| 1. |
- Ahlström, Anders
(författare)
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Emergency stop simulation using a finite element model developed for large blade deflections
- 2006
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Ingår i: Wind Energy. - : Wiley. - 1095-4244 .- 1099-1824. ; 9:3, s. 193-210
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Tidskriftsartikel (refereegranskat)abstract
- Predicting the load in every possible situation is necessary in order to build safe and optimized structures. A highly dynamical case where large loads are developed is an emergency stop. Design simulation tools that can cope with the upcoming non-linearities will be especially important as the turbines get bigger and more flexible. The model developed here uses the advanced commercial finite element system MSC.Marc, focused on non-linear design and analysis, to predict the structural response. The aerodynamic model named AERFORCE, used to transform the wind to loads on the blades, is a blade element momentum model. A comparison is made between measured and calculated loads for the Tjaere-borg wind turbine during emergency braking of the rotor. The simulation results correspond well with measured data. The conclusion is that the aeroelastic tool is likely to perform well when simulating more flexible turbines.
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| 2. |
- Ahlström, Anders
(författare)
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Influence of wind turbine flexibility on loads and power production
- 2006
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Ingår i: Wind Energy. - : Wiley. - 1095-4244 .- 1099-1824. ; 9:3, s. 237-249
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Tidskriftsartikel (refereegranskat)abstract
- Most aeroelastic codes used today assume small blade deflections and application of loads on the undeflected structure. However, with the design of lighter and more flexible wind turbines, this assumption is not obvious. By scaling the system mass and stiffness properties equally, it is possible to compare wind turbines of different degrees of slenderness and at the same time keep system frequencies the some in an undeformed state. The developed model uses the commercial finite element system MSC. Marc, focused on non-linear design and analysis, to predict the structural response. The aerodynamic model AERFORCE, used to transform the wind to loads on the blades, is a blade element momentum model. A comparison is made between different slenderness ratios in three wind conditions below rated wind speed. The results show that large blade deflections have a major influence on power production and the resulting structural loads and must be considered in the design of very slender turbines.
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| 3. |
- Aihara, Aya, 1989-, et al.
(författare)
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A numerical study of strut and tower influence on the performance of vertical axis wind turbines using computational fluid dynamics simulation
- 2022
-
Ingår i: Wind Energy. - : John Wiley & Sons. - 1095-4244 .- 1099-1824. ; 25:5, s. 897-913
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents the influence of the strut and the tower on the aerodynamic force of the blade for the vertical axis wind turbine (VAWT). It has been known that struts degrade the performance of VAWTs due to the inherent drag losses. In this study, three-dimensional Reynolds-averaged Navier-Stokes simulations have been conducted to investigate the effect of the strut and the tower on the flow pattern around the rotor region, the blade force distribution, and the rotor performance. A comparison has been made for three different cases where only the blade; both the blade and the strut; and all of the blade, the strut, and the tower are considered. A 12-kW three-bladed H-rotor VAWT has been studied for tip speed ratio of 4.16. This ratio is relatively high for this turbine, so the influence of the strut is expected to be crucial. The numerical model has been validated first for a single pitching blade and full VAWTs. The simulations show distinguished differences in the force distribution along the blade between two cases with and without struts. Since the wake from the struts interacts with the blades, the tangential force is reduced especially in the downwind side when the struts are considered. The calculated power coefficient is decreased by 43 %, which shows the importance of modeling the strut effect properly for accurate prediction of the turbine performance. The simulations also indicate that including the tower does not yield significant difference in the force distribution and the rotor power.
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| 4. |
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| 5. |
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| 6. |
- Bangalore, Pramod, 1983, et al.
(författare)
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An artificial neural network based condition monitoring method for wind turbines, with application to the monitoring of the gearbox
- 2017
-
Ingår i: Wind Energy. - : Wiley. - 1099-1824 .- 1095-4244. ; 20:8, s. 1421-1438
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Tidskriftsartikel (refereegranskat)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.
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| 7. |
- Barthelmie, Rebecca, et al.
(författare)
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ENDOW (efficient development of offshore wind farms): modelling wake and boundary layer interactions
- 2004
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Ingår i: Wind Energy. - : Wiley. - 1095-4244 .- 1099-1824. ; 7, s. 225-245
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Tidskriftsartikel (refereegranskat)abstract
- While experience gained through the offshore wind energy projects currently operating is valuable, a major uncertainty in estimating power production lies in the prediction of the dynamic links between the atmosphere and wind turbines in offshore regimes. The objective of the ENDOW project was to evaluate, enhance and interface wake and boundary layer models for utilization offshore. The project resulted in a significant advance in the state of the art in both wake and marine boundary layer models, leading to improved prediction of wind speed and turbulence profiles within large offshore wind farms. Use of new databases from existing offshore wind farms and detailed wake profiles collected using sodar provided a unique opportunity to undertake the first comprehensive evaluation of wake models in the offshore environment. The results of wake model performance in different wind speed, stability and roughness conditions relative to observations provided criteria for their improvement. Mesoscale model simulations were used to evaluate the impact of thermal flows, roughness and topography on offshore wind speeds. The model hierarchy developed under ENDOW forms the basis of design tools for use by wind energy developers and turbine manufacturers to optimize power output from offshore wind farms through minimized wake effects and optimal grid connections. The design tools are being built onto existing regional-scale models and wind farm design software which was developed with EU funding and is in use currently by wind energy developers.
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| 8. |
- Bergström, Hans, et al.
(författare)
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A Study of Valley Winds Using the MIUU Meso-scale Model
- 2006
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Ingår i: Wind Energy. - : Wiley. - 1095-4244 .- 1099-1824. ; 9, s. 109-129
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Tidskriftsartikel (refereegranskat)abstract
- High winds are sometimes found in low-altitude terrain in mountain valleys. The reasons for this and the possibilities to find such sites from modelled wind fields are investigated. The higher-order closure MIUU model developed at Uppsala University is used for a general study of channelling of winds in mountain valleys. The importance of length, width and depth of the valley is investigated. Comparisons are made with wind measurements.
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| 9. |
- Bollen, M. H. J., et al.
(författare)
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Voltage dips at the terminals of wind power installations
- 2005
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Ingår i: Wind Energy. - : Wiley. - 1099-1824 .- 1095-4244. ; 8:3, s. 307-318
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Tidskriftsartikel (refereegranskat)abstract
- This article gives an overview of the kind of voltage dips that can be expected at the terminals of a wind power installation. The overview is based on the study of those dips at the terminals of industrial installations and provides a guideline for the testing of wind power installations against voltage dips. For voltage dips due to faults, a classification into different types is presented. Five types appear at the terminals of sensitive equipment and thus have to be included when testing the wind power installation against disturbances coming from the grid. A distinction is made between installations connected at transmission level and those connected at distribution level. For the latter the phase angle jump has to be considered. Dips due to other causes (motor, transformer and capacitor switching) are briefly discussed as well as the voltage recovery after a dip. Finally some thoughts are presented on the way in which voltage tolerance requirements should be part of the design process for wind power installations. Copyright
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| 10. |
- Chougule, A., et al.
(författare)
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Spectral tensor parameters for wind turbine load modeling from forested and agricultural landscapes
- 2015
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Ingår i: Wind Energy. - : John Wiley & Sons. - 1095-4244 .- 1099-1824. ; 18:3, s. 469-481
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Tidskriftsartikel (refereegranskat)abstract
- A velocity spectral tensor model was evaluated from the single-point measurements of wind speed. The model contains three parameters representing the dissipation rate of specific turbulent kinetic energy, a turbulence length scale and the turbulence anisotropy. Sonic anemometer measurements taken over a forested and an agricultural landscape were used to calculate the model parameters for neutral, slightly stable and slightly unstable atmospheric conditions for a selected wind speed interval. The dissipation rate above the forest was nine times that at the agricultural site. No significant differences were observed in the turbulence length scales between the forested and agricultural areas. Only a small difference was observed in the turbulence anisotropy at the two sites, except near the surface, where the forest turbulence was more isotropic. The turbulence anisotropy remained more or less constant with height at the forest site, whereas the turbulence became more isotropic with height for the agricultural site. Using the three parameters as inputs, we quantified the performance of the model in coherence predictions for vertical separations. The model coherence of all the three velocity components was overestimated for the analyzed stability classes at both sites. As expected from the model approximations, the model performed better at both sites for neutral stability than slightly stable and unstable conditions. The model prediction of coherence of the along-wind and vertical components was better than that of the cross-wind component. No significant difference was found between the performance of the model at the forested and the agricultural areas.
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| 11. |
- Conrady, Kristina, et al.
(författare)
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Impact of low-level wind maxima below hub height on wind turbine sound propagation
- 2020
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Ingår i: Wind Energy. - : John Wiley and Sons Ltd. - 1095-4244 .- 1099-1824. ; 23:8, s. 1767-1775
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Tidskriftsartikel (refereegranskat)abstract
- An analysis of the effect of low-level wind maxima (LLWM) below hub height on sound propagating from wind turbines has been performed at a site in northern Sweden. The stably stratified boundary layer, which is typical for cold climates, commonly features LLWM. The simplified concept for the effects of refraction, based on the logarithmic wind profile or other approaches where the wind speed is continuously increasing with height, is often not applicable there. Long-term meteorological measurements in the vicinity of a wind farm were therefore used to identify LLWM. Sound measurements were conducted simultaneously to the meteorological measurements. LLWM below hub height decrease the sound level close to the surface downwind of the wind farm. This effect increases with increasing strength of the LLWM. The occurrence of LLWM as well as strength and height of the LLWM are dependent on the wind direction.
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| 12. |
- Conrady, Kristina, et al.
(författare)
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Impact of snow on sound propagating from wind turbines
- 2018
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Ingår i: Wind Energy. - : Wiley. - 1095-4244 .- 1099-1824. ; 21:12, s. 1282-1295
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Tidskriftsartikel (refereegranskat)abstract
- The impact of snow on sound propagating from a wind farm in northern Sweden has been investigated. Simultaneous acoustic and meteorological measurements, combined with daily snow observations, have been analysed for the snow season in 2013 to 2014. Such measurements are crucial since significant knowledge gaps exist, especially for conditions in cold climates, in the implementation of atmospheric boundary layer complexity in sound propagation models. The effect of snow on sound propagation is shown to be dependent on the snow quality. Moreover, snow on trees (upplega) also has an influence on sound propagation. Compared with conditions without snow on trees, the average sound level is approximately 2 dBA lower. The effect is more distinct for higher frequencies compared with lower frequencies.
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| 13. |
- Cui, Yue, et al.
(författare)
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A fault detection framework using RNNs for condition monitoring of wind turbines
- 2021
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Ingår i: Wind Energy. - : John Wiley & Sons. - 1095-4244 .- 1099-1824.
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Tidskriftsartikel (refereegranskat)abstract
- This paper proposes a fault detection framework for the condition monitoring of wind turbines. The framework models and analyzes the data in supervisory control and data acquisition systems. For log information, each event is mapped to an assembly based on the IEA Reliawind taxonomy. For operation data, recurrent neural networks are applied to model normal behaviors, which can learn the long-time temporal dependencies between various time series. Based on the estimation results, a two-stage threshold method is proposed to determine the current operation status. The method evaluates the shift values deviating from the estimated behaviors and their duration time to attenuate the effect of minor fluctuations. The generated results from the framework can help to understand when the turbine deviates from normal operations. The framework is validated with the data from an onshore wind park. The numerical results show that the framework can detect operational risks and reduce false alarms.
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| 14. |
- Ebenhoch, Raphael, et al.
(författare)
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A linearized numerical model of wind-farm flows
- 2017
-
Ingår i: Wind Energy. - : John Wiley & Sons. - 1095-4244 .- 1099-1824. ; 20:5, s. 859-875
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Tidskriftsartikel (refereegranskat)abstract
- A fast and reasonably accurate numerical three-dimensional wake model able to predict the flow behaviour of a wind farm over a flat terrain has been developed. The model is based on the boundary-layer approximation of the Navier-Stokes equations, linearized around the incoming atmospheric boundary layer, with the assumption that the wind turbines provide a small perturbation to the velocity field. The linearization of the actuator-disc theory brought additional insights that could be used to understand the behaviour, as well as the limitations, of a flow model based on linear methods: for instance, it is shown that an adjustment of the turbine's thrust coefficient is necessary in order to obtain the same wake velocity field provided by the actuator disc theory within the used linear framework. The model is here validated against two independent wind-tunnel campaigns with a small and a large wind farm aimed at the characterization of the flow above and upstream of the farms, respectively. The developed model is, in contrary to current engineering wake models, able to account for effects occurring in the upstream flow region, thereby including more physical mechanisms than other simplified approaches. The conducted simulations (in agreement with the measurement results) show that the presence of a wind farm affects the approaching flow far more upstream than generally expected and definitely beyond the current industrial standards. Despite the model assumptions, several velocity statistics above wind farms have been properly estimated providing an insight into the transfer of momentum inside the turbine rows.
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| 15. |
- Eriksson, Ola, 1979-, et al.
(författare)
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Large-eddy simulations of the evolution of imposed turbulence in forced boundary layers in a very long domain
- 2020
-
Ingår i: Wind Energy. - : Wiley. - 1095-4244 .- 1099-1824. ; 23:6, s. 1482-1493
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Tidskriftsartikel (refereegranskat)abstract
- The technique of using imposed turbulence in combination with a forced boundary layer in order to model the atmospheric boundary layer is analyzed for a very long domain using large-eddy simulations with different combinations of prescribed velocity profiles and pregenerated turbulence fields based on the Mann model. The ambient flow is first studied in the absence of wind turbines. The velocity profiles undergo a transition throughout the domain with a velocity increase of 10% to 15% close to the ground far downstream in the domain. The turbulence characteristics close to the turbulence plane are, as expected, similar to those of the added Mann turbulence. The turbulence will then undergo a transition throughout the domain to finally reach a balance with the shear profile at a certain downstream distance. This distance is found to depend on the turbulence level of the added Mann turbulence planes. A lower Mann turbulence level generally results in a shorter "balancing" distance. Secondly, a row of 10 turbines is imposed in the simulations at different distances from the plane of turbulence in order to determine how the distance affects wake conditions and power production levels. Our results show that a "balancing" distance is needed between the turbulence plane and the first turbine in the row in order to ensure nonchanging ambient conditions throughout the turbine row. This introduces an increase in the computational costs. The computational cost for the forced boundary technique is normally lower compared with using precursor simulations, for longer domains; however, this needs to be verified further.
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| 16. |
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| 17. |
- Guth, Stephen, et al.
(författare)
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Statistical modeling of fully nonlinear hydrodynamic loads on offshore wind turbine monopile foundations using wave episodes and targeted CFD simulations through active sampling
- 2024
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Ingår i: Wind Energy. - : John Wiley & Sons. - 1095-4244 .- 1099-1824. ; 27:1, s. 75-100
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Tidskriftsartikel (refereegranskat)abstract
- Accurately determining hydrodynamic force statistics is crucial for designing offshore engineering structures, including offshore wind turbine foundations, due to the significant impact of nonlinear wave-structure interactions. However, obtaining precise load statistics often involves computationally intensive simulations. Furthermore, the estimation of statistics using current practices is subject to ongoing discussion due to the inherent uncertainty involved. To address these challenges, we present a novel machine learning framework that leverages data-driven surrogate modeling to predict hydrodynamic loads on monopile foundations while reducing reliance on costly simulations and facilitate the load statistics reconstruction. The primary advantage of our approach is the significant reduction in evaluation time compared to traditional modeling methods. The novelty of our framework lies in its efficient construction of the surrogate model, utilizing the Gaussian process regression machine learning technique and a Bayesian active learning method to sequentially sample wave episodes that contribute to accurate predictions of extreme hydrodynamic forces. Additionally, a spectrum transfer technique combines computational fluid dynamics (CFD) results from both quiescent and extreme waves, further reducing data requirements. This study focuses on reducing the dimensionality of stochastic irregular wave episodes and their associated hydrodynamic force time series. Although the dimensionality reduction is linear, Gaussian process regression successfully captures high-order correlations. Furthermore, our framework incorporates built-in uncertainty quantification capabilities, facilitating efficient parameter sampling using traditional CFD tools. This paper provides comprehensive implementation details and demonstrates the effectiveness of our approach in delivering reliable statistics for hydrodynamic loads while overcoming the computational cost constraints associated with classical modeling methods.
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| 18. |
- Göransson, Lisa, 1982, et al.
(författare)
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A comparison of variation management strategies for wind power integration in different electricity system contexts
- 2018
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Ingår i: Wind Energy. - : Wiley. - 1099-1824 .- 1095-4244. ; 21:10, s. 837-854
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Tidskriftsartikel (refereegranskat)abstract
- Variation management strategies improve the capability of the electricity system to meet variations both in the electricity demand and in the generation that relies on variable energy sources. In this work, we introduce a new, functionality-based, categorization of variation management strategies: shifting (eg, batteries), absorbing (eg, power-to-gas), and complementing (dispatchable generation, including reservoir hydropower) strategies. A dispatch model with European coverage (EU-27 plus Norway and Switzerland) is applied to compare the benefits of shifting and absorbing strategies on wind integration in regions with different amounts of complementing strategies in place. The benefits are measured in terms of the wind value factor, wind owner revenue, and average short-term generation cost. The results of the modeling show that the reduction in average short-term generation cost and the increase in revenue earned by the wind owner from shifting strategies, such as the use of batteries, are more substantial at low wind shares than at high wind shares. The opposite situation is found for absorbing strategies, such as power-to-gas, which are found to be more efficient at reducing the average generation cost and increasing profit for the wind owner as the wind share increases. In regions that have access to complementing strategies in the form of reservoir hydropower, variation management has a weak ability to reduce the average short-term generation cost, although it can increase significantly the revenue accrued by the wind power owner.
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| 19. |
- Göransson, Lisa, 1982, et al.
(författare)
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Cost-optimized allocation of wind power investments: a Nordic-German perspective
- 2013
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Ingår i: Wind Energy. - : Wiley. - 1099-1824 .- 1095-4244. ; 16:4, s. 587-604
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Tidskriftsartikel (refereegranskat)abstract
- Using a linear cost minimization model with a 1h time resolution, we investigated the influence of geographic allocation of wind power on large-scale wind power investments, taking into account wind conditions, distance to load, and the nature of the power system in place (i.e. power generation and transmission capacities). We employed a hypothetical case in which a 20% wind power share of total electricity demand is applied to the NordicGerman power system. Free, i.e. geographically unrestricted, allocation of new wind power capacity is compared with a case in which national planning frameworks impose national limitations on wind power penetration levels. Given the cost assumptions made in the present work, the prospect of increasing the wind power capacity factor from 20 to 30% could motivate investments in transmission capacity from northern Scandinavia to continental Europe. The results obtained using the model show that the distribution of wind farms between regions with favorable wind conditions is dependent upon two factors: (i) the extent to which existing lines can be used to transmit the electricity that results from the new wind power and (ii) the correlation for wind power generation between the exporting region and the wind power generation already in place. In addition, the results indicate that there is little difference, i.e. just over 1%, in total yearly cost between the free allocation of new wind power and an allocation that complies with national planning frameworks. However, on a national level, there are significant differences with respect to investments in transmission and wind power capacities and the replacement of conventional power generation.
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| 20. |
- Göransson, Lisa, 1982, et al.
(författare)
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Large scale integration of wind power: moderating thermal power plant cycling
- 2011
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Ingår i: Wind Energy. - : Wiley. - 1099-1824 .- 1095-4244. ; 14:1, s. 91-105
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Tidskriftsartikel (refereegranskat)abstract
- Power plant cycling in thermal plants typically implies high costs and emissions. It is, therefore, important to find ways to reduce the influence of variations in wind power generation on these plants without forsaking large amounts of wind power. Using a unit commitment model, this work investigates the possibility to reduce variations by means of a moderator, such as a storage unit or import/export capacity. The relation between the reduction in CO2-emissions and the power rating of the moderator is investigated, as well as the benefit of a moderator which handles weekly variations compared with a moderator which has to be balanced on a daily basis. It is found that a daily balanced moderator yields a decrease in emissions of about 2% at 20% wind power grid penetration. The reduction in emissions is mainly due to an avoidance of start-up and part load emissions and a moderator of modest power rating is sufficient to achieve most of this decrease. In the case of a weekly balanced moderator, emissions are reduced as the moderator power rating increases. At 40% wind power grid penetration, a weekly balanced moderator reduces emissions with up to 11%. The major part of this reduction is due to the avoidance of wind power curtailment. The simulated benefit (CO2-emissions and costs) from adding a general moderator is compared with emissions from Life Cycle Assessment (LCA) studies and cost data of five available moderator technologies; transmission capacity, pumped hydro power, compressed air energy storage, flow batteries and sodium sulphur batteries. Copyright (C) 2010 John Wiley & Sons, Ltd.
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| 21. |
- Hodel, Henrik, 1995, et al.
(författare)
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Which wind turbine types are needed in a cost-optimal renewable energy system?
- 2024
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Ingår i: Wind Energy. - 1099-1824 .- 1095-4244. ; 27:6, s. 549-568
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Tidskriftsartikel (refereegranskat)abstract
- Previous research has indicated that wind power plants can be designed to have less-variable power generation, thereby mitigating the drop in economic value that typically occurs at high wind power penetration rates. This study investigates the competitiveness of adapted turbine design and the interplay with other flexibility measures, such as batteries and hydrogen storage, for managing variations. The analysis covers seven turbine designs for onshore and offshore wind generation, with different specific power ratings and hub heights. Various flexibility measures (batteries, hydrogen storage and transmission expansion) are included in the optimization of investment and dispatch of the electricity system of northern Europe. Three driving forces for turbine design selection are identified: (1) lowest cost of electricity generation; (2) annual wind production per land area and (3) improved generation profile of wind power. The results show that in regions with good wind resources and limited availability of variation management, it is cost-efficient to reduce the variability of wind power production by adapting the turbine design. This remains the case when variation management is available in the form of batteries, hydrogen storage and transmission system expansion. Moreover, it is more cost-effective to improve variability by changing the specific power rating rather than the turbine hub height.
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| 22. |
- Holttinen, Hannele, et al.
(författare)
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Impacts of large amounts of wind power on design and operation of power systems, results of IEA collaboration
- 2011
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Ingår i: Wind Energy. - : Wiley. - 1095-4244 .- 1099-1824. ; 14:2, s. 179-192
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Tidskriftsartikel (refereegranskat)abstract
- There are dozens of studies made and ongoing related to wind integration. However, the results are not easy to compare. IEA WIND R&D Task 25 on 'Design and Operation of Power Systems with Large Amounts of Wind Power' collects and shares information on wind generation impacts on power systems, with analyses and guidelines on methodologies. In the state-of-the-art report (October, 2007), and the final report of the 3 years period (July, 2009) the most relevant wind power grid integration studies have been analysed especially regarding methodologies and input data. Several issues that impact on the amount of wind power that can be integrated have been identified. Large balancing areas and aggregation benefits of wide areas help in reducing the variability and forecast errors of wind power as well as help in pooling more cost effective balancing resources. System operation and functioning electricity markets at less than day-ahead time scales help reduce forecast errors of wind power. Transmission is the key to aggregation benefits, electricity markets and larger balancing areas. Best practices in wind integration studies are described. There is also benefit when adding wind power to power systems: it reduces the total operating costs and emissions as wind replaces fossil fuels and this should be highlighted more in future studies.
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| 23. |
- Hyvärinen, Ann, et al.
(författare)
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A wind-tunnel study of the wake development behind wind turbines over sinusoidal hills
- 2018
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Ingår i: Wind Energy. - : John Wiley & Sons. - 1095-4244 .- 1099-1824. ; 21:8, s. 605-617
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Tidskriftsartikel (refereegranskat)abstract
- In the present work, the wake development behind small-scale wind turbines is studied when introducing local topography variations consisting of a series of sinusoidal hills. Additionally, wind-tunnel tests with homogeneous and sheared turbulent inflows were performed to understand how shear and ambient turbulence influence the results. The scale of the wind-turbine models was about 1000times smaller than full-size turbines, suggesting that the present results should only be qualitatively extrapolated to real-field scenarios. Wind-tunnel measurements were made by means of stereoscopic particle image velocimetry to characterize the flow velocity in planes perpendicular to the flow direction. Over flat terrain, the wind-turbine wake was seen to slowly approach the ground while it propagated downstream. When introducing hilly terrain, the downward wake deflection was enhanced in response to flow variations induced by the hills, and the turbulent kinetic energy content in the wake increased because of the speed-up seen over the hills. The combined wake observed behind 2 streamwise aligned turbines was more diffused and when introducing hills, it was more prone to deflect towards the ground compared to the wake behind an isolated turbine. Since wake interactions are common at sites with multiple turbines, this suggested that it is important to consider the local hill-induced velocity variations when onshore wind farms are analysed. Differences in the flow fields were seen when introducing either homogeneous or sheared turbulent inflow conditions, emphasizing the importance of accounting for the prevailing turbulence conditions at a given wind-farm site to accurately capture the downstream wake development.
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| 24. |
- Ivanell, Stefan, et al.
(författare)
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Actuator disc modelling of wake interaction in Horns Rev wind farm
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Ingår i: Wind Energy. - 1095-4244 .- 1099-1824.
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Large eddy simulations of the Navier-Stokes equations are performed tosimulate the Horns Rev off shore wind farm 15 km outside the Danish westcoast. The aim is to achieve a better understanding of the wake interactioninside the farm. The simulations are performed by combining the in-housedeveloped computer code EllipSys3D with the actuator-disc methodology. Inthe actuator-disc method the blades are represented by a disc at which bodyforces representing the aerodynamic loading are introduced. The body forcesare determined by computing local angles of attack and tabulated aerofoil coefficients.The advantage of using the actuator-disc technique is that it is notnecessary to resolve blade boundary layers since the computational resourcesare devoted to simulating the dynamics of the flow structures.In the present study approximately 13.6 million mesh points are used toresolve the wake structure in the park. The park contains 80 wind turbinesdistributed over an area of about 20km2. Since it is not possible to simulate allturbines, the 2 central columns of turbines have been simulated with periodicboundary conditions. This corresponds to an infinitely wide farm with 10turbines in downstream direction. Simulations were performed within plusminus 15 degrees of the turbine alignment, making the wide farm approximationreasonable.The results from the CFD simulations are evaluated and the downstreamevolution of the velocity field is depicted. Special interest is given to whatextent the production is dependent on the inflow angle and turbulence level.The study shows that the applied method captures the main productionvariation within the wind farm. The result further demonstrates that levelsof production correlate well with measurements. However, in some cases thevariation of the measurement data is caused by variation of measurement conditionswith inflow angles.
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| 25. |
- Ivanell, Stefan, et al.
(författare)
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Analysis of numerically generated wake structures
- 2009
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Ingår i: Wind Energy. - : Wiley. - 1095-4244 .- 1099-1824. ; 1, s. 63-80
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Tidskriftsartikel (refereegranskat)abstract
- Direct numerical simulations of the Navier-Stokes equations are performed to achieve a better understanding of the behaviour of wakes generated by wind turbines. The simulations are performed by combining the in-house developed computer code EllipSys3D with the actuator-line methodology. In the actuator-line method, the blades are represented by lines along which body forces representing the loading are introduced. The body forces are determined by computing local angles of attack and using tabulated aerofoil coefficients. The advantage of using the actuator-line technique is that it is not needed to resolve blade boundary layers and instead the computational resources are devoted to simulating the dynamics of the flow structures. In the present study, approximately 5 million mesh points are used to resolve the wake structure in a 120-degree domain behind the turbine. The results from the computational fluid dynamics (CFD) simulations are evaluated and the downstream evolution of the velocity field is depicted. Special interest is given to the structure and position of the tip vortices. Further, the circulation from the wake flow field is computed and compared to the distribution of circulation on the blades.
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| 26. |
- Ivanell, Stefan, et al.
(författare)
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Stability analysis of the tip vortices of a wind turbine
- 2010
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Ingår i: Wind Energy. - : Wiley. - 1095-4244 .- 1099-1824. ; 13:8, s. 705-715
-
Tidskriftsartikel (refereegranskat)abstract
- The aim of the present project is to obtain a better understanding of thestability properties of wakes generated by wind turbine rotors. To accomplishthis a numerical study on the stability of the tip vortices of the Tjaereborgwind turbine has been carried out. The numerical model is based on large eddysimulations of the Navier-Stokes equations using the actuator line method togenerate the wake and the tip vortices. To determine critical frequencies theflow is disturbed by inserting harmonic perturbations.The results show that the instability is dispersive and that growth arisesonly for some specific frequencies and type of modes. The study also providesevidence of a relationship between the turbulence intensity and the length ofthe near wake. The relationship however needs to be calibrated against measurements.
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| 27. |
- Jauch, C., et al.
(författare)
-
International Comparison of Requirements for Connection of Wind Turbines to Power Systems
- 2005
-
Ingår i: Wind Energy. - : Wiley. - 1095-4244 .- 1099-1824. ; 8:3, s. 295-306
-
Tidskriftsartikel (refereegranskat)abstract
- Power production from wind turbines has increased considerably during the lost decade. Therefore today's wind turbines, which are typically set up in wind farms, have a significant influence on the operation of power systems. The efficient and secure operation of power systems is supported by grid codes, which are sets of requirements for all network users (suppliers, customers, etc.). In Europe, several transmission network operators have introduced special grid connection requirements for wind farms. These requirements are mainly based on existing grid codes, initially written for conventional power plants usually equipped with synchronous generators. This article presents a comparison of grid connection requirements for wind farms issued, or proposed as a draft, by transmission network operators in Denmark, Sweden, Germany, Scotland and Ireland.
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| 28. |
- Johansson, Maria, et al.
(författare)
-
Intention to respond to local wind turbines: The role of attitudes and visual perception
- 2007
-
Ingår i: Wind Energy. - : Wiley. - 1099-1824 .- 1095-4244. ; 10:5, s. 435-451
-
Tidskriftsartikel (refereegranskat)abstract
- Wind power projects frequently face public opposition during the siting process. A deeper understanding of social factors has therefore been called for. Based on psychological theories, the present research was initiated in order to study the relative importance of individual aspects related to visual perception and attitudinal factors in public intention to oppose local wind turbines. In an empirical study, 80 people assessed the visual characteristics of wind turbines on site and rated their emotional state. They also completed a questionnoire covering attitudes towards the turbines, perception of significant others'opinions of local wind turbines, and perceived possibilities to oppose as well as intention to oppose local turbines. Moreover, socio-demographics, factors related to the place of residence and general attitude towards wind power, were investigated. In a multiple regression analysis explaining 50% of the variance, it was found that intention to oppose was related to only a few perceptual and attitudinal factors, i.e. the perceived unity of the environment, the personal attitude towards the effects of wind turbines on landscape aesthetics and recreation, and the general attitude towards wind power. Of minor importance was the attitude towards the effects of wind turbines on people's daily quality of life. It seems important to involve the public in the discussion of how wind power installations can be integrated into the landscape without threatening the visual beauty and the recreational value of the natural and cultural environment. Moreover, social intervention is required to promote positive attitudes towards wind turbines. Copyright (c) 2007 John Wiley & Sons, Ltd.
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| 29. |
- Kiviluoma, Juha, et al.
(författare)
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Variability in Large-Scale Wind Power Generation
- 2016
-
Ingår i: Wind Energy. - : John Wiley & Sons. - 1095-4244 .- 1099-1824. ; 19:9, s. 1649-1665
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Tidskriftsartikel (refereegranskat)abstract
- The article demonstrates the characteristics of wind power variability and net load variability in multiple power systems based on real data from multiple years. Demonstrated characteristics include probability distribution for different ramp durations, seasonal and diurnal variability, and low net load events. In some characteristics the power systems are different, but in others they are significantly similar. Somewhat surprisingly there seems to be no straightforward correlation between wind power penetration level and variability. As long as there are several wind power plants with enough geographical spread, most of the smoothing impact is captured. Wind power variability is mainly explained by the extent of geographical spread, but also higher capacity factor causes higher variability. It was also shown how wind power ramps are auto correlated and dependent on the operating output level. In most cases wind power did not have strong diurnal or seasonal variations in the variability. However, there can be exceptions depending on the latitude and on the local characteristics of the wind resource.
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| 30. |
- Kleusberg, Elektra, et al.
(författare)
-
Parametric dependencies of the yawed wind-turbine wake development
- 2020
-
Ingår i: Wind Energy. - : John Wiley and Sons Ltd. - 1095-4244 .- 1099-1824. ; 23:6, s. 1367-1380
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Tidskriftsartikel (refereegranskat)abstract
- Yaw misalignment is currently being treated as one of the most promising methods for optimizing the power of wind farms. Therefore, detailed knowledge of the impact of yaw on the wake development is necessary for a range of operating conditions. This study numerically investigates the wake development behind a single yawed wind turbine operating at different tip-speed ratios and yaw angles using the actuator-line method in the spectral-element code Nek5000. It is shown that depending on the tip-speed ratio, the blade loading varies along the azimuth, resulting in a wake that is asymmetric in both the horizontal and vertical directions. Large tip-speed ratios as well as large yaw angles are shown to decrease the vertical asymmetry of the yaw-induced counter-rotating vortex pair. Both parameters have the effect that they increase the spanwise force induced by yaw relative to the wake rotation. However, while the strength of the counter-rotating vortex pair in the far wake increases with yaw angle, it is shown to decrease with the tip-speed ratio. The vertical shift in the wake center is found to be highly dependent on the yaw angle and the tip-speed ratio. These detailed insights into the yawed wake are important when optimizing potential downstream turbines.
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| 31. |
- Kleusberg, Elektra, et al.
(författare)
-
Tip-vortex breakdown of wind turbines subject to shear
- 2019
-
Ingår i: Wind Energy. - : John Wiley & Sons. - 1095-4244 .- 1099-1824. ; 22:12, s. 1789-1799
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Tidskriftsartikel (refereegranskat)abstract
- Sheared velocity profiles pervade all wind-turbine applications, thus making it important to understand their effect on the wake. In this study, a single wind turbine is modeled using the actuator-line method in the incompressible Navier–Stokes equations. The tip vortices are perturbed harmonically, and the growth rate of the response is evaluated under uniform inflow and a linear velocity profile. Whereas previous investigations of this kind were conducted in the rotating frame of reference, this study evaluates the excitation response in the fixed frame of reference, thus necessitating a frequency transformation. It is shown that increasing the shear decreases the spatial growth rate in the upper half of the wake while increasing it in the lower half. When scaled with the local tip vortex parameters, the growth rate along the entire azimuth collapses to a single value for the investigated wavenumbers. We conclude that even though the tip-vortex breakdown is asymmetric in sheared flow, the scaled growth rates follow the behavior of axisymmetric helical vortices. An excitation amplitude reduction by an order of magnitude extends the linear growth region of the wake by one radius for uniform inflow. In the sheared setup, the linear growth region is extended further in the top half than in the bottom half because of the progressive distortion of the helical tip vortices. An existing model to determine the stable wake length was shown to be in close agreement with the observed numerical results when adjusted for shear.
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| 32. |
- Lindgren, Elin, et al.
(författare)
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Minimizing regulation costs in multi-area systems with uncertain wind power forecasts
- 2008
-
Ingår i: Wind Energy. - : Wiley. - 1095-4244 .- 1099-1824. ; 11:1, s. 97-108
-
Tidskriftsartikel (refereegranskat)abstract
- In a power system where there are many fast and/or large changes in generation or consumption, e.g. in a system with large amounts of wind power, it is more complicated to handle the frequency control efficiently. Minimizing regulation costs for the system operator in such a system requires the possibility to simulate the frequency control in the time range from minutes to a few hours. In this paper, it is shown how the frequency control during normal operation can be optimized using a multi-area model, without exceeding frequency limits or transmission capacity. The model has also been expanded to include uncertainties in wind power forecasts, which may lead to an increase of the regulating costs. The optimization model is applied to numerical examples to show the impact of wind power on costs for regulating power and to show the value of better wind speed forecasts. No load forecast errors are taken into account.
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| 33. |
- Malz, Elena, 1990, et al.
(författare)
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The value of airborne wind energy to the electricity system
- 2022
-
Ingår i: Wind Energy. - : Wiley. - 1099-1824 .- 1095-4244. ; 25:2, s. 281-299
-
Tidskriftsartikel (refereegranskat)abstract
- Airborne wind energy (AWE) is a new power generation technology that harvests wind energy at high altitudes using tethered wings. The potentially higher energy yield, combined with expected lower costs compared to traditional wind turbines (WTs), motivates interest in further developing this technology. However, commercial systems are currently unavailable to provide more detailed information on costs and power generation. This study estimates the economic value of AWE in the future electricity system, and by that indicates which cost levels are required for AWE to be competitive. A specific focus is put on the relation between AWE systems (AWESs) and WTs. For this work, ERA-5 wind data are used to compute the power generation of the wind power technologies, which is implemented in a cost-minimizing electricity system model. By forcing a certain share of the annual electricity demand to be supplied by AWESs, the marginal system value (MSV) of AWE is investigated. The MSV is found to be affected by the AWE share, the wind resource, and the temporal distribution of the AWES's electricity generation. The MSV of AWE is location- and system-dependent and ranges between 1.4 and 2.2 (Formula presented.) at a low share of AWE supply (0%–30%). At higher shares, the MSV drops. The power generation of WTs and AWESs are related, implying that the wind technologies present a similar power source and can be used interchangeably. Thus, the introduction of AWESs will have a low impact on the cost-optimal wind power share in the electricity system, unless an AWES cost far below the system-specific MSV is attained.
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| 34. |
- Matevosyan, Julija, et al.
(författare)
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Short Term Hydro Power Planning Coordinated with Wind Power in Areas with Congestion Problems
- 2007
-
Ingår i: Wind Energy. - : Wiley. - 1095-4244 .- 1099-1824. ; 10:3, s. 195-208
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper, a day-ahead planning algorithm for a multi-reservoir hydropower system coordinated with wind power is developed Coordination applies to real situations, where wind power and hydropower are owned by different utilities, sharing the same transmission lines, although hydropower has priority for transmission capacity. Coordination is thus necessary to minimize wind energy curtailments during congestion situations. The planning algorithm accounts for the uncertainty of wind power forecast. Only planning for the spot market is considered. Once the production bid is placed on the market, it cannot be changed. The solution of the stochastic optimization problem should, therefore, fulfill the transmission constraints for all wind power production scenarios. An evaluation algorithm is also developed to quantify the impact from the coordinated planning in the long run. The developed planning algorithm and the evaluation algorithm are applied in a case study. The results are compared with uncoordinated operation. The results of the case study show that coordination with wind power brings additional income to the hydropower utility and leads to significant reduction of wind energy curtailments.
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| 35. |
- Matsfelt, Johanna, 1990, et al.
(författare)
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Large eddy simulation: A study of clearings in forest and their effect on wind turbines
- 2021
-
Ingår i: Wind Energy. - : Wiley. - 1099-1824 .- 1095-4244. ; 24:12, s. 1388-1406
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Tidskriftsartikel (refereegranskat)abstract
- In this article, the Ryningsnas site in Sweden is investigated using large eddy simulation with three different clearing setups: a homogeneous forest, that is, no clearing, the current clearing, that is, the existing clearing at the location, and an extended clearing. Neutral stratification is simulated, and the wind turbines are modelled by a two-way-coupled actuator line model. From the simulations, the electrical generator power was found to be the highest for the current clearing. But the fatigue loads were both higher and lower than the homogeneous forest depending on which part of the wind turbine that was investigated. The extended clearing nearly always had the lowest fatigue loads but unfortunately also the lowest electrical generator power. Further optimization of the clearings and the wind turbine locations in relation to them is needed to find the sweet spot where the fatigue loads are lower and the electrical generator power is higher.
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| 36. |
- Max, Lena, 1980, et al.
(författare)
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Control of a wind farm with an internal direct current collection grid
- 2012
-
Ingår i: Wind Energy. - : Wiley. - 1099-1824 .- 1095-4244. ; 15:4, s. 547-561
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Tidskriftsartikel (refereegranskat)abstract
- For a high-voltage, direct current connected wind farm, an internal direct current (DC) collection grid is a possible further development that can reduce the weight of the transformers significantly, with average losses for the DC system of 3%. For the internal DC grid, the DC/DC converters control the power flow and thereby also the voltages. In this paper, the control of the DC/DC converters in the wind farm is investigated in detail. The control strategy is presented, and suitable time constants are chosen depending on the switching frequency of the converters. Also, the required capacitor size to keep the voltage variations within 5% of the rated value in the case without communication within the wind farm is derived. It is shown that the control is stable and can handle faults on the external grid without any communication within the wind farm.
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| 37. |
- Max, Lena, 1980, et al.
(författare)
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Fault handling for a wind farm with an internal DC collection grid
- 2012
-
Ingår i: Wind Energy. - : Wiley. - 1099-1824 .- 1095-4244. ; 15:2, s. 259-273
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Tidskriftsartikel (refereegranskat)abstract
- As a result of the increased wind power penetration, there are increased requirements for robustness of the wind farms during disturbances and faults. The wind farms are required to stay connected during short grid faults, and also, the disturbances as a result of internal faults should be minimized. In this paper, the fault ride-through properties during external faults for a wind farm with an internal direct current (DC) grid are investigated. A safe detection of the fault is derived, as well as the reconnection procedure after the fault has been cleared. Also, the operation during internal faults has been investigated, focusing on the faults in the DC bus. It has been shown that the wind farm with an internal DC grid can operate through faults in the external grid and reconnect well within the requirements stated in the grid codes. Further, methods for detection of internal faults are derived, and it was shown that a faulted part can be disconnected and the non-faulted parts can be back in operation within 100?ms using a DC breaker in each radial and within 300?ms without DC breakers.
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| 38. |
- Medici, D., et al.
(författare)
-
Measurements behind model wind turbines : Further evidence of wake meandering
- 2008
-
Ingår i: Wind Energy. - : Wiley. - 1095-4244 .- 1099-1824. ; 11:2, s. 211-217
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Tidskriftsartikel (refereegranskat)abstract
- The frequency of wind turbine wake meandering was studied using wind turbine models with one, two and three blades. The one-bladed turbine did not give rise to any meandering motion, whereas meandering was observed for both the two- and three-bladed turbines at high enough rotational speeds. It was shown that both the thrust of the turbine and the tip-speed ratio influence the meandering.
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| 39. |
- Medici, Davide, et al.
(författare)
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Measurements on a wind turbine wake: 3D effects and bluff-body vortex shedding
- 2006
-
Ingår i: Wind Energy. - : Wiley. - 1095-4244 .- 1099-1824. ; 9:3, s. 219-236
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Tidskriftsartikel (refereegranskat)abstract
- The velocity held in the wake of a two-bladed wind turbine model (diameter 180 mm) has been studied under different conditions using a two-component hot wire. All three velocity components were measured both for the turbine rotor normal to the oncoming flow as well as with the turbine inclined to the freestream direction (the yaw angle was varied from 0 degrees to 20 degrees). The measurements showed, as expected, a wake rotation in the opposite direction to that of the turbine. A yawed turbine is found to clearly deflect the wake flow to the side, showing the potential of controlling the woke by yawing the turbine. An unexpected feature of the flow was that spectra from the time signals showed the appearance of a low-frequency fluctuation both in the wake and in the flow outside the wake. This fluctuation was found both with and without freestream turbulence and also with a yawed turbine. The frequency expressed as a Strouhal number was shown to be independent of the freestream velocity or turbulence level, but the low frequency was only observed when the tip speed ratio (or equivalently the drag coefficient) was high. The shedding frequency changed also with the yaw angle. This is in agreement with the idea that the turbine sheds structures as a bluff body. The phenomenon, noticeable in all the velocity components, was further investigated using two-point cross-correlations of the velocity signals.
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| 40. |
- Medici, D., et al.
(författare)
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The upstream flow of a wind turbine : blockage effect
- 2011
-
Ingår i: Wind Energy. - : Wiley. - 1095-4244 .- 1099-1824. ; 14:5, s. 691-697
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Tidskriftsartikel (refereegranskat)abstract
- The flow upstream a wind turbine is studied in order to investigate blockage effects. We use rotating wind turbine models in a wind tunnel, where velocity measurements have been made both with hot-wire anemometry up to approximately 4.5 diameters (D) upstream the turbine, as well as laser particle image velocimetry measurements close to the turbine rotor. Also, numerical simulations have been carried out by means of a finite volume code. The measurements show, among other things, that the flow is affected more than 3 D upstream the rotor plane.
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| 41. |
- Mendoza, Victor, et al.
(författare)
-
Aerodynamic performance of a dual turbine concept characterized by a relatively close distance between rotors
- 2023
-
Ingår i: Wind Energy. - : John Wiley & Sons. - 1095-4244 .- 1099-1824. ; 26:6, s. 521-537
-
Tidskriftsartikel (refereegranskat)abstract
- In this work, a closely spaced dual turbine concept is studied. The distance between the two side-by-side hubs is 1.05D$$ D $$, where D$$ D $$ is the rotor diameter. This configuration has a potential benefit for offshore wind developments in which power density can be maximized. The main goal is to evaluate the overall aerodynamic performance, blade loads, and wake structure of a reference wind turbine generator operating within this dual turbine configuration and to compare the effects against those for the typical single turbine configuration. For this purpose, an actuator line model has been employed together with the large eddy simulation approach for predicting the turbulence effects. This model was implemented by using the open-source computational fluid dynamics toolbox OpenFOAM. Results show a better performance for the dual turbine concept. Under same operating conditions, the aerodynamic power of each turbine within the dual concept is higher than the power of the stand alone turbine, particularly at lower operating wind speeds (approximately 2% to 3% of extra power per turbine). Comparison between the two configurations shows similar character of the tangential and normal forces acting on the blades in terms of magnitude and fluctuation, eliminating potential concerns regarding fatigue and blade design. The largest difference in the tangential and normal root bending moments are approximately 3% and 2%, respectively, between single and dual turbine configurations. Finally, wake recovery analysis shows a downwind velocity deficit that is not enhanced streamwise in the dual turbine configuration with no considerable difference after 7D$$ D $$.
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| 42. |
- Mendoza, Victor, et al.
(författare)
-
Improving farm efficiency of interacting vertical‐axis wind turbines through wake deflection using pitched struts
- 2019
-
Ingår i: Wind Energy. - : Wiley. - 1095-4244 .- 1099-1824. ; 22:4, s. 538-546
-
Tidskriftsartikel (refereegranskat)abstract
- This work presents a numerical study of the obtained performance and the resulting flow field between two interacting large scale vertical-axis wind turbines (VAWTs), under the influence of a deflected wake through the struts pitching of the upwind turbine. The configuration consists of two VAWTs aligned in the direction of the incoming flow in which a wide range of fixed struts pitching angles in the upwind turbine have been investigated. The main goal is to evaluate the influence of the wake deflection on the turbines performance while they are operating at their optimal tip speed ratio (TSR), and to reproduce the most relevant phenomena involved in the flow pattern of the interacting wake. Arrangements with cross-stream offsets have also been tested for quantifying the contribution of this modification into the overall performance. For this purpose, an actuator line model (ALM) has been implemented using the open-source CFD library OpenFOAM in order to solve the governing equations and to calculate the resulting flow. The Large eddy simulation (LES) approach is considered to reproduce the turbulence flow effects. A preliminary study to identify the optimal TSR of the interacting downwind turbine has been investigated.
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| 43. |
- Mendoza, Victor, et al.
(författare)
-
Near-Wake Flow Simulation of a Vertical Axis Turbine Using an Actuator Line Model
- 2019
-
Ingår i: Wind Energy. - : Wiley. - 1095-4244 .- 1099-1824. ; 22:2, s. 171-188
-
Tidskriftsartikel (refereegranskat)abstract
- In the present work, the near‐wake generated for a vertical axis wind turbine (VAWT) was simulated using an actuator line model (ALM) in order to validate and evaluate its accuracy. The sensitivity of the model to the variation of the spatial and temporal discretization was studied and showed a bigger response to the variation in the mesh size as compared with the temporal discretization. The large eddy simulation (LES) approach was used to predict the turbulence effects. The performance of Smagorinsky, dynamic k‐equation, and dynamic Lagrangian turbulence models was tested, showing very little relevant differences between them. Generally, predicted results agree well with experimental data for velocity and vorticity fields in representative sections. The presented ALM was able to characterize the main phenomena involved in the flow pattern using a relatively low computational cost without stability concerns, identified the general wake structure (qualitatively and quantitatively), and the contribution from the blade tips and motion on it. Additionally, the effects of the tower and struts were investigated with respect to the overall structure of the wake, showing no significant modification. Similarities and discrepancies between numerical and experimental results are discussed. The obtained results from the various simulations carried out here can be used as a practical reference guideline for choosing parameters in VAWTs simulations using the ALM.
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| 44. |
- Mendoza, Victor, et al.
(författare)
-
Performance and wake comparison of horizontal and vertical axis wind turbines under varying surface roughness conditions
- 2019
-
Ingår i: Wind Energy. - : Wiley. - 1095-4244 .- 1099-1824. ; 22:4, s. 458-472
-
Tidskriftsartikel (refereegranskat)abstract
- A numerical study of both a horizontal axis wind turbine (HAWT) and a vertical axis wind turbine (VAWT) with similar size and power rating is presented. These large scale turbines have been tested when operating stand-alone at their optimal tip speed ratio (TSR) within a neutrally stratified atmospheric boundary layer (ABL). The impact of three different surface roughness lengths on the turbine performance is studied for the both turbines. The turbines performance, the response to the variation in the surface roughness of terrain, and the most relevant phenomena involved on the resulting wake were investigated. The main goal was to evaluate the differences and similarities of these two different types of turbine when they operate under the same atmospheric flow conditions. An actuator line model (ALM) was used together with the large eddy simulation (LES) approach for predicting wake effects, and it was implemented using the open-source computational fluid dynamics (CFD) library OpenFOAM to solve the governing equations and to compute the resulting flow fields. This model was first validated using wind tunnel measurements of power coefficients and wake of interacting HAWTs, and then employed to study the wake structure of both full scale turbines. A preliminary study test comparing the forces on a VAWT blades against measurements was also investigated. These obtained results showed a better performance and shorter wake (faster recovery) for an HAWT compared with a VAWT for the same atmospheric conditions.
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| 45. |
- Mucke, Tanja, et al.
(författare)
-
Atmospheric turbulence and its influence on the alternating loads on wind turbines
- 2011
-
Ingår i: Wind energy. - : Wiley. - 1095-4244 .- 1099-1824. ; 14:2, s. 301-316
-
Tidskriftsartikel (refereegranskat)abstract
- Analysis of measurements on atmospheric turbulence with respect to the statistics of velocity increments reveals that the statistics are not Gaussian but highly intermittent. Here, we demonstrate that the higher quantity of extreme events in atmospheric wind fields transfers to alternating loads on the airfoil and on the main shaft in the form of torque fluctuations. For this purpose, alternating loads are discussed with respect to their increment statistics. Our conjecture is that the anomalous wind statistics are responsible for load changes, which may potentially contribute to additional loads and may cause additional fatigue. Our analysis is performed on three different wind field data sets: measured fields, data generated by a standard wind field model and data generated by an alternative model based on continuous time random walks, which grasps the intermittent structure of atmospheric turbulence in a better way. Our findings suggest that fluctuations in the loads might not be reflected properly by the standard wind field models.
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| 46. |
- Möllerström, Erik, 1984-, et al.
(författare)
-
Turbulence influence on wind energy extraction for a medium size vertical axis wind turbine
- 2016
-
Ingår i: Wind Energy. - Chichester : John Wiley & Sons. - 1095-4244 .- 1099-1824. ; 19:11, s. 1963-1973
-
Tidskriftsartikel (refereegranskat)abstract
- The relation between power performance and turbulence intensity for a VAWT H-rotor is studied using logged data from a 14 month (discontinuous) period with the H-rotor operating in wind speeds up to 9 m/s. The turbine, designed originally for a nominal power of 200 kW, operated during this period mostly in a restricted mode due to mechanical concerns, reaching power levels up to about 80 kW. Two different approaches are used for presenting results, one that can be compared to power curves consistent with the International Electrotechnical Commission (IEC) standard and one that allows isolating the effect of turbulence from the cubic variation of power with wind speed. Accounting for this effect, the turbine still shows slightly higher efficiency at higher turbulence, proposing that the H-rotor is well suited for wind sites with turbulent winds. The operational data are also used to create a Cp(λ) curve, showing slightly lower Cp compared with a curve simulated by a double multiple streamtube model. Copyright © 2016 John Wiley & Sons, Ltd.
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| 47. |
- Nebenführ, Bastian, 1984, et al.
(författare)
-
Prediction of wind-turbine fatigue loads in forest regions based on turbulent LES inflow fields
- 2017
-
Ingår i: Wind Energy. - : Wiley. - 1099-1824 .- 1095-4244. ; 20:6, s. 1003-1015
-
Tidskriftsartikel (refereegranskat)abstract
- Large-eddy simulations (LES) were used to predict the neutral atmospheric boundary layer over a sparse and a dense forest, as well as over grass-covered flat terrain. The forest is explicitly represented in the simulations through momentum sink terms. Turbulence data extracted from the LES served then as inflow turbulence for the simulation of the dynamic structural response of a generic wind turbine. In this way, the impact of forest density, wind speed and wind-turbine hub height on the wind-turbine fatigue loads was studied. Results show for example significantly increased equivalent fatigue loads above the two forests. Moreover, a comparison between LES turbulence and synthetically generated turbulence in terms of load predictions was made and revealed that synthetic turbulence was able to excite the same spectral peaks as LES turbulence but lead to consistently lower equivalent fatigue loads. Copyright (c) 2016 John Wiley & Sons, Ltd.
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| 48. |
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| 49. |
- Nilsson, Karl, et al.
(författare)
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Large-eddy simulations of the Lillgrund wind farm
- 2015
-
Ingår i: Wind Energy. - : John Wiley & Sons. - 1095-4244 .- 1099-1824. ; 18:3, s. 449-467
-
Tidskriftsartikel (refereegranskat)abstract
- The power production of the Lillgrund wind farm is determined numerically using large-eddy simulations and compared with measurements. In order to simulate realistic atmospheric conditions, pre-generated turbulence and wind shear are imposed in the computational domain. The atmospheric conditions are determined from data extracted from a met mast, which was erected prior to the establishment of the farm. In order to allocate most of the computational power to the simulations of the wake flow, the turbines are modeled using an actuator disc method where the discs are imposed in the computational domain as body forces which for every time step are calculated from tabulated airfoil data. A study of the influence of imposed upstream ambient turbulence is performed and shows that higher levels of turbulence results in slightly increased total power production and that it is of great importance to include ambient turbulence in the simulations. By introducing ambient atmospheric turbulence, the simulations compare very well with measurements at the studied inflow angles. A final study aiming at increasing the farm production by curtailing the power output of the front row turbines and thus letting more kinetic energy pass downstream is performed. The results, however, show that manipulating only the front row turbines has no positive effect on the farm production, and therefore, more complex curtailment strategies are needed to be tested.
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| 50. |
- Nilsson, Karl, et al.
(författare)
-
Validation of the actuator line method using near wake measurements of the MEXICO rotor
- 2015
-
Ingår i: Wind Energy. - : John Wiley & Sons. - 1095-4244 .- 1099-1824. ; 18:3, s. 499-514
-
Tidskriftsartikel (refereegranskat)abstract
- The purpose of the present work is to validate the capability of the actuator line method to compute vortex structures in the near wake behind the MEXICO experimental wind turbine rotor. In the MEXICO project/MexNext Annex, particle image velocimetry measurements have made it possible to determine the exact position of each tip vortex core in a plane parallel to the flow direction. Determining center positions of the vortex cores makes it possible to determine the trajectory of the tip vortices, and thus the wake expansion in space, for the analyzed tip speed ratios. The corresponding cases, in terms of tip speed ratios, have been simulated by large-eddy simulations using a Navier - Stokes code combined with the actuator line method. The flow field is analyzed in terms of wake expansion, vortex core radius, circulation and axial and radial velocity distributions. Generally, the actuator line method generates significantly larger vortex cores than in the experimental cases, but predicts the expansion, the circulation and the velocity distributions with satisfying results. Additionally, the simulation and experimental data are used to test three different techniques to compute the average axial induction in the wake flow. These techniques are based on the helical pitch of the tip vortex structure, 1D momentum theory and wake expansion combined with mass conservation. The results from the different methods vary quite much, especially at high values of λ.
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