MW Blade Research Articles

Definition of a Baseline Rotor Design for a 25MW Floating Offshore Wind Turbine

Abstract Wind turbine sizes have grown rapidly in recent years with machine ratings of 15-16 MW available from multiple manufacturers for offshore wind turbines. While the industry advances large-scale turbine designs, offshore developers have initially focused on siting in shallow waters on bottom-fixed foundations; however, eyes are also on deep-water locations where floating systems are required. The present study presents the definition for an initial baseline rotor design at 25 MW scale, designed for a floating offshore system. The purpose of this paper is to document this 25 MW blade definition for use as a reference design for future floating wind technology development by industrial and academic researchers and developers. As this is an initial reference design, some opportunities (and plans) for further mass or cost reduction are also noted. In summary, the paper documents the initial baseline rotor design including aerodynamic and structural design of the rotor blades, along with key details about the control system and floating system designs for the floating 25 MW wind turbine system.

Aero-structural coupled optimization of a rotor blade for an upscaled 25 MW reference wind turbine

One major challenge of the wind turbine industry is the reduction of the levelized cost of energy (LCoE) while following the strong demand for a higher annual energy production (AEP). To meet these goals, larger wind turbine sizes are required. The common method of upscaling existing wind turbine designs comes along with the problem of faster growing blade masses and costs compared to the AEP. Investigations in new technologies to improve the structural efficiency of larger blades can be supported by aero-structural coupled optimizations. The present work introduces a two-step aero-structural coupled design process to capture the multi-disciplinary trade-offs between costs and AEP, aiming at minimizing LCoE for a 25 MW wind turbine. In a first step, a preliminary aero-structural optimisation is carried out using simplified and fast methods. The output is then refined with respect to additional design criteria with an advanced optimization process, including an aero-servo-elastic coupled loads analysis. The process is applied to a 25 MW blade, upscaled from the IEA 15 MW reference wind tubine. Based on the results of an utilization analysis, the structural design is adapted, and a stiffness optimization is performed. The optimum airfoil positions are identified to reduce the amount of material while limiting losses in the aerodynamic performance. The obtained blade designs facilitate a consistent AEP compared to the upscaled reference design. A mass reduction of 35% could be achieved, which results in a reduced LCoE of 1.7% compared to the purely upscaled blade design.

2.5MW급 블레이드 설계 및 전산해석에 관한 연구

2.5MW급 블레이드 설계 및 전산해석에 관한 연구

The Impact of Bend–Twist Coupling on Structural Characteristics and Flutter Limit of Ultra-Long Flexible Wind Turbine Composite Blades

Flutter is an instability phenomenon that can occur in wind turbine blades due to fluid–structure interaction, particularly for longer and more flexible blades. Aeroelastic tailoring through bend–twist coupling is an effective method to enhance the aeroelastic performance of blades. In this study, we investigate the impact of bend–twist coupling on the structural performance and flutter limit of the IEA 15 MW blade, which is currently the longest reference wind turbine blade, and determine the optimal layup configuration that maximizes the flutter speed. The blade is modeled by NuMAD and iVABS, and the cross-section properties are obtained by PreComb and VABS. The accuracy of the blade model is verified in terms of stiffness and frequency. The bend–twist coupling is implemented by changing the fiber angle of the skin and spar cap considering symmetric and asymmetric layups. The flutter limits of both the baseline and the bend–twist coupled blade are evaluated based on HAWC2. The results show that the angle of spar cap carbon fiber has a greater effect on the blade’s structural properties and flutter speed than the skin fiber. Varying the spar cap carbon fiber angle increases the flutter speed, with the effect being more significant for the symmetric layup, up to 9.66% at a fiber angle of 25 degrees. In contrast, the variation in skin fiber angle has a relatively small impact on flutter speed—within ±3%.

Analysis of variations in annual energy production based on types of suction side erosion at the blade tip of a wind turbine using numerical simulation

ABSTRACT Wind turbine blades are prone to damage by airborne particulate matter; hence, periodic blade inspection is necessary for maintenance. However, quantitative data on wind turbine performance reduction due to erosion are lacking. Herein, erosion conditions were classified into four levels based on the erosion area in long-term operated wind turbine blades. Transient three-dimensional computational fluid dynamics analysis was performed by applying the suction side erosion shape to the tip airfoil of the National Renewable Energy Laboratory’s 5 MW blade. In the early stage, irregular erosion of the airfoil’s leading edge resulted in the development of a reverse pressure gradient inside the boundary layer and a larger recirculation area of the trailing edge. Thus, the lift coefficient of the airfoil decreased by up to 30%, whereas the drag coefficient increased by over 100%. The wind turbine output curve calculated using aeroelastic software showed that the worst erosion condition decreased output by 2.54%. Annual energy production had declined by up to 2% based on the annual average wind speed; therefore, immediately repairing erosion can minimize the loss of energy production.

Multi-Megawatt Horizontal Axis Wind Turbine Blade Optimization Based on PSO Method

Blade optimization methods are crucial for wind turbine design. In this research, a new set of values for the parameters of the Particle Swarm Optimization (PSO) method is proposed, and its effects on the enhancement of the power generation of the NREL WP-Baseline 1.5 MW horizontal axis wind turbine are investigated. First, the PSO parameters are tuned, and the convergence speed and the optimal accuracy of the objective function are improved. Then, the Class/Shape Transformation (CST) method is employed, and an appropriate order of the shape function polynomial is selected. In the third step, the WP-Baseline 1.5 MW blade is optimized according to the tuned PSO parameters, and the airfoil is represented by CST algorithms. Later, a CFD model, including 37 million cells and an IDDES turbulence model, was validated and used for a comparison of the power generation of the original and optimized blades. The optimized blade produced more power for all wind speeds above 4.5 m/s, with a maximum of 13.8% at 10 m/s and +7.25% at the rated wind speed (11.5 m/s). It should be noted that since the algorithms, tunings, and techniques adopted in the present study were general, the presented method can be used as a systematic approach for the aerodynamics shape optimization of multi-megawatt HAWTs.

Multiple limit cycle amplitudes in high-fidelity predictions of standstill wind turbine blade vibrations

Abstract. In this study, vortex-induced vibrations (VIVs) on the IEA 10 MW blade are investigated using two methodologies in order to assess strengths and weaknesses of the two simulation types. Both fully coupled fluid–structure interaction (FSI) simulations and computational fluid dynamics (CFD) with forced motion of the blade are used and compared. It is found that for the studied cases with high inclination angles, the forced-motion simulations succeed in capturing the power injection by the aerodynamics, despite the motion being simplified. From the fully coupled simulations, a dependency on initial conditions of the vibrations was found, showing that cases which are stable if unperturbed might go into large VIVs if provoked initially by, for instance, inflow turbulence or turbine operations. Depending on the initial vibration amplitudes, multiple limit cycle levels can be triggered, for the same flow case, due to the non-linearity of the aerodynamics. By fitting a simple damping model for the specific blade and mode shape from FSI simulations, it is also demonstrated that the equilibrium limit cycle amplitudes between power injection and dissipation can be estimated using forced-motion simulations, even for the multiple stable vibration cases, with good agreement with fully coupled simulations. Finally, a time series generation from forced-motion simulations and the simple damping model is presented, concluding that CFD amplitude sweeps can estimate not only the final limit cycle oscillation amplitude, but also the vibration build-up time series.

Study of Centrifugal Stiffening on the Free Vibrations and Dynamic Response of Offshore Wind Turbine Blades

Due to their large and increasing size and the corrosive nature of salt water and high wind speeds, offshore wind turbines are required to be more robust, more rugged and more reliable than their onshore counterparts. The dynamic characteristics of the blade and its response to applied forces may be influenced dramatically by rotor rotational speed, which may even threaten the stability of the wind turbine. An accurate and computationally efficient structural dynamics model is essential for offshore wind turbines. A comprehensive model that takes the centrifugal stiffening effect into consideration could make rapid and accurate decisions with live data sensed from the structure. Moreover, this can enhance both the performance and reliability of wind turbines. When a rotating blade deflects in its plane of rotation or perpendicular to it, the centrifugal force exerts an inertia force that increases the natural frequencies and changes the mode shapes, leading to changes in the dynamic response of the blade. However, in the previous literature, studies of centrifugal stiffening are rarely found. This study investigates the influence of centrifugal stiffening on the free vibrations and dynamic response of offshore wind turbine blades. The National Renewable Energy Laboratory (NREL) 5 MW blade benchmark was considered to study the effect of angular speed in the flap-wise and edge-wise directions. The results demonstrate that the angular speed directly affects the modal features, which directly impacts the dynamic response. The results also show that the angular velocity effect in the flap-wise direction is more significant than its effect in the edge-wise direction.

Experimental evidence of coupled-mode flutter in a two-meter-long non-rotating wind turbine blade

We have conducted a series of experiments on a 30:1 scale wind turbine blade and observed coupled-mode flutter. This observation validates the theoretical studies that predict larger wind turbine blades are susceptible to coupled-mode flutter. To conduct these experiments, we built a scaled-down model of the 61-meter NREL 5 MW blade. The blade was approximately 2 meters long and was built such that the ratios between its first torsional natural frequency and the flapwise natural frequencies were similar to those of the full-scale blade. This was important, because the theoretical predictions suggest that coupled-mode flutter occurs when one of the flapwise modes and the first torsional mode of the blade coalesce. In our experiments, as the wind speed was increased, turbulence induced vibrations were observed initially in the form of oscillations of the blade’s first flapwise mode, and then its second flapwise mode, with a region of multi-modal oscillations in between. It was observed that the first torsional natural frequency of the blade decreased with increasing wind speed and the second flapwise natural frequency increased slightly. Then at a critical wind speed, the second flapwise mode and the first torsional mode merged into a flutter mode, indicating coupled-mode flutter has occurred at this wind speed.

Sensitivity Study of the Influence of Blade Sectional Stiffness Parameters on the Aeroelastic Response of Wind Turbines

With the development of wind turbines as a result of large-scale and offshore trends, the wind turbine size is becoming increasingly larger. The passive control technique is used to alleviate the increasing loads on the blade for the sake of improving the durability of the wind turbine. The ply design of shells considering the coupling effect of bending and torsion is one of the passive control techniques. The bending torsion coupling stiffness is one of the parameters of the blade section stiffness matrix. In order to fully understand the influence of each blade stiffness parameter on the aeroelastic responses of wind turbines and to consider the influence of structural characteristics on the aeroelastic responses in blade design, the influences and sensitivity of each stiffness parameter in the 6 × 6 stiffness matrix of the blade sections on the aeroelastic responses of the wind turbines are systematically studied under steady wind condition. The aerodynamic forces in the aeroelastic model are calculated by an AeroDyn module based on blade element momentum theory, and the structural dynamic responses of the blade are calculated using generalized Timoshenko beam theory and geometric exact beam theory. The NREL baseline 5 MW wind turbine and blade properties are used in this study, where the diagonal stiffness parameters and non-diagonal stiffness parameters of the matrixes of each blade section are scaled according to certain principles. The results show that the axial stiffness, the flap-wise stiffness, and the torsional stiffness in the diagonal are sensitive to the root loads and tip displacement of the blade. The flap-wise bending torsion coupling stiffness, the flap-wise shear-torsion coupling stiffness, and the edge-wise shear-torsion coupling stiffness in the non-diagonal are also sensitive to the aeroelastic responses. For completeness, the effects of other stiffness parameters on the aeroelastic responses are also analyzed and discussed.

Design Optimization of a Multi-Megawatt Wind Turbine Blade with the NPU-MWA Airfoil Family

A new airfoil family, called NPU-MWA (Northwestern Polytechnical University Multi-megawatt Wind-turbine A-series) airfoils, was designed to improve both aerodynamic and structural performance, with the outboard airfoils being designed at high design lift coefficient and high Reynolds number, and the inboard airfoils being designed as flat-back airfoils. This article aims to design a multi-megawatt wind turbine blade in order to demonstrate the advantages of the NPU-MWA airfoils in improving wind energy capturing and structural weight reduction. The distributions of chord length and twist angle for a 5 MW wind turbine blade are optimized by a Kriging surrogate model-based optimizer, with aerodynamic performance being evaluated by blade element-momentum theory. The Reynolds-averaged Navier–Stokes equations solver was used to validate the improvement in aerodynamic performance. Results show that compared with an existing NREL (National Renewable Energy Laboratory) 5 MW blade, the maximum power coefficient of the optimized NPU 5 MW blade is larger, and the chord lengths at all span-wise sections are dramatically smaller, resulting in a significant structural weight reduction (9%). It is shown that the NPU-MWA airfoils feature excellent aerodynamic and structural performance for the design of multi-megawatt wind turbine blades.

Comparison of numerical modelling techniques for impact investigation on a wind turbine blade

Wind turbine blades are exposed to numerous impact risks throughout their lifetimes. The impact risks range from bird collisions during operation to impacts with surrounding structures at the time of transportation and installation. Impact loads on the fibre composite blades can induce several complex, simultaneously interacting and visually undetectable damage modes and have a high potential to reduce the local and global blade stiffness. An assessment of such impact-induced damages is therefore necessary and usually involves high computational costs using numerical procedures, especially when analysing large composite components. To minimise this computational expense, different numerical impact modelling techniques are utilised, primarily shell-element-based approaches and multiscale-modelling-based global-local approaches. In this article, a comparison between (1) pure shell, (2) shell-to-solid coupling, and (3) submodelling finite element modelling techniques using Abaqus/Explicit is presented for a case where an impactor hits the leading edge of a blade. A high-fidelity local solid finite element model is developed for the leading edge of a DTU 10 MW blade at the region of impact and its stiffness is compared with baseline. A user material subroutine VUMAT for the intralaminar damage mode based on the Hashin failure criterion is formulated and then validated via an experiment from the literature. Finally, based on different numerical modelling techniques, impact investigations are performed, and the impact responses, damage to the blade and computational analysis durations are compared. It is found that the submodelling-based global-local approach is the most efficient analysis technique for this case, capturing failure modes including delamination, core crushing and local surface indentation in the blade. The findings of this study can be used to develop accurate and computationally efficient tools for modelling impact-induced damage to a blade.

Deep neural network-based wind speed forecasting and fatigue analysis of a large composite wind turbine blade

The purpose of this paper is to analyze the modern deep neural networks such as nonlinear autoregressive network with external inputs and a recurrent neural network called long short-term memory for wind speed forecast for long-term and use the prediction for fatigue analysis of a large 5 MW wind turbine blade made of composite materials. The use of machine learning algorithms of advanced neural network applied for engineering problems is increasing recently. The present paper therefore brings as important connection between these latest machine learning methods and engineering analysis of complex wind turbine blades which are also the focus of researchers in renewable system design and analysis. First, a nonlinear autoregressive network with external inputs neural network model using Levenberg–Marquardt back propagation feed forward algorithm is developed with 5 years of environment parameters as input. Similarly, a long short-term memory based model is developed and compared. The chosen long short-term memory model is used for developing two-year wind speed forecast. This wind pattern is used to create time varying loads on blade sections and cross-verified with National Renewable Energy Laboratory tools. A high-fidelity CAD model of the NREL 5 MW blade is developed and the fatigue analysis of the blade is carried out using the stress life approach with load ratio based on cohesive zone modeling. The blade is found to have available life of about 23.6 years. Thus, an integrated methodology is developed involving high-fidelity modeling of the composite material blade, wind speed forecasting using multiple environmental parameters using latest deep learning methods for machine learning, dynamic wind load calculation, and fatigue analysis for National Renewable Energy Laboratory blade.

A parametric study on the final blade installation process for monopile wind turbines under rough environmental conditions

Single blade installation is a method for installing wind turbine blades. If a jack-up vessel is used during an o shore installation, the wind turbine blade is mainly subjected to wind loads and experiences resonant motions, and the monopile is subjected to wave-induced vibrations. The blade mating process can be challenging if large relative motions occur between the blade root and the monopile top. This study numerically models a blade installation system that consists of a pre-installed monopile and nacelle assembly, and a 5 MW blade with tugger lines. By analyzing the blade-root and the hub motion radii from time-domain simulations, we evaluate the effects of mean wind speed, wind turbulence, significant wave height, wave spectrum peak period, wind-wave misalignment, and water depth on the blade installation. For the alignment phase, the blade-root motion is critical, especially when the mean wind speed and turbulence are high. The hub motion can be important when the monopile resonant responses are prominent. The relative in-plane motions rather than the hub or the blade motion alone should be considered during the assessment. For the mating phase, the high-frequency components of the responses are important in general. Because of the dominant flange-hole motions at the monopile top, an increase in water depth reduces the success rate of mating.

An integrated dynamic analysis method for simulating installation of single blades for wind turbines

Installation of blades for wind turbines is challenging due to large lifting height and high precision. Assessment of blade dynamic responses during installation needs advanced simulation tools which are limited at present. This paper aims at developing an integrated simulation tool SIMO-Aero for single blade installation for both onshore and offshore wind turbines. Based on the cross-flow principle, the aerodynamic model is established by accounting for the effect of wind turbulence and dynamic stall. Then it is coupled with SIMO to achieve the integrated simulation tool SIMO-Aero which can account for blade aerodynamics, vessel hydrodynamics and system mechanical couplings. The aerodynamic code is verified by code-to-code comparisons with HAWC2. Furthermore, SIMO-Aero is applied in case studies on the wind-induced dynamic responses of a DTU 10 MW blade during installation using a jack-up crane vessel which is assumed to be rigid, including the crane, and rigidly fixed to the seabed. The characteristics of system dynamic responses prior to mating the blade onto the hub are studied. It is shown that the blade motions are dominated by the pendulum motion. Critical parameters of the installation process are identified. The extreme responses of critical parameters are further studied under turbulent winds and wind gusts.

Integrated aero-structural optimization design of pre-bend wind turbine blades

In the optimization design of a pre-bend wind turbine blade, there is a coupling relationship between blade aerodynamic shape and structural layup. The evaluation index of a wind turbine blade not only shows on conventional ones, such as Annual energy production (AEP), cost, and quality, but also includes the size of the loads on the hub or tower. Hence, the design of pre-bend wind turbine blades is a true multi-objective engineering task. To make the integrative optimization design of the pre-bend blade, new methods for the blade’s pre-bend profile design and structural analysis for the blade sections were presented, under dangerous working conditions, and considering the fundamental control characteristics of the wind turbine, an integrated aerodynamic-structural design technique for pre-bend blades was developed based on the Multi-objective particle swarm optimization algorithm (MOPSO). By using the optimization method, a three-dimensional Pareto-optimal set, which can satisfy different matching requirements from overall design of a wind turbine, was obtained. The most suitable solution was chosen from the Pareto-optimal set and compared with the original 1.5 MW blade. The results show that the optimized blade have better performance in every aspect, which verifies the feasibility of this new method for the design of pre-bend wind turbine blades.

The environmental impact of wind turbine blades

The first generation of wind turbine (WT) blades are now reaching their end of life, signalling the beginning of a large problem for the future. Currently most waste is sent to landfill, which is not an environmentally desirable solution. Awareness of this issue is rising, but no studies have fully assessed the eco impact of WT blades. The present study aims to provide a macroscopic quantitative assessment of the lifetime environmental impact of WT blades. The first stage has been to analyse global data to calculate the amount of WT blade materials consumed in the past. The life cycle environmental impact of a single WT blade has then been estimated using eco data for raw materials, manufacturing processes, transportation, and operation and maintenance processes. For a typical 45.2 meter 1.5 MW blade this is 795 GJ (CO2 footprint 42.1 tonnes), dominated by manufacturing processes and raw materials (96% of the total. Based on the 2014 installed capacity, the total mass of WTB is 78 kt, their energy consumption is 82 TJ and the carbon dioxide footprint is 4.35 Mt. These figures will provide a basis for suggesting possible solutions to reduce WTB environmental impact.

Flutter of Variations on a 5 MW Swept Wind Turbine Blade1

Previous research has shown that as wind turbine rotor designs increase in diameter they approach flutter instabilities. Recent blade designs incorporate blade sweep to lower fatigue loads and increase rotor diameter for more energy production; however, the flutter margin may be reduced with sweep. In this work, flutter speeds are predicted and compared for turbines with straight and swept blades. For this study, a wind turbine analysis tool specifically developed for blade sweep (CurveFAST) was used, in addition to a tool developed for helicopter analysis: rotorcraft comprehensive analysis system (RCAS). The results showed that CurveFAST generally predicted a higher flutter speed and RCAS predicted a lower flutter speed compared to previous research. The difference may be attributed to the unsteady aerodynamics modeling. The results for an extended radius and swept 5 MW turbine showed that the flutter speed was most sensitive to the position of the blade center of mass axis. Using the selected design parameters, it was not possible to design a swept 5 MW blade with adequate flutter margin that would produce 5% more energy with the same loads as a baseline straight rotor. Further study of swept blades is required to determine if there is a size limit above which flutter cannot be avoided.

Developement and verification of a performance based optimal design software for wind turbine blades

In this research, we developed software for designing the optimum shape of multi-MW wind turbine blades and analyzing the performance, and it features aerodynamic shape design, performance analysis, pitch–torque analysis and shape optimization for wind turbine blades. In order to verify the accuracy of the performance analysis results of the software developed in this research, we chose the 5 MW blade, designed by NREL, as the comparison model and compared with the analysis results of well known commercial software (GH-Bladed). The calculated performance analysis results of GH-Bladed and our software were consistent in all values of CP in all λ ranges. Also, to confirm applicability of the optimum design module, the optimum design of the new 5 MW blade was performed using the initial design data of the comparison model and found that solidity was smaller in our design even though it produced the same output and efficiency. Through optimization of blade design, efficiency increased by 1% while the thrust coefficient decreased by 7.5%.

Optimization Design of Blade Shapes for Wind Turbines

For the optimization design of wind turbines, the new normal and tangential induced factors of wind turbines are given considering the tip loss of the normal and tangential forces based on the blade element momentum theory and traditional aerodynamic model. The cost model of the wind turbines and the optimization design model are developed. In the optimization model, the objective is the minimum cost of energy and the design variables are the chord length, twist angle and the relative thickness. Finally, the optimization is carried out for a 2 MW blade by using this optimization design model. The performance of blades is validated through the comparison and analysis of the results. The reduced cost shows that the optimization model is good enough for the design of wind turbines. The results give a proof for the design and research on the blades of large scale wind turbines and also establish the foundation for further research and industrial application of wind turbines.