to different operational strategies. The results show that by decreasing the turbine rotational speed and accepting a slight energy conversion decrease during the icing event, the performance can improve up to 6% when full operation is restored compared to the baseline operational strategy. Whereas, sustaining the rotational speed during the icing event can generate a 3% of performance loss afterwards compared to the same baseline. The developed workflow can be used for optimising performance of wind turbines by accounting for environmental conditions, the duration of the icing event, and performance after the icing event itself, thus constituting a valuable tool to maximise profitability of wind turbines in cold climates.

Uncertainty propagation through an aeroelastic wind turbine model using polynomial surrogates

Abstract Polynomial surrogates are used to characterize the energy production and lifetime equivalent fatigue loads for different components of the DTU 10 MW reference wind turbine under realistic atmospheric conditions. The variability caused by different turbulent inflow fields are captured by creating independent surrogates for the mean and standard deviation of each output with respect to the inflow realizations. A global sensitivity analysis shows that the turbulent inflow realization has a bigger impact on the total distribution of equivalent fatigue loads than the shear coefficient or yaw miss-alignment. The methodology presented extends the deterministic power and thrust coefficient curves to uncertainty models and adds new variables like damage equivalent fatigue loads in different components of the turbine. These surrogate models can then be implemented inside other work-flows such as: estimation of the uncertainty in annual energy production due to wind resource variability and/or robust wind power plant layout optimization. It can be concluded that it is possible to capture the global behavior of a modern wind turbine and its uncertainty under realistic inflow conditions using polynomial response surfaces. The surrogates are a way to obtain power and load estimation under site specific characteristics without sharing the proprietary aeroelastic design.

Modal properties and stability of bend–twist coupled wind turbine blades

Abstract. Coupling between bending and twist has a significant influence on the aeroelastic response of wind turbine blades. The coupling can arise from the blade geometry (e.g. sweep, prebending, or deflection under load) or from the anisotropic properties of the blade material. Bend–twist coupling can be utilized to reduce the fatigue loads of wind turbine blades. In this study the effects of material-based coupling on the aeroelastic modal properties and stability limits of the DTU 10 MW Reference Wind Turbine are investigated. The modal properties are determined by means of eigenvalue analysis around a steady-state equilibrium using the aero-servo-elastic tool HAWCStab2 which has been extended by a beam element that allows for fully coupled cross-sectional properties. Bend–twist coupling is introduced in the cross-sectional stiffness matrix by means of coupling coefficients that introduce twist for flapwise (flap–twist coupling) or edgewise (edge–twist coupling) bending. Edge–twist coupling can increase or decrease the damping of the edgewise mode relative to the reference blade, depending on the operational condition of the turbine. Edge–twist to feather coupling for edgewise deflection towards the leading edge reduces the inflow speed at which the blade becomes unstable. Flap–twist to feather coupling for flapwise deflections towards the suction side increase the frequency and reduce damping of the flapwise mode. Flap–twist to stall reduces frequency and increases damping. The reduction of blade root flapwise and tower bottom fore–aft moments due to variations in mean wind speed of a flap–twist to feather blade are confirmed by frequency response functions.

Aerodynamic design methodology for wind tunnel tests of wind turbine rotors

This paper illustrates the methodology and the experimental verification of the design of a 1/75 aero-elastic scaled rotor of the DTU 10 MW reference wind turbine for wind tunnel tests. The aerodynamic design was focused on the minimization of the difference, in terms of thrust coefficient, with respect to the full scale reference. From the Selig low-Reynolds airfoils database, the SD7032 one was chosen for this purpose and a corresponding constant section wing was tested at DTU red wind tunnel, providing force and distributed pressure coefficients for the design, in the Reynolds range 30−250×103 and for different angles of attack. The aero-elastic design algorithm was set to define the optimal spanwise thickness over chord ratio (t/c), the chord length and the twist, in order to match at least the first flapwise scaled natural frequency. An aluminium mould for the carbon fibre autoclave process was CNC manufactured based on B-Splines CAD definition of the external geometry given as an output of the design procedure. Wind tunnel tests at were carried out Politecnico di Milano on the whole 1/75 wind turbine scale model, confirming the successful aerodynamic design and manufacturing approaches. The experimental modal analysis carried out to verify the structural consistency of the scaled blade is also reported.

Microtab Design and Implementation on a 5 MW Wind Turbine

Microtabs (MT) consist of a small tab placed on the airfoil surface close to the trailing edge and perpendicular to the surface. A study to find the optimal position to improve airfoil aerodynamic performance is presented. Therefore, a parametric study of a MT mounted on the pressure surface of an airfoil has been carried out. The aim of the current study is to find the optimal MT size and location to increase airfoil aerodynamic performance and to investigate its influence on the power output of a 5 MW wind turbine. Firstly, a computational study of a MT mounted on the pressure surface of the airfoil DU91W(2)250 has been carried out and the best case has been found according to the largest lift-to-drag ratio. This airfoil has been selected because it is typically used on wind turbine, such as the 5 MW reference wind turbine of the National Renewable Energy Laboratory (NREL). Second, Blade Element Momentum (BEM) based computations have been performed to investigate the effect of the MT on the wind turbine power output with different wind speed realizations. The results show that, due to the implementation of MTs, a considerable increase in the turbine average power is achieved.

Toward Identifying Aeroelastic Mechanisms in Near-Wake Instabilities of Floating Offshore Wind Turbines

A free-vortex-wake aeroelastic framework evaluates the impact of blade elasticity on the near-wake formation and its linear stability for onshore and offshore configurations of the National Renewable Energy Laboratory (NREL) 5 MW reference wind turbine. Numerical results show that motion of the flexible rotor further destabilizes its tip-vortices through earlier onset of mutual inductance relative to the rigid rotor results for onshore and offshore turbines. The near-wake growth rate is demonstrated to depend on the azimuthal position of the rotor for all cases considered, which appears to not have been reported previously for wake stability analyses in the rotorcraft literature.

Identification of loading conditions resulting in roller slippage in gearbox bearings of large wind turbines

AbstractThe dynamic loads on the rollers inside the bearings of large wind turbine gearboxes operating under transient conditions are presented with a focus on identifying conditions leading to slippage of rollers. The methodology was developed using a multi‐body model of the drivetrain coupled with aeroelastic simulations of the wind turbine system. A 5 MW reference wind turbine is considered for which a three‐stage planetary gearbox is designed on the basis of upscaling of an actual 750 kW gearbox unit. Multi‐body dynamic simulations are run using the ADAMS software using a detailed model of the gearbox planetary bearings to investigate transient loads inside the planet bearing. It was found that assembly and pre‐loading conditions have significant influence on the bearing's operation. Also, the load distribution in the gearbox bearings strongly depends on wind turbine operation. Wind turbine start‐up and shut‐down under normal conditions are shown to induce roller slippage, as characterized by loss of contacts and impacts between rollers and raceways. The roller impacts occur under reduced initial pre‐load on opposite sides of the load zone followed by stress variation, which can be one of the potential reasons leading to wear and premature bearing failures. Copyright © 2017 John Wiley & Sons, Ltd.