MW Wind Turbine Research Articles

A new similarity criterion for wind tunnel testing of floating wind turbines achieving dynamic aerodynamic characteristics similarity

Floating offshore wind turbines have rapidly advanced in recent years, prompting numerous experiments. Small-scale turbines require blade redesign with low Reynolds number airfoils due to the scale effect. Consequently, the chord length of these blades no longer corresponds to the length scale, leading to dissimilar dynamic aerodynamic phenomena during platform movement. To address this, a new criterion is proposed for wind tunnel tests of floating offshore wind turbines. The criterion incorporates reduced frequency correction and optimal tip speed ratio (TSR) matching. It ensures hydrodynamic similarity through Froude number similarity and avoids excessive low wind speed through wind speed correction. Optimal TSR matching ensures the model and prototype turbines operate within similar power curve intervals. Reduced frequency corrects the platform sine motion parameters, providing the model turbine with a dynamic aerodynamic effect similar to the prototype. The National Renewable Energy Laboratory’s 5 MW wind turbine is chosen as the prototype, and the model wind turbine blades are designed using NACA 4412 airfoil and the criterion of thrust coefficient similarity. The sub-module of the FAST, AeroDyn is employed to validate the new criterion for different surge motions between the NREL 5 MW and the model. Under the new similarity criterion, the model turbine exhibits a relative root mean square error of 4.61 % and 1.54 % in power coefficient and thrust coefficient, a significant improvement compared to the current similarity criterion's relative root mean square error of 24.74 % and 8.36 %. The results reveal a circular trend variation in both thrust coefficient and power coefficient with position. Notably, the new criterion demonstrates a closer alignment with the dynamic aerodynamic characteristics of the prototype when contrasted with the current similarity criterion. Furthermore, this model design, tailored for the NREL 5 MW turbine, can be applied to model experiments with various turbine configurations. It has been successfully verified for the IEA 15 MW turbine, exhibiting promising simulation results. The proposed similarity criterion is suitable for wind tunnel testing, addressing the problem that the current similarity criterion cannot reflect the dynamic aerodynamic characteristics of prototype wind turbines. Additionally, it enables experiments on different prototype configurations using a single model turbine, reducing wind tunnel testing costs.

Study on Mooring Design of 15 MW Floating Wind Turbines in South China Sea

Wind turbines and floating platform upsizing are major trends in the current offshore wind development. However, harsh environmental conditions increase the risk of anchor dragging and mooring failure when deploying large offshore floating wind turbines. Therefore, it is necessary to design a mooring system for the specific deployment site. This study aims to perform the mooring system design of a floating offshore wind turbine (FOWT) operated in the South China Sea, which is a combination of the IEA 15 MW wind turbine and UMaine VolturnUS-S floating platform. Hydrodynamic coefficients were calculated based on the potential flow theory, considering the environmental loads in the South China Sea. Additionally, the hydrodynamic coefficients were imported into AQWA to calculate the time-domain mooring tension. The mooring design parameters, such as mooring line length, nominal sizes, and anchor point, were determined using the criterion of anchor uplift, maximum breaking strength, and fatigue life, respectively. The design criterion required that the anchor uplift is not more than the allowable value, the long-term breaking limit of mooring with a 100-year return period should be less than the maximum breaking limit, and the fatigue damage accumulation in 50 years should be safe. The mooring design procedure provides a reference for mooring system design and safe operation of large floating wind turbines in the South China Sea.

Feedforward pitch control for a 15 MW wind turbine using a spinner-mounted single-beam lidar

Abstract. Feedforward blade pitch control is one of the most promising lidar-assisted control strategies due to its significant improvement in rotor speed regulation and fatigue load reduction. A high-quality preview of the rotor-effective wind speed is a key element of control benefits. In this work, a single-beam lidar is simulated in the spinner of a bottom-fixed IEA 15 MW wind turbine. Both continuous-wave (CW) and pulsed lidar systems are considered. The single-beam lidar can rotate with the wind turbine rotor and scan the inflow with a circular pattern, which mimics a multiple-beam nacelle lidar at a lower cost. Also, the spinner-based lidar has an unimpeded view of the inflow without intermittent blockage from the rotating blade. The focus distance and the cone angle of the spinner-based single-beam lidar are optimized for the best wind preview quality based on a rotor-effective wind speed coherence model. Then, the control benefits of using the optimized spinner-based lidar are evaluated for an above-rated wind speed in OpenFAST with an embedded lidar simulator and virtual four-dimensional Mann turbulence fields considering the wind evolution. Results are compared against those using a single-beam nacelle-based lidar. We found that the optimum scanning configurations of both CW and pulsed spinner-based single-beam lidars lead to a lidar scan radius of 0.6 of the rotor radius. Also, results show that a single-beam lidar mounted in the spinner provides many more control benefits (i.e. better rotor speed regulations and higher reductions in the damage equivalent loads on the tower base and blade roots) than the one based on the nacelle. The spinner-based single-beam lidar has a similar performance to a four-beam nacelle lidar when used for feedforward control.

Dynamic response analysis of the TetraSpar floater in waves: Experiment and numerical reproduction

The initial proof of concept model scale test campaign for the TetraSpar floating wind turbine substructure is presented here along with a detailed response analysis and numerical reproduction. The tests were conducted at scale 1:60 in wind and waves with the pitch-regulated DTU 10 MW wind turbine. The floater was tested in two configurations: semi-submersible and spar. The experimental setup and program is described in detail followed by system identification for natural frequencies and damping. The responses of the floater in the two configurations to hydrodynamic loading are analysed and compared. The analysis includes irregular sea states and focused wave groups at both 0° and 30° heading. The hydrodynamic damping of the floaters was quantified in decay tests, showing a clear linear and second-order component. It was observed that the semi-submersible configuration had significantly larger motion response than the spar configuration in ultimate limit state wave conditions. Emphasis is placed on the mooring loads and the tensions in the support lines for the ballasted keel. The increased ballast of the spar keel led to larger loads in these support lines. Further, second- and higher-order wave forcing were observed in responses of both configurations. A numerical model based on first-order radiation-diffraction theory, second-order Newman loads and additional Morison viscous forcing is set up. The model damping is calibrated against the measurements at each sea state. It is demonstrated that after this calibration, the model is able to reproduce the floater response and tower top accelerations with good accuracy, both in the linear range and at the natural floater frequencies, with heave in the storm sea state as the exception. The dynamic tensions in the keel lines are found to depend strongly on the lines projection to the inline wave direction. Also this behaviour is reproduced accurately by the model, although with some under-prediction in one of the lines in the rated wind sea state, which is linked to differences in the experimental pre-tension for the six lines.

An Investigation on the Effect of Watertightness Criteria on the Structural Assessment of Prestressed Concrete Substructures of Floating Wind Turbines

The effect of watertightness criteria on structural assessment was investigated in this study through the design of a semi-submersible floater for a 10 MW wind turbine. A structural assessment was carried out for the ultimate, fatigue, and serviceability limit states, where six cases of combinations of the loading conditions and structural requirements were assumed for the assessment of watertightness. The different assessment conditions for watertightness resulted in different designs of prestressing tension, which also caused differences in the design margins for the ultimate limit states as well as the SN diagram used for the assessment of the fatigue limit states. The maximum stress acting on the member of the floating wind turbine differed by about 7.6 to 16.2% for the 50-year and 1-year environmental conditions. The small difference between the stresses for the two return periods can be attributed to the large component of the wind turbine thrust in the variable loads. When the partial load factor of 1.0 was used, the structural requirement in the prestressing design caused larger differences compared to the return period of the loading conditions in the watertightness assessment. The use of the partial load factor of 0.5 for the prestressing design also caused a large difference in the results of the structural assessment, indicating the necessity of further investigation of the determination of the partial load factor for floating offshore wind turbines.

Numerical Investigation of a Floating-Type Support Structure (Tri-Star Floater) for 9.5 MW Wind Turbine Generators

A numerical investigation of floating-type substructures for wind turbine generators was conducted by using time-domain simulation. A Tri-Star floater for 8–10 MW generators, which was developed by Samsung Heavy Industries (SHI), was chosen as the floating substructure. To make the anchor system, catenary mooring lines, considering redundancy, were installed on the floater. The main sources of external force on the wind turbine generator are wind, waves, and currents. To consider severe environmental conditions, Design Load Cases (DLCs) 1.6 and 6.1 of the IEC guidelines (IEC 61400-3-1) were chosen. From the measured environmental data for the installation site, the main parameters for the simulation conditions were obtained. The tilt angle and horizontal movement of the floater and the mooring tension for the different mooring systems were checked. The response of the floater during the failure of the mooring was also studied, and the critical failure of the mooring was confirmed. During the failure of the mooring, the redundancy system worked well, in which the movement of the floater was constrained within the criteria for all scenarios.

Experimental evaluation of fatigue in wind turbine blades with wake effects

Large investments in the implementation of wind turbines were made in the last decades, which increased the importance of this source in the national electricity systems. The management of this asset can strongly benefit from the development of cost effective monitoring strategies. These may contribute to the extension of the lifespan of existing wind turbines, to the reduction of insurance costs by reducing the operation risk and to the optimization of the design, leading to an increase in the competitiveness of this energy source. This paper is dedicated to the implementation and validation of tools to assess the fatigue condition of wind turbine blades from strains measurements with a set of fiber optic strain gages installed at the blade roots. It describes the data processing needed to obtain accurate experimental estimations of the bending moments applied to the blade roots and the procedure to obtain the accumulated fatigue damage. The instrumentation of two wind turbine rotors aligned with the most frequent wind direction allowed quantifying the interaction between neighboring wind turbines and its effect on fatigue life consumption. The paper presents unique experimental data simultaneously collected in two 1.8 MW wind turbines of an onshore wind farm during 14 months that permitted a very clear illustration of wake effects on fatigue consumption.

Prediction of wind turbine noise: Comparison of three standardized prediction methods

Noise from wind turbines seems to be relatively more annoying than noise from other sources. Yet wind turbines are considered important for the shift to renewable energy sources. In the planning process of new wind farms, it is important to accurately predict the noise impact on nearby communities in order to minimize noise conflicts. Noise from an existing 2 MW wind turbine has been measured in a number of positions under various wind conditions. The results have been compared with predictions using sound propagation modules according to ISO 9613-2, CnossosEU and Nord 2000. The work is part of the ongoing Polish-Norwegian research project HETMAN, Healthy Society towards optimal management of wind turbine noise. The actual results presented in this paper should therefore be considered preliminary.

Real-time yaw-misalignment calibration and field-test verification of wind turbine via machine learning methods

It has become a general consensus that nacelle-mounted LiDAR can be used to calibrate the yaw misalignment or drive the real-time yaw motions for wind turbines, which would improve the power-generation efficiency. The advantage of LiDAR utilization is that the accuracy of inflow wind measurement would be greatly improved, while its disadvantage is that the cost remains high and the data validity is not sufficiently high. In this paper, an efficient machine learning method for estimating LiDAR measurement is developed to establish the real-time yaw calibration framework and sustain the LiDAR rolling utilization. Firstly, the correlation of LiDAR measurement with SCADA features is analyzed to estimate LiDAR measurement using only SCADA data. Secondly, several machine learning algorithms are studied for performance comparison, and the dependence of each algorithm on data size is also analyzed. Experimental results show that, the proposed XGBoost algorithm has high accuracy, requires less data, and can quickly calibrate the yaw misalignment. Finally, the field testing is held for a commercial 2 MW wind turbine to verify the effectiveness. The field-test results show that the proposed method is feasible for industrial applications and can improve the annual theoretical power generation by 3.66% compared to the situation without calibration, which also provides an executable and economical solution for LiDAR replacement planning.

A hybrid 3DSE-CNN-2DLSTM model for compound fault detection of wind turbines

In recent years, intelligent fault detection methods have achieved dramatic results for wind power generation. However, a majority of the available intelligent detected methods can only detect single faults. The supervisory control and data acquisition (SCADA) system is widely applied in wind turbines (WTs). SCADA data is a timestream of variable data. Considering spatial correlation and temporal correlation among SCADA data, this study proposes a WT compound fault detection model combining a three-dimensional squeeze-excitation convolutional neural network (3DSE-CNN), and a two-dimensional long and short-term memory network (2DLSTM). The proposed 3DSE-CNN-2DLSTM model consists of three parts: data preprocessing, spatiotemporal feature extraction, and fault detection. First, one-dimensional data is converted into a 2D image by a sliding window method. Then, in the proposed 3DSE-CNN model, the SE attention module is applied to enhance the convolutional channel information and extract spatial features. The proposed 2DLSTM model extracts spatiotemporal fusion features. Finally, the label of the fault category is obtained by argmax function. To validate 3DSE-CNN-2DLSTM, a 3 MW WT SCADA dataset from the south coast of Ireland is applied in the experiment. The proposed 3DSE-CNN-2DLSTM model achieves specific compound fault detection and improves detection accuracy.

Robust model predictive control of wind turbines based on Bayesian parameter self-optimization

This paper studies the effect of different turbulent wind speeds on the operation of wind turbines. The proportion of wind power in the field of new energy generation has increased massively and has gained wide application and attention. However, the smooth operation and the stability of the output power of the wind power generation system are susceptible to wind speed fluctuations. To tackle this problem, this paper takes a 5 MW horizontal axis wind turbine as the research object that proposes a parameter adaptive robust control method to achieve self-optimization of controller parameters by means of Bayesian optimization. The 5 MW wind turbine model is utilized to verify the feasibility of the algorithm by combining the wind speed types commonly found in a high-altitude region in northwestern. The simulation results validate the effectiveness of the proposed scheme. The outcomes demonstrate that Bayesian optimization can significantly decrease the effects of wind speed instability. The output power increases by 1.9% on average at low wind speed and stabilizes on 5 MW at high wind speed. Therefore, the stable controller for wind power output is the robust model predictive controller with parameter improvement.

Assessment of industrial-scale green hydrogen production using renewable energy

The global pivot towards sustainable energy solutions necessitates a closer examination of green hydrogen production using renewable energy sources. This study aimed to assess the feasibility and efficiency of green hydrogen production on an industrial scale using solar and wind energy in Diyala city, Iraq. Experimental weather data, including solar irradiance, ambient temperature, and wind speed, were meticulously collected throughout 2022. The analysis indicated that, for wind energy, the optimum electrolyser capacity that matched a 1.5 MW wind turbine achieved a hydrogen production of 11,963 kg/year, with associated costs of $8.87/kg. In contrast, when focusing on solar energy, the ideal electrolyser capacity harmonizing with a 2 MW solar photovoltaic generated a notably higher hydrogen output of 94,432 kg/year at a more competitive cost of $6.33/kg. These findings underscore the potential economic advantages of solar-based green hydrogen production over wind-based methods in Diyala city. Furthermore, the significant difference in hydrogen production yields between the two methods emphasizes the need to optimize renewable infrastructure based on location-specific renewable resources. This study offers valuable insights into tailoring green hydrogen production strategies in regions with similar climatic conditions to Diyala and serves as a blueprint for future renewable energy-driven hydrogen production initiatives.

Influence of wind and seismic ground motion directionality on the dynamic response of four-legged jacket-supported Offshore Wind Turbines

This paper aims at investigating the influence of the direction of wind and seismic ground motion on the structural response of four-legged jacket support substructures for bottom-fixed Offshore Wind Turbines located in areas with non-negligible seismic risk. To do that, a parametric analysis that considers thirteen seismic shaking directions, seven wind directions and four earthquake records is carried out. The results are presented in terms of accelerations, axial forces, bending moments and shear forces in the jacket substructure. The NREL 5 MW wind turbine supported on the jacket designed for the phase I of the OC4 project is considered. The response of the system is simulated using an OpenFAST model that takes into account multi-support seismic input, soil–structure interaction and kinematic interaction. The results show that load combinations with aligned wind and ground motion directions are never the worst-case scenario. On the contrary, the largest accelerations are found when the shaking direction acts along the side-to-side direction, and the most significant internal forces are usually found when the ground motion is aligned with one of the diagonals of the base of the jacket and not aligned with the wind direction. The paper also discusses the specific ranges of the angle of misalignment between wind and seismic shaking directions within which the maximum internal forces are expected to be found.

Wind turbine main bearing rating lives as determined by IEC 61400‐1 and ISO 281: A critical review and exploratory case study

AbstractThis paper studies the rating lives of wind turbine main bearings, as determined by the IEC 61400‐1 and ISO 281 standards. A critical review of relevant bearing life theory and turbine design requirements is provided, including discussion on possible shortcomings such as the existence (or not) of the bearing fatigue load limit and the validity of assuming linear damage accumulation. A detailed exploratory case study is then undertaken to determine rating lives for two models of main bearing in a 1.5 MW wind turbine. Rating life assessment is carried out under different conditions, including various combinations of main bearing temperature, wind field characteristics, lubricant viscosity, and contamination levels. Rating lives are found to be sufficiently above the desired 20‐year design life for both bearing models under expected operating conditions. For the larger bearing, operational loads are shown to be below or close to the bearing fatigue load limit a vast majority of the time. Key sensitivities for rating life values are temperature and contamination. Overall, the results of this study suggest that an ISO 281 rating life assessment does not account for reported rates of main bearing failures in 1 to 3 MW wind turbines. It is recommended that a similar analysis be undertaken for ISO/TS 16281 rating lives, along with further efforts to identify principal root causes of main bearing failures in future work, possibly leading to a new application standard specific to this component. It is also recommended that the impacts of partial wake impingement on main bearing rating lives are investigated.

A Bayesian deep learning framework for reliable fault diagnosis in wind turbine gearboxes under various operating conditions

Vibration-based fault diagnostics combined with deep learning approaches has promising applications in detecting and diagnosing faults in wind turbine gearboxes. Specifically when time series vibration data is transformed to a 2-dimensional cyclic spectral coherence maps, the accuracy of deep neural networks in classifying faults increases. Nevertheless, standard deep learning techniques are vulnerable to inaccurate predictions when tested with new data originating from unseen faults or unusual operating conditions. To address some of these shortcomings in the context of wind turbine gearboxes, this paper investigates fault diagnostics using Bayesian convolutional neural network which provide accurate results with uncertainty bounds reducing wrong overconfident classifications. The performance of Bayesian and standard neural networks is compared using a simulation-based dataset of acceleration signals generated from a multibody dynamic model of a 5 MW wind turbine. The framework proposed in this paper has relevance to fault detection and diagnosis in other rotating machinery applications.

Optimized design of wind turbine airfoil aerodynamic performance and structural strength based on surrogate model

Though offshore wind turbines have great potential for applications in ocean energy harvesting, their subject to complex unsteady aerodynamic loads. To obtain wind turbine airfoils with both aerodynamic performance and structural strength, an airfoil optimization method was proposed by adding the torsion constant to the objective function under the premise of improving the lift-to-drag ratio. The optimization method characterizes the airfoil geometric profile by using Class/Shape function transformation (CST) parameterization. The airfoil aerodynamics and the structural properties were solved by the computational fluid dynamics (CFD) method and Matlab program, respectively. To reduce the number of CFD calculations, a Kriging surrogate model was established. The combination of an optimized Latin hypercube sampling method and expected improvement (EI) points infill criteria is to improve surrogate modeling efficiency. The NREL 5 MW wind turbine blade was used as the research object. The NACA64618 with 18% relative thickness, the DU91-W2-250 with 25% and the DU97-W-350 with 35% were optimized by assigning different weights to aerodynamic and structural objectives based on airfoil position in the blade. The results showed that the aerodynamic performance of the optimized airfoils was improved. Subsequently, the three optimized airfoils were replaced in the same positions of the blade for lay-up design. The blade structural properties were calculated by finite element method (FEM) and it found that the blade torsion angle was reduced, which indicated that adding structural objectives in the airfoil optimization could enhance the blade structural performance.

Time-dependent reliability-based design optimization of main shaft bearings in wind turbines involving mixed-integer variables

Main shaft bearings are essential transmission components in wind turbines. The failure of the main shaft bearings is inevitably accompanied by catastrophic consequences. In this regard, this paper presents a time-dependent reliability-based design optimization (TRBDO) approach for main shaft bearings involving mixed-integer variables. The maximization of fatigue life and the elastohydrodynamic film thickness under the load spectrum are selected as optimization objectives. The time-dependent reliability and geometric correlation are determined as nonlinear constraints. An efficient two-stage enrichment strategy is introduced to handle the time-dependent probabilistic constraint with mixed-integer design variables. A mixed-integer nonlinear optimization method is developed based on a meta-heuristic algorithm to solve the formulated TRBDO problem. Eventually, the effectiveness and robustness of the proposed approach are demonstrated by a real application in a 5 MW wind turbine.

Lateral Ultimate Capacity of Monopile Foundations for Offshore Wind Turbines: Effects of Monopile Geometry and Soil Stiffness Properties

Offshore Wind Turbines (OWT) with increasingly higher energy output are being developed to meet energy demands, posing greater challenges for their foundation design. Several foundation types are used to support these turbines, with monopiles (MPs) accounting for 80% of the installed capacity. In this study, three-dimensional (3D) nonlinear finite element models (FEM) were employed to investigate the behaviour of a monopile foundation supporting a 5MW wind turbine subjected to lateral loading. The results indicate that the MP behaviour depends on the pile length to diameter (L/D) ratio and the soil shear strength. Inspection of the bending-moment profiles at the lateral ultimate capacity indicated that the monopiles can behave in a flexible manner, even with low L/D ratios. The L/D ratio affected the MP normalized lateral ultimate capacity to varying degrees, and the biggest effect was for soft clays, amounting to an approximately five-fold increase for L/D values of 3.33 to 13.33. Lesser effects were found for stiff clays.

A novel wake control strategy for a twin-rotor floating wind turbine: Mitigating wake effect

As one of the most promising alternatives to large wind turbines, twin-rotor floating wind turbines enables a significant reduction in the cost of power generation. An appropriate wake control strategy for twin-rotor floating wind turbines can contribute to the reduction of wake effect in wind farms. In the current study, aerodynamics and wake characteristics of twin-rotor floating wind turbine under four different yaw configurations are demonstrated. In detail, the variation of power and thrust of the twin-rotor at different yaw angles (0°, 5°, 10°, 15°, 20°, 30°) are analyzed, and the effect of the surge motion amplitude on the torque and thrust of twin-rotor in yaw are considered, and the wake dynamics under four different configurations are compared. The power gain of a twin-rotor floating wind turbine is about 5 % compared to a DTU 10 MW wind turbine at yaw angle of 15°. When the twin-rotor wind turbine is in wake channeling configuration, optimum wake recovery performance with the lowest level of turbulent kinetic energy is achieved. This study provides a scenario for optimizing the framework of floating wind farms.

Research on the strengthening and retrofitting methods of existing wind turbine foundations with embedded‐ring

AbstractHigher demand for the load‐bearing ability of supporting foundations is required if its upper wind turbine is substituted by a larger one. Presently, there is a lack of strengthening and retrofitting methods for existing wind turbine foundations. A combined‐connection retrofitting method by adding anchor bolts to the existing wind turbine foundation with embedded‐ring is prompted in this study. Furthermore, a strengthening measure by adding a stiffening beam, expanding the foundation plate, and adding rock bolts to increase the ability to resist basal bending moment is proposed. Finite element analysis is conducted to analyze the effectiveness of the proposed strengthening and retrofitting methods for a 2 MW wind turbine foundation. The result shows that the peak stress of embedded‐ring is decreased by 36% by using the internal combined‐connection, and the peak tensile stress of concrete is reduced by 4.0%. When the external combined‐connection is adopted, the peak stress of the embedded‐ring is reduced by 69.3%, and the peak tensile stress of the concrete is reduced by 26.0%. It illustrates that the external combined‐connection method is more effective to increase the load‐bearing capability of the connection than the internal combined‐connection method. Additionally, the proposed strengthening measure by adding a stiffening beam, expanding the foundation plate, and adding rock bolts is effective to decrease the responses of foundation. The maximum tensile stress of concrete, the maximum compressive stress of concrete, and the maximum stress of the embedded‐ring are reduced by 19.8%, 31.3%, and 22.2%, respectively. To summarize, the proposed strengthening and retrofitting methods are capable of ensuring the safety and stability of higher‐power wind turbine operation.