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Topical Collection "Wind Turbines"

Editor

Collection Editor
Prof. Dr. Frede Blaabjerg
Highly Cited - Clarivate Analytics (formerly Thomson Reuters)

Department of Energy Technology, Aalborg University, Aalborg 9220, Denmark
Website | E-Mail
Fax: +45 9815 1411
Interests: power electronics and its applications in motor drives; wind turbines; PV systems; harmonics; reliability of power electronic systems

Topical Collection Information

Dear Colleagues,

This issue is a continuation of the previous successful Special Issue “Wind Turbines 2013”. Similarly, this issue also focuses on recent advances in the wind energy sector on a wide range of topics, including: wind resource mapping, wind intermittency issues, aerodynamics, foundations, aeroelasticity, wind turbine technologies, control of wind turbines, diagnostics, generator concepts including gearless concepts, power electronic converters, grid interconnection, ride-through operation, protection, wind farm layouts - optimization and control, reliability, operations and maintenance, effects of wind farms on local and global climate, wind power stations, smart-grid and micro-grid related to wind turbine operation.

Prof. Dr. Frede Blaabjerg
Collection Editor

Manuscript Submission Information

Manuscripts for the topical collection can be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. All papers will be peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on this website. The topical collection considers regular research articles, short communications and review articles. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page.

Please visit the Instructions for Authors page before submitting a manuscript. The article processing charge (APC) for publication in this open access journal is 1600 CHF (Swiss Francs).

Related Special Issues

Published Papers (81 papers)

2018

Jump to: 2017, 2016, 2015, 2014

Open AccessArticle Assessment of Wind Energy Potential as a Power Generation Source: A Case Study of Eight Selected Locations in Northern Cyprus
Energies 2018, 11(10), 2697; https://doi.org/10.3390/en11102697
Received: 8 September 2018 / Revised: 1 October 2018 / Accepted: 4 October 2018 / Published: 10 October 2018
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Abstract
This paper presents a techno-economic assessment of the wind power potential for eight locations distributed over the Northern part of Cyprus. The wind speed data were collected from the meteorological department located in Lefkoşa, Northern Cyprus.Ten distribution models were used to analyze the
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This paper presents a techno-economic assessment of the wind power potential for eight locations distributed over the Northern part of Cyprus. The wind speed data were collected from the meteorological department located in Lefkoşa, Northern Cyprus.Ten distribution models were used to analyze the wind speed characteristics and wind energy potential at the selected locations. The maximum-likelihood method was used for calculating the parameters of the distribution functions.The power law model is utilized to determine the mean wind speed at various heights. In addition, the wind power density for each location was estimated. Furthermore, the performances of different small-scale vertical axis 3–10 kW wind turbines were evaluated to find those that were suitable and efficient for power generation in the studied locations.The results showed that the annual mean wind speed in the regions is greater than 2 m/s at a height of 10 m. Moreover, it is indicated that Generalized Extreme Value distribution provided the best fit to the actual data for the regions of Lefkoşa, Ercan, Girne, Güzelyurt, and Dipkarpaz. However, the Log-Logistic, Weibull, and Gamma distributions gave a better fit to the actual data of Gazimağusa, YeniBoğaziçi, and Salamis, respectively. The Rayleigh distribution does not fit the actual data from all regions. Furthermore, the values of wind power densityat the areas studied ranged from 38.76 W/m2 to 134.29 W/m2 at a height of 50 m, which indicated that wind energy sources in these selected locations are classified as poor. Meanwhile, based on the wind analysis, small-scale wind turbine use can be suitable for generating electricity in the studied locations. Consequently, an Aeolos-V2 with a rating of 5 kW was found to be capable of producing the annual energy needs of an average household in Northern Cyprus. Full article
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Open AccessArticle Combined Blade-Element Momentum—Lifting Line Model for Variable Loads on Downwind Turbine Towers
Energies 2018, 11(10), 2521; https://doi.org/10.3390/en11102521
Received: 20 July 2018 / Revised: 4 September 2018 / Accepted: 13 September 2018 / Published: 21 September 2018
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Abstract
Downwind rotors are a promising concept for multi-megawatt scale large wind turbines due to their advantages in safety and cost reduction. However, they have risks from impulsive loads when one of the blades passes across the tower wake, where the wind speed is
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Downwind rotors are a promising concept for multi-megawatt scale large wind turbines due to their advantages in safety and cost reduction. However, they have risks from impulsive loads when one of the blades passes across the tower wake, where the wind speed is lower and locally turbulent. Although the tower shadow effects on the tower loads have been discussed in former studies, there is currently no appropriate model for the blade-element and momentum theory so far. This study formulates the tower shadow effects on the tower load variation induced by blades using the lifting line theory, which does not require any empirical parameters. The method is verified via computational fluid dynamics for a 2 MW(megawatt), 3-bladed downwind turbine. The amplitude and the phase of the variation are shown to be accurate in outboard sections, where the rotor-tower clearance is large (>3.0 times of the tower diameter) and the ratio of the blade chord length is small (<0.5 times of the tower diameter), in both of rated and cut-out conditions. Full article
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Open AccessArticle Combined Pitch and Trailing Edge Flap Control for Load Mitigation of Wind Turbines
Energies 2018, 11(10), 2519; https://doi.org/10.3390/en11102519
Received: 24 August 2018 / Revised: 17 September 2018 / Accepted: 18 September 2018 / Published: 21 September 2018
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Abstract
Using active control methods for load mitigation in wind turbines could greatly reduce the cost of per kilowatt hour of wind power. In this work, the combined pitch and trailing edge flap control (CPFC) for load mitigation of wind turbines is investigated. The
[...] Read more.
Using active control methods for load mitigation in wind turbines could greatly reduce the cost of per kilowatt hour of wind power. In this work, the combined pitch and trailing edge flap control (CPFC) for load mitigation of wind turbines is investigated. The CPFC includes an individual pitch control (IPC) loop and a trailing edge flap control (TEFC) loop, which are combined by a load frequency division control algorithm. The IPC loop is mainly used to mitigate the low frequency loads, and the TEFC loop is mainly used to mitigate the high frequency loads. The CPFC adopts both an azimuth angle feed-forward and a loads feedback control strategy. The azimuth angle feed-forward control strategy should mitigate the asymmetrical loads caused by observable disturbances. and the loads feedback control strategy should decrease asymmetrical loads by closed loop control. A simulation is carried out on the joint platform of FAST and MATLAB. The simulation results show that the damage equivalent load (DEL) of blade root out-of-plane bending moment is reduced by 53.7% while using CPFC, compared to collective pitch control (CPC); and the standard deviation of blade tip out-of-plane deflection is reduced by 50.2% while using CPFC, compared to CPC. The results demonstrate that the CPFC can mitigate the fatigue loads of wind turbines as anticipated. Full article
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Open AccessArticle Using Multiple Fidelity Numerical Models for Floating Offshore Wind Turbine Advanced Control Design
Energies 2018, 11(9), 2484; https://doi.org/10.3390/en11092484
Received: 30 July 2018 / Revised: 2 September 2018 / Accepted: 12 September 2018 / Published: 18 September 2018
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Abstract
This paper summarises the tuning process of the Aerodynamic Platform Stabiliser control loop and its performance with Floating Offshore Wind Turbine model. Simplified Low-Order Wind turbine numerical models have been used for the system identification and control tuning process. Denmark Technical University’s 10
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This paper summarises the tuning process of the Aerodynamic Platform Stabiliser control loop and its performance with Floating Offshore Wind Turbine model. Simplified Low-Order Wind turbine numerical models have been used for the system identification and control tuning process. Denmark Technical University’s 10 MW wind turbine model mounted on the TripleSpar platform concept was used for this study. Time-domain simulations were carried out in a fully coupled non-linear aero-hydro-elastic simulation tool FAST, in which wind and wave disturbances were modelled. This testing yielded significant improvements in the overall Floating Offshore Wind Turbine performance and load reduction, validating the control technique presented in this work. Full article
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Open AccessArticle Dimensional Analysis of Power Prediction of a Real-Scale Wind Turbine Based on Wind-Tunnel Torque Measurement of Small-Scaled Models
Energies 2018, 11(9), 2374; https://doi.org/10.3390/en11092374
Received: 15 August 2018 / Revised: 31 August 2018 / Accepted: 5 September 2018 / Published: 8 September 2018
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Abstract
A preliminary study of a horizontal-axis wind turbine (HAWT) design is carried out using a wind tunnel to obtain its aerodynamic characteristics. Utilization of data from the study to develop large-scale wind turbines requires further study. This paper aims to discuss the use
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A preliminary study of a horizontal-axis wind turbine (HAWT) design is carried out using a wind tunnel to obtain its aerodynamic characteristics. Utilization of data from the study to develop large-scale wind turbines requires further study. This paper aims to discuss the use of wind turbine data obtained the wind-tunnel measurements to estimate the characteristics of wind turbines that have field size. One should measure the torque of two small-scale turbines inside the wind tunnel. The first small-scale turbine has a radius of 0.14 m, and the radius of the second small turbine is 0.19 m. Torque measurement results from both turbines were analyzed using the Buckingham π theorem to obtain a correlation between torsion and diameter variations. The obtained correlation equation was used to estimate the field measurement of turbine power with a radius of 1.2 m. The resulting correlation equation can be applied to approximate the energy generated by the turbine using the size of the field well in the operating area and the tip-speed ratio (λ) of the turbine design. Full article
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Open AccessArticle Conceptual Synthesis of Speed Increasers for Wind Turbine Conversion Systems
Energies 2018, 11(9), 2257; https://doi.org/10.3390/en11092257
Received: 30 June 2018 / Revised: 9 August 2018 / Accepted: 22 August 2018 / Published: 27 August 2018
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Abstract
Most wind turbines (WT) are of the single-rotor type, which means they are simple, reliable and durable, but unlikely to convert more than 40% of the available wind energy. Different solutions are proposed to minimize WT energy loss and improve performance, such as
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Most wind turbines (WT) are of the single-rotor type, which means they are simple, reliable and durable, but unlikely to convert more than 40% of the available wind energy. Different solutions are proposed to minimize WT energy loss and improve performance, such as the use of speed increasers, counter-rotating wind rotors or counter-rotating electric generators. Downsizing the design, saving weight and reducing the cost of WT conversion systems, while increasing their efficiency, have posed constant challenges to WT designers. Nevertheless, very little research in the field is concerned with, and partially recommends, the design of conversion systems. Therefore, the aim of this paper is to propose a specific algorithm for the conceptual synthesis of speed increasers integrated in WT conversion systems, starting with an inventory of all combinations of the main components of a conversion system that prove compatible for efficient functioning. The algorithm is structured in two sections: the first one includes a four-step approach to WT system design, while the second one follows a three-step procedure for identifying the speed increaser concept. Twenty-two variants of speed increasers are further generated and analyzed, four of which are innovative solutions proposed by the authors. The paper also provides guidelines for identifying the WT conversion system concept according to the circumstances of its application. Full article
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Open AccessArticle An Analytical Model for the Regeneration of Wind after Exiting a Wind Farm
Energies 2018, 11(8), 2071; https://doi.org/10.3390/en11082071
Received: 15 July 2018 / Revised: 4 August 2018 / Accepted: 7 August 2018 / Published: 8 August 2018
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Abstract
The simplest model for an atmospheric boundary layer assumes a uniform steady wind over a certain depth, of order 1 km, with the forces of friction, pressure gradient and Coriolis in balance. A linear model is here employed for the adjustment of wind
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The simplest model for an atmospheric boundary layer assumes a uniform steady wind over a certain depth, of order 1 km, with the forces of friction, pressure gradient and Coriolis in balance. A linear model is here employed for the adjustment of wind to this equilibrium, as the wake of a very wide wind farm. A length scale is predicted for the exponential adjustment to equilibrium. Calculation of this length scale is aided by knowledge of the angle for which the wind would normally cross the isobars in environmental conditions in the wake. Full article
Open AccessArticle Assessment and Performance Evaluation of a Wind Turbine Power Output
Energies 2018, 11(8), 1992; https://doi.org/10.3390/en11081992
Received: 12 June 2018 / Revised: 25 June 2018 / Accepted: 26 June 2018 / Published: 1 August 2018
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Abstract
Estimation errors have constantly been a technology bother for wind power management, often time with deviations of actual power curve (APC) from the turbine power curve (TPC). Power output dispersion for an operational 800 kW turbine was analyzed using three averaging tine steps
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Estimation errors have constantly been a technology bother for wind power management, often time with deviations of actual power curve (APC) from the turbine power curve (TPC). Power output dispersion for an operational 800 kW turbine was analyzed using three averaging tine steps of 1-min, 5-min, and 15-min. The error between the APC and TPC in kWh was about 25% on average, irrespective of the time of the day, although higher magnitudes of error were observed during low wind speeds and poor wind conditions. The 15-min averaged time series proved more suitable for grid management and energy load scheduling, but the error margin was still a major concern. An effective power curve (EPC) based on the polynomial parametric wind turbine power curve modeling technique was calibrated using turbine and site-specific performance data. The EPC reduced estimation error to about 3% in the aforementioned time series during very good wind conditions. By integrating statistical wind speed forecasting methods and site-specific EPCs, wind power forecasting and management can be significantly improved without compromising grid stability. Full article
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Open AccessArticle Load Estimation of Offshore Wind Turbines
Energies 2018, 11(7), 1895; https://doi.org/10.3390/en11071895
Received: 6 May 2018 / Revised: 16 July 2018 / Accepted: 16 July 2018 / Published: 20 July 2018
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Abstract
The influence of 3 MW Hywind-II wind turbine wakes from an upstream offshore floating wind turbine on a downstream turbine with a separation distance of seven rotor diameters was studied for a site in the Gulf of Maine. The turbines and the platforms
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The influence of 3 MW Hywind-II wind turbine wakes from an upstream offshore floating wind turbine on a downstream turbine with a separation distance of seven rotor diameters was studied for a site in the Gulf of Maine. The turbines and the platforms were subjected to atmospheric boundary layer flows. Various sensitivity studies on fatigue loads with respect to the positions of the downstream turbine were performed and validated with a large-eddy simulation tool. In particular, the effect of various lateral positions of the downstream turbine relative to the upstream turbine were considered using time-series turbine wake data generated from the large-eddy simulation tool which served as an input to an aero-elastic wind turbine model to assess the loads. The load response from the rotor, tower, and the floating platform for the downstream turbine were sensitive to the lateral offset positions where turbines that were partially exposed to upstream turbine wakes yielded significant increases in the cyclic load range. For the given set of lateral positions for the downstream turbine, the largest damage equivalent load occurred when the turbine was one rotor diameter to the left of the centerline, when looking upstream, which is the position of the turbine fully exposed to upstream turbine wake. On the other hand, the fatigue load on the downstream turbine placed on the right side of the position fully exposed to the upstream turbine wake, yielded lower stress due to the non-symmetric shape of the turbine wake. The configuration associated with the largest damage equivalent loads was further investigated in a large-eddy simulation, modeling both the upstream and downstream turbines. It was found that the energy spectra at the blade rotational frequency were a magnitude order higher for the downstream turbine, especially for surge, heave, pitch, and yaw motion of the platform. The increase of the damage equivalent load for the flapwise blade root moment was 45% compared to the upstream turbine, which can potentially reduce the turbine service life time. Full article
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Open AccessArticle Computational Fluid Dynamics Approach to Predict the Actual Wind Speed over Complex Terrain
Energies 2018, 11(7), 1694; https://doi.org/10.3390/en11071694
Received: 24 May 2018 / Revised: 20 June 2018 / Accepted: 22 June 2018 / Published: 29 June 2018
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Abstract
This paper proposes a procedure for predicting the actual wind speed for flow over complex terrain with CFD. It converts a time-series of wind speed data acquired from field observations into a time-series data of actual scalar wind speed by using non-dimensional wind
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This paper proposes a procedure for predicting the actual wind speed for flow over complex terrain with CFD. It converts a time-series of wind speed data acquired from field observations into a time-series data of actual scalar wind speed by using non-dimensional wind speed parameters, which are determined beforehand with the use of CFD output. The accuracy and reproducibility of the prediction procedure were investigated by simulating the flow with CFD with the use of high spatial resolution (5 m) surface elevation data for the Noma Wind Park in Kagoshima Prefecture, Japan. The errors of the predicted average monthly wind speeds relative to the observed values were less than approximately 20%. Full article
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Open AccessArticle LES Investigation of Terrain-Induced Turbulence in Complex Terrain and Economic Effects of Wind Turbine Control
Energies 2018, 11(6), 1530; https://doi.org/10.3390/en11061530
Received: 25 May 2018 / Revised: 8 June 2018 / Accepted: 10 June 2018 / Published: 12 June 2018
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Abstract
In the present study, numerical wind simulation was conducted by reproducing the realistic topography near wind turbine sites with high spatial resolutions and using the Large-Eddy Simulation (LES) technique. The topography near wind turbine sites serves as the origin of the terrain-induced turbulence.
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In the present study, numerical wind simulation was conducted by reproducing the realistic topography near wind turbine sites with high spatial resolutions and using the Large-Eddy Simulation (LES) technique. The topography near wind turbine sites serves as the origin of the terrain-induced turbulence. The obtained numerical simulation results showed that the terrain-induced turbulence is generated at a small terrain feature located upstream of the wind turbine. The generated terrain-induced turbulence affects the wind turbine directly. The wind speed and wind direction at the wind turbine site are significantly changed with time. In the present study, a combination of the series of wind simulation results and on-site operation experience led to a decision to adopt an “automatic shutdown program”. Here, an “automatic shutdown program” means the automatic suspension of wind turbine operation based on the wind speed and wind direction meeting the conditions associated with significant effects of terrain-induced turbulence at a wind turbine site. The adoption of the “automatic shutdown program” has successfully led to a large reduction in the number of occurrences of wind turbine damage, thus, creating major positive economic effects. Full article
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Open AccessArticle Computational Fluid Dynamics (CFD) Investigation of Wind Turbine Nacelle Separation Accident over Complex Terrain in Japan
Energies 2018, 11(6), 1485; https://doi.org/10.3390/en11061485
Received: 22 May 2018 / Revised: 5 June 2018 / Accepted: 5 June 2018 / Published: 7 June 2018
Cited by 2 | PDF Full-text (22838 KB) | HTML Full-text | XML Full-text
Abstract
We have developed an unsteady and non-linear wind synopsis simulator called RIAM-COMPACT (Research Institute for Applied Mechanics, Kyushu University, COMputational Prediction of Airflow over Complex Terrain) to simulate the airflow on a micro scale, i.e., a few tens of km or less. In
[...] Read more.
We have developed an unsteady and non-linear wind synopsis simulator called RIAM-COMPACT (Research Institute for Applied Mechanics, Kyushu University, COMputational Prediction of Airflow over Complex Terrain) to simulate the airflow on a micro scale, i.e., a few tens of km or less. In RIAM-COMPACT, the large-eddy simulation (LES) has been adopted for turbulence modeling. LES is a technique in which the structures of relatively large eddies are directly simulated and smaller eddies are modeled using a sub-grid scale model. In the present study, we conducted numerical wind diagnoses for the Taikoyama Wind Farm nacelle separation accident in Japan. The simulation results suggest that all six wind turbines at Taikoyama Wind Farm are subject to significant influence from separated flow (terrain-induced turbulence) which is generated due to the topographic irregularities in the vicinity of the wind turbines. A proposal was also made on reconstruction of the wind farm. Full article
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Open AccessArticle Interaction of Wind Turbine Wakes under Various Atmospheric Conditions
Energies 2018, 11(6), 1442; https://doi.org/10.3390/en11061442
Received: 3 May 2018 / Revised: 29 May 2018 / Accepted: 30 May 2018 / Published: 4 June 2018
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Abstract
We present a numerical study of two utility-scale 5-MW turbines separated by seven rotor diameters. The effects of the atmospheric boundary layer flow on the turbine performance were assessed using large-eddy simulations. We found that the surface roughness and the atmospheric stability states
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We present a numerical study of two utility-scale 5-MW turbines separated by seven rotor diameters. The effects of the atmospheric boundary layer flow on the turbine performance were assessed using large-eddy simulations. We found that the surface roughness and the atmospheric stability states had a profound effect on the wake diffusion and the Reynolds stresses. In the upstream turbine case, high surface roughness increased the wind shear, accelerating the decay of the wake deficit and increasing the Reynolds stresses. Similarly, atmospheric instabilities significantly expedited the wake decay and the Reynolds stress increase due to updrafts of the thermal plumes. The turbulence from the upstream boundary layer flow combined with the turbine wake yielded higher Reynolds stresses for the downwind turbine, especially in the streamwise component. For the downstream turbine, diffusion of the wake deficits and the sharp peaks in the Reynolds stresses showed faster decay than the upwind case due to higher levels of turbulence. This provides a physical explanation for how turbine arrays or wind farms can operate more efficiently under unstable atmospheric conditions, as it is based on measurements collected in the field. Full article
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Open AccessArticle Experimental Investigation of Static Stall Hysteresis and 3-Dimensional Flow Structures for an NREL S826 Wing Section of Finite Span
Energies 2018, 11(6), 1418; https://doi.org/10.3390/en11061418
Received: 13 April 2018 / Revised: 21 May 2018 / Accepted: 30 May 2018 / Published: 1 June 2018
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Abstract
Flow characteristics of an S826 airfoil at different Reynolds numbers, ranging from 40,000–400,000 (based on airfoil chord length) and angles of attack from −10–25 degrees are thoroughly investigated in a low-speed wind tunnel. The airfoil’s lift and drag polars are first measured, and
[...] Read more.
Flow characteristics of an S826 airfoil at different Reynolds numbers, ranging from 40,000–400,000 (based on airfoil chord length) and angles of attack from −10–25 degrees are thoroughly investigated in a low-speed wind tunnel. The airfoil’s lift and drag polars are first measured, and with a focus on pitching the airfoil in upstroke and downstroke orders, static stall hysteresis is identified in selected experiments at Reynolds numbers below 100,000 near the stall angle and subsequently investigated. Experiments using wire-generated free stream turbulence are conducted, and the hysteresis effects are shown to disappear when introducing a free stream turbulence of less than 2.5%. Further, spanwise flow is detected by comparing lift and drag values measured using both surface pressure integration at one cross section as well as integral force gauge measurement, and the surface oil flow visualization technique is subsequently used to study the 3D flow topologies formed on the airfoil. The formation of distinct stall cells on the suction side of the airfoil is observed at Reynolds numbers above 100,000 near the stall angle. By repeating the experiments, stall cells are proven to be reproduceable, although the identical geometries are necessarily not retained in abscence of surface impurities such as tapes. The effect of disturbances on the stall cells is investigated by utilizing roughness elements on the airfoil surface, and it is found that while such disturbances tend to change the shape of the stall cells, they do not contribute to the creation, nor destruction of the cells. Polar and visualisation measurements are also used to study flow separation, and it is observed that the separation location, as well as the laminar separation bubble, moves towards the leading edge when increasing the angle of attack. Full article
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Open AccessArticle Comparison of Shell and Solid Finite Element Models for the Static Certification Tests of a 43 m Wind Turbine Blade
Energies 2018, 11(6), 1346; https://doi.org/10.3390/en11061346
Received: 2 May 2018 / Revised: 14 May 2018 / Accepted: 16 May 2018 / Published: 25 May 2018
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Abstract
A commercial 43 m wind turbine blade was tested under static loads. During these tests, loads, displacements, and local strains were recorded. In this work, the blade was modeled using the finite element method. Both a segment of the spar structure and the
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A commercial 43 m wind turbine blade was tested under static loads. During these tests, loads, displacements, and local strains were recorded. In this work, the blade was modeled using the finite element method. Both a segment of the spar structure and the full-scale blade were modeled. In both cases, conventional outer mold layer shell and layered solid models were created by means of an in-house developed software tool. First, the boundary conditions and settings for modeling the tests were explored. Next, the behavior of a spar segment under different modeling methods was investigated. Finally, the full-scale blade tests were conducted. The resulting displacements and longitudinal and transverse strains were investigated. It was found that for the considered load case, the differences between the shell and solid models are limited. Thus, it is concluded that the shell representation is sufficiently accurate. Full article
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Open AccessArticle Comparative Study of the Aerodynamic Performance of the New MEXICO Rotor under Yaw Conditions
Energies 2018, 11(4), 833; https://doi.org/10.3390/en11040833
Received: 12 February 2018 / Revised: 18 March 2018 / Accepted: 2 April 2018 / Published: 4 April 2018
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Abstract
The influence of yaw misalignment on the aerodynamic performance of the New MEXICO rotor is investigated using blade-resolved Computational Fluid Dynamics (CFD) approaches with three wind speeds considered at a fixed yaw angle of 30 degrees. The air-load predictions and near wake characteristics
[...] Read more.
The influence of yaw misalignment on the aerodynamic performance of the New MEXICO rotor is investigated using blade-resolved Computational Fluid Dynamics (CFD) approaches with three wind speeds considered at a fixed yaw angle of 30 degrees. The air-load predictions and near wake characteristics from the numerical results are compared and discussed against the most recent wind tunnel test data. The nacelle impact, dynamic stall phenomenon and wake characteristics are analyzed, demonstrating the yaw effects and numerical issues raised from Reynolds-Averaged Navier Stokes (RANS) and Detached Eddy Simulation (DES) computations. Full article
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Open AccessArticle Wind Turbine Condition Monitoring Strategy through Multiway PCA and Multivariate Inference
Energies 2018, 11(4), 749; https://doi.org/10.3390/en11040749
Received: 20 February 2018 / Revised: 22 March 2018 / Accepted: 23 March 2018 / Published: 26 March 2018
Cited by 2 | PDF Full-text (592 KB) | HTML Full-text | XML Full-text
Abstract
This article states a condition monitoring strategy for wind turbines using a statistical data-driven modeling approach by means of supervisory control and data acquisition (SCADA) data. Initially, a baseline data-based model is obtained from the healthy wind turbine by means of multiway principal
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This article states a condition monitoring strategy for wind turbines using a statistical data-driven modeling approach by means of supervisory control and data acquisition (SCADA) data. Initially, a baseline data-based model is obtained from the healthy wind turbine by means of multiway principal component analysis (MPCA). Then, when the wind turbine is monitorized, new data is acquired and projected into the baseline MPCA model space. The acquired SCADA data are treated as a random process given the random nature of the turbulent wind. The objective is to decide if the multivariate distribution that is obtained from the wind turbine to be analyzed (healthy or not) is related to the baseline one. To achieve this goal, a test for the equality of population means is performed. Finally, the results of the test can determine that the hypothesis is rejected (and the wind turbine is faulty) or that there is no evidence to suggest that the two means are different, so the wind turbine can be considered as healthy. The methodology is evaluated on a wind turbine fault detection benchmark that uses a 5 MW high-fidelity wind turbine model and a set of eight realistic fault scenarios. It is noteworthy that the results, for the presented methodology, show that for a wide range of significance, α [ 1 % , 13 % ] , the percentage of correct decisions is kept at 100%; thus it is a promising tool for real-time wind turbine condition monitoring. Full article
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Open AccessArticle A New Analytical Wake Model for Yawed Wind Turbines
Energies 2018, 11(3), 665; https://doi.org/10.3390/en11030665
Received: 26 January 2018 / Revised: 28 February 2018 / Accepted: 13 March 2018 / Published: 15 March 2018
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Abstract
A new analytical wake model for wind turbines, considering ambient turbulence intensity, thrust coefficient and yaw angle effects, is proposed from numerical and analytical studies. First, eight simulations by the Reynolds Stress Model are conducted for different thrust coefficients, yaw angles and ambient
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A new analytical wake model for wind turbines, considering ambient turbulence intensity, thrust coefficient and yaw angle effects, is proposed from numerical and analytical studies. First, eight simulations by the Reynolds Stress Model are conducted for different thrust coefficients, yaw angles and ambient turbulence intensities. The wake deflection, mean velocity and turbulence intensity in the wakes are systematically investigated. A new wake deflection model is then proposed to analytically predict the wake center trajectory in the yawed condition. Finally, the effects of yaw angle are incorporated in the Gaussian-based wake model. The wake deflection, velocity deficit and added turbulence intensity in the wake predicted by the proposed model show good agreement with the numerical results. The model parameters are determined as the function of ambient turbulence intensity and thrust coefficient, which enables the model to have good applicability under various conditions. Full article
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Open AccessArticle Flow Control over the Blunt Trailing Edge of Wind Turbine Airfoils Using Circulation Control
Energies 2018, 11(3), 619; https://doi.org/10.3390/en11030619
Received: 16 February 2018 / Revised: 4 March 2018 / Accepted: 8 March 2018 / Published: 10 March 2018
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Abstract
A new partial circulation control (PCC) method is implemented on the blunt trailing edge DU97-Flatback airfoil, and compared with the traditional full circulation control (FCC) based on numerical analysis. When the Coanda jet is deactivated, PCC has an attractive advantage over FCC, since
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A new partial circulation control (PCC) method is implemented on the blunt trailing edge DU97-Flatback airfoil, and compared with the traditional full circulation control (FCC) based on numerical analysis. When the Coanda jet is deactivated, PCC has an attractive advantage over FCC, since the design of PCC doesn’t degrade aerodynamic characteristics of the baseline flatback section, in contrast to FCC, which is important in practical use in case of failure of the circulation control system. When the Coanda jet is activated, PCC also outperforms FCC in several respects. PCC can produce much higher lift coefficients than FCC over the entire range of angles of attack as well as the entire range of jet momentum coefficients under investigation, but with slightly higher drag coefficients. The flow field of PCC is less complex than that of FCC, indicating less energy dissipation in the main flow and hence less power expenditure for the Coanda jet. The aerodynamic figure of merit (AFM) and control efficiency for circulation control are defined, and results show that PCC has much higher AFM and control efficiency than FCC. It is demonstrated that PCC outperforms FCC in terms of effectiveness, efficiency and reliability for flow control in the blunt trailing edge wind turbine application. Full article
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Open AccessArticle On the Evolution of the Integral Time Scale within Wind Farms
Energies 2018, 11(1), 93; https://doi.org/10.3390/en11010093
Received: 27 November 2017 / Revised: 19 December 2017 / Accepted: 28 December 2017 / Published: 2 January 2018
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Abstract
A wind-tunnel investigation was carried out to characterize the spatial distribution of the integral time scale (Tu) within, and in the vicinity of, two model wind farms. The turbine arrays were placed over a rough wall and operated under high
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A wind-tunnel investigation was carried out to characterize the spatial distribution of the integral time scale ( T u ) within, and in the vicinity of, two model wind farms. The turbine arrays were placed over a rough wall and operated under high turbulence. The two layouts consisted of aligned units distinguished only by the streamwise spacing ( Δ x T ) between the devices, set at five and ten rotor diameters d T (or S x = Δ x T / d T = 5 and 10). They shared the same spanwise spacing between turbines of 2.5 d T ; this resulted in arrays of 8 × 3 and 5 × 3 horizontal-axis turbines. Hotwire anemometry was used to characterize the instantaneous velocity at various vertical and transverse locations along the central column of the wind farms. Results show that T u was modulated by the wind farm layout. It was significantly reduced within the wind farms and right above them, where the internal boundary layer develops. The undisturbed levels above the wind farms were recovered only at ≈ d T / 2 above the top tip. This quantity appeared to reach adjusted values starting the fifth row of turbines in the S x = 5 wind farm, and earlier in the S x = 10 counterpart. Within the adjusted zone, the distribution of T u at hub height exhibited a negligible growth in the S x = 5 case; whereas it underwent a mild growth in the S x = 10 wind farm. In addition, the flow impinging the inner turbines exhibited T u / T i n c u < 1 , where T i n c u is the integral time scale of the overall incoming flow. Specifically, T u β T i n c u at z = z h u b , where β < 1 within standard layouts of wind farms, in particular β 0.5 and 0.7 for S x = 5 and 10. Full article
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2017

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Open AccessArticle Machine Learning for Wind Turbine Blades Maintenance Management
Energies 2018, 11(1), 13; https://doi.org/10.3390/en11010013
Received: 28 October 2017 / Revised: 14 December 2017 / Accepted: 18 December 2017 / Published: 21 December 2017
Cited by 1 | PDF Full-text (2529 KB) | HTML Full-text | XML Full-text
Abstract
Delamination in Wind Turbine Blades (WTB) is a common structural problem that can generate large costs. Delamination is the separation of layers of a composite material, which produces points of stress concentration. These points suffer greater traction and compression forces in working conditions,
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Delamination in Wind Turbine Blades (WTB) is a common structural problem that can generate large costs. Delamination is the separation of layers of a composite material, which produces points of stress concentration. These points suffer greater traction and compression forces in working conditions, and they can trigger cracks, and partial or total breakage of the blade. Early detection of delamination is crucial for the prevention of breakages and downtime. The main novelty presented in this paper has been to apply an approach for detecting and diagnosing the delamination WTB. The approach is based on signal processing of guided waves, and multiclass pattern recognition using machine learning. Delamination was induced in the WTB to check the accuracy of the approach. The signal is denoised by wavelet transform. The autoregressive Yule–Walker model is employed for feature extraction, and Akaike’s information criterion method for feature selection. The classifiers are quadratic discriminant analysis, k-nearest neighbors, decision trees, and neural network multilayer perceptron. The confusion matrix is employed to evaluate the classification, especially the receiver operating characteristic analysis by: recall, specificity, precision, and F-score. Full article
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Open AccessEditor’s ChoiceArticle Flow Adjustment Inside and Around Large Finite-Size Wind Farms
Energies 2017, 10(12), 2164; https://doi.org/10.3390/en10122164
Received: 1 December 2017 / Revised: 13 December 2017 / Accepted: 14 December 2017 / Published: 18 December 2017
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Abstract
In this study, large-eddy simulations are performed to investigate the flow inside and around large finite-size wind farms in conventionally-neutral atmospheric boundary layers. Special emphasis is placed on characterizing the different farm-induced flow regions, including the induction, entrance and development, fully-developed, exit and
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In this study, large-eddy simulations are performed to investigate the flow inside and around large finite-size wind farms in conventionally-neutral atmospheric boundary layers. Special emphasis is placed on characterizing the different farm-induced flow regions, including the induction, entrance and development, fully-developed, exit and farm wake regions. The wind farms extend 20 km in the streamwise direction and comprise 36 wind turbine rows arranged in aligned and staggered configurations. Results show that, under weak free-atmosphere stratification ( Γ = 1 K/km), the flow inside and above both wind farms, and thus the turbine power, do not reach the fully-developed regime even though the farm length is two orders of magnitude larger than the boundary layer height. In that case, the wind farm induction region, affected by flow blockage, extends upwind about 0.8 km and leads to a power reduction of 1.3% and 3% at the first row of turbines for the aligned and staggered layouts, respectively. The wind farm wake leads to velocity deficits at hub height of around 3.5% at a downwind distance of 10 km for both farm layouts. Under stronger stratification ( Γ = 5 K/km), the vertical deflection of the subcritical flow induced by the wind farm at its entrance and exit regions triggers standing gravity waves whose effects propagate upwind. They, in turn, induce a large decelerating induction region upwind of the farm leading edge, and an accelerating exit region upwind of the trailing edge, both extending about 7 km. As a result, the turbine power output in the entrance region decreases more than 35% with respect to the weakly stratified case. It increases downwind as the flow adjusts, reaching the fully-developed regime only for the staggered layout at a distance of about 8.5 km from the farm edge. The flow acceleration in the exit region leads to an increase of the turbine power with downwind distance in that region, and a relatively fast (compared with the weakly stratified case) recovery of the farm wake, which attains its inflow hub height speed at a downwind distance of 5 km. Full article
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Open AccessReview Structural Reliability Analysis of Wind Turbines: A Review
Energies 2017, 10(12), 2099; https://doi.org/10.3390/en10122099
Received: 7 November 2017 / Revised: 5 December 2017 / Accepted: 6 December 2017 / Published: 11 December 2017
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Abstract
The paper presents a detailed review of the state-of-the-art research activities on structural reliability analysis of wind turbines between the 1990s and 2017. We describe the reliability methods including the first- and second-order reliability methods and the simulation reliability methods and show the
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The paper presents a detailed review of the state-of-the-art research activities on structural reliability analysis of wind turbines between the 1990s and 2017. We describe the reliability methods including the first- and second-order reliability methods and the simulation reliability methods and show the procedure for and application areas of structural reliability analysis of wind turbines. Further, we critically review the various structural reliability studies on rotor blades, bottom-fixed support structures, floating systems and mechanical and electrical components. Finally, future applications of structural reliability methods to wind turbine designs are discussed. Full article
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Open AccessArticle Force Measurements on a VAWT Blade in Parked Conditions
Energies 2017, 10(12), 1954; https://doi.org/10.3390/en10121954
Received: 24 October 2017 / Revised: 17 November 2017 / Accepted: 20 November 2017 / Published: 24 November 2017
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Abstract
The forces on a turbine at extreme wind conditions when the turbine is parked is one of the most important design cases for the survivability of a turbine. In this work, the forces on a blade and its support arms have been measured
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The forces on a turbine at extreme wind conditions when the turbine is parked is one of the most important design cases for the survivability of a turbine. In this work, the forces on a blade and its support arms have been measured on a 12 kW straight-bladed vertical axis wind turbine at an open site. Two cases are tested: one during electrical braking of the turbine, which allows it to rotate slowly, and one with the turbine mechanically fixed with the leading edge of the blade facing the main wind direction. The force variations with respect to wind direction are investigated, and it is seen that significant variations in forces depend on the wind direction. The measurements show that for the fixed case, when subjected to the same wind speed, the forces are lower when the blade faces the wind direction. The results also show that due to the lower forces at this particular wind direction, the average forces for the fixed blade are notably lower. Hence, it is possible to reduce the forces on a turbine blade, simply by taking the dominating wind direction into account when the turbine is parked. The measurements also show that a positive torque is generated from the blade for most wind directions, which causes the turbine to rotate in the electrically-braked case. These rotations will cause increased fatigue loads on the turbine blade. Full article
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Open AccessArticle Numerical Investigation of the Tip Vortex of a Straight-Bladed Vertical Axis Wind Turbine with Double-Blades
Energies 2017, 10(11), 1721; https://doi.org/10.3390/en10111721
Received: 5 September 2017 / Revised: 23 October 2017 / Accepted: 23 October 2017 / Published: 27 October 2017
Cited by 2 | PDF Full-text (17063 KB) | HTML Full-text | XML Full-text
Abstract
Wind velocity distribution and the vortex around the wind turbine present a significant challenge in the development of straight-bladed vertical axis wind turbines (VAWTs). This paper is intended to investigate influence of tip vortex on wind turbine wake by Computational Fluid Dynamics (CFD)
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Wind velocity distribution and the vortex around the wind turbine present a significant challenge in the development of straight-bladed vertical axis wind turbines (VAWTs). This paper is intended to investigate influence of tip vortex on wind turbine wake by Computational Fluid Dynamics (CFD) simulations. In this study, the number of blades is two and the airfoil is a NACA0021 with chord length of c = 0.265 m. To capture the tip vortex characteristics, the velocity fields are investigated by the Q-criterion iso-surface (Q = 100) with shear-stress transport (SST) k-ω turbulence model at different tip speed ratios (TSRs). Then, mean velocity, velocity deficit and torque coefficient acting on the blade in the different spanwise positions are compared. The wind velocities obtained by CFD simulations are also compared with the experimental data from wind tunnel experiments. As a result, we can state that the wind velocity curves calculated by CFD simulations are consistent with Laser Doppler Velocity (LDV) measurements. The distribution of the vortex structure along the spanwise direction is more complex at a lower TSR and the tip vortex has a longer dissipation distance at a high TSR. In addition, the mean wind velocity shows a large value near the blade tip and a small value near the blade due to the vortex effect. Full article
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Open AccessArticle Sensitivity Analysis to Control the Far-Wake Unsteadiness Behind Turbines
Energies 2017, 10(10), 1599; https://doi.org/10.3390/en10101599
Received: 19 August 2017 / Revised: 9 October 2017 / Accepted: 10 October 2017 / Published: 13 October 2017
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Abstract
We explore the stability of wakes arising from 2D flow actuators based on linear momentum actuator disc theory. We use stability and sensitivity analysis (using adjoints) to show that the wake stability is controlled by the Reynolds number and the thrust force (or
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We explore the stability of wakes arising from 2D flow actuators based on linear momentum actuator disc theory. We use stability and sensitivity analysis (using adjoints) to show that the wake stability is controlled by the Reynolds number and the thrust force (or flow resistance) applied through the turbine. First, we report that decreasing the thrust force has a comparable stabilising effect to a decrease in Reynolds numbers (based on the turbine diameter). Second, a discrete sensitivity analysis identifies two regions for suitable placement of flow control forcing, one close to the turbines and one far downstream. Third, we show that adding a localised control force, in the regions identified by the sensitivity analysis, stabilises the wake. Particularly, locating the control forcing close to the turbines results in an enhanced stabilisation such that the wake remains steady for significantly higher Reynolds numbers or turbine thrusts. The analysis of the controlled flow fields confirms that modifying the velocity gradient close to the turbine is more efficient to stabilise the wake than controlling the wake far downstream. The analysis is performed for the first flow bifurcation (at low Reynolds numbers) which serves as a foundation of the stabilization technique but the control strategy is tested at higher Reynolds numbers in the final section of the paper, showing enhanced stability for a turbulent flow case. Full article
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Open AccessArticle Techno-Economic Assessment of Wind Energy Potential at Three Locations in South Korea Using Long-Term Measured Wind Data
Energies 2017, 10(9), 1442; https://doi.org/10.3390/en10091442
Received: 25 August 2017 / Revised: 12 September 2017 / Accepted: 15 September 2017 / Published: 19 September 2017
Cited by 5 | PDF Full-text (8029 KB) | HTML Full-text | XML Full-text
Abstract
The present study deals with wind energy analysis and the selection of an optimum type of wind turbine in terms of the feasibility of installing wind power system at three locations in South Korea: Deokjeok-do, Baengnyeong-do and Seo-San. The wind data measurements were
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The present study deals with wind energy analysis and the selection of an optimum type of wind turbine in terms of the feasibility of installing wind power system at three locations in South Korea: Deokjeok-do, Baengnyeong-do and Seo-San. The wind data measurements were conducted during 2005–2015 at Deokjeok-do, 2001–2016 at Baengnyeong-do and 1997–2016 at Seo-San. In the first part of this paper wind conditions, like mean wind speed, wind rose diagrams and Weibull shape and scale parameters are presented, so that the wind potential of all the locations could be assessed. It was found that the prevailing wind directions at all locations was either southeast or southwest in which the latter one being more dominant. After analyzing the wind conditions, 50-year and 1-year extreme wind speeds (EWS) were estimated using the graphical method of Gumbel distribution. Finally, according to the wind conditions at each site and international electro-technical commission (IEC) guidelines, a set of five different wind turbines best suited for each location were shortlisted. Each wind turbine was evaluated on the basis of technical parameters like monthly energy production, annual energy production (AEP) and capacity factors (CF). Similarly, economical parameters including net present value (NPV), internal rate of return (IRR), payback period (PBP) and levelized cost of electricity (LCOE) were considered. The analysis shows that a Doosan model WinDS134/3000 wind turbine is the most suitable for Deokjeok-do and Baengnyeong-do, whereas a Hanjin model HJWT 87/2000 is the most suitable wind turbine for Seo-San. Economic sensitivity analysis is also included and discussed in detail to analyze the impact on economics of wind power by varying turbine’s hub height. Full article
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Open AccessArticle Analyses of the Extensible Blade in Improving Wind Energy Production at Sites with Low-Class Wind Resource
Energies 2017, 10(9), 1295; https://doi.org/10.3390/en10091295
Received: 24 July 2017 / Revised: 24 August 2017 / Accepted: 27 August 2017 / Published: 30 August 2017
Cited by 6 | PDF Full-text (5804 KB) | HTML Full-text | XML Full-text
Abstract
This paper describes the feasibility analysis of an innovative, extensible blade technology. The blade aims to significantly improve the energy production of a wind turbine, particularly at locations with unfavorable wind conditions. The innovative ‘smart’ blade will be extended at low wind speed
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This paper describes the feasibility analysis of an innovative, extensible blade technology. The blade aims to significantly improve the energy production of a wind turbine, particularly at locations with unfavorable wind conditions. The innovative ‘smart’ blade will be extended at low wind speed to harvest more wind energy; on the other hand, it will be retracted to its original shape when the wind speed is above the rated wind speed to protect the blade from damages by high wind loads. An established aerodynamic model is implemented in this paper to evaluate and compare the power output of extensible blades versus a baseline conventional blade. The model was first validated with a monitored power production curve based on the wind energy production data of a conventional turbine blade, which is subsequently used to estimate the power production curve of extended blades. The load-on-blade structures are incorporated as the mechanical criteria to design the extension strategies. Wind speed monitoring data at three different onshore and offshore sites around Lake Erie are used to predict the annual wind energy output with different blades. The effects of extension on the dynamic characteristics of blade are analyzed. The results show that the extensive blade significantly increases the annual wind energy production (up to 20% to 30%) with different blade extension strategies. It, therefore, has the potential to significantly boost wind energy production for utility-scale wind turbines located at sites with low-class wind resource. Full article
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Open AccessArticle Windbreak Effects Within Infinite Wind Farms
Energies 2017, 10(8), 1140; https://doi.org/10.3390/en10081140
Received: 9 May 2017 / Revised: 14 July 2017 / Accepted: 27 July 2017 / Published: 3 August 2017
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Abstract
Building upon a recent study that showed windbreaks to be effective in increasing the power output of a wind turbine, the potential of windbreaks in a large wind farm is explored using simplified formulations. A top-down boundary layer approach is combined with methods
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Building upon a recent study that showed windbreaks to be effective in increasing the power output of a wind turbine, the potential of windbreaks in a large wind farm is explored using simplified formulations. A top-down boundary layer approach is combined with methods of estimating both the roughness effects of windbreaks and the induced inviscid speed-up for nearby turbines to investigate power production impact for several layouts of infinite wind farms. Results suggest that the negative impact of windbreak wakes for an infinite wind farm will outweigh the local inviscid speed-up for realistic inter-turbine spacings, with the break-even point expected at a spacing of ∼25 rotor diameters. However, the possibility that windbreaks may be applicable in finite and other wind farm configurations remains open. Inspection of the windbreak porosity reveals an impact on the magnitude of power perturbation, but not whether the change is positive or negative. Predictions from the boundary-layer approach are validated with power measurements from large-eddy simulations. Full article
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Open AccessArticle A DC Microgrid Coordinated Control Strategy Based on Integrator Current-Sharing
Energies 2017, 10(8), 1116; https://doi.org/10.3390/en10081116
Received: 10 July 2017 / Revised: 23 July 2017 / Accepted: 25 July 2017 / Published: 1 August 2017
Cited by 6 | PDF Full-text (4429 KB) | HTML Full-text | XML Full-text
Abstract
The DC microgrid has become a new trend for microgrid study with the advantages of high reliability, simple control and low losses. With regard to the drawbacks of the traditional droop control strategies, an improved DC droop control strategy based on integrator current-sharing
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The DC microgrid has become a new trend for microgrid study with the advantages of high reliability, simple control and low losses. With regard to the drawbacks of the traditional droop control strategies, an improved DC droop control strategy based on integrator current-sharing is introduced. In the strategy, the principle of eliminating deviation through an integrator is used, constructing the current-sharing term in order to make the power-sharing between different distributed generation (DG) units uniform and reasonable, which can reduce the circulating current between DG units. Furthermore, at the system coordinated control level, a hierarchical/droop control strategy based on the DC bus voltage is proposed. In the strategy, the operation modes of the AC main network and micro-sources are determined through detecting the DC voltage variation, which can ensure the power balance of the DC microgrid under different operating conditions. Meanwhile, communication is not needed between different DG units, while each DG unit needs to sample the DC bus voltage, which retains the plug-and-play feature of the DC microgrid. The proposed control strategy is validated by simulation on a DC microgrid with permanent magnet synchronous generator-based wind turbines, solar arrays and energy storage batteries, which can be applied to small commercial or residential buildings. Full article
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Open AccessArticle Optimal Dynamic Analysis of Electrical/Electronic Components in Wind Turbines
Energies 2017, 10(8), 1111; https://doi.org/10.3390/en10081111
Received: 23 May 2017 / Revised: 28 June 2017 / Accepted: 18 July 2017 / Published: 31 July 2017
Cited by 4 | PDF Full-text (3931 KB) | HTML Full-text | XML Full-text
Abstract
Electrical and electronic components are very important subcomponents in modern industrial wind turbines. Complex multimegawatt wind turbines are continuously being installed both onshore and offshore, continuously increasing the demand for sophisticated electronic and electrical components. In this work, most critical electrical and electronic
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Electrical and electronic components are very important subcomponents in modern industrial wind turbines. Complex multimegawatt wind turbines are continuously being installed both onshore and offshore, continuously increasing the demand for sophisticated electronic and electrical components. In this work, most critical electrical and electronic components in industrial wind turbines have been identified and the applicability of appropriate condition monitoring processes simulated. A fault tree dynamic analysis has been carried out by binary decision diagrams to obtain the system failure probability over time and using different time increments to evaluate the system. This analysis allows critical electrical and electronic components of the converters to be identified in different conditions. The results can be used to develop a scheduled maintenance that improves the decision making and reduces the maintenance costs. Full article
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Open AccessArticle A New Miniature Wind Turbine for Wind Tunnel Experiments. Part II: Wake Structure and Flow Dynamics
Energies 2017, 10(7), 923; https://doi.org/10.3390/en10070923
Received: 10 May 2017 / Revised: 24 June 2017 / Accepted: 29 June 2017 / Published: 4 July 2017
Cited by 3 | PDF Full-text (2697 KB) | HTML Full-text | XML Full-text
Abstract
An optimized three-bladed horizontal-axis miniature wind turbine, called WiRE-01, with the rotor diameter of 15 cm is designed and fully characterized in Part I of this study. In the current part of the study, we investigate the interaction of the turbine with a
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An optimized three-bladed horizontal-axis miniature wind turbine, called WiRE-01, with the rotor diameter of 15 cm is designed and fully characterized in Part I of this study. In the current part of the study, we investigate the interaction of the turbine with a turbulent boundary layer. The comparison of the spectral density of the thrust force and the one of the incoming velocity revealed new insights on the use of turbine characteristics to estimate incoming flow conditions. High-resolution stereoscopic particle image-velocimetry (S-PIV) measurements were also performed in the wake of the turbine operating at optimal conditions. Detailed information on the velocity and turbulence structure of the turbine wake is presented and discussed, which can serve as a complete dataset for the validation of numerical models. The PIV data are also used to better understand the underlying mechanisms leading to unsteady loads on a downstream turbine at different streamwise and spanwise positions. To achieve this goal, a new method is developed to quantify and compare the effect of both turbulence and mean shear on the moment of the incoming momentum flux for a hypothetical turbine placed downstream. The results show that moment fluctuations caused by turbulence are bigger under full-wake conditions, whereas those caused by mean shear are clearly dominant under partial-wake conditions. Especial emphasis is also placed on how the mean wake flow distribution is affected by wake meandering. Conditional averaging based on the instantaneous position of the wake center revealed that when the wake meanders laterally to one side, a high-speed region exists on the opposite side. The results show that, due to this high-speed region, large lateral meandering motions do not lead to the expansion of the mean wake cross-section in the lateral direction. Full article
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Open AccessArticle A New Miniature Wind Turbine for Wind Tunnel Experiments. Part I: Design and Performance
Energies 2017, 10(7), 908; https://doi.org/10.3390/en10070908
Received: 10 May 2017 / Revised: 24 June 2017 / Accepted: 28 June 2017 / Published: 3 July 2017
Cited by 2 | PDF Full-text (1423 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Miniature wind turbines, employed in wind tunnel experiments to study the interaction of turbines with turbulent boundary layers, usually suffer from poor performance with respect to their large-scale counterparts in the field. Moreover, although wakes of wind turbines have been extensively examined in
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Miniature wind turbines, employed in wind tunnel experiments to study the interaction of turbines with turbulent boundary layers, usually suffer from poor performance with respect to their large-scale counterparts in the field. Moreover, although wakes of wind turbines have been extensively examined in wind tunnel studies, the proper characterization of the performance of wind turbines has received relatively less attention. In this regard, the present study concerns the design and the performance analysis of a new three-bladed horizontal-axis miniature wind turbine with a rotor diameter of 15 cm. Due to its small size, this turbine, called WiRE-01, is particularly suitable for studies of wind farm flows and the interaction of the turbine with an incoming boundary-layer flow. Especial emphasis was placed on the accurate measurement of the mechanical power extracted by the miniature turbine from the incoming wind. In order to do so, a new setup was developed to directly measure the torque of the rotor shaft. Moreover, to provide a better understanding on the connection between the mechanical and electrical aspects of miniature wind turbines, the performance of three different direct-current (DC) generators was studied. It is found that electrical outputs of the tested generators can be used to provide a rather acceptable estimation of the mechanical input power. Force and power measurements showed that the thrust and power coefficients of the miniature turbine can reach 0.8 and 0.4 , respectively, which are close to the ones of large-scale turbines in the field. In Part II of this study, the wake structure and dynamic flow characteristics are studied for the new miniature turbine immersed in a turbulent boundary-layer flow. Full article
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Open AccessArticle Wind Turbine Synchronous Reset Pitch Control
Energies 2017, 10(6), 770; https://doi.org/10.3390/en10060770
Received: 6 April 2017 / Revised: 22 May 2017 / Accepted: 27 May 2017 / Published: 1 June 2017
Cited by 1 | PDF Full-text (1696 KB) | HTML Full-text | XML Full-text
Abstract
Reset controllers are commonly used to smooth the transient response of systems. We use this technique to improve a standard baseline pitch controller for offshore wind turbines (WTs). The introduction of this strategy enhances the overall performance of the WT. In particular, the
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Reset controllers are commonly used to smooth the transient response of systems. We use this technique to improve a standard baseline pitch controller for offshore wind turbines (WTs). The introduction of this strategy enhances the overall performance of the WT. In particular, the fore-aft and side-to-side accelerations of the WT tower are significantly reduced, whilst a steadier power output is obtained, in comparison to the standard baseline pitch controller. Furthermore, our designed pitch control’s main advantage, with respect to the baseline, is its ease of implementation and reduced complexity as it does not require a gain-scheduling technique, nor pitch position measurement (thus, it is insensitive to pitch sensor faults). The proposed approach has been simulated on the NREL 5-MW prototype offshore turbine model, mounted on a jacket support. The simulations are carried out using the aero-hydro-servo-elastic simulator FAST, and key observations are thoroughly discussed. Full article
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Open AccessArticle Sliding Mode Control of a Variable- Speed Wind Energy Conversion System Using a Squirrel Cage Induction Generator
Energies 2017, 10(5), 604; https://doi.org/10.3390/en10050604
Received: 25 January 2017 / Revised: 19 April 2017 / Accepted: 19 April 2017 / Published: 1 May 2017
Cited by 4 | PDF Full-text (657 KB) | HTML Full-text | XML Full-text
Abstract
This paper deals with the control of a variable-speed wind energy conversion (WEC) system using a squirrel cage induction generator (SCIG) connected to the grid through a back-to-back three phase (AC-DC-AC) power converter. The sliding mode control technique is used to control the
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This paper deals with the control of a variable-speed wind energy conversion (WEC) system using a squirrel cage induction generator (SCIG) connected to the grid through a back-to-back three phase (AC-DC-AC) power converter. The sliding mode control technique is used to control the WEC system. The objective of the controllers is to force the states of the system to track their desired states. One controller is used to regulate the generator speed and the flux so that maximum power is extracted from the wind. Another controller is used to control the grid side converter, which controls the DC bus voltage and the active and reactive powers injected into the grid. The performance of the controlled wind energy conversion system is verified through MATLAB simulations, which show that the controlled system performs well. Full article
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Open AccessArticle Automatic Tracking of the Modal Parameters of an Offshore Wind Turbine Drivetrain System
Energies 2017, 10(4), 574; https://doi.org/10.3390/en10040574
Received: 20 February 2017 / Revised: 13 April 2017 / Accepted: 19 April 2017 / Published: 22 April 2017
Cited by 1 | PDF Full-text (4761 KB) | HTML Full-text | XML Full-text
Abstract
An offshore wind turbine (OWT) is a complex structure that consists of different parts (e.g., foundation, tower, drivetrain, blades, et al.). The last decade, there has been continuous trend towards larger machines with the goal of cost reduction. Modal behavior is an important
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An offshore wind turbine (OWT) is a complex structure that consists of different parts (e.g., foundation, tower, drivetrain, blades, et al.). The last decade, there has been continuous trend towards larger machines with the goal of cost reduction. Modal behavior is an important design aspect. For tackling noise, vibration, and harshness (NVH) issues and validating complex simulation models, it is of high interest to continuously track the vibration levels and the evolution of the modal parameters (resonance frequencies, damping ratios, mode shapes) of the fundamental modes of the turbine. Wind turbines are multi-physical machines with significant interaction between their subcomponents. This paper will present the possibility of identifying and automatically tracking the structural vibration modes of the drivetrain system of an instrumented OWT by using signals (e.g., acceleration responses) measured on the drivetrain system. The experimental data has been obtained during a measurement campaign on an OWT in the Belgian North Sea where the OWT was in standstill condition. The drivetrain, more specifically the gearbox and generator, is instrumented with a dedicated measurement set-up consisting of 17 sensor channels with the aim to continuously track the vibration modes. The consistency of modal parameter estimates made at consequent 10-min intervals is validated, and the dominant drivetrain modal behavior is identified and automatically tracked. Full article
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Open AccessArticle Fatigue Reliability Analysis of Wind Turbine Cast Components
Energies 2017, 10(4), 466; https://doi.org/10.3390/en10040466
Received: 14 December 2016 / Revised: 23 February 2017 / Accepted: 28 March 2017 / Published: 2 April 2017
PDF Full-text (1714 KB) | HTML Full-text | XML Full-text
Abstract
The fatigue life of wind turbine cast components, such as the main shaft in a drivetrain, is generally determined by defects from the casting process. These defects may reduce the fatigue life and they are generally distributed randomly in components. The foundries, cutting
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The fatigue life of wind turbine cast components, such as the main shaft in a drivetrain, is generally determined by defects from the casting process. These defects may reduce the fatigue life and they are generally distributed randomly in components. The foundries, cutting facilities and test facilities can affect the verification of properties by testing. Hence, it is important to have a tool to identify which foundry, cutting and/or test facility produces components which, based on the relevant uncertainties, have the largest expected fatigue life or, alternatively, have the largest reliability to be used for decision-making if additional cost considerations are added. In this paper, a statistical approach is presented based on statistical hypothesis testing and analysis of covariance (ANCOVA) which can be applied to compare different groups (manufacturers, suppliers, test facilities, etc.) and to quantify the relevant uncertainties using available fatigue tests. Illustrative results are presented as obtained by statistical analysis of a large set of fatigue data for casted test components typically used for wind turbines. Furthermore, the SN curves (fatigue life curves based on applied stress) for fatigue assessment are estimated based on the statistical analyses and by introduction of physical, model and statistical uncertainties used for the illustration of reliability assessment. Full article
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Open AccessArticle Power Loss Analysis for Wind Power Grid Integration Based on Weibull Distribution
Energies 2017, 10(4), 463; https://doi.org/10.3390/en10040463
Received: 3 January 2017 / Revised: 28 March 2017 / Accepted: 29 March 2017 / Published: 2 April 2017
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Abstract
The growth of electrical demand increases the need of renewable energy sources, such as wind energy, to meet that need. Electrical power losses are an important factor when wind farm location and size are selected. The capitalized cost of constant power losses during
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The growth of electrical demand increases the need of renewable energy sources, such as wind energy, to meet that need. Electrical power losses are an important factor when wind farm location and size are selected. The capitalized cost of constant power losses during the life of a wind farm will continue to high levels. During the operation period, a method to determine if the losses meet the requirements of the design is significantly needed. This article presents a Simulink simulation of wind farm integration into the grid; the aim is to achieve a better understanding of wind variation impact on grid losses. The real power losses are set as a function of the annual variation, considering a Weibull distribution. An analytical method has been used to select the size and placement of a wind farm, taking into account active power loss reduction. It proposes a fast linear model estimation to find the optimal capacity of a wind farm based on DC power flow and graph theory. The results show that the analytical approach is capable of predicting the optimal size and location of wind turbines. Furthermore, it revealed that the annual variation of wind speed could have a strong effect on real power loss calculations. In addition to helping to improve utility efficiency, the proposed method can develop specific designs to speeding up integration of wind farms into grids. Full article
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Open AccessFeature PaperArticle Wind Turbine Power Curve Design for Optimal Power Generation in Wind Farms Considering Wake Effect
Energies 2017, 10(3), 395; https://doi.org/10.3390/en10030395
Received: 2 December 2016 / Revised: 14 March 2017 / Accepted: 15 March 2017 / Published: 20 March 2017
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Abstract
In modern wind farms, maximum power point tracking (MPPT) is widely implemented. Using the MPPT method, each individual wind turbine is controlled by its pitch angle and tip speed ratio to generate the maximum active power. In a wind farm, the upstream wind
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In modern wind farms, maximum power point tracking (MPPT) is widely implemented. Using the MPPT method, each individual wind turbine is controlled by its pitch angle and tip speed ratio to generate the maximum active power. In a wind farm, the upstream wind turbine may cause power loss to its downstream wind turbines due to the wake effect. According to the wake model, downstream power loss is also determined by the pitch angle and tip speed ratio of the upstream wind turbine. By optimizing the pitch angle and tip speed ratio of each wind turbine, the total active power of the wind farm can be increased. In this paper, the optimal pitch angle and tip speed ratio are selected for each wind turbine by the exhausted search. Considering the estimation error of the wake model, a solution to implement the optimized pitch angle and tip speed ratio is proposed, which is to generate the optimal control curves for each individual wind turbine off-line. In typical wind farms with regular layout, based on the detailed analysis of the influence of pitch angle and tip speed ratio on the total active power of the wind farm by the exhausted search, the optimization is simplified with the reduced computation complexity. By using the optimized control curves, the annual energy production (AEP) is increased by 1.03% compared to using the MPPT method in a case-study of a typical eighty-turbine wind farm. Full article
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Open AccessArticle Wind Farm Wake: The 2016 Horns Rev Photo Case
Energies 2017, 10(3), 317; https://doi.org/10.3390/en10030317
Received: 22 December 2016 / Revised: 24 February 2017 / Accepted: 27 February 2017 / Published: 7 March 2017
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Abstract
Offshore wind farm wakes were observed and photographed in foggy conditions at Horns Rev 2 on 25 January 2016 at 12:45 UTC. These new images show highly contrasting conditions regarding the wind speed, turbulence intensity, atmospheric stability, weather conditions and wind farm wake
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Offshore wind farm wakes were observed and photographed in foggy conditions at Horns Rev 2 on 25 January 2016 at 12:45 UTC. These new images show highly contrasting conditions regarding the wind speed, turbulence intensity, atmospheric stability, weather conditions and wind farm wake development as compared to the Horns Rev 1 photographs from 12 February 2008. The paper examines the atmospheric conditions from satellite images, radiosondes, lidar and wind turbine data and compares the observations to results from atmospheric meso-scale modelling and large eddy simulation. Key findings are that a humid and warm air mass was advected from the southwest over cold sea and the dew-point temperature was such that cold-water advection fog formed in a shallow layer. The flow was stably stratified and the freestream wind speed was 13 m/s at hub height, which means that most turbines produced at or near rated power. The wind direction was southwesterly and long, narrow wakes persisted several rotor diameters downwind of the wind turbines. Eventually mixing of warm air from aloft dispersed the fog in the far wake region of the wind farm. Full article
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2016

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Open AccessArticle Experimental and Simulation Studies of Strength and Fracture Behaviors of Wind Turbine Bearing Steel Processed by High Pressure Torsion
Energies 2016, 9(12), 1033; https://doi.org/10.3390/en9121033
Received: 3 November 2016 / Revised: 30 November 2016 / Accepted: 2 December 2016 / Published: 8 December 2016
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Abstract
White structure flaking (WSF) has been found to be one of the failure modes in bearing steels under rolling contacts through the formation of cracks associated with a microstructural change called white etching area (WEA). In the present research, the effects of the
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White structure flaking (WSF) has been found to be one of the failure modes in bearing steels under rolling contacts through the formation of cracks associated with a microstructural change called white etching area (WEA). In the present research, the effects of the high-pressure torsion (HPT) process on the microstructure and mechanical properties of an AISI 52100 alloy are studied. An annealed AISI 52100 was subjected to high-pressure torsion at room temperature under a pressure of up to ~6 GPa for up to three turns. Finite-element modeling (FEM) was used to simulate the process under high-pressure torsion and quasi-constrained conditions to reveal the material property changes occurring in HPT. Scanning electron microscopy and microhardness testing after processing were used to investigate the microstructural and mechanical property evolution of the steel. Strain induced microstructural transformations occur and affect the mechanical properties in a similar way to the well-known white etching area (WEA) found beneath the surface of wind turbine bearings. Here, HPT is used to study the feasibility of creating microstructural changes that are similar to WEA. This paper presents the preliminary results of using HPT to produce WEAs. Full article
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Open AccessArticle Inverse Aerodynamic Optimization Considering Impacts of Design Tip Speed Ratio for Variable-Speed Wind Turbines
Energies 2016, 9(12), 1023; https://doi.org/10.3390/en9121023
Received: 5 September 2016 / Revised: 23 November 2016 / Accepted: 28 November 2016 / Published: 3 December 2016
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Abstract
Because of the slow dynamic behavior of the large-inertia wind turbine rotor, variable-speed wind turbines (VSWTs) are actually unable to keep operating at the design tip speed ratio (TSR) during the maximum power point tracking (MPPT) process. Moreover, it has been pointed out
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Because of the slow dynamic behavior of the large-inertia wind turbine rotor, variable-speed wind turbines (VSWTs) are actually unable to keep operating at the design tip speed ratio (TSR) during the maximum power point tracking (MPPT) process. Moreover, it has been pointed out that although a larger design TSR can increase the maximum power coefficient, it also greatly prolongs the MPPT process of VSWTs. Consequently, turbines spend more time operating at the off-design TSRs and the wind energy capture efficiency is decreased. Therefore, in the inverse aerodynamic design of VSWTs, the static aerodynamic performance (i.e., the maximum power coefficient) and the dynamic process of MPPT should be comprehensively modeled for determining an appropriate design TSR. In this paper, based on the inverse design method, an aerodynamic optimization method for VSWTs, fully considering the impacts of the design TSR on the static and dynamic behavior of wind turbines is proposed. In this method, to achieve higher wind energy production, the design TSR, chord length and twist angle are jointly optimized, which is structurally different from the conventional separated design procedure. Finally, the effectiveness of the proposed method is validated by simulation results based on the Bladed software. Full article
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