bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis of Power-Maximizing Region 2 Controllers for Wind and Marine Turbines
Authors: Stockhouse David , Zalkind Daniel , Ross Hannah , Pao Lucy
Published: June 2024

Wind and marine energy are rapidly growing and complementary technologies that share some techniques for simplified modeling and control, particularly in below-rated flow speeds. A turbine operator has several choices of controller for maximizing power in Region 2. The simple and ubiquitous KΩ2 control law is often effective but limited in its flexibility. Alternative controllers use reference tracking to split the control objectives into a low-bandwidth optimal tip-speed ratio tracking loop to maximize steady-state power and a higher-bandwidth proportional-integral control loop to reject inflow turbulence. Several options exist for identifying the slowly varying optimal set point during operation, based on estimating the inflow velocity or filtering the power or torque signals. This study compares the trade-offs between performance and other design priorities for a few choices of reference-tracking controller in the literature for reference wind and marine turbines. Analysis is performed in the frequency domain using the linearization of each controller, and the impact of turbulent disturbances on the closed-loop system is described. The controllers are simulated in OpenFAST to analyze their performance with higher-order nonlinear turbine dynamics.
Impact of floating turbine motion on nacelle lidar turbulence measurements
Authors: Peña Alfredo , Angelou Nikolas , Mann Jakob
Published: June 2024

We determine the impact of floating turbine motion on turbulence measurements from a four-beam lidar by emulating its scanning configuration and retrievals within atmospheric turbulence boxes. Since the elevation angle of the lidar beams is small and the two bottom lidar beams point closely to the horizontal plane, we also evaluate the turbulence estimation abilities of a two-beam lidar. For the two-beam nacelle lidar, the variance of the individual beams is close to the target u-variance and closer than that we compute by reconstructing the u-velocity component with the two lidar beams radial velocities. By using floating turbine motion measurements from Hywind, we show that the floating turbine motion impacts turbulence estimations of the nacelle lidar. Roll does not have a clear impact on nacelle-lidar turbulence, whereas both the beam and the u-reconstructed variances increase with pitch amplitude.
An evaluation of different measurement strategies to measure wind turbine near wake flow with small multicopter UAS
Authors: Wildmann N , Kistner J
Published: June 2024

Wind turbine wake flow, especially in the near wake, that is up to one rotor diameter D downstream, is subject to interaction between tip vortices and ambient turbulence. These interactions are important to understand wake decay, but most difficult to measure with common instrumentation. Small uncrewed aerial systems (UAS) can help to measure at such locations where no masts can be installed. We contrast two measurement strategies, the hover flight with multiple UAS and cross-section flights with single UAS. We show that both strategies have advantages; the cross-section flights provide a full picture of the width and wind speed deficit across the rotor diameter whereas multi-UAS hover flights can provide more reliable turbulence intensity and turbulent flux measurements at specific locations. With both strategies, tip vortices can be detected and qualified to characterize the state of wake decay at different positions. A fit to the vortex models Lamb-Oseen and Burnham-Hallock allows to estimate circulation and core radius of the vortices. For best characterization of the wake, we recommend to combine the hover and cross-section flight strategies in future.
Experimental study on the joint load-bearing behaviour of segmented concrete towers under combined loading
Authors: Fürll F , Klein F , Götze M , Betz T , Marx S
Published: June 2024

Hybrid towers have become increasingly established for tall wind turbines. The concrete part of these towers consists of several stacked circular ring segments. To reduce the amount of fabrication and erection effort, the joints between the segments are usually designed as dry ground joints. This paper presents experimental investigations aimed at determining the torsional load-bearing capacity of such dry ground concrete joints subjected to combined loading of prestressing, bending and torsion. Based on previous investigations, a test set-up consisting of three circular ring segments was developed. The torsional load-bearing capacity was determined by monitoring the tangential joint displacements in detail. The test results demonstrate that an increasing bending load leads to a uniform decrease in the torsional load-bearing capacity. The comparison with the current calculation model shows significant differences and confirms the weaknesses of the model regarding its inadequate interaction with the bending load. In contrast, numerical simulations agree well with the test results.
The Underwater Berlin Research Turbine: A Wind Turbine Model for Wake Investigations in a Water Towing Tank
Authors: Krumbein S. , Jentzsch M. , Saverin J. , Nayeri C. N. , Paschereit C. O.
Published: June 2024

The design of a three-bladed horizontal axis research wind turbine for wake investigations is described. The turbine has a rotor diameter of 1.3 m and will be operated in a large water towing tank at a submergence depth of 2.5 m, yielding 3.3 % blockage. The test rig is configured to allow for underwater-stereo-particle-image-velocimetry measurements of the near and the far wake at chordwise Reynolds numbers approaching 700 000. The low-Reynolds number airfoil SG6040 is selected to constitute the rotor blades. Cavitation-free operation is assured by assigning a rated angle of attack of 1°. The blades are designed to maintain a constant circulation along the blade span and to minimize tip deflections. The blade pitch angles are adjustable. Various blade designs, with and without passive flow control devices, can be tested by replacing individual blade sections. This eliminates the need for multiple sets of blades. Sensors for acquiring the turbine’s thrust, torque, rotational speed, the azimuthal positions of the blades, and the imposed blade root bending moments are incorporated into the design. The wind turbine simulation suite QBlade is utilized to simulate the turbine characteristics. A model of the entire test rig is derived, representing its structural properties. Results from the analytical verification of the structural model are provided. QBlade is utilized to analyze the test rig design by imposing design loads and calculating the modal properties.
Extracting Atmospheric Stability Information from Dual-Doppler Radar Scans in the AWAKEN Campaign
Authors: Nadolsky J S , Schroeder J L , Hirth B D
Published: June 2024

Dual-Doppler (DD) derived wind direction (β) fields are used in a novel approach to assess atmospheric stability by way of power spectral density (PSD) and bulk statistical analyses. Data on atmospheric stability, thermodynamic profiles, and wind behavior, as acquired during the AWAKEN campaign, are examined for a measurement time period featuring two stability transitions to analyze differences in flow structure as a function of stability regime. Analysis of the β fields yielded promising results when compared to stability regime and can be applied to other cases.
Deviation of drive torque and rotational speed of a wind turbine drivetrain between test bench and wind turbine simulations
Authors: Krause A , Jacobs G , Röder J , Zweiffel M. , Roth J
Published: June 2024

Wind turbines are a major contributor to a clean and independent energy mix in the future and therefore need to have as little downtime as possible. Downtime can be minimised by increasing the robustness of the wind turbine. The drivetrain is one of the major contributors of downtime in wind turbines. One way of increasing the robustness of wind turbine drivetrains is extensive drivetrain testing and loop-back optimization of the design and control. On the drivetrain test bench torque and other wind loads are applied. To ensure that the drivetrain reaction to the loads is the same as of the erected wind turbine the accuracy of the application of the loads needs to be sufficient. Only if the reaction loads of the drivetrain are similar to the rotor loads of the erected wind turbine a robustness increase can be achieved via the loop-back optimization after the tests. This paper therefore quantifies the deviations occurring in torque and rotational speed of the hub between the erected wind turbine and the test bench for a specific wind turbine. To do so the results of a test bench simulation are compared to results of an aerodynamic simulation of the wind turbine. For the investigated wind turbine drivetrain, the deviation in drive torque increases slightly with higher windspeeds. The torque values are maximum 2 % higher at the wind turbine compared to the rated torque. The rotational speed values are represented very well regarding the time series as well as the occurring frequencies. The speed values deviate maximum 0.5 % compared to the rated speed. The deviations are a result of differences in an underestimation of the drivetrain efficiency in the aerodynamic simulation and the inertia of the drivetrain on the test bench and wind turbine. Since the deviations are small the abstraction losses at the test bench regarding torque and speed are acceptable.
Error analysis of low-fidelity models for wake steering based on field measurements
Authors: Letizia Stefano , Doubrawa Paula , Martinez-Tossas Luis A. , Maric Emina
Published: June 2024

The observations collected by two scanning lidars deployed on the roof of a 2.8-MW turbine undergoing a series of imposed yaw offsets are analyzed. The wake lateral displacement detected by the rear-facing lidar correlates well with the yaw offset sensed by the forward-facing lidar. We find that the high-frequency part of the yaw offset signal is connected to wake meandering, whereas the low frequency component is a good predictor for wake displacement due to yaw misalignment. Conditionally averaged wake velocity data for different yaw offsets are used as benchmarks for the validation of a linearized Reynolds-averaged Navier-Stokes and an empirical wake model. A mean error as low as 2% and a good prediction of the wake trajectory are achieved, provided that the wake recovery rate matches the observations.
Comparison of line-of-sight wind speed measurements from an X-band radar and a long-range scanning lidar
Authors: Hung Lin-Ya , Gottschall Julia , Lilén Vöhringer A. , Hirth Brian D. , Schroeder John
Published: June 2024

As a still novel wind measurement technology, a dual-Doppler X-band wind radar system has been a substantial element of the full-scale onshore campaignAWAKEN(The American WAKE experimeNt). In order to select suitable further applications in the future and, particularly, the most optimal use cases in the wind industry for this technology, a line-of-sight wind speed verification campaign using a co-located scanning lidar as reference was set up as part of theAWAKENcampaign. The wind radar scanned in azimuth sector or plan position indicator (PPI) mode, with multiple elevations (volumetric PPI scan), while the scanning lidar remained fixed in a specific position during the verification campaign. Considering the individual spatial and temporal resolutions of the two systems, the closest points from the radar scanned volumes were compared with measurements from the scanning lidar after threshold-based and statistical data quality control. For a linear regression with 30-minute resolution data collected at 2 km range, a coefficient of determination ofR2= 0.99 was found. Radar mean values, binned according to reference wind speed, do lie in part within the reference uncertainty but not consistently for the investigated range of line-of-sight wind speeds. Part of the reference uncertainty is hereby also associated with the procedure of comparison but kept as low as possible by optimizing the verification setup and procedure.
Redefining Digital Twins – A Wind Energy Operations and Maintenance Perspective
Authors: Tuton Eamonn , Ma Xinhui , Dethlefs Nina
Published: June 2024

Digital Twin (DT) technology has seen an explosion in popularity, with wind energy no exception. This is particularly true for Operations & Maintenance (O&M) applications. However, this expanded use has been accompanied by loose, conflicting, definitions that threaten to reduce the term to a buzzword and prevent the technology from meeting its full potential. A number of attempts have been made to better define and classify DTs, however, these either oversimplify the term or tighten criteria, leading to the exclusion of many DT applications. A new definition framework dubbed the Digital Twin Family Tree is therefore proposed. This widens “Digital Twin” to a general umbrella term for the technology, accompanied by specific definitions. DT Tags are also used to provide individualised characteristics for implementations. A sector-specific definition was devised for component and system monitoring and predictions in wind energy O&M dubbed a CS-DT and suitable DT Tags created. The proposed framework was used to review existing research in literature, demonstrating the potential for increased understanding, explainability, and accessibility of DTs for expert and non-expert stakeholders.
Data-driven met-ocean model for offshore wind energy applications
Authors: Yousefi Kianoosh , Hora Gurpreet S. , Yang Hongshuo , Giometto Marco
Published: June 2024

In recent years, the global transition towards green energy, driven by environmental concerns and increasing electricity demands, has remarkably reshaped the energy landscape. The transformative potential of marine wind energy is particularly critical in securing a sustainable energy future. To achieve this objective, it is essential to have an accurate understanding of wind dynamics and their interactions with ocean waves for the proper design and operation of offshore wind turbines (OWTs). The accuracy of met-ocean models depends critically on their ability to correctly capture sea-surface drag over the multiscale ocean surface—a quantity typically not directly resolved in numerical models and challenging to acquire using either field or laboratory measurements. Although skin friction drag contributes considerably to the total wind stress, especially at moderate wind speeds, it is notoriously challenging to predict using physics-based approaches. The current work introduces a novel approach based on a convolutional neural network (CNN) model to predict the spatial distributions of skin friction drag over wind-generated surface waves using wave profiles, local wave slopes, local wave phases, and the scaled wind speed. The CNN model is trained using a set of high-resolution laboratory measurements of air-side velocity fields and their respective surface viscous stresses obtained over a range of wind-wave conditions. The results demonstrate the capability of our model to accurately estimate both the instantaneous and area-aggregate viscous stresses for unseen wind-wave regimes. The proposed CNN-based wall-layer model offers a viable pathway for estimating the local and averaged skin friction drag in met-ocean simulations.
Challenges in simulating the wind over a coastal complex-shaped site: Madeira Island
Authors: Coimbra I L , Palma J M L M
Published: June 2024

The wind behaviour on Madeira Island is shaped by the intricate coastline and mountainous terrain, features that are not easily represented in numerical models. Thus, this study addressed some challenges and possible solutions in setting up a meso-to-microscale model chain, with the WRF (1-km mesh) and VENTOS®/M (50-m mesh in the high-resolution area) models, in Madeira’s coastal and complex terrain. Wind measurements from four meteorological towers (located in the eastern peninsula–T0978/E–and on the SE–T0521/SE, S–T0522/S, and N–T0960/N–coasts) served as references to assess the simulations. First, due to WRF’s landmask position, land areas were misclassified as sea (and vice versa). This issue was addressed by altering the domain position to better allocate the landmask on the east peninsula, resulting in improved near-surface wind simulations at T0978/E (reducing RMSE and bias by 19% and 67%). Secondly, WRF’s default interpolation of the SST variable did not account for missing and masked data. As such, a different SST interpolation method was employed, leading to improved near-surface wind simulations at T0960/N (reducing RMSE and bias by 11% and 84%) and T0522/S (10% and 16% reduction) masts, but higher errors at T0978/E (7% and 45% increase). The negative influence arose from an incorrect speedup with the new interpolation method. Thirdly, the impact of SST_SKIN, which influences the temperature distribution at the skin level, was evaluated in WRF. Activating SST_SKIN led to a slight improvement in the near-surface wind simulation only at T0521/SE (2% and 6% RMSE and bias reduction), probably due to the dominant smaller-scale nature of the atmospheric circulation in the area, which contrasts with the circulation at the other towers, dominated by the trade winds (N and E masts) and the Island’s wake (S mast). When using the WRF outputs as boundary conditions, these effects on the microscale runs were less pronounced than on the mesoscale results. Nonetheless, the RMSE and bias of the near-surface wind simulation in VENTOS®/M were reduced by 6% and 9% at T0978/E.
Wake Modelling Based on the Stokes Stream Function and the Toroidal-Poloidal Decomposition
Authors: Warncke Norbert G.W. , Lokken Thomas S. , Leo Ilaria
Published: June 2024

The Stokes stream function and its generalisation, the Toroidal-Poloidal decomposition, are well-suited for the derivation of axisymmetric engineering wake models, as they are solenoidal and fulfil conservation of mass of an incompressible fluid by construction. The presented model consists of a time-independent contribution based on Laguerre basis functions with a simple parametrisation that is well-suited for the diffusion-dominated far wake. A time-dependent part can be added based on a poloidal component that can model either helical or toroidal vortex structures, and its parametrisation can be closed towards the total thrust force on the rotor. Despite requiring an external solver, the presented model shows potential as a drop-in replacement for the Ainslie wake model that is widely used in the Wind Energy industry as part of the Dynamic Wake Meandering model.
Uncertainty Quantification Analysis in the Blade Element Momentum Method
Authors: Cao G. , Fei Z. , Vogel C.R.
Published: June 2024

The blade element momentum (BEM) method is a widely used non-linear model for the efficient performance evaluation and design of wind turbines. The aim of this paper is to quantify the uncertainty related to BEM model inputs and sub-models, and investigate how these propagate through the model. Uncertainties related to viscous dissipation in the wake, aerofoil force coefficients, and tip-loss models are considered. Global sensitivity to these parameters is analysed using non-intrusive polynomial chaos expansion, which provides a structured method for uncertainty propagation and sensitivity quantification. Sobol indices are employed to rank the relative importance of each factor to the overall uncertainty in the system. By analysing the NREL 5 MW and DTU 10 MW reference wind turbines, we observe that the different rotors may exhibit different levels of sensitivity to input parameters. The effect of viscous mixing in the turbine wake is found to have a significant impact on predicted rotor performance. Uncertainty in tip-loss model coefficients is also found to generally be important, particularly when evaluating spanwise variations in rotor loads. Uncertainty quantification has the potential to improve understanding of BEM modelling, and provide guidance on the importance of sub-model improvements.
Design and optimization of a modular Electro-Thermal Ice Protection System for a Horizontal Axis Wind Turbine
Authors: Gallia Mariachiara , Caccia Francesco , Guardone Alberto
Published: June 2024

In this work, an Electro-Thermal Ice Protection System (IPS) design was refined and optimized on the NREL 5 MW reference wind turbine. These improvements involved optimization of the distribution of heat flux in both the stream-wise and span-wise directions to minimize power consumption and avoid run-back ice formation. The design conditions focus on a 3-hour rime ice accretion event. The chosen optimization algorithm is a derivative-free method called Mesh Adaptive Direct Search. his optimization effort results in a 55% reduction in power consumption necessary to maintain a completely clean blade surface. The power and energy losses associated with this new optimized design are compared with those from various ice shapes and with the clean blade conditions established in the initial design phase from previous work. The new optimized design allows to keep the blade clean from ice with much larger energy saving. Indeed, only initially, during the first two hours of ice accretion, the energy losses due to this IPS design are higher than without IPS. Beyond this short initial period, substantial energy savings are achieved, especially considering that after the accretion event, the blade remains ice-free, and there are no more power losses.
Static response of wind turbine blades: Comparison of low- and high-fidelity numerical models
Authors: Antunes A M , Verelst D R , Riva R , Kim T , Haselbach P U , Zahle F , Berring P
Published: June 2024

As wind turbine blades become longer and more flexible, the structural models employed in current aeroelastic simulation tools, usually based on beam formulations, are challenged. Effects previously considered negligible in the response of these beam-like structures might now become significant to obtain accurate predictions of their behaviour. To assess this, a test matrix covering key geometric and material characteristics of wind turbine blades is defined and three test cases are analysed as an initial stage of this study. For each case, the static deflection of the beam model from the aeroelastic tool HAWC2 is compared against higher-fidelity shell/solid finite element models. Three structures are studied: an isotropic box beam, a composite box beam and a 12.6 m wind turbine blade from DTU. A good match is observed for the box beam test cases, which study the effects of out-of-plane and distortion warping and bend-twist coupling. Important aspects of high-fidelity models (such as end effects) are highlighted. The study of the DTU 12.6 m blade shows a significant difference (15%) in the torsion natural frequencies of the beam model, though a good agreement in twist angle under torsional loading. More pronounced differences in twist angle are observed when a load distribution based on the steady state response is applied, likely resulting from challenges in modelling complex structural features and three-dimensional effects.
Impact of low-frequency fluctuations on loads of a fixed-bottom offshore reference wind turbine
Authors: Syed Abdul Haseeb , Hannesdóttir Ásta , Mann Jakob
Published: June 2024

Marine atmospheric turbulence is predominantly characterized by large-scale structures associated with low-frequency wind turbulence. However, conventional turbulence models for assessing wind turbine loads often fall short of adequately integrating these low-frequency wind fluctuations. In this study, we used a model for low-frequency wind fluctuations to generate wind fields representative of offshore wind conditions in the North Sea. The IEA 15 MW reference wind turbine was subjected to three distinct incoming wind fields: high-frequency (3D) turbulence, low-frequency (2D) turbulence combined with high-frequency (3D) turbulence, and 3D turbulence scaled by target or measured standard deviation of wind components. We found that increased damage equivalent loads by incorporating the low-frequency wind fields are only significant for the tower base fore-aft and blade root fore-aft moments. The impact of low-frequency wind turbulence on the damage equivalent loads was more pronounced for below-rated wind speeds. The scaled 3D turbulence yielded the highest lifetime damage equivalent loads, i.e., in the range of 8-18% more than the unscaled 3D turbulence and combined 2D +3D turbulence. This method leads to an overestimation of loads and, hence, more expensive turbines. Incorporating 2D+3D turbulence in wind fields resulted in a 3.8% increase in lifetime damage equivalent loads for the tower base fore-aft and blade root flapwise moments compared to unscaled 3D turbulence. These findings underscore the importance of considering low-frequency wind fluctuations in the turbine design process, especially for a more accurate estimation of loads.
Aeroelastic code comparison using the IEA 22MW reference turbine
Authors: Collier W , Ors D , Barlas T , Zahle F , Bortolotti P , Marten D , Jensen C S L , Branlard E , Zalkind D , Lønbæk K
Published: June 2024

Reference wind turbine designs and the associated aeroelastic models are widely used in both research and industry. Reference models representing future concepts are of particular interest. Current state of the art aeroelastic tools are relied upon to design the next generation of large wind turbines. However, modelling assumptions may be invalidated by upcoming very large turbines, and different aeroelastic tools may give inconsistent results. A 22MW turbine model has been defined as part of International Energy Agency (IEA) Wind Task 55 on Reference Wind Turbines and Farms to represent future turbines to be deployed in the 2030s. In this study, an aeroelastic model of this turbine has been created in four tools; Bladed, HAWC2, OpenFAST, and QBlade. Code comparisons are presented for steady state operation, linear stability analysis, and time domain power production simulations in steady and turbulent wind. Generally, the codes show a good agreement, but with some differences present in the linear stability analysis, periodic azimuthal variation, and time domain simulations. The models are a good basis for further study with the IEA 22MW turbine, and further code comparison exercises.
Section force calculation in flexible substructures modelled by wind turbine design tool Bladed
Authors: Nim Erik , Roadnight John , Moharram Hassan , Collier William , Jensen Chr. Sigurd L.
Published: June 2024

Aeroelastic simulation tools are widely used to predict the coupled response for onshore, offshore and floating wind turbines. A key output of the analysis is the section forces in the flexible substructures, such as blades, towers, and floating foundations. Individual substructures are typically modelled as a single linear flexible body, which is a part of a multibody system used for modelling the complete wind turbine. However, in some floating foundation concepts, in particular those with pre-tensioned cables, geometric non-linearities in the substructure can have a significant effect. With the conventional method for calculating section forces, such non-linearities are not captured accurately. In this paper, a new equilibrium-based method for calculation of section forces is presented, as part of the wind turbine design tool Bladed. This method relies on equilibrium between the section forces and the applied loads. The section forces are determined in terms of Lagrange multipliers, which represent the internal constraint forces in the structural elements. This method is valid even when significant geometric non-linearities are present, with reduced order models, and for statically indeterminate substructures. A case study is presented demonstrating the self-consistency of the new method for the OC4 semi-submersible floating platform, modified to include pre-tensioned cables. A verification against ANSYS is also presented for some elementary cases.
Towards an automated framework for Aero-Servo-Elastic Large Eddy Simulation of wind turbine wakes
Authors: Patel A , Muller E , Houtin-Mongrolle F
Published: June 2024

During the last two decades, wind turbine wakes have been extensively studied in academia by implementing the Actuator Line method in numerous Large Eddy Simulation solvers tailored for atmospheric flows. However, and while being computationally affordable for ad hoc deep investigations, this approach remains barely used in industry. One of the leading causes is the complexity of the simulation process, which still involves several aspects to be carefully looked at to get valuable results in output. This paper aims to present a workflow that merges and automates the different steps required to conduct aero-servo-elastic Large Eddy Simulations of wind turbines. In particular, a strategy based on an Accurate Conservative Level Set function is used to flag the regions where wakes propagate. This allows to automatically derive refinement zones that cover the wakes at all times. This generic procedure can be seamlessly applied to various farm layouts and inflow conditions. To display the capabilities of the workflow, it is applied to several configurations, including one to seven wind turbines for different inflow conditions. It is observed that lower wind speeds require larger mesh to capture the wake dynamics adequately. Overall, the workflow offers the added advantage of significantly reducing the required human effort while standardizing the process. This is important from an industrial perspective, wherein parametric studies are usually carried out as part of the design process.
Wind-field characterization using synthetic lidar measurements and proper orthogonal decomposition
Authors: Soto Sagredo E. , Rinker J.M. , Andersen S.J. , Forrest J.P.
Published: June 2024

This paper presents a simple least-square method combined with Proper Orthogonal Decomposition to reconstruct full-rotor flow, using synthetic measurements from a pulsed lidar mounted on the turbine hub. The proposed lidar effectively overcomes blade blockage effects, enhancing data availability. Conducted at a wind speed of 11.4 m/s with 10% turbulence intensity, the study assesses wind-field reconstruction accuracy with the proposed method by examining the influence of mode count and measurement range selection. Comparisons with a baseline, derived from averaging line-of-sight across the rotor plane, reveal that including more modes generally improves reconstruction performance, achieving up to 57% error reduction in the wind-field reconstruction over the baseline. However, this benefit is constrained by the availability of measurements at each time step; limited data coupled with an increased number of modes can lead to overfitting, escalating errors. The method demonstrated here offers advantages in characterizing turbine responses, particularly in capturing low-frequency content in the wind-flow. Yet, channels like tower base moment necessitate a substantially higher number of modes for accurate characterization. Overall, this approach shows potential for real-time wind-flow estimation in lidar-assisted control applications.
Comparison of steady-state analytical wake models implemented in wind farm analysis software
Authors: Mudafort Rafael , Quick Julian , Schulte Jonas
Published: June 2024

A common set of mathematical wind turbine wake models are implemented in a few, well-adopted computational tools for wind farm wake modelling. Although the referenced mathematical formulations are common, implementation details may lead to differences in results. This study presents a systematic comparison of the implementation of mathematical wake models in open source, Python-based wind turbine wake modelling software, and a set of the models are directly compared. Despite aligning only the mathematical model parameters and retaining the default computational model parameters, good agreement is found across most of the model implementations, and additional agreement is expected upon further parameters alignment.
Maintenance & failure data analysis of an offshore wind farm
Authors: Moros D , Berrabah N , Searle K D , Ashton I G
Published: June 2024

Offshore Wind (OW) continues to grow globally at a rapid pace, with growth estimates of 630GW by 2050. To facilitate this rapid growth, costs must continue to be reduced. Reducing operations and maintenance (O&M) costs, which are estimated at 30% of the lifetime costs of wind farms, offers opportunity. This could be achieved by moving current maintenance strategies to a prescriptive strategy. Prescriptive strategies use the turbine monitoring data to determine component remaining useful lifetimes or predict failure windows and then provide an optimised maintenance plan. The first stage of a framework, that can be applied to operational assets, for improving maintenance schedules with failure predictions is presented. Analysis of the SCADA system and the maintenance logs, at an operational offshore wind farm (OWF), with the purpose of identifying turbine failure rates, availabilities and losses and costs from maintenance and failures has been performed. The analysis has revealed two types of maintenance actions, one is cost of maintenance driven and the other cost of downtime driven. It is proposed that, given different characteristics, they should be approached differently in the context of failure predictions. It is also revealed that electrical components are critical to the failure rate and energy losses due to maintenance at the OWF. Electrical components represent approximately 28% of all failures and nearly 40% of revenue loss due downtimes from failure for the period analysed. The power converters drive most electrical failures and are of key commercial interest to the farm. As a result, the power converters should be the target for future prognostic model development. The analysis also shows that with perfect prediction and maintenance scheduling, this OWF could generate an extra 0.26% revenue and a generic 1GW OWF could generate an extra 0.6% extra energy and approximately £1.4m in revenue. This analysis did not reveal the benefit of taking fewer maintenance actions, which should be assessed in future work. Producing a combined prognostic maintenance scheduling method will generate extra wind farm revenues, reduce the number of maintenance actions taken and facilitate the work of maintenance teams.
Experimental and CFD analysis of a floating offshore wind turbine under imposed motions
Authors: Taruffi Federico , Combette Robin , Viré Axelle
Published: June 2024

The rotor of a floating offshore wind turbine experiences intricate aerodynamics due to significant motion in the floating foundation, necessitating a holistic understanding through a synergistic blend of experimental and numerical methodologies. This study investigates rotor loads and the emergence of unsteady phenomena for a floating offshore wind turbine under motion. The approach compares a wind tunnel experimental campaign on a moving scale model with large-eddy simulations. Importantly, both experimental and numerical setups were co-designed simultaneously to match conditions and allow a fair comparison. The experimental setup features a 1:148 scale model of the DTU 10MW reference wind turbine on a six degrees of freedom robotic platform, tested in a wind tunnel. Numerically, the LES code YALES2, employing an actuator line approach undergoing imposed motions, is used. Harmonic motions on one degree of freedom in surge and pitch directions are explored at various frequencies. Thrust force variation aligns with quasi-steady theory for both numerical and experimental results at low frequencies. However, higher frequencies reveal the rise of unsteady phenomena in experiments. Large-eddy simulations, coupled with an actuator line approach, provide additional insights into the near- and mid-wake response to imposed motions. This co-design approach between numerical and experimental tests enhances the comprehension of aerodynamic behaviour in floating offshore wind turbines, offering valuable insights for future designs.
Can mesoscale models capture the effect from cluster wakes offshore?
Authors: Gomez Miguel Sanchez , Deskos Georgios , Lundquist Julie K. , Juliano Timothy W.
Published: June 2024

Long wakes from offshore wind turbine clusters can extend tens of kilometers downstream, affecting the wind resource of a large area. Given the ability of mesoscale numerical weather prediction models to capture important atmospheric phenomena and mechanisms relevant to wake evolution, they are often used to simulate wakes behind large wind turbine clusters and their impact over a wider region. Yet, uncertainty persists regarding the accuracy of representing cluster wakes via mesoscale models and their wind turbine parameterizations. Here, we evaluate the accuracy of the Fitch wind farm parameterization in the Weather Research and Forecasting model in capturing cluster-wake effects using two different options to represent turbulent mixing in the planetary boundary layer. To this end, we compare operational data from an offshore wind farm in the North Sea that is fully or partially waked by an upstream array against high-resolution mesoscale simulations. In general, we find that mesoscale models accurately represent the effect of cluster wakes on front-row turbines of a downstream wind farm. However, the same models may not accurately capture cluster-wake effects on an entire downstream wind farm, due to misrepresenting internal-wake effects.
Efficient design space exploration for spar Floating Offshore Wind Turbines: the trilemma of the ultimate, fatigue, and serviceability limit states
Authors: Patryniak Katarzyna , Yeter Baran , Collu Maurizio , Coraddu Andrea
Published: June 2024

Floating Offshore Wind Turbines (FOWT) can harness the abundant wind resource in deep-water offshore conditions. However, they face challenges in harsh, unsheltered marine environments. The mean hydro- and aerodynamic loads coupled with fluctuating stochastic wind and wave loads contribute to varied failure mechanisms. Therefore, the serviceability, ultimate, and fatigue limit states are vital in ensuring the safety and reliability of FOWT. This paper investigates how specific loads and states drive the design of a spar-type support structure, utilising a computationally efficient frequency-domain model. This approach combines quasi-static aerodynamic and mooring models with a potential-theory-based radiation-diffraction solver. The serviceability criteria concern the platform and tower top displacements and accelerations. The ultimate and fatigue limit states are assessed for the tower base, the waterline section, and the mooring lines, including the effects of yielding under the bending moment and compressive axial load, column buckling, and tension-tension effects in the mooring lines. The full factorial design of experiments is employed to investigate the non-trivial relationships between the limit states and the various features of the support structure. The results demonstrate that the design of the spar platform above the waterline is mainly driven by fatigue, which results from significant dynamic tilt and increased stress concentration at the platform-tower intersection. On the other hand, the catenary mooring lines’ design is mainly driven by the requirements of maximum offset (serviceability limit state) and fatigue.
Atmospheric flow simulation over stationary waves with various steepness using WRF-LES
Authors: Hamzeloo S , Peña A , Larsén X G , Hahmann A N
Published: June 2024

We investigated the capability of the Weather Research and Forecasting (WRF) model to perform large-eddy simulations (LESs) of the structure of the marine atmospheric boundary layer and turbulence exchange between the sea surface and the atmosphere. The WRF-LES model results were compared to results from Sullivan (2008) [1] for two cases: flow over a flat surface and over an idealized stationary sinusoidal surface. We also investigated the sensitivity of the WRF-LES model in simulating turbulence through the marine atmosphere under a range of horizontal grid spacings and wave steepnesses. All simulations were performed in a conventionally neutral atmosphere with a geostrophic wind of 5 m s−1, aligned with the direction of wave propagation. The simulated wind vertical profiles from the WRF-LES model exhibit similar behavior to those of Sullivan (2008) over both wavy and flat surfaces, but the differences in Sullivan’s vertical wind profiles for the two cases are considerably larger in comparison to those derived from WRF-LES. Furthermore, spectral analysis clearly shows the effect of horizontal grid spacing. The impact of wave steepness on turbulence distribution is clearly noticeable in the vertical profiles of resolved velocity variances and covariances. The impact is largest close to the surface, but it is important throughout the bulk of the boundary layer.
Data analysis of the TetraSpar demonstrator measurements
Authors: Singh Deepali , Haugen Erik , Laugesen Kasper , Chauhan Ayush , Viré Axelle
Published: June 2024

Floating offshore wind turbines can extract energy from deep offshore locations, typically unfit for fixed bottom designs. The complex interaction between the structural behavior of the floating offshore wind turbine and the stochastic site conditions, however, is an active area of research. Characterizing the relationship between the environmental conditions and loads may help design reduced-order models, surrogate models, and physics-based engineering models for floating wind turbines. This study uses data from the TetraSpar prototype equipped with a 3.6 MW Siemens Gamesa wind turbine. One-to-one simulations performed using an aero-servo-hydro-elastic software are included for comparison. Various tools, including linear correlation, mutual information, feature ordering using conditional independence, and sensitivity analysis using a data-driven variogram fit, are used for the assessment. This study is also helpful in validating the engineering model for future global sensitivity analysis using elementary effects or Sobol indices that require a rigid sampling of features and can, therefore, only be calculated with simulation tools. We find a good agreement between the experiments and simulations. The 10-min. damage equivalent loads on the tower show a correlation, particularly with the wind speed statistics and the significant wave height.
Turbine-Level Possible Power Prediction using Farm-Wide Spatial Information through Similarity-Based Multivariate Gaussians
Authors: Ávila Francisco Jara , Daenens Simon , Vervlimmeren Ivo , Verstraeten Timothy , Helsen Jan
Published: June 2024

Wind farms usually comprise several turbines of the same type in proximity to one another. Therefore, similarities exist between the power production of specific turbines within the wind farm over time. Considering this, it is possible to find a way to express the similarity between turbines and exploit their properties to find a formulation of the expected behavior of a turbine. With this estimation of possible power output, one can analyze the losses generated by curtailments or transients with a higher precision. Based on this, a probabilistic model is proposed that can be used to calculate energy losses due to maintenance or environmental reasons. On top of that, heavy deviations in the behavior of specific wind turbines can be detected on high-frequency (1-second) data. Overall, the goal of this work is to predict possible power on high-frequency SCADA data using a statistical white-box modeling approach. The presented method is based on a probabilistic framework, constructing a system of linear combinations that permits analytically tracking the expected behavior of a turbine, even if it is non-operational for a specific amount of time. The methodology includes two parts, the first one is the use of data-driven power curves, and the second one consists of an inferential framework based on the environmental conditions of the farm. Results show that the method presented performs better than the manufacturer’s power curves under specific wind speeds and wind directions.
Impact of motions on floating wind turbine power production
Authors: Garcia-Sagrado A , Schlipf D , Perez Brovia S , Burstein J , Yoshinaga T
Published: June 2024

Floating offshore wind opens new possibilities for harnessing wind energy in deeper waters where it is not feasible to install traditional fixed-bottom turbines. Accessing deeper waters enables the utilization of stronger and more consistent wind resources, potentially leading to higher energy production. However, one of the challenges of floating offshore wind is the impact of increased motions on floating turbine’s power. This paper addresses this challenge by investigating wind field reconstruction and motion compensation algorithms when using a nacelle lidar to characterize floating turbine inflow wind speed. The fully instrumented TetraSpar floating demonstrator with a 3.6 MW wind turbine and a nacelle lidar, offers a unique opportunity to investigate the effect of motions on power production. Observations from real measurements are complemented with numerical simulations, highlighting that motion-compensating for mean tilt angle is an effective correction for 10 min average power performance measurements. Results showed that mean tilt angles causing the lidar beams to shift upwards, result in overestimation of the estimated hub height wind speed if no motion compensation is applied. The paper also assesses the impact of motions induced by different sea states on power production.
Offshore Wind Energy Validation Experiment Hierarchy
Authors: Maniaci D C , Naughton J , Haupt S , Jonkman J , Robertson A , Churchfield M , Johnson N , Hsieh A , Cheung L , Herges T , Kelley C
Published: June 2024

This paper provides a summary of planning work for experiments that will be necessary to address the long-term model validation needs required to meet offshore wind energy deployment goals. Conceptual experiments are identified and laid out in a validation hierarchy for both wind turbine and wind plant applications. Instrumentation needs that will be required for the offshore validation experiments to be impactful are then listed. The document concludes with a nominal vision for how these experiments can be accomplished.
Power smoothing by kite tether force control for megawatt-scale airborne wind energy systems
Authors: Hummel JIS , Pollack TSC , Eijkelhof D , Van Kampen E , Schmehl R
Published: June 2024

Airborne wind energy is an emerging technology that uses tethered flying devices to capture stronger and more steady winds at higher altitudes. Compared to smaller systems, megawatt-scale systems are substantially affected by gravity during flight operation, resulting in power fluctuations. MegAWES, a 3 MW reference model, experiences power fluctuations between -5.8 MW and +20.5 MW every 12.5 seconds during the traction phase when using its baseline controller at a wind speed of 22 m/s. The baseline controller does not have a power limit, leading to high peak power, and aims to keep the tether force constant, causing it to consume power when the kite is flying upwards. In this paper, we implement an optimal torque controller in the MegAWES framework and show that this eliminates the power consumption during the traction phase. Furthermore, we propose a kite tether force controller that allows setting a power limit when combined with the 2-phase reeling strategy, which decreases the peak power. Our new architecture reduces the power output range by 75% to between +3.7 MW and +9.4 MW in strong wind conditions.