Modern Large Wind Turbines Research Articles

A fluid–structure interaction model for large wind turbines based on flexible multibody dynamics and actuator line method

With higher demand of affordable electricity and rapid development of wind turbine technology, scale of wind turbines has been upsizing. As a consequence, blades grow longer and slender. Large geometrically nonlinear deformation and vibration of the blades lead to more challenges in the structural design. In the aerodynamic analysis, traditional blade element momentum theory cannot predict the complicated behavior of these modern blades while fully resolved Navier–Stokes equation methods cost much time modeling and refinement. To overcome these challenges, this paper proposes a new numerical method to analyze turbine aeroelastic performance and fluid–structure interaction based on flexible multibody dynamics and large eddy simulation with anisotropic actuator line method. By comparing the results with various existing numerical methods, we show that the newly proposed method can effectively predict the performance of the wind turbine, such as deformation and power output. We find that blade flexibility poses noticeable impact on the performance of large scale turbines in normal working conditions. The location of nascent vortex is postponed by blade deflection. The max difference between rigid and flexible blades in downstream velocity distributions reaches 10% and occurs at a radial distance around 0.55D. The new method also takes the fluid–structure coupling effect and momentum interaction into consideration and eliminates the non-physical oscillations caused by uncoupled methods. Also, blades deflection and vibration are found to accelerate momentum exchange, which facilitates the wake recovery process. Furthermore,compared to the rigid-blade model, the turbulent kinetic energy obtained by the newly developed flexible-blade model is higher in the low-frequency region, in which large-scale vortices develop and the turbulent mixing effect is strong. This paper is expected to provide a framework for researchers and technology developers to design or estimate the performance of modern large wind turbines.

Wind speed reconstruction from three synchronized short-range WindScanner lidars in a large wind turbine inflow field campaign and the associated uncertainties

Modern large wind turbines require high-resolution wind measurements as input to aerodynamic and aeroelastic simulations for modelling and validation purposes. Within the HighRe project, we aim at studying the aerodynamic effects at high Reynolds numbers by measuring four-dimensional wind fields (vx ,vy ,vz ,t) using three short-range WindScanners (SRWS). The systems were set up at the test site in Bremerhaven (Testfeld BHV) to perform an inflow wind field measurement campaign. In order to get a better understanding of the measurements, we describe the propagation of uncertainties in SRWS parameters to the measured wind field and propose an uncertainty model for a measurement setup with three SRWS lidars. In this study, we first evaluate the wind reconstruction and derive an uncertainty model for the wind components vx , vy , and vz , which are mainly dependent on the input parameters, e.g., focus range, elevation angle, and azimuth angle. The effective intersection diameter at the intersection of three beams was found to be in the order of 2-5m. As expected, a high uncertainty was observed at lower heights in the vz -component due to low elevation angles. This uncertainty evaluation forms the basis for comparing scanning patterns with regard to their accuracy in providing four-dimensional measurements.

Modelling of anisotropic beam for rotating composite wind turbine blade by using finite-difference time-domain (FDTD) method

Most modern large wind turbine blades are made of composite materials which are naturally anisotropic. Modern wind turbine blade design, such as BTC design tends to further enhance the anisotropy of a composite blade. As a result, the modelling of an anisotropic rotating wind turbine blade is an important topic in the wind energy industry. In this paper, the governing equations of an anisotropic rotating beam is derived using Newtonian theory. These governing equations are discretized and solved using a finite-difference time-domain (FDTD) method. This methodology is shown to be highly computationally efficient owing to the fact that the governing equations are solved element by element alternately and explicitly, so only a few operations are required per grid point. The anisotropic beam model developed in this paper is validated using four test cases: (1) modal analysis of an anisotropic box beam; (2) dynamic simulation of a spin-up maneuver; (3) simulation of the NREL 5 MW wind turbine blade; and, (4) simulation of the WindPACT wind turbine blade. The validation is conducted in terms of the predicted natural frequencies and tip displacements for both inertial and non-inertial frames. It is shown that the proposed model can be extended to deal with the case of large rotations. • A novel FDTD scheme is proposed for structural modelling of wind turbine blade. • The proposed FDTD scheme is simple to implement and computationally efficient. • Various aspects of FDTD scheme have been validated using three test cases. • Very good agreement of natural frequencies/mode shapes were obtained from validation.

Østerild: A natural laboratory for atmospheric turbulence

Understanding atmospheric turbulence is without a doubt one of the most complex subjects in meteorology. However, its behavior can be more easily investigated by analysis of high-quality measurements of the velocity and temperature fluctuations. Here, we show that at the large test station of wind turbines in Østerild, northern Denmark, measurements from a 250-m lightning mast provide unique insights into the behavior of atmospheric turbulence within the range of heights where modern large wind turbines operate. We illustrate that for the predominant westerly winds at the site, the flow can be assumed to be close to homogeneous. This allows the analysis of the behavior with atmospheric stability and height of the wind speed and direction and of different turbulence measures, i.a., velocity variances and covariances, as well as turbulence spectra and their characteristics. It is shown, e.g., that for a wide range of atmospheric stability conditions, turbulence parameters up to 241 m are directly related to mean flow characteristics, which will aid in improving the simulation of turbulence in, i.a., aeroelastic simulations. These measurements can be used for the evaluation of a wide range of models describing the meteorological conditions and the atmospheric flow and its turbulent behavior.

Development of semi-active vibration control strategy for horizontal axis wind turbine tower using multiple magneto-rheological tuned liquid column dampers

Vibration control is an essential component of safe and sustained operation of modern large wind turbine towers. As the blade-tower assembly offers time-dependent system matrices that periodically evolves with the rotational speed of the turbine, control theories (e.g. linear quadratic Gaussian) can not be directly adopted in this case. With this in view, the present study aims to develop a semi-active strategy using multiple magneto-rheological tuned liquid column dampers (MR-TLCDs) to mitigate excessive vibration of the horizontal axis wind turbine towers. The design of the controller is achieved by the multi-blade coordinate transformation that converts the system matrices into a non-rotating framework followed by time averaging to enable the application of linear control law. Parameters of the non-linear MR-TLCDs are optimally tuned to dissipate maximum energy. Aerodynamic loads are evaluated using modified blade element momentum theory. The performance of the proposed control strategy is assessed against different wind speeds. Clipping laws are employed to keep the semi-active control force in the feasible range. Two different clipping laws are compared in this paper. Finally, sensitivity analysis is carried out to demonstrate the performance envelope and robustness of the proposed controller.

Individual pitch controller based on fuzzy logic control for wind turbine load mitigation

With the increasing size of modern large wind turbine (WT), the effects of dynamic loading on the structures become an important influence factor. There are mitigation measures for WT control systems to reduce these imbalance structural loads and regulate power. It has motivated the development of blade individual pitch control (IPC) methodologies. This study focuses on the design of fuzzy logic controller (FLC) for IPC. The controllers are designed in order to optimise a trade-off among several control objectives such as blade root moment and generator torque. Three different FLC had been used in the controllers, the first one related to blade pitch angle and electromagnetic torque control variables to meet specified objectives for operation region, the second control model and the third model related to d-q axis blade moment in non-rotating frame of reference. Likewise, the optimisation criteria of FLC consider for each controller objective to mitigate fatigue loads and regulate output power. Finally, the effectiveness of proposed method is verified by simulation results for three-bladed NREL 2 MW WT. The results proved that the fatigue loads in the turbine are reduced obviously.

Modelling the effect of freestream turbulence on dynamic stall of wind turbine blades

Large-eddy simulations of flow over a pitching airfoil are conducted to study the effect of freestream turbulence on the aerodynamic characteristics. A primary field of applications of this study is wind turbine aerodynamics. The wind turbines operate in yaw in large scale variations of wind direction due to very large turbulence eddies, and the blades operate in a periodically oscillating condition. The pitching frequency of the airfoil corresponds to a typical rotating frequency of modern large wind turbines. A divergence-free synthetic turbulence inflow is applied at the upstream region of the pitching airfoil to investigate the effect of small-scale freestream turbulence on dynamic stall. Phase-averaged lift, drag and moment of the pitching airfoil show good agreement with experimental data in the literature. Characteristic phenomena of dynamic stall, such as leading edge vortex motions, are analysed and quantified. The effect of the small-scale upstream turbulence is significant on the lift coefficient during the downstroke. The power spectral density of the streamwise velocity sampled from one point in the wake shows that the inertial sub-range tends to extend towards the pitching mode for the turbulent inflow, while there is a distinctive spectral gap for the laminar inflow.

Effect of Wind Shear on Amplitude Modulation of Wind Turbine Noise

Wind turbines normally produce a swishing sound due to directivity of broadband trailing edge noise from the blades. The sound level variation of this ‘Normal Amplitude Modulation’ (NAM) is usually only a few decibels. However, in some cases periods of increased swish or thumping are reported. The source mechanism for this phenomenon, here denoted as ‘Other Amplitude Modulation’ (OAM), is presently not clear. In order to identify potential causes for OAM, a simulation study is performed into the effects of wind shear on the noise from a modern large wind turbine. The results show that, as long as the flow over the blades is attached, wind shear has practically no effect on amplitude modulation. However, strong wind shear can lead to local stall during the upper part of the revolution. This can yield noise characteristics which are very similar to those of OAM. Thus, it can be concluded that local stall is a plausible explanation for OAM. A sensitivity study indicates that the occurrence of wind shear induced amplitude modulation mainly depends on the size of the stall region and the stall noise behaviour of the airfoils. Apart from wind shear, other causes for a non-uniform inflow, such as yaw or topography, may also lead to local stall and OAM.

Prediction and analysis of infra and low-frequency noise of upwind horizontal axis wind turbine using statistical wind speed model

Despite increasing concern about low-frequency noise of modern large horizontal-axis wind turbines (HAWTs), few studies have focused on its origin or its prediction methods. In this paper, infra- and low-frequency (the ILF) wind turbine noise are closely examined and an efficient method is developed for its prediction. Although most previous studies have assumed that the ILF noise consists primarily of blade passing frequency (BPF) noise components, these tonal noise components are seldom identified in the measured noise spectrum, except for the case of downwind wind turbines. In reality, since modern HAWTs are very large, during rotation, a single blade of the turbine experiences inflow with variation in wind speed in time as well as in space, breaking periodic perturbations of the BPF. Consequently, this transforms acoustic contributions at the BPF harmonics into broadband noise components. In this study, the ILF noise of wind turbines is predicted by combining Lowson’s acoustic analogy with the stochastic wind model, which is employed to reproduce realistic wind speed conditions. In order to predict the effects of these wind conditions on pressure variation on the blade surface, unsteadiness in the incident wind speed is incorporated into the XFOIL code by varying incident flow velocities on each blade section, which depend on the azimuthal locations of the rotating blade. The calculated surface pressure distribution is subsequently used to predict acoustic pressure at an observing location by using Lowson’s analogy. These predictions are compared with measured data, which ensures that the present method can reproduce the broadband characteristics of the measured low-frequency noise spectrum. Further investigations are carried out to characterize the IFL noise in terms of pressure loading on blade surface, narrow-band noise spectrum and noise maps around the turbine.

Adaptive neuro-fuzzy methodology for noise assessment of wind turbine.

Wind turbine noise is one of the major obstacles for the widespread use of wind energy. Noise tone can greatly increase the annoyance factor and the negative impact on human health. Noise annoyance caused by wind turbines has become an emerging problem in recent years, due to the rapid increase in number of wind turbines, triggered by sustainable energy goals set forward at the national and international level. Up to now, not all aspects of the generation, propagation and perception of wind turbine noise are well understood. For a modern large wind turbine, aerodynamic noise from the blades is generally considered to be the dominant noise source, provided that mechanical noise is adequately eliminated. The sources of aerodynamic noise can be divided into tonal noise, inflow turbulence noise, and airfoil self-noise. Many analytical and experimental acoustical studies performed the wind turbines. Since the wind turbine noise level analyzing by numerical methods or computational fluid dynamics (CFD) could be very challenging and time consuming, soft computing techniques are preferred. To estimate noise level of wind turbine, this paper constructed a process which simulates the wind turbine noise levels in regard to wind speed and sound frequency with adaptive neuro-fuzzy inference system (ANFIS). This intelligent estimator is implemented using Matlab/Simulink and the performances are investigated. The simulation results presented in this paper show the effectiveness of the developed method.

Research status on aero-acoustic noise from wind turbine blades

This paper describes the noise mechanisms and categories of modern large wind turbine and main noise sources. Then the latest progresses in wind turbine noise researches are described from three aspects: noise prediction model, detection of noise sources by microphone array technique and methods for noise reduction. Although the turbine is restricted to horizontal axis wind turbines, the noise prediction model and reduction methods also can be applied to other turbines when the noise mechanisms are similar. Microphone array technique can be applied to locate any kind of noise sources.

Research Status and Development Trend on Large Scale Wind Turbine Blades

Blade is one of key components of wind turbines. The wind rotor consisting of blades and hub is the energy capture mechanism which can transform the wind energy into mechanical energy. At the same time,blades are the force sources and bearing parts. Thus,blades play a key role for the safety of wind turbines. The industry development situation of modern large-scale wind turbines and its blades is analyzed. Then,a comprehensive summary is carried out including the aerodynamic shape and structural design,operation monitoring and control technology,the development trend and faced problems. The status and development trend of modern large scale wind turbine blades are grasped in general. It is specially pointed out that some fields will have further development such as the personalized airfoil design of anti-typhoon blades,low wind blades,bionic blades and low noise blades,design of novel wind turbine blade with sensing function of external environment load and structural state change. The formed blade design,manufacture and operation monitoring theory of wind turbines will be further improved and developed by service practice of large quantities of wind turbine in full life-cycle.

Evaluating winds and vertical wind shear from Weather Research and Forecasting model forecasts using seven planetary boundary layer schemes

ABSTRACTThe existence of vertical wind shear in the atmosphere close to the ground requires that wind resource assessment and prediction with numerical weather prediction (NWP) models use wind forecasts at levels within the full rotor span of modern large wind turbines. The performance of NWP models regarding wind energy at these levels partly depends on the formulation and implementation of planetary boundary layer (PBL) parameterizations in these models. This study evaluates wind speeds and vertical wind shears simulated by the Weather Research and Forecasting model using seven sets of simulations with different PBL parameterizations at one coastal site over western Denmark. The evaluation focuses on determining which PBL parameterization performs best for wind energy forecasting, and presenting a validation methodology that takes into account wind speed at different heights.Winds speeds at heights ranging from 10 to 160 m, wind shears, temperatures and surface turbulent fluxes from seven sets of hindcasts are evaluated against observations at Høvsøre, Denmark. The ability of these hindcast sets to simulate mean wind speeds, wind shear, and their time variability strongly depends on atmospheric static stability. Wind speed hindcasts using the Yonsei University PBL scheme compared best with observations during unstable atmospheric conditions, whereas the Asymmetric Convective Model version 2 PBL scheme did so during near‐stable and neutral conditions, and the Mellor–Yamada–Janjic PBL scheme prevailed during stable and very stable conditions. The evaluation of the simulated wind speed errors and how these vary with height clearly indicates that for wind power forecasting and wind resource assessment, validation against 10 m wind speeds alone is not sufficient. Copyright © 2012 John Wiley & Sons, Ltd.

Blade sections for wind turbine and tidal current turbine applications-current status and future challenges

The designers of horizontal axis wind turbines and tidal current turbines are increasingly focusing their attention on the design of blade sections appropriate for specific applications. In modern large wind turbines, the blade tip is designed using a thin airfoil for high lift : drag ratio, and the root region is designed using a thick version of the same airfoil for structural support. A high lift to drag ratio is a generally accepted requirement; however, although a reduction in the drag coefficient directly contributes to a higher aerodynamic efficiency, an increase in the lift coefficient does not have a significant contribution to the torque, as it is only a small component of lift that increases the tangential force while the larger component increases the thrust, necessitating an optimization. An airfoil with a curvature close to the leading edge that contributes more to the rotation will be a good choice; however, it is still a challenge to design such an airfoil. The design of special purpose airfoils started with LS and SERI airfoils, which are followed by many series of airfoils, including the new CAS airfoils. After nearly two decades of extensive research, a number of airfoils are available; however, majority of them are thick airfoils as the strength is still a major concern. Many of these still show deterioration in performance with leading edge contamination. Similarly, a change in the freestream turbulence level affects the performance of the blade. A number of active and passive flow control devices have been proposed and tested to improve the performance of blades/turbines. The structural requirements for tidal current turbines tend to lead to thicker sections, particularly near the root, which will cause a higher drag coefficient. A bigger challenge in the design of blades for these turbines is to avoid cavitation (which also leads to thicker sections) and still obtain an acceptably high lift coefficient. Another challenge for the designers is to design blades that give consistent output at varying flow conditions with a simple control system. The performance of a rotating blade may be significantly different from a non-rotating blade, which requires that the design process should continue till the blade is tested under different operating conditions. Copyright © 2012 John Wiley & Sons, Ltd.

Structural Optimization of 3 MW Wind Turbine Blades Using a Two-Step Procedure

Modern large wind turbines, utilized to harness the kinetic energy of the wind, are rated at megawatts in output power. The design of large wind turbine blades must consider both their aerodynamic efficiency and structural robustness. This paper presents a two-step procedure for the optimum design of composite wind turbine blades. The results of the first step are the aerodynamically optimal cord lengths and twist angles of airfoils for the blade cross-sections along the blade spanwise direction. The second step yields optimal material distribution for the composite blade. A 3 MW wind turbine with blades having cross-sections of NREL S818, S825 and S826 airfoil types is demonstrated as the design example. Loaded by maximum forces and moments extracted from simulated time series, a parameterized finite element model of the aerodynamically optimized blade is created using the ANSYS software. The optimization results show that the initial blade model is an infeasible design due to a high level of the maximum stress, exceeding the upper limit of the stress constraint, but eventually the process converges to a feasible solution with the expense of increased total mass of the blade.

Collision Risk of Birds with Modern Large Wind Turbines

We studied collision rate of birds with modern, large 1.65 MW wind turbines in three wind farms in The Netherlands during three months in autumn and winter. Collision rate, after correction for retrieval and disappearance rate, was 0.08 birds per turbine per day on average (range 0.05–0.19). Collision risk, i.e. the number of victims relative to the flight intensity of birds at the wind farms, was 0.14% on average. For nocturnal migrants, risk was as low as 0.01%, while the risk was 0.16% for local birds flying at night. In absolute numbers, the overall collision risk was similar to the 0.06–0.28% found for earlier-generation lower turbines that have a smaller rotor surface. However, given the comparatively large rotor surface and altitude range of the modern turbines, the risk was c. threefold lower than for smaller turbines. A large fraction of collision victims were diurnally active and local birds that were foraging in the area, rather than nocturnal migrants. Flight intensities of this group should b...

Prediction of Wind Turbine Noise and Validation against Experiment

A semi-empirical prediction method for trailing edge noise is applied to calculate the noise from two modern large wind turbines. The prediction code only needs the blade geometry and the turbine operating conditions as input. Using detailed acoustic array and directivity measurements, a thorough validation of the predictions is carried out. The predicted noise source distribution in the rotor plane (as a function of frequency and observer position) shows the same characteristics as in the experiments: due to trailing edge noise directivity and convective amplification, practically all noise (emitted to the ground) is produced during the downward movement of the blades, causing an amplitude modulation of broadband aerodynamic blade noise at the blade passing frequency (‘swish’). Good agreement is also found between the measured and predicted spectra, in terms of levels and spectral shape. For both turbines, the deviation between predicted and measured overall sound levels (as a function of rotor power) is less than 1–2 dB, which is smaller than the scatter in the experimental data. Using a smoothed analytical trailing edge noise directivity function, the turbine noise directivity is predicted within 1–2 dB, and the swish amplitude in different directions within 1 dB. This semi-empirical directivity function shows similar characteristics as the theoretical directivity function for a flat plate, except for regions close to the plane of the blade. The validated prediction code is then applied to calculate noise footprints of the wind turbine as a function of rotor azimuth. These footprints show that for cross-wind directions the average level is lower than in the up- and downwind directions, but the variation in level is larger. Even at large distance, swish amplitudes up to 5 dB can be expected for cross-wind directions.

SIMULATION ANALYSIS OF DFIG CHARACTERISTICS UNDER d–q CONTROL STRATEGIES IN STATOR-VOLTAGE-ORIENTED FRAME

The technology employed for electromechanical energy conversion in large wind turbines deviates from that used in traditional generation equipment. Induction machines, rather than synchronous generators, are used in most commercial wind turbines. The doubly-fed induction generator (DFIG) is a special variable-speed induction machine widely utilized in modern large wind turbines. Unlike a conventional fixed-speed induction machine, a DFIG transfers power to the grid through both the stator and rotor paths, and its characteristics depend strongly on how the generator is controlled. To enhance control design and analysis, this paper investigates DFIG steady-state characteristics through computer simulation. The paper presents a steady-state model that allows (1) DFIG characteristic study under decoupled d-q control strategies in a stator-voltage-oriented frame and (2) power computation of both the stator and rotor paths. The steady-state model is validated through a transient simulation system by using SimPowerSystem. Extensive simulation-based studies are performed to investigate how DFIG characteristics are affected by different d-q control conditions in the stator-voltage-oriented frame.

Characteristic Study of Vector-controlled Doubly-fed Induction Generator in Stator-flux-oriented Frame

The doubly-fed induction generator (DFIG) is a “special” variable-speed induction machine that is widely utilized in modern large wind turbine generators. This article focuses on the DFIG characteristic study in the stator-flux-oriented frame. Compared to traditional approaches, the specific features of this article are (1) a DFIG steady-state model in the d-q reference frame, (2) simulation-based characteristic study under general d-q control strategies in the stator-flux-oriented frame, (3) a numerical iteration technique, (4) power simulation from both the stator and rotor paths, and (5) an integrated study that enables assessment of various DFIG parametric data simultaneously in an integrative environment. In addition, transient simulation using the MatLab SimPowerSystem is developed to validate the effectiveness of steady-state results and to further investigate a DFIG in a transient environment. Extensive simulation-based analysis is performed to inspect how DFIG characteristics are affected by different d-q control conditions in the stator-flux-oriented frame, such as torque-speed and real and reactive power versus speed characteristics.

Statistical Model of Extreme Shear

In order to continue cost-optimization of modern large wind turbines, it is important to continuously increase the knowledge of wind field parameters relevant to design loads. This paper presents a general statistical model that offers site-specific prediction of the probability density function (PDF) of turbulence driven short-term extreme wind shear events, conditioned on the mean wind speed, for an arbitrary recurrence period. The model is based on an asymptotic expansion, and only a few and easily accessible parameters are needed as input. The model of the extreme PDF is supplemented by a model that, on a statistically consistent basis, describes the most likely spatial shape of an extreme wind shear event. Predictions from the model have been compared with results from an extreme value data analysis, based on a large number of full-scale measurements recorded with a high sampling rate. The measurements have been extracted from ”Database on Wind Characteristics” (http:∕∕www.winddata.com∕), and they refer to a site characterized by a flat homogeneous terrain. The comparison has been conducted for three different mean wind speeds in the range of 15-19m∕s, and model predictions and experimental results are consistent, given the inevitable uncertainties associated with the model as well as with the extreme value data analysis.