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Showing papers in "Wind Energy in 2017"


Journal ArticleDOI
TL;DR: In this article, the authors present a wind plant modeling and optimization tool that enables the maximization of wind plant annual energy production (AEP) using yaw-based wake steering control and layout changes.
Abstract: This paper presents a wind plant modeling and optimization tool that enables the maximization of wind plant annual energy production (AEP) using yaw-based wake steering control and layout changes. The tool is an extension of a wake engineering model describing the steady-state effects of yaw on wake velocity profiles and power productions of wind turbines in a wind plant. To make predictions of a wind plant's AEP, necessary extensions of the original wake model include coupling it with a detailed rotor model and a control policy for turbine blade pitch and rotor speed. This enables the prediction of power production with wake effects throughout a range of wind speeds. We use the tool to perform an example optimization study on a wind plant based on the Princess Amalia Wind Park. In this case study, combined optimization of layout and wake steering control increases AEP by 5%. The power gains from wake steering control are highest for region 1.5 inflow wind speeds, and they continue to be present to some extent for the above-rated inflow wind speeds. The results show that layout optimization and wake steering are complementary because significant AEP improvements can be achieved with wake steering in a wind plant layout that is already optimized to reduce wake losses. Copyright © 2016 John Wiley & Sons, Ltd.

173 citations


Journal ArticleDOI
TL;DR: In this paper, the performance of four prominent drive train configurations over a range of sites distinguished by their distance to shore was investigated, and the results predicted that turbines with a permanent magnet generator and a fully rated power converter will have a higher availability and lower operation and maintenance costs than turbines with doubly-fed induction generators.
Abstract: Different configurations of gearbox, generator and power converter exist for offshore wind turbines. This paper investigated the performance of four prominent drive train configurations over a range of sites distinguished by their distance to shore. Failure rate data from onshore and offshore wind turbine populations was used where available or systematically estimated where no data was available. This was inputted along with repair resource requirements to an offshore accessibility and operation and maintenance model to calculate availability and operation and maintenance costs for a baseline wind farm consisting of 100 turbines. The results predicted that turbines with a permanent magnet generator and a fully rated power converter will have a higher availability and lower operation and maintenance costs than turbines with doubly-fed induction generators. This held true for all sites in this analysis. It was also predicted that in turbines with a permanent magnet generator, the direct drive configuration has the highest availability and lowest operation and maintenance costs followed by the turbines with 2 stage and 3 stage gearboxes.

104 citations


Journal ArticleDOI
TL;DR: In this article, a wind turbine model is placed in a virtual wind tunnel to reproduce the "Blind test" experiment performed at the Norwegian University of Science and Technology (NTNU) closed-loop wind tunnel.
Abstract: Large eddy simulations (LES) of the flow past a wind turbine with and without tower and nacelle have been performed at 2 tip speed ratios (TSR, λ=ωR/U∞), λ=3 and 6, where the latter corresponds to design conditions. The turbine model is placed in a virtual wind tunnel to reproduce the “Blind test 1” experiment performed at the Norwegian University of Science and Technology (NTNU) closed-loop wind tunnel. The wind turbine was modeled using the actuator line model for the rotor blades and the immersed boundary method for the tower and nacelle. The aim of the paper is to highlight the impact of tower and nacelle on the turbine wake. Therefore, a second set of simulations with the rotating blades only (neglecting the tower and nacelle) has been performed as reference. Present results are compared with the experimental measurements made at NTNU and numerical simulations available in the literature. The tower and nacelle not only produce a velocity deficit in the wake but they also affect the turbulent kinetic energy and the fluxes. The wake of the tower interacts with that generated by the turbine blades promoting the breakdown of the tip vortex and increasing the mean kinetic energy flux into the wake. When tower and nacelle are modeled in the numerical simulations, results improve significantly both in the near wake and in the far wake.

101 citations


Journal ArticleDOI
TL;DR: An overview of the most important publications that discuss the application of ANN for condition monitoring in wind turbines and a method utilizing the Mahalanobis distance is presented, which improves the anomaly detection by considering the correlation between ANN model errors and the operating condition.
Abstract: Major failures in wind turbines are expensive to repair and cause loss of revenue due to long downtime. Condition-based maintenance, which provides a possibility to reduce maintenance cost, has been made possible because of the successful application of various condition monitoring systems in wind turbines. New methods to improve the condition monitoring system are continuously being developed. Monitoring based on data stored in the supervisory control and data acquisition (SCADA) system in wind turbines has received attention recently. Artificial neural networks (ANNs) have proved to be a powerful tool for SCADA-based condition monitoring applications. This paper first gives an overview of the most important publications that discuss the application of ANN for condition monitoring in wind turbines. The knowledge from these publications is utilized and developed further with a focus on two areas: the data preprocessing and the data post-processing. Methods for filtering of data are presented, which ensure that the ANN models are trained on the data representing the true normal operating conditions of the wind turbine. A method to overcome the errors from the ANN models due to discontinuity in SCADA data is presented. Furthermore, a method utilizing the Mahalanobis distance is presented, which improves the anomaly detection by considering the correlation between ANN model errors and the operating condition. Finally, the proposed method is applied to case studies with failures in wind turbine gearboxes. The results of the application illustrate the advantages and limitations of the proposed method.

97 citations


Journal ArticleDOI
TL;DR: A forecasting methodology that explores a set of different sparse structures for the vector autoregression (VAR) model using the Least Absolute Shrinkage and Selection Operator (LASSO) framework to create a scalable forecasting method supported by parallel computing and fast convergence.
Abstract: The deployment of smart grids and renewable energy dispatch centers motivates the development of forecasting techniques that take advantage of near real-time measurements collected from geographically distributed sensors. This paper describes a forecasting methodology that explores a set of different sparse structures for the vector autoregression (VAR) model using the Least Absolute Shrinkage and Selection Operator (LASSO) framework. The alternating direction method of multipliers is applied to fit the different VAR-LASSO variants and create a scalable forecasting method supported by parallel computing and fast convergence, which can be used by system operators and renewable power plant operators. A test case with 66 wind power plants is used to show the improvement in forecasting skill from exploring distributed sparse structures. The proposed solution outperformed the conventional autoregressive and vector autoregressive models, as well as a sparse-VAR model from the state of the art.LASSO Vector Autoregression Structures for Very Short-term Wind Power Forecasting

87 citations


Journal ArticleDOI
TL;DR: In this paper, the authors present an aerodynamic shape optimization framework consisting of a Reynolds-averaged Navier Stokes solver coupled with a numerical optimization algorithm, a geometry modeler, and a mesh perturbation algorithm.
Abstract: Computational fluid dynamics (CFD) is increasingly used to analyze wind turbines, and the next logical step is to develop CFD-based optimization to enable further gains in performance and reduce model uncertainties. We present an aerodynamic shape optimization framework consisting of a Reynolds-averaged Navier Stokes solver coupled to a numerical optimization algorithm, a geometry modeler, and a mesh perturbation algorithm. To efficiently handle the large number of design variables, we use a gradient-based optimization technique together with an adjoint method for computing the gradients of the torque coefficient with respect to the design variables. To demonstrate the effectiveness of the proposed approach, we maximize the torque of the NREL VI wind turbine blade with respect to pitch, twist, and airfoil shape design variables while constraining the blade thickness. We present a series of optimization cases with increasing number of variables, both for a single wind speed and for multiple wind speeds. For the optimization at a single wind speed performed with respect to all the design variables (1 pitch, 11 twist, and 240 airfoil shape variables), the torque coefficient increased by 22.4% relative to the NREL VI design. For the multiple-speed optimization, the torque increased by an average of 22.1%. Depending on the CFD mesh size and number of design variables, the optimization time ranges from 2 to 24h when using 256 cores, which means that wind turbine designers can use this process routinely. Copyright © 2016 John Wiley & Sons, Ltd.

78 citations


Journal ArticleDOI
TL;DR: In this paper, a theoretical approach is followed to determine the most suitable value of e, based on an analytical solution to the linearized inviscid flow response to a Gaussian force.
Abstract: The actuator line model (ALM) is a commonly used method to represent lifting surfaces such as wind turbine blades within large-eddy simulations (LES). In the ALM, the lift and drag forces are replaced by an imposed body force that is typically smoothed over several grid points using a Gaussian kernel with some prescribed smoothing width e. To date, the choice of e has most often been based on numerical considerations related to the grid spacing used in LES. However, especially for finely resolved LES with grid spacings on the order of or smaller than the chord length of the blade, the best choice of e is not known. In this work, a theoretical approach is followed to determine the most suitable value of e, based on an analytical solution to the linearized inviscid flow response to a Gaussian force. We find that the optimal smoothing width eopt is on the order of 14%-25% of the chord length of the blade, and the center of force is located at about 13%-26% downstream of the leading edge of the blade for the cases considered. These optimal values do not depend on angle of attack and depend only weakly on the type of lifting surface. It is then shown that an even more realistic velocity field can be induced by a 2-D elliptical Gaussian lift-force kernel. Some results are also provided regarding drag force representation. Copyright © 2017 John Wiley & Sons, Ltd.

75 citations


Journal ArticleDOI
TL;DR: In this article, the authors proposed a model-based receding horizon control to enable a wind farm to provide secondary frequency regulation for a power grid, which can reduce the opportunity cost associated with lost revenue in the bulk power market that is typically associated with providing frequency regulation services.
Abstract: In this study, we propose the use of model-based receding horizon control to enable a wind farm to provide secondary frequency regulation for a power grid. The controller is built by first proposing a time-varying one-dimensional wake model, which is validated against large eddy simulations of a wind farm at startup. This wake model is then used as a plant model for a closed-loop receding horizon controller that uses wind speed measurements at each turbine as feedback. The control method is tested in large eddy simulations with actuator disk wind turbine models representing an 84-turbine wind farm that aims to track sample frequency regulation reference signals spanning 40 min time intervals. This type of control generally requires wind turbines to reduce their power set points or curtail wind power output (derate the power output) by the same amount as the maximum upward variation in power level required by the reference signal. However, our control approach provides good tracking performance in the test system considered with only a 4% derate for a regulation signal with an 8% maximum upward variation. This performance improvement has the potential to reduce the opportunity cost associated with lost revenue in the bulk power market that is typically associated with providing frequency regulation services. Copyright © 2017 John Wiley & Sons, Ltd.

75 citations


Journal ArticleDOI
TL;DR: In this article, the spatial and temporal correlation of wind power generation across several European Union countries was examined to understand how wind ‘travels’ across Europe, and the results of the analysis were then compared with two other studies focused on the Nordic region and the United States of America.
Abstract: Studies have shown that a large geographic spread of installed capacity can reduce wind power variability and smooth production. This could be achieved by using electricity interconnections and storage systems. However, interconnections and storage are not totally flexible, so it is essential to understand the wind power correlation in order to address power system constraints in systems with large and growing wind power penetrations. In this study the spatial and temporal correlation of wind power generation across several European Union countries was examined to understand how wind ‘travels’ across Europe. Three years of historical hourly wind power generation data from ten countries were analysed. The results of the analysis were then compared with two other studies focused on the Nordic region and the United States of America. The findings show that similar general correlation characteristics do exist between European country pairs. This is of particular importance when planning and operating interconnector flows, storage optimisation and cross-border power trading.

65 citations


Journal ArticleDOI
TL;DR: In this paper, the power production of an onshore wind farm is investigated through supervisory control and data acquisition data, while the wind field is monitored through scanning light detection and ranging measurements and meteorological data acquired from a met-tower located in proximity to the turbine array.
Abstract: Power production of an onshore wind farm is investigated through supervisory control and data acquisition data, while the wind field is monitored through scanning light detection and ranging measurements and meteorological data acquired from a met-tower located in proximity to the turbine array. The power production of each turbine is analysed as functions of the operating region of the power curve, wind direction and atmospheric stability. Five different methods are used to estimate the potential wind power as a function of time, enabling an estimation of power losses connected with wake interactions. The most robust method from a statistical standpoint is that based on the evaluation of a reference wind velocity at hub height and experimental mean power curves calculated for each turbine and different atmospheric stability regimes. The synergistic analysis of these various datasets shows that power losses are significant for wind velocities higher than cut-in wind speed and lower than rated wind speed of the turbines. Furthermore, power losses are larger under stable atmospheric conditions than for convective regimes, which is a consequence of the stability-driven variability in wake evolution. Light detection and ranging measurements confirm that wind turbine wakes recover faster under convective regimes, thus alleviating detrimental effects due to wake interactions. For the wind farm under examination, power loss due to wake shadowing effects is estimated to be about 4% and 2% of the total power production when operating under stable and convective conditions, respectively. However, cases with power losses about 60-80% of the potential power are systematically observed for specific wind turbines and wind directions. Copyright © 2017 John Wiley & Sons, Ltd.

64 citations


Journal ArticleDOI
TL;DR: In this article, a numerical implementation of the geometrically exact beam theory based on the Legendre-spectral-finite element (LSFE) method is presented, and the special treatment of three-dimensional rotation parameters is reviewed.
Abstract: This paper presents a numerical implementation of the geometrically exact beam theory based on the Legendre-spectral-finite-element (LSFE) method. The displacement-based geometrically exact beam theory is presented, and the special treatment of three-dimensional rotation parameters is reviewed. An LSFE is a high-order finite element with nodes located at the Gauss–Legendre–Lobatto points. These elements can be an order of magnitude more computationally efficient than low-order finite elements for a given accuracy level. The new module, BeamDyn, is implemented in the FAST modularization framework for dynamic simulation of highly flexible composite-material wind turbine blades within the FAST aeroelastic engineering model. The framework allows for fully interactive simulations of turbine blades in operating conditions. Numerical examples are provided to validate BeamDyn and examine the LSFE performance as well as the coupling algorithm in the FAST modularization framework. BeamDyn can also be used as a stand-alone high-fidelity beam tool. Copyright © 2017 John Wiley & Sons, Ltd.

Journal ArticleDOI
TL;DR: In this article, the benefits of a vertically staggered (VS) wind farm, in which vertical-axis and horizontal-axis wind turbines are collocated in a large wind farm was addressed.
Abstract: In this study, we address the benefits of a vertically staggered (VS) wind farm, in which vertical-axis and horizontal-axis wind turbines are collocated in a large wind farm. The case study consists of 20 small vertical-axis turbines added around each large horizontal-axis turbine. Large-eddy simulation is used to compare power extraction and flow properties of the VS wind farm versus a traditional wind farm with only large turbines. The VS wind farm produces up to 32% more power than the traditional one, and the power extracted by the large turbines alone is increased by 10%, caused by faster wake recovery from enhanced turbulence due to the presence of the small turbines. A theoretical analysis based on a top-down model is performed and compared with the large-eddy simulation. The analysis suggests a nonlinear increase of total power extraction with increase of the loading of smaller turbines, with weak sensitivity to various parameters, such as size, and type aspect ratio, and thrust coefficient of the vertical-axis turbines. We conclude that vertical staggering can be an effective way to increase energy production in existing wind farms. Copyright © 2016 John Wiley & Sons, Ltd.

Journal ArticleDOI
TL;DR: In this article, the effect of wind farm configuration and atmospheric stability on the power generated by large wind farms was investigated using large-eddy simulation (LES) and three stabilities (neutral and moderately unstable and stable).
Abstract: Large-eddy simulation (LES) has been used previously to study the effect of either configuration or atmospheric stability on the power generated by large wind farms. This is the first study to consider both stability and wind farm configuration simultaneously and methodically with LES. Two prevailing wind directions, two layouts (turbines aligned versus staggered with respect to the wind) and three stabilities (neutral and moderately unstable and stable) were evaluated. Compared with neutral conditions, unstable conditions led to reduced wake losses in one configuration, to enhanced wake losses in two and to unchanged wake losses in one configuration. Conversely, stable conditions led to increased wake losses in one, decreased wake losses in two and unchanged wake losses in one configuration. Three competing effects, namely, rates of wake recovery due to vertical mixing, horizontal spread of wakes and localized regions of acceleration caused by multiple upstream wakes, were identified as being responsible for the observed trends in wake losses. The detailed flow features responsible for these non-linear interactions could only be resolved by the LES. Existing analytical models ignore stability and non-linear configuration effects, which therefore need to be incorporated. Copyright © 2017 John Wiley & Sons, Ltd.


Journal ArticleDOI
TL;DR: In this article, an active tuned mass damper (ATMD) is employed for damping of tower vibrations of fixed offshore wind turbines, where the additional actuator force is controlled using feedback from the tower displacement and the relative velocity of the damper mass.
Abstract: An active tuned mass damper (ATMD) is employed for damping of tower vibrations of fixed offshore wind turbines, where the additional actuator force is controlled using feedback from the tower displacement and the relative velocity of the damper mass. An optimum tuning procedure equivalent to the tuning procedure of the passive tuned mass damper combined with a simple procedure for minimizing the control force is employed for determination of optimum damper parameters and feedback gain values. By time domain simulations conducted in an aeroelastic code, it is demonstrated that the ATMD can be used to further reduce the structural response of the wind turbine compared with the passive tuned mass damper and this without an increase in damper mass. A limiting factor of the design of the ATMD is the displacement of the damper mass, which for the ATMD, increases to compensate for the reduction in mass. Copyright © 2016 John Wiley & Sons, Ltd.


Journal ArticleDOI
TL;DR: In this paper, the influence of yaw inflow conditions on the performance of a single generic 2.4MW wind turbine was analyzed using a detached eddy simulation approach, and it was shown that the reduction of power output follows a cosine to the power of X function of the yaw angle.
Abstract: This article deals with the influence of yawed inflow conditions on the performance of a single generic 2.4MW wind turbine. It presents the results of studies performed at the Institute of Aerodynamics and Gas Dynamics by means of computational fluid dynamics, using a fully meshed wind turbine with all boundary layers being resolved. The block-structured flow solver FLOWer is used; a dual-time stepping method for temporal discretization and a second-order Jameson–Schmidt–Turkel method for the calculation of the convective fluxes are applied. All simulations are carried out using a detached eddy simulation approach. In detail, two different wind speeds and a yaw angle range between −50° and +50° are evaluated in the paper. Based on these data, it is shown that the reduction of power output follows a cosine to the power of X function of the yaw angle. Furthermore, the growing azimuthal non-uniformity of the load distributions with increasing yaw angle magnitude is analysed by spanwise load distributions. As a central influence on the load distributions, the advancing and retreating blade effect is identified. Moreover, the deflection of the wake as a result of the inflow is investigated, and the deflection angles are compared with a modelling approach. A connection line between wake deflection and load asymmetry is drawn. The results are of particular importance for wind park situations with downstream turbines facing the distorted inflow created from upstream ones. Copyright © 2016 John Wiley & Sons, Ltd.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated a two-bladed configuration by keeping the structural and aerodynamic characteristics of each blade fixed (to avoid a complete blade re-design) to examine potential rotor mass reduction.
Abstract: Downwind force angles are small for current turbines systems (1–5 MW) such that they may be readily accommodated by conventional upwind configurations. However, analysis indicates that extreme-scale systems (10–20 MW) will have larger angles that may benefit from downwind-aligned configurations. To examine potential rotor mass reduction, the pre-alignment concept was investigated a two-bladed configuration by keeping the structural and aerodynamic characteristics of each blade fixed (to avoids a complete blade re-design). Simulations for a 13.2 MW rated rotor at steady-state conditions show that this concept-level two-bladed design may yield 25% rotor mass savings while also reducing average blade stress over all wind speeds. These results employed a pre-alignment on the basis of a wind speed of 1.25 times the rated wind speed. The downwind pre-aligned concept may also reduce damage equivalent loads on the blades by 60% for steady rated wind conditions. Even higher mass and damage equivalent load savings (relative to conventional upwind designs) may be possible for larger systems (15–20 MW) for which load-alignment angles become even larger. However, much more work is needed to determine whether this concept can be translated into a practical design that must meet a wide myriad of other criteria. Copyright © 2017 John Wiley & Sons, Ltd.

Journal ArticleDOI
TL;DR: In this article, a power loss analysis of a 1.5MW wind turbine subject to various icing conditions was performed using a combination of ice accretion experiments and wind tunnel tests, and the results showed that wind turbine aerodynamic loads in Region II led to power losses ranging from 16% to 22% for freezing fog conditions and 26% for a freezing drizzle condition.
Abstract: The objective of this work is a quantitative analysis of power loss of a representative 1.5-MW wind turbine subject to various icing conditions. Aerodynamic performance data are measured using a combination of ice accretion experiments and wind tunnel tests. Atmospheric icing conditions varying in static temperature, droplet diameter and liquid water content are generated in an icing facility to simulate a 45-min icing event on a DU 93-W-210 airfoil at flow conditions pertinent to 80% blade span on a 1.5-MW wind turbine. Iced airfoil shapes are molded for preservation and casted for subsequent wind tunnel testing. In general, ice shapes are similar in 2D profile, but vary in 3D surface roughness elements and in the ice impingement length. Both roughness heights and roughness impingement zones are measured. A 16% loss of airfoil lift at operational angle of attack is observed for freezing fog conditions. Airfoil drag increases by 190% at temperatures near 0° C, 145% near 10° C and 80% near 20° C. For a freezing drizzle icing condition, lift loss and drag rise are more severe at 25% and 220%, respectively. An analysis of the wind turbine aerodynamic loads in Region II leads to power losses ranging from 16% to 22% for freezing fog conditions and 26% for a freezing drizzle condition. Differences in power loss between icing conditions are correlated to variance in temperature, ice surface roughness and ice impingement length. Some potential control strategies are discussed for wind turbine operators attempting to minimize revenue loss in cold-climate regions. Copyright © 2016 John Wiley & Sons, Ltd.

Journal ArticleDOI
TL;DR: In this paper, an actuator line model is implemented in an atmospheric boundary layer large eddy simulation code, with offline coupling to a high-resolution blade-scale unsteady Reynolds-averaged Navier-Stokes model.
Abstract: Because of several design advantages and operational characteristics, particularly in offshore farms, vertical axis wind turbines (VAWTs) are being reconsidered as a complementary technology to horizontal axial turbines. However, considerable gaps remain in our understanding of VAWT performance since cross-flow rotor configurations have been significantly less studied than axial turbines. This study examines the wakes of VAWTs and how their evolution is influenced by turbine design parameters. An actuator line model is implemented in an atmospheric boundary layer large eddy simulation code, with offline coupling to a high-resolution blade-scale unsteady Reynolds-averaged Navier–Stokes model. The large eddy simulation captures the turbine-to-farm scale dynamics, while the unsteady Reynolds-averaged Navier–Stokes captures the blade-to-turbine scale flow. The simulation results are found to be in good agreement with three existing experimental datasets. Subsequently, a parametric study of the flow over an isolated VAWT, carried out by varying solidities, height-to-diameter aspect ratios and tip speed ratios, is conducted. The analyses of the wake area and velocity and power deficits yield an improved understanding of the downstream evolution of VAWT wakes, which in turn enables a more informed selection of turbine designs for wind farms. Copyright © 2016 John Wiley & Sons, Ltd.

Journal ArticleDOI
TL;DR: A novel method for generating probabilistic wind power scenarios using epi-spline basis functions that allows users to control for the degree to which extreme errors are captured, embodied in the joint Sandia–University of California Davis Prescient software package for assessing and analyzing stochastic operations strategies.
Abstract: Forecasts of available wind power are critical in key electric power systems operations planning problems, including economic dispatch and unit commitment. Such forecasts are necessarily uncertain, limiting the reliability and cost-effectiveness of operations planning models based on a single deterministic or “point” forecast. A common approach to address this limitation involves the use of a number of probabilistic scenarios, each specifying a possible trajectory of wind power production, with associated probability. We present and analyze a novel method for generating probabilistic wind power scenarios, leveraging available historical information in the form of forecasted and corresponding observed wind power time series. We estimate nonparametric forecast error densities, specifically using epi-spline basis functions, allowing us to capture the skewed and nonparametric nature of error densities observed in real-world data. We then describe a method to generate probabilistic scenarios from these basis functions that allows users to control for the degree to which extreme errors are captured. We compare the performance of our approach to the current state-of-the-art considering publicly available data associated with the Bonneville Power Administration, analyzing aggregate production of a number of wind farms over a large geographic region. Finally, we discuss the advantages of our approach in the context of specific power systems operations planning problems: stochastic unit commitment and economic dispatch. Our methodology is embodied in the joint Sandia–University of California Davis Prescient software package for assessing and analyzing stochastic operations strategies.

Journal ArticleDOI
TL;DR: In this paper, a row of wind turbine rotors with a mutual spacing of three diameters is simulated using both Reynolds averaged Navier-Stokes (RANS) simulations and a simple inviscid vortex model.
Abstract: A row of wind turbine rotors with a mutual spacing of three diameters is simulated using both Reynolds averaged Navier-Stokes (RANS) simulations and a simple inviscid vortex model. The angle between the incoming wind and the line connecting the turbines is varied between 45 and 90 degrees. The simulations show that the power production of the turbines deviate significantly compared with a corresponding isolated turbine even though there is no direct wake-turbine interaction at the considered wind directions. Nevertheless, both models indicate marked alterations in the upstream flow, which directly link to the turbines' power adjustments. Thus, turbines which are placed laterally relative to the prevailing wind (as seen at various test sites) have, at least numerically, a mutual effect on each other. Therefore, they might not necessarily produce the same power as a stand-alone turbine. Copyright © 2016 John Wiley & Sons, Ltd.

Journal ArticleDOI
TL;DR: In this paper, the authors show that Swedish wind turbines constructed before 2007 lose 0.15 capacity factor percentage points per year, corresponding to a lifetime energy loss of 6% and that gradual increase of downtime accounts for around one third of the deterioration.
Abstract: We show that Swedish wind turbines constructed before 2007 lose 0.15 capacity factor percentage points per year, corresponding to a lifetime energy loss of 6%. A gradual increase of downtime accounts for around one third of the deterioration and worsened efficiency for the remaining. Although the performance loss in Sweden is considerably smaller than previously reported in the UK, it is statistically significant and calls for a revision of the industry practice for wind energy calculations. The study is based on two partly overlapping datasets, comprising 1,100 monthly and 1,300 hourly time series spanning 5–25 years each.

Journal ArticleDOI
TL;DR: In this paper, a computational fluid dynamics model for simulations of rotor under floating platform induced motions is developed, where the rotor motion is realized using arbitrary mesh interface, and wind flows are modelled by incompressible Navier-Stokes flow solver appended by the k−−−ω shear stress transport turbulence model to resolve turbulence quantities.
Abstract: Forfloating offshore wind turbines, rotors are under coupled motions of rotating and platform-induced motions because of hydrodynamics impacts. Notably, the coupled motion of platform pitching and rotor rotating induces unsteadiness and nonlinear aerodynamics in turbine operations; thus having a strong effect on the rotor performances including thrust and power generation. The present work aims at developing a computational fluid dynamics model for simulations of rotor under floating platform induced motions. The rotor motion is realized using arbitrary mesh interface, and wind flows are modelled by incompressible Navier-Stokes flow solver appended by the k − ω shear stress transport turbulence model to resolve turbulence quantities. In order to investigate the fully coupled motion of floating wind turbine, the six degree of freedom solid body motion solver is extended to couple with multiple motions, especially for the motion of rotor coupled with the prescribed surge-heave-pitch motion of floating platform. The detailed methodology of multiple motion coupling is also described and discussed in this work. Both steady and unsteady simulations of offshore floating wind turbine are considered in the present work. The steady aerodynamic simulation of offshore floating wind turbine is implemented by the multiple reference frames approach and for the transient simulation, the rotor motion is realized using arbitrary mesh interface. A rigorous benchmark of the present numerical model is performed by comparing to the reported literatures. The detailed elemental thrust and power comparisons of wind turbine are carried out by comparing with the results from FAST developed by National Renewable Energy Laboratory and various existing numerical data with good agreement. The proposed approach is then applied for simulations of National Renewable Energy Laboratory 5MW turbine in coupled platform motion at various wind speeds under a typical load case scenario. Transient effect of flows over turbines rotor is captured with good prediction of turbine performance as compared with existing data from FAST. Copyright © 2016 John Wiley & Sons, Ltd.

Journal ArticleDOI
TL;DR: In this article, the authors studied the induction zone in front of different wind turbine rotors by means of steady-state Navier-Stokes simulations combined with an actuator disk approach.
Abstract: The induction zone in front of different wind turbine rotors is studied by means of steady-state Navier-Stokes simulations combined with an actuator disk approach. It is shown that, for distances beyond 1 rotor radius upstream of the rotors, the induced velocity is self-similar and independent of the rotor geometry. On the basis of these findings, a simple analytical model of the induction zone of wind turbines is proposed.

Journal ArticleDOI
TL;DR: In this article, the authors generalize the analysis to include effects of cable and maintenance costs upon optimal wind turbine spacing in very large wind farms under various economic criteria, and investigate the influence of the type of wind farm layout.
Abstract: Wind turbine spacing is an important design parameter for wind farms. Placing turbines too close together reduces their power extraction because of wake effects and increases maintenance costs because of unsteady loading. Conversely, placing them further apart increases land and cabling costs, as well as electrical resistance losses. The asymptotic limit of very large wind farms in which the flow conditions can be considered ‘fully developed’ provides a useful framework for studying general trends in optimal layouts as a function of dimensionless cost parameters. Earlier analytical work by Meyers and Meneveau (Wind Energy 15, 305–317 (2012)) revealed that in the limit of very large wind farms, the optimal turbine spacing accounting for the turbine and land costs is significantly larger than the value found in typical existing wind farms. Here, we generalize the analysis to include effects of cable and maintenance costs upon optimal wind turbine spacing in very large wind farms under various economic criteria. For marginally profitable wind farms, minimum cost and maximum profit turbine spacings coincide. Assuming linear-based and area-based costs that are representative of either offshore or onshore sites we obtain for very large wind farms spacings that tend to be appreciably greater than occurring in actual farms confirming earlier results but now including cabling costs. However, we show later that if wind farms are highly profitable then optimization of the profit per unit area leads to tighter optimal spacings than would be implied by cost minimization. In addition, we investigate the influence of the type of wind farm layout. © 2016 The Authors Wind Energy Published by John Wiley & Sons Ltd

Journal ArticleDOI
TL;DR: In this paper, the authors investigate the risk of cost overruns and underruns occurring in the construction of 51 onshore and offshore wind farms commissioned between 2000 and 2015 in 13 countries and find that across the entire dataset, the mean cost escalation per project is 6.5% or about $63 million per windfarm.
Abstract: This article investigates the risk of cost overruns and underruns occurring in the construction of 51 onshore and offshore wind farms commissioned between 2000 and 2015 in 13 countries. In total, these projects required about $39 billion in investment and reached about 11 GW of installed capacity. We use this original dataset to test six hypotheses about construction cost overruns related to (i) technological learning, (ii) fiscal control, (iii) economies of scale, (iv) configuration, (v) regulation and markets and (vi) manufacturing experience. We find that across the entire dataset, the mean cost escalation per project is 6.5% or about $63 million per windfarm, although 20 projects within the sample (39%) did not exhibit cost overruns. The majority of onshore wind farms exhibit cost underruns while for offshore wind farms the results have a larger spread. Interestingly, no significant relationship exists between the size (in total MWor per individual turbine capacity) of a windfarm and the severity of a cost overrun. Nonetheless, there is an indication that the risk increases for larger wind farms at greater distances offshore using new types of turbines and foundations. Overall, the mean cost escalation for onshore projects is 1.7% and 9.6% for offshore projects, amounts much lower than those for other energy infrastructure.

Journal ArticleDOI
TL;DR: In this paper, the structural response of composite blades is investigated numerically and three-dimensional strains are investigated by reconstructing the root transition region of the blade using solid brick elements in a finite element analysis.
Abstract: Full-scale structural tests enable an in-depth understanding of how composite blades respond to specific applied loads. Blade strength can be validated, and necessary modifications can be made to improve structural performance and/or reduce blade weight. This study revisits the structural collapse of a 52.3 m composite blade with new research content. Specifically, the present work examines the chain of events captured in the video record of the blade collapse and provides direct phenomenological evidence of how the blade collapsed in its ultimate limit state. In addition, three-dimensional strains are investigated by reconstructing the root transition region of the blade using solid brick elements in a finite element analysis. The strain components responsible for particular failure characteristics are identified. The structural response of the blade is investigated numerically. Interactive failure phenomena associated with strains, local buckling and material failure are examined in detail. The study shows that local buckling of the sandwich panels with unbalanced construction drives progressive failure of the composite materials and eventually leads to blade collapse owing to significant failure of the load-carrying spar cap. Design modifications of the blade are proposed and validated with the test of a new blade. With respect to the latest DNV GL standard, this study notes a possible method to predict delamination and skin/core debonding failures. This study also recommends the use of three-dimensional solid elements in finite element analysis, especially when the strength and failure of large blades are of concern. Copyright © 2017 John Wiley & Sons, Ltd.

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TL;DR: In this article, the authors describe a new blade condition monitoring system, which combines the lightning safety of optical fibers with the reliability and cost-effectiveness of electronic sensors, and present results on the reliability of a newly developed prototype based on optically powered sensors.
Abstract: Blade condition monitoring systems with fiber-optic sensors attract much attention because they are resistant to lightning strikes, a major issue with increasing blade lengths. However, fiber-optic sensor systems are more complex and more expensive than their electronic counterparts. We describe a new blade condition monitoring system, which combines the lightning safety of optical fibers with the reliability and cost-effectiveness of electronic sensors. The optical fibers transport data from the blades to the hub, and in addition, they provide the electrical power for operating the sensor units in the blades. To achieve full protection against lightning-induced electromagnetic fields, an appropriate shielding of the sensor units is required. We present results on the reliability of a newly developed prototype based on optically powered sensors. In a field trial, the unit monitored successfully the blade vibrations of a 1.5 MW wind turbine for a period of 23 months. Copyright © 2016 John Wiley & Sons, Ltd.

Journal ArticleDOI
TL;DR: In this paper, the authors quantified the influence of mooring dynamic models on the calculation of fatigue and ultimate loads with integrated tools and compared its performance with a lower computational cost quasi-static model.
Abstract: The calculation of loads for floating offshore wind turbines requires time-domain integrated simulation tools where most of the physical concepts involved in the system dynamics are considered The loads at the different components are used for the structural calculation and influence the design noticeably This study quantifies the influence of mooring dynamic models on the calculation of fatigue and ultimate loads with integrated tools and compares its performance with a lower computational cost quasi-static mooring model Three platforms representing the principal topologies (spar, semisubmersible and tension-leg platform) were assumed to be installed at the same 200 m depth location in the Irish coast For each platform, the fatigue and ultimate loads were computed with an integrated floating wind turbine simulation code using both, a quasi-static and a fully dynamic moorings model More than 3500 simulations for each platform and mooring model were launched and post-processed according to the IEC 61400-3 guideline in an exercise similar to what a certification entity may require to an offshore wind turbine designer The results showed that the impact of mooring dynamics in both fatigue and ultimate loads increases as elements located closer to the platform are evaluated; the blade and the shaft loads are only slightly modified by the mooring dynamics in all the platform designs; the tower base loads can be significantly affected depending on the platform concept; and the mooring lines tensions strongly depend on the lines dynamics, both in fatigue and extreme loads for all the platform concepts evaluated Copyright © 2016 John Wiley & Sons, Ltd