bluuebunny/crossref_metadata_2025 huggingface dataset:
Reference dynamic power cable of floating offshore wind turbine for thermal and simple fatigue evaluation
Authors: Abrahamsen A B , Faria B R , Lund R S , Verelst D R , Roques J , Hochet M , Dykes K
Published: June 2024

With the increased interest in floating offshore wind turbines, there is an increased focus on evaluating the lifetime of the auxiliary systems, such as the mooring lines and the dynamic power cables, because these systems are in-mature compared to bottom-mounted offshore wind farms. This paper proposes a conceptual reference dynamic power cable suitable for the 15 MW NREL reference turbine mounted on the UMaine floater installed at 82 m water depth. This provides a basis for discussing the life evaluation of dynamic cables obtained from aeroelastic simulation of the floating offshore wind turbine platform exposed to the environmental conditions of the South Brittany site in France. An electromagnetic-thermal finite element simulation is used to determine the current capacity of the cable and aeroelastic simulations are used for a simple assessment of the main failure modes as maximum cable tension, minimum bending radius and fatigue damage accumulation.
Dynamic Thermal Behavior of Natural Ester Liquid-Filled Transformer for Offshore Wind Turbine in Cold Climate
Authors: Al-Abadi A , Gamil A
Published: June 2024

The current study presents a dynamic thermal model (DTM) which accounts for variation of liquid properties with temperature to predict the development of temperature pattern of the entire transformer for offshore wind application during energizing process. The DTM is benchmarked against dynamic heat run test which is applied on a natural ester filled transformer designed for offshore wind application. The transformer is equipped with fiber-optic sensors (FOS) to observe the temperature distribution of the winding and the liquid during heating up and cooling down processes. The investigations involve in-depth analysis of the thermal behavior of the winding and liquid at different locations to give physical interpretation of the dynamic thermal response of different transformer components during dynamic loading. The results are further validated with transient computational fluid dynamics (CFD) simulations. Eventually, the study gives guidelines and recommendations of dynamic loading of transformer filled with natural ester in cold climate.
Experiments on Interaction between Six Vertical-Axis Wind Turbines in Pairs or Trios
Authors: Jodai Yoshifumi , Tokuda Haruki , Hara Yutaka
Published: June 2024

The interaction between six closely placed vertical-axis wind turbines (VAWTs) in a parallel-pair arrangement (layout 1), a staggered-pair arrangement (layout 2) and a parallel-trio arrangement (layout 3) were investigated. Six miniature wind turbine models were used in the wind tunnel experiments. The rotor gap g
1 within each turbine pair or trio is set fixed values. In the layout 1, three paired turbines are in line perpendicular to the main stream. In the layout 2, two paired turbines are in line perpendicular to the main stream on a front rail and remaining paired turbines are on a rear rail. In the layout 3, two trios of turbines are in line perpendicular to the main stream. Increases in the averaged turbine power up to 106% of the single turbine power PSI
with the decrease in an inter-cluster gap g
2, have been demonstrated in the layout 1. The power of a downstream central turbine pair significantly decreases with increasing a streamwise spacing s in the layout 2. In the layout 3, the averaged turbine power reaches 0.95PSI
for Case A. Reducing an inter-cluster interval (g
2 or s) improves a wind-farm performance in a unit footprint area in all the layouts tested.
Numerical Investigation of Atmospheric Boundary Layer Control in Wind Farms with Multirotor Systems
Authors: Martins Flavio A. C. , Ferreira Carlos S. , Van Zuijlen Alexander
Published: June 2024

This study investigates the near-wake aerodynamics of actuator disks (multirotor devices) paired with lift-generating devices (rotor-sized wings, dubbed ABL-control devices). These rotor-sized wings generate vortical structures that enhance the vertical momentum flux from above the atmospheric boundary layer (ABL) into the wind farm, aiding wake recovery. Using three-dimensional actuator surface models based on Momentum theory, the study employs steady-state Reynolds-averaged Navier–Stokes computations in OpenFOAM to address the current proof-of-concept model. The numerical results of this paper are validated with a comparison against the experimental results of a scaled multirotor device in a wind tunnel. The performance of the ABL-controlling devices is evaluated through the wind farm’s total pressure and vertical momentum flux. Results indicate that ABL-control significantly accelerates wake recovery, with designs featuring two or four ABL-control devices achieving 95% total pressure recovery at x/D ≈ 5, one order of magnitude shorter than the baseline setup without ABL-control.
Validation of the structural response of a 70kW VAWT
Authors: Malcolm DJ , Ajala-Inyang O
Published: June 2024

This paper describes a project for predicting and measuring the structural response of a 70 kW vertical axis wind turbine (VAWT), while offering insights for optimizing efficiency and ensuring longevity. We employ the traditional frequency response approach and model the turbine behavior using its stationary modal shapes. Results for three key sensor locations – lower central mast bending, upper blade flapwise bending, and central horizontal arm out-of-plane bending – reveal that the codes accurately predict the critical resonance and capture the dominant harmonic amplitudes. In addition, it is shown how a statistical treatment of the steady state solution can lead to predictions of the fatigue cycle distribution enabling accurate life predictions. The program demonstrates the potential to significantly improve the design and operation of VAWT using simple, steady state, simulation codes for capturing VAWT aerodynamics. The model demonstrates sufficient accuracy while offering computational efficiency compared to more complex models.
Lift-Induced Wake Re-Energization for a VAWT-Based Multi-Rotor System
Authors: Broertjes T J , Bensason D , Sciacchitano A , Ferreira C
Published: June 2024

This study explores the performance and near-wake dynamics of a VAWT-based Multi-Rotor System in both its original configuration and in the presence of external lift-generating devices, specifically employed for wake control operations. The wake of a scaled VAWT-based MRS was measured in a wind tunnel using Particle Tracking Velocimetry. Lift-generating devices, including a 3-element cascading wing on top and a single-element wing in the middle of the MRS, were used to enhance wake control and deflection. Measurements in the near-wake revealed notable differences between configurations with and without these devices. Without them, the wake remained concentrated in the actuator surface’s projected downstream area, with minimal crossflow diffusion. Conversely, the configuration with lift-generating devices exhibited significant wake deformation, including axial expansion and lateral contraction, promoting streamwise momentum recovery. As a result, increased power recovery was found downstream of such a system compared to the clean MRS. These findings underscore the potential of such systems, particularly when equipped with lift-generating devices, to manipulate wake dynamics and enhance wind farm efficiency, thereby advancing innovative wind energy solutions.
Experimental study of the impact of blade-tip mounted rotors on the X-Rotor vertical-axis wind turbine
Authors: Bensason David , Sciacchitano Andrea , Ferreira Carlos
Published: June 2024

The Horizon 2020 European Commission-funded project - X-ROTOR - proposes a radical rethink of the traditional vertical-axis wind turbine geometry. The X-Rotor vertical axis wind turbine relies on blade-tip mounted rotors, referred to as secondary rotors, for power generation and takeoff. This study examines the aerodynamic effects of secondary rotors on a scaled X-Rotor model’s loading in an open-jet wind tunnel. Particle image velocimetry measurements are taken at two cross-stream planes within the volume of rotation of a scaled turbine model at two phase-locked positions. The measurements are compared with cases without secondary rotors present to understand the local impact of the blade-tip mounted devices on the wake and vortex strengths. The results indicate an accelerated turbulent diffusion of the trailing tip-vortex of the X-Rotor, and the subsequent local in-plane velocity gradients induced by the trailing tip-vortex are diminished. These insights and experimental database contribute to the development and validation of numerical models of the X-Rotor with blade-tip mounted rotors.
NREL 15-kW: An Advanced Horizontal-Axis Reference Turbine for Distributed Wind
Authors: Gupta A , Bortolotti P , Summerville B
Published: June 2024

Distributed wind energy can play a significant role in the renewable energy landscape. A recent study conducted by the National Renewable Energy Laboratory (NREL) identified the lack of availability and utilization of reference wind turbine models as a major hindrance in the development of the distributed wind energy technology sector, even though such models are widely used in offshore and land-based wind research. Therefore, NREL is developing three reference wind turbine architectures, also called archetypes, for distributed wind energy applications. This paper describes the detailed design and modeling of the first of the three reference turbine archetypes, called the NREL 15-kW turbine. It is a passive-yaw, upwind turbine with a rated power of 15 kW.
Using intrusive approaches as a step towards accounting for stochasticity in wind turbine design
Authors: Branlard E. , Frontin C. , Maack J. , Laird D.
Published: June 2024

Current wind turbine design methods require tens of thousands of time-domain simulations and use different random seeds to account for the stochasticity of the environmental conditions. The account of stochasticity is nonintrusive because the sampling method calls a deterministic model multiple times without changing its underlying equations. In this work, we investigate and demonstrate using simple proof of concepts how intrusive approaches can be used to directly account for stochasticity in the equations representing a mechanical system. Our long term goal is to apply such methodology to the design of wind turbines without requiring an excessive number of simulations. Intrusive methods manipulate stochastic variables directly to provide the probability density functions (PDFs) of the states and outputs at any time as functions of the PDFs of the inputs. We illustrate how different methods can be used with a reduced-order model of a wind turbine with one degree of freedom and for linear and nonlinear models. We discuss how the methods can be extended and what it will take to apply them to a level of fidelity similar to current state-of-the-art wind turbine design tools.
Power Density Analysis of Wind Turbine Main Bearing Units by Holistic Optimization of Material, Manufacturing and Design of the Main Shaft
Authors: Hollas Christian , Jacobs Georg , Züch Vitali , Röder Julian , Reinisch Niklas , Gouverneur Moritz , Bailly David , Babashahi Maryam , Gramlich Alexander
Published: June 2024

Given the continuous increases in wind turbine (WT) rated power and size, the nacelle weight and logistic handling costs increases significantly. To support heavier nacelles, stronger towers are needed which again increases material costs, so a need for nacelle power density increase arises. One solution to this problem is to increase the power density of the cast or forged WT main shaft. The power density in cast main shafts is limited by the low tensile strength of cast iron. High tensile strength steels, which theoretically increase power density, are used in state-of-the-art forged main shafts. However, their inner shaft diameter is kept small to reduce drilling costs. Since the loads of WT main shafts are dominated by the bending moments of the rotors, a high section modulus corresponds to a high power density. Material near the centre of the shaft therefore decreases the shaft power density. Hollow forging combines high tensile strength steel with a variable inner shaft diameter, enabling shaft designs with increased power density. Additionally, the use of air-hardening ductile (AHD) steel eliminates the need for costly heat treatment if the wall thickness is thin enough. The paper presents a holistic system model for the predesign of main bearing units (MBU) considering various materials and manufacturing methods. The model enables a feasibility assessment of hollow forged main shafts by comparing the resulting MBU weights across a wide range of WT power ratings. The MBU is selected instead of solely analysing the main shaft to account for the bearing and bearing housing weights, which depend on the main shaft geometry. The results show increased MBU power density of up to 23% for hollow forged shafts compared to forged shafts of the same material. Furthermore, when the shaft is hollow forged from AHD steel, the increase is even greater, up to 52%.
Floaters upscaling for multi-megawatt floating wind turbines
Authors: Di Carlo S , Fontanella A , Bontumasi S , Pietro F Di , Ambrosini S , Muggiasca S , Belloli M
Published: June 2024

The floating wind industry requires wind turbines with increasing power ratings, which will require bigger platforms. However, few floater concepts are publicly available for numerical simulation of future wind turbines. To overcome this limit, this work proposes a fast and straightforward upscaling methodology to upscale already available floating platforms to greater wind turbine power ratings. The methodology does not aim to optimize the floater design, but it can generate realistic floater data for preliminary numerical simulations. The methodology is applied here to four floaters: a semisubmersible, a barge, a TLP and a spar. The platforms are upscaled for a 15 MW wind turbine and a specific deployment site. The upscaling methodology is adapted for each floater since some differences arise from one platform to the other. Whenever possible a comparison with already existing 15 MW substructures is proposed and commented.
How do technological choices affect the economic and environmental performance of offshore wind farms?
Authors: Kainz S , Guilloré A , Bottasso C L
Published: June 2024

The ongoing energy transition towards fully sustainable energy systems requires designing wind farms looking beyond the sole levelized cost of energy, in order to concurrently ensure not only the economic profitability but also the environmental friendliness of future plants. Within this new approach to design, it becomes necessary to understand the effects that various possible technological choices have on both the economic and the environmental performance of wind farms. This study presents a framework designed to support these coupled economic-environmental assessments. The capabilities of the code are showcased by analysing the impact of different choices in terms of support structure type, specific power, tower height, powertrain type, and array and export voltage level for an exemplary offshore farm, chosen here as the IEA Wind 740-10-MW Reference Offshore Wind Plant with irregular layout. While the effects of many technological choices on the cost of energy are already well understood by industry, the present analysis shows that — at least in this specific case — climate change impacts are mainly driven by steel production, due to the massive amount of required material, but also, interestingly, by vessel activities. A low specific power, tall towers, and a high export cable voltage appear to offer the greatest potential for the concurrent improvement of the environmental and economic performance of the plant.
The Petiole of the Miriti Palm: A sustainable material for wind turbine blades?
Authors: Gomes I. Dos S. , Vaz J. R. P. , Wong J. C. H. , Wood D. H.
Published: June 2024

The Miriti Palm (Mauritia flexuosa) grows abundantly in the Amazon Region of Brazil. The petiole (PMP) that supports the leaves, has a density of about one-half of Balsa wood (BW), which is used in the manufacture of wind turbine blades. A further possible advantage of PMP is that harvesting does not kill the palm tree, in contrast to the harvesting of BW. Because the mechanical properties of PMP have not been measured, we determined the shear and tensile properties of 16 samples of PMP and BW to allow a preliminary assessment of PMP as a possible material for blades. The absolute shear and tensile strengths for BW are higher, but specific properties (normalized by the density) are similar and can favour PMP. Direct substitution of BW by PMP would reduce the weight of a typical large blade by around 2%.
Determination of delamination-related material properties and sensitivities in a ply drop composite using a semi-numerical approach
Authors: Balzani Claudio , Rajjoub Basem , Schulz Andreas , Sayer Florian
Published: June 2024

The aims of this paper are to investigate damage initiation and propagation in the vicinity of a ply drop focusing on delamination, to extract delamination-related material properties without conducting classical single and mixed mode fracture tests, and to analyze the sensitivities of delamination evolution with respect to these material properties. Quasi-static tensile tests were conducted on the coupon level for a UD composite with a ply drop. A combined numerical/experimental approach was followed to identify the material properties associated with delamination evolution. For that purpose, a finite element model was created using the cohesive zone model. The delamination-related properties were characterized by fitting the numerical model to the experiments by calibrating the cohesive zone parameters such that the global coupon response and the delamination extent for different load levels were in good agreement. Based on the identified parameter set, a sensitivity analysis was carried out to raise awareness of the influence of material properties on damage evolution. Damage around the ply drop was governed by mode II delamination. Several sensitivities were found to be non-linear with respect to a variation of input parameters. The methodology can be used in future work focusing on different ply drop configurations, and the results so far can serve as a database for benchmark simulations of delamination evolution in the vicinity of ply drops.
Multi-physics system modelling based on bond graph theory for offshore hydrogen production
Authors: Wiens Marcus , Luxa Aline , Wendt Jan , Meyer Tobias
Published: June 2024

Hydrogen is expected to take a major role in the energy sector. To address the increasing demand, specially designed systems for dedicated hydrogen production, e.g., an offshore hydrogen wind turbine, are developed. Especially in these multi-physics systems, each aspect of the entire system requires expert knowledge for modelling and sometimes specific simulation tools. A key challenge lies in the integration of component models into the full system model. We propose a method based on bond graph theory to identify single components and their interfaces for dynamic simulation. This top-down approach leads to a structured processes for the development of multi-physics systems with a diverse team. We present a design of a new 15 MW green hydrogen offshore wind turbine to study the performance in turbulent wind conditions. The single components are decoupled in their development and therefore, can be tested independently or combined with other components. All component models are utilized as Functional Mock-Up Units and combined in the full model with Co-Simulation. The development method is applicable to other complex energy systems.
Optimal hybrid power plants for electric vehicle charging demand
Authors: Irmas Cameron , Brunik Kaitlin , Clark Caitlyn E.
Published: June 2024

Transmission constraints, increasing motivations to decarbonize, and concerns over peak electric vehicle (EV) load impacts on local grids have driven electric customers to consider behind-the-meter, hybrid power plant generation and storage at the distributed-grid level for EV charging. In this study, we develop capabilities to optimize hybrid power plant component capacities for EV charging. We then demonstrate these capabilities in a case study for Boulder, Colorado, using public EV charging data as well as wind and solar resource data.

Our results show system designs that balance the cost of energy with load-meeting and peak shaving performance. Within the case study, systems designed for wind, solar photovoltaic (PV), and storage resulted in lower cost of energy than those optimized for PV and storage only. This indicates that in areas where wind resource exists, hybrid power plants that include wind, PV, and battery assets can better meet EV charging loads (including peak loads that are prone to overloading local grids) than PV and battery assets alone. Future work to address limitations in this paper include extending cost modeling to include performance losses (e.g., based on operations or weather) and charging station costs to estimate levelized cost of charging, and quantifying uncertainty and error in our aggregation methods for estimating EV charging loads at the hourly timescale.
Assessing wind turbine lightning risk using data-driven methodology
Authors: Sumetha-Aksorn Potchara , Dykes Katherine
Published: June 2024

This paper presents a methodology to conduct a risk assessment of lightning damage for wind farms using a data-driven methodology. Methods exist to address risks related to fire but this approach goes further to address lightning damage and related impacts to operations and maintenance comprehensively - monetizing the costs of these impacts. The process begins with lightning data analysis and strike rate calculation. Then, the lightning damage data is analyzed to gain insight into the repair cost and impact. Lastly, the process-based cost model takes lightning strike rate and impact data, resulting in risk provision. The paper presents two case studies, one for onshore wind farms and one for offshore wind farms. The relative importance of operations and maintenance costs versus lost revenues due to downtime (liquidated damages) differ between the two cases due to underlying technical drivers (turbine sizes, wind farm performance, lightning prevalence) as well as market drivers (labor costs, electricity prices). Thus, the study demonstrates both the viability of the method in providing monetary estimates of lightning damage costs over the lifteime of a wind farm as well as the importance of site-specific analysis and models to ensure accurate estimation of those costs.
End-to-end wind turbine design under uncertainties: a practical example
Authors: Dimitrov NK , Kelly M , McWilliam M. , Guiton M , Cousin A , Joulin PA , Mayol ML , Munoz-Zuniga M , Franceschini L , Lovera A , Fekhari E , Ardillon E , Peyrard C , Bakhoday-Paskyabi M , Marelli S , Schar S , Vanem E , Agrell C , Gramstad O , Wang H
Published: June 2024

This paper illustrates the process of design under uncertainty on a practical case study of an offshore wind farm. We document the entire process through selection and quantification of relevant uncertainties, definition of probabilistic limit states, reliability computation algorithms, as well as illustrating the impacts of the analysis through a design utilization study. The brief introduction in this study draws information and summarizes outcomes from the extensive works that took part within the EU H2020 HIPERWIND project. The results from the study show that significant material savings can be achieved by introducing probabilistic design methodologies, and particularly with the help of an integrated modelling approach where the entire structure (turbine, tower & foundation) is considered as a whole.
Hybrid power plant design for low-carbon hydrogen in the United States
Authors: Grant E , Brunik K , King J , Clark C E
Published: June 2024

In this study, we provide a nationwide techno-economic analysis of clean hydrogen production powered by a hybrid renewable energy plant for over 50,000 locations in the United States. We leverage the open-source Hybrid Optimization Performance Platform (HOPP) tool to simulate the hourly performance of an off-grid wind-solar plant integrated with a 1-GW polymer exchange membrane electrolyzer system. The levelized cost of hydrogen is calculated for varying technology costs, and tax credits to explore cost sensitivities independent of plant design, performance, and site selection. Our findings suggest that strategies for cost reduction include selecting sites with abundant wind resources, complementary wind and solar resources, and optimizing the sizing of wind and solar assets to maximize the hybrid plant capacity factor. These strategies are linked to increased hydrogen production and reduced electrolyzer stack replacements, thereby lowering the overall cost of hydrogen.
Time-Series Based Surrogate Model For Wind Farm Performance Prediction
Authors: Scheurich Frank , Hedevang Emil , López-Caballero Miguel , Bernard Valentin , Enevoldsen Peder Bay , Pedersen Søren Markkilde , Kirkegaard Jeppe Funk
Published: June 2024

Aeroelastic codes are state-of-the-art simulation tools in both industry and academia for the modelling of wind turbine loads and power output. Although these codes are widely used for the analysis of individual turbines, they are in general computationally too expensive for the calculation of all turbines within a wind farm. Engineering models that are computationally cheaper but also provide a lower fidelity are therefore typically used for wind farm power performance predictions. In this paper, an alternative approach to simulate wind farm performance is presented: the use of a data-driven surrogate model that is trained on time series that were generated by the in-house aeroelastic tool BHawC. This surrogate model provides results with potentially higher fidelity than more simplistic engineering models but is computationally much cheaper than BHawC simulations.
Role of rotational and divergence effects induced by wind turbines wakes on near-ground air warming
Authors: Boumendil Paul , Joulin Pierre-Antoine , Toumi Hiba Zahia , Rodier Quentin , Masson Valéry
Published: June 2024

Various studies, involving numerical simulations, satellite measurements, and field measurement campaigns, have demonstrated that wind turbine wakes can lead to changes in near-ground air temperature. To investigate these phenomena an Actuator Disk with Rotation (ADR) is implemented within the LES non-hydrostatic atmospheric model Meso-NH and validated using the NewMEXICO wind tunnel measurements and the VERTEX field measurement campaign. The NewMEXICO case allows the aerodynamic validation of the approach, especially the effect of the wake on wind speed components. On the VERTEX case, meteorological effects of wind turbines are studied under various atmospheric stability conditions through the application of homogeneous sensible heat fluxes at the ground level. Results show wake-related warming and cooling near the ground for stable and unstable conditions respectively, and no significant change for neutral conditions. These results show a good tendency agreement with the VERTEX campaign’s measurements. The comparison between an Actuator Disk with No Rotation (ADNR) and an ADR simulation show the dual contribution of the rotation of the wake and the growth of the stream tube on the temperature change in the wake but also on the near-ground air temperature.
An improved wind farm parametrization for inhomogeneous inflow
Authors: Van Der Laan M P , Baungaard M , Meyer Forsting A , Réthoré P-E
Published: June 2024

Energy losses due to wind farm clustering and wind farm interaction are rarely well represented in the wind farm design process because of the lack of fast models that can accurately account for neighboring wind farm wakes. A recently developed solution is the actuator wind farm (AWF) model, which is a Reynolds-averaged Navier-stokes (RANS) based wind farm parametrization that models a wind farm as a distributed thrust force and applies a global wind farm thrust coefficient controller. We propose an improved version of the AWF model, where each turbine employs a local thrust force controller and uses turbine thrust and power coefficients as input to better handle inhomogeneous inflow conditions. The proposed AWF model shows improved performance compared to the original AWF model in terms of predicted wind turbine power of a downstream wind farm that operates in a partial wake of an upstream wind farm, without significantly increasing the computational effort. However, the annual energy production (AEP) wake losses of a large wind farm cluster are nearly unaffected by using local or global control and input because the largest impact is found near the cut-in wind speed, which does not contribute much to the AEP wake losses.
Field validation of a yaw misalignment observer for wind farm control
Authors: Bertelè M , Sucameli CR , Bottasso CL
Published: June 2024

In this work, a load-based yaw misalignment observer was tested and validated with turbine and mast data collected during a wake steering and characterization campaign. A shallow feed-forward neural network was used to map the relation between the yaw misalignment and the in-and out-of-plane blade load harmonics for a 3.5 MW machine, and its performance was analysed over about 108 full days of useful data. Confirming previous findings, this simple neural network was able to accurately estimate the yaw misalignment, with an average 10-minute absolute error of at most 4°.

The performance of the yaw misalignment observer was compared to the one of the standard onboard wind vane during the wake steering campaign. Results indicate that the wind vane may significantly overestimate the misalignment for large angles, possibly on account of the wake rotation and flow distortion effects caused by the nacelle. The observer on the other hand, sampling the flow at the rotor disk and not behind it, is not affected by such phenomena and could therefore provide a more accurate measurement of the misalignment angle, possibly improving the performance of wake steering. When the turbine is already equipped with load sensors, this is obtained without the need to install and maintain extra hardware, which instead is the case with spinner-mounted anemometers or lidars.
Evaluating wind-farm power generation using a new direct integration of axisymmetric turbine wake
Authors: Ali Karim , Stallard Tim , Ouro Pablo
Published: June 2024

Engineering wake models enable fast calculation of wind-farm power generation including wake-turbine interactions, which is critical for annual energy yield calculations and for farm optimisation either at the design phase or during operation. The operating point of a turbine is determined based on its rotor-averaged wind speed, which is impacted by upstream wakes. The rotor-averaged deficit is typically calculated numerically by using a set of averaging points across the rotor disk, for which a Gaussian shape function is commonly evaluated. This numerical approach can have uncertainty based on the distribution of the averaging points. Also, a large number of averaging points can increase computational cost, which is detrimental for studies that rely on many individual calculations of farm power generation, e.g. layout optimisation. To avoid these drawbacks, we present a novel analytical solution to the circular-disk integration of a two-dimensional axisymmetric Gaussian function depicting upstream wakes for an arbitrary lateral offset between the rotor and the wake source. The analytical expression is compared against numerical averaging of an upstream wake, and is applied to the Horns Rev 1 wind farm showing excellent accuracy. Wind-farm layout optimisation models, especially methods based on gradient-descent optimisation, can benefit from the presented expression significantly, as this provides an analytically differentiable formulation for the rotor-averaged wind speed.
Evaluation of wind farm performance over heterogeneously rough terrain using large eddy simulation
Authors: Kethavath Naveen N. , Ghaisas Niranjan S.
Published: June 2024

We evaluate the effect of an abrupt change in the surface aerodynamic roughness height on a wind farm sited on it using the large eddy simulation (LES). Compared to a wind farm sited on a uniformly rough surface, the alteration in aerodynamic surface roughness from a rough to smooth value leads to substantial changes in the first-order and second-order turbulent statistics. Specifically, the rough-to-smooth surface roughness transition leads to an acceleration of the flow downstream of it, which affects the wake recovery and wind farm power production. Different velocity deficits are formulated considering different definitions of “upstream” velocity. The usual deficit, i.e., the difference between the overall wind farm upstream velocities and downstream of a turbine, attains negative values near the ground, rendering it difficult to model within the usual Gaussian radial-shape framework. An alternative definition, i.e., the difference in velocity at the same location with and without turbines on a heterogeneous surface, consistently yields positive values and is amenable to Gaussian shape-based modelling. The power generation decreases as the step change in surface roughness progressively moves into the wind farm. Maximum power is produced when all turbines are placed downstream of the surface roughness jump and minimum power is generated for a homogeneously rough surface when the entire wind farm is placed on the rough surface.
Assessing the effect of turbine size on the coherent structures in the wake using DMD
Authors: Bernardi C. , Manganelli F. , Leonardi S. , Cherubini S. , De Palma P.
Published: June 2024

The aim of this study is to evaluate and quantify the effect of the incoming flow on the coherent structures characterizing the wake of two NREL reference wind turbines with different dimensions: the NREL 5-MW and the NREL 15-MW. The Dynamic Mode Decomposition (DMD) approach has been used to detect dynamically-relevant flow structures. We employ the Sparsity-Promoting version of the DMD (SPDMD) algorithm for ranking the most relevant modes, in order to extract a limited subset of relevant flow features that optimally approximate the original data sequence. The dataset on which the SPDMD is based consists of ordered snapshots obtained by Large Eddy Simulations (LES), where the Actuator Line Method (ALM) simulates the rotor and the Immersed Boundary Method (IBM) models the tower and nacelle. To ensure a realistic comparison, the two turbines are subjected to two turbulent inflows with the same mean Atmospheric Boundary Layer (ABL) obtained through a precursor simulation. The results reveal significant differences in the dependence of the absolute value of the DMD amplitudes on the angular frequency for the two turbines. However, the coherent structures appear to have the same shape for the main modes, although the tip vortex structures have a higher dynamic relevance for the NREL 15-MW turbine. This is due to the larger length scales imposed by the rotor and the lower turbulence intensity at the taller rotor height.
Analysis of high-resolution convection-permitting regional climate simulations with respect to the impact of spatial resolution on simulated wind climate
Authors: Frisius Thomas , Cetin Irem Isik , Keup-Thiel Elke , Rechid Diana
Published: June 2024

In this study, climate model simulations are evaluated with regard to the wind energy resource in Germany. Since accurate determination of local wind requires a high resolution, we consider simulations of Convection Permitting Regional Climate Models (CP-RCMs) with 3km resolution. We analysed RCP8.5 scenario simulations carried out i) within the EUCP project using the RCM REMO in convection permitting mode for the Central European domain (CEU-3) and ii) within the CORDEX FPS-Convection project that provides a model ensemble of CP-RCMs for the Pan-Alpine region (ALP-3). The models are forced with RCMs at intermediate resolution which are by itself driven with General Circulation Models (GCMs) from the Coupled Model Intercomparison Project – Phase 5 (CMIP5) for the two time slices 1996-2005 (’historical’) and 2041-2050 (‘near-future’). Wind energy yield is calculated at 100 m height by using 3 MW wind turbine parameters and a height correction method. The results are compared with observations from four different met-masts at different locations. The RCMs at intermediate resolution are closer to observations than the CP-RCM REMO while the ensemble of CORDEX FPS-Convection reveals that the difference between model and observations is highly model dependent. A significant climate trend in the comparison of ’historical’ to ‘near future’ cannot be seen for Germany in both the EUCP and CORDEX FPS-Convection simulations.
Enhanced shear and veer in the Taiwan Strait during typhoon passage
Authors: Müller S , Larsén X G , Verelst D
Published: June 2024

During typhoon passage extreme wind conditions pose a challenge to the structural integrity of wind turbines. Particularly, wind shear and wind veer can influence wind turbine loads. This study investigates how Taiwan’s central mountain range affects wind shear and wind veer in the Taiwan Strait during three westward-moving typhoon cases. The typhoons are simulated with the Weather Research and Forecasting (WRF) model. We find that wind speed, shear, and veer vary over different regions in the Taiwan Strait. In large areas, the simulated wind shear is larger than modeled over the open ocean. In particular, mountain blockage leads to a spatially confined area of several 1000 km2 downstream of Taiwan, that exhibits over several hours strongly modified shear and veer in all three analyzed cases. Shear exponents up to 0.75 and veer between 0.2 and 0.6° m−1 suggest that turbine loads are impacted by the vertical change in the wind field in this area. The shear exponent and wind veer vary strongly with height in the downstream area of Taiwan’s central mountain range. The location of the area with large wind shear and wind veer differs between the three simulated typhoon cases and primarily depends on the latitude of the typhoon track relative to the central mountain range.
Rotor Performance and Wake Interaction of Controlled Dual Surging FOWT Rotors in Tandem
Authors: Li YuanTso , Yu Wei , Sarlak Hamid
Published: June 2024

Using Large Eddy Simulation (LES) with Actuator Line Model (ALM), this work investigates the system of two surging wind turbine rotors operating under realistic turbulent inflow conditions (TI = 5.3%). The two rotors are placed in tandem with a spacing of 5D and the surging motions are harmonic. A widely used torque controlling strategy, MPPT (Maximum Power Point Tracking), is implemented to ensure a maximium power extraction under all conditions. The rotor performances as well as the field data are surveyed to examine the effectiveness and impacts of the controller. It is found that the power performances of the surging rotors are benefited by the controller with a small margin (∼ 1%) when the surging motions are moderate. The results also show that the controller reacts much slower than the considered surging frequency, making the power performances of the rotors worse than the quasi-steady predictions (targeted values) and complicating the system dynamics. In general, the implementation of the controller has minor impacts on the wake characteristics; however, the strengths of Surging Induced Periodic Coherent Structures (SIPCS) are found to be enhanced.
Recommendations on setup in simulating atmospheric gravity waves under conventionally neutral boundary layer conditions
Authors: Khan Mehtab A , Watson Simon J , Allaerts Dries J N , Churchfield Matthew
Published: June 2024

Wind farm-induced atmospheric gravity waves have been the subject of recent research as they can impact wind farm performance. Pressure variations associated with gravity waves can contribute to the global blockage effect and wind farm wake recovery. Therefore, accurate numerical simulation of flow fields, where wind-farm-induced gravity waves may be produced, is important. Three main considerations in such simulations are the overall domain size, the use of Rayleigh damping near domain boundaries to dampen gravity waves, and advection damping at the inlet to prevent spurious oscillations. Often these considerations are treated ad hoc rather than systematically. This work aims to test and extend the systematic modelling of internal gravity waves proposed in a preliminary investigation to modelling of both internal and trapped gravity waves. The preliminary study identifies the length scales to set the domain and damping layer sizes and the time scale to configure the Rayleigh damping coefficient but under linearly stratified conditions. Large eddy simulations of flow through a wind farm canopy are performed under conventionally neutral boundary layer (CNBL) conditions to test the validity of proposed setups for CNBL conditions. Background atmospheric parameters, such as Froude number (Fr), inversion height (Hi
), and inversion layer Froude number (Fri
) control most of the atmospheric gravity wave characteristics. We validated for CBNL conditions that the effective wavelengths of the internal gravity waves are the correct length scale to configure the domain size and damping layer thickness. Likewise, the optimum damping coefficient to dampen the internal gravity waves relates to the free atmosphere’s buoyancy frequency or buoyant perturbations’ time scale. We infer that the damping coefficient in the inversion layer may relate to the inversion buoyancy frequency to effectively dampen the trapped gravity waves. Moreover, the advection damping length is linked to the horizontal wavelength of the trapped gravity waves in the inversion layer to prevent spurious waves at the inlet by retaining wave energy accumulation.
An open-source framework for the development, deployment and testing of wind farm control strategies
Authors: Sucameli C R , Campagnolo F , Petrović V , Hulsman P , Bottasso C L
Published: June 2024

Recent advances in wind farm control, particularly in wake steering strategies, have attracted significant interest from both academia and industry. Wake steering consists in misaligning an upstream turbine to redirect its wake, mitigating its impact on downstream turbines. Implementing these strategies on real-time systems presents challenges, especially in collaborative research projects, for example because of the handling of confidential operational data and the complexity of deploying source code on proprietary hardware. This paper introduces a MATLAB-based framework designed to facilitate the development, deployment, and testing of wake steering strategies on real-time Bachmann PLC systems. The modular architecture of the framework is designed to address common challenges in research projects, such as model deployment, data confidentiality, adaptability to runtime changes, and more in general for the facilitation of multi-partner collaborations. The framework was used to perform multiple wake steering experiments on two 3.5 MW turbines.
Ranking multi-fidelity model performances in reproducing internal and external wake impacts at neighbouring offshore wind farms
Authors: Freitas S , Rowen M , Navarro Diaz G , Erbslöh S
Published: June 2024

Performances of multi-fidelity numerical models in reproducing the impacts of internal and external wakes on power production are evaluated against SCADA data from two operational offshore wind farms in the North Sea. Results obtained for two engineering wake models, Jensen and Cumulative Curl (default and tuned settings), and a higher-fidelity Reynolds-averaged Navier-Stokes (RANS) setup are presented here. Tuned engineering models were configured against RANS results for an idealized wind farm layout consisting of several turbine setups. Input data for the model evaluation scenarios was obtained from a met mast located between the wind farms and binned according to atmospheric stability, wind speed and direction, and turbulence intensity. Results show that the higher fidelity of RANS is most advantageous for inflows that generate more complex internal wakes (e.g., deep array effects) and in replicating external wake impacts. The tuned setups of the engineering models, modified according to stability, outperform their default setups. Default model properties are defined in accordance with developer recommendations. One of the tuned models shows close farm-level agreements with RANS and even exceeds its performances in some of the cases of lower flow complexity.
Wind Turbine Loads in the Near Wake
Authors: Alletto Michael , Woesmann Jan , Letzel Marcus O. , Heyen Malte
Published: June 2024

This work is analyzing the the engineering wake models Frandsen and TNO for small inter-turbine distances. Therefore, loads generated by actuator line LES (AL-LES) wind fields will be compared with the loads generated by the mentioned engineering models. The results are checked for plausibility by comparing the current results with the available literature. Load comparisons for non-dimensional distances x/D ranging from 1.75 to 4 and for seven different wind speeds in order to consider the influence of the thrust coefficient were made. For each speed a low and a high turbulence intensity case is considered. Two latest generation turbines are investigated (one with a diameter of 138 m and one with a diameter of 175 m).Our studies reveal that the fatigue loads computed with the Frandsen and TNO model are conservative compared to the loads generated by AL-LES wind fields. The Wöhler averaged (m=10 and a sector of 120°) flap wise blade root bending moments are at least 150% higher for the TNO and Frandsen model compared to the loads generated by AL-LES wind fields for distances lower than 2.3D and C

T
values higher than 0.7. This indicates that current sector management rules applied by means of the results of the Frandsen and TNO model are too restrictive. A possible way to decrease the conservatism without posing at risk the structural integrity of the turbines is the following: The wake added turbulence intensity can be kept constant for regions in the parameter space with very high conservatism (e.g. for distances smaller than 2.3D and C

T
values higher than 0.7).
A Machine Learning Approach to Correct Turbulence Intensity measured by Floating Lidars
Authors: Rapisardi Giacomo , Araújo Da Silva Marcos Paulo , Miquel Adrià
Published: June 2024

In this work we introduce a supervised Machine Learning (ML) model to correct the Turbulence Intensity (TI) measured by Floating LiDAR Systems (FLS) in offshore environment. The model was developed using data from 46 EOLOS-FLS200 validation campaigns (≈ 4.6 years) carried out at three reference sites in the North Sea. It is based on Gradient Boosting Decision Tree (GBDT) and accounts for wind characteristics, atmospheric conditions, buoy motion, and wave features. Numerical analyses pronounced a consistent improvement in both coefficient of determination (R
2) and Mean Bias Error yielded by the ML-corrected TI. In addition, TI estimates in accordance with the state-of-the-art best practices were successfully obtained, even when evaluating the ML model in a site out of the training dataset, which demonstrates the model’s robustness.