Today's Wind Research Articles

New engineering wake model for wind farm applications

Engineering wake model is widely used in today's wind farm design because of its fast performance. In the literature, there were good developments in the track of engineering wake models. However, its accuracy is still not ideal. In this paper, two novel analytical wake models (one uses a Gaussian shape and the other uses a cosine shape) are developed to predict the wind velocity distribution in the wake region of a wind turbine by applying the momentum and mass conservation laws. Next, the two models are validated using published experimental data and large-eddy simulation (LES) data, and also compared to the analytical wake model of Ishihara and Qian which is considered to be the state-of-the-art engineering wake model in the literature. In all considered cases, the relative errors between the new models and experimental or LES data are much lower than those between the Ishihara and Qian model and experimental or LES data. From the comparisons, it is also seen that the new model with a Gaussian shape is slightly better in far-field wake prediction in most cases while the new model with a cosine shape is slightly better in a few cases. Because of their good performance, these two models are recommended in wind farm design including both wind farm layout optimization and wind farm control.

Renewable energy in Russia: A critical perspective

AbstractPartly explaining the low uptake of energy production from renewable energy sources, Russia accesses huge oil, natural gas, coal, and uranium resources and hosts advanced nuclear energy, oil, and natural gas industries. Yet, the combined effect of the exceedingly low cost of electricity generation via today's photovoltaic modules and wind turbines combined with energy storage in Li‐ion battery and hydrogen obtained via water electrolysis will shortly have a profound impact on Russia's economy and manufacturing industry. The buildup of Russia's clean energy technology industry will require proper planning, rationalization efforts, and the development of creative and effective policies, which will include new educational initiatives in today's new energy technologies and energy management.

Influence of structural design variations on economic viability of offshore wind turbines: An interdisciplinary analysis

Offshore wind energy is a seminal technology to achieve the goals set for renewable energy deployment. However, today's offshore wind energy projects are mostly not yet sufficiently competitive. The optimization of offshore wind turbine substructures with regard to costs and reliability is a promising approach to increase competitiveness. Today, interdisciplinary analyses considering sophisticated engineering models and their complex economic effects are not widespread. Existing approaches are deterministic. This research gap is addressed by combining an aero-elastic wind turbine model with an economic viability model for probabilistic investment analyses. The impact of different monopile designs on the stochastic cost-efficiency of an offshore wind farm is investigated. Monopiles are varied with regard to diameters and wall thicknesses creating designs with increased lifetimes but higher capital expenditures (durable designs) and vice versa (cheaper designs). For each substructure, the aero-elastic wind turbine model yields distributions for the fatigue lifetime and electricity yield and different capital expenditures, which are applied to the economic viability model. For other components, e.g. blades, constant lifetimes and costs are assumed. The results indicate that the gain of increased stochastic lifetimes exceeds the benefit of reduced initial costs, if the overall lifetime is not governed by other turbine components' lifetimes.

Spatio-temporal asymmetry of local wind fields and its impact on short-term wind forecasting.

The massive amounts of spatio-temporal data collected in today's wind farms have created a necessity for accurate spatio-temporal models. Despite the growing recognition for non-separable spatio-temporal models, a significant reliance on separable, symmetric models is still the norm in today's renewable industry. We discover that the broad use of separable models is due to the handling of wind data in a setting that does not reveal their fine-scale spatio-temporal structure. The contribution of this research is two-fold. First, we devise a special pair of spatio-temporal "lens" that allows us to see the fine-scale spatio-temporal variations and interactions, and subsequently, we conclude that local wind fields exhibit strong signs of non-separability and asymmetry. Using one year of turbine-specific wind measurements, we show that asymmetry can in fact be detected in more than 93% of the time. Second, making use of the spatio-temporal lens, we propose an enhanced procedure for short-term wind speed forecast. Substantial improvements in forecast accuracy in both wind speed and wind power were observed. When combined with certain intelligent methods such as support vector machine, additional improvements are possible.

A study on PD activities of oil-impregnated paper under pulse voltages using gas analysis

Gassing problems and premature insulation failures in wind turbine transformers indicate reliability concerns about the insulation of such components. This fact necessitates investigating degradation of transformer insulation systems under high-dV/dt pulse voltages that exist in today's wind energy plants. For this purpose, PD activity, generally assumed to be the main long-term cause of insulation deterioration, needs to be analyzed. This work evaluates the effects of impulse voltage parameters such as rate of rise and repetition frequency on inception voltage and intensity of PD activities. PD parameter measurement under high-dV/dt voltages is challenging due to interferences from fast oscillations, and difficulties of PD energy measurements. To avoid such issues, which are related to electromagnetic detection methods under pulse energization, this work uses a chemical approach to compare PD activities, based on the rate of hydrogen generation in a controlled test chamber with oil/paper samples. Gas monitoring of the oil containing impregnated paper samples reveals that a faster rate of rise increases PD energy as well as inception voltage. Results based on experiments conducted in small oil chambers show good correlation between the amount of hydrogen detected and PD energy level; hence it can be used as a benchmark for comparing PD energy levels under various impulse waveforms.

The Effect of Aerodynamic Evaluators on the Multi-Objective Optimization of Flatback Airfoils

With the long lengths of today's wind turbine rotor blades, there is a need to reduce the mass, thereby requiring stiffer airfoils, while maintaining the aerodynamic efficiency of the airfoils, particularly in the inboard region of the blade where structural demands are highest. Using a genetic algorithm, the multi-objective aero-structural optimization of 30% thick flatback airfoils was systematically performed for a variety of aerodynamic evaluators such as lift-to-drag ratio (Cl/Cd), torque (Ct), and torque-to-thrust ratio (Ct/Cn) to determine their influence on airfoil shape and performance. The airfoil optimized for Ct possessed a 4.8% thick trailing-edge, and a rather blunt leading-edge region which creates high levels of lift and correspondingly, drag. It's ability to maintain similar levels of lift and drag under forced transition conditions proved it's insensitivity to roughness. The airfoil optimized for Cl/Cd displayed relatively poor insensitivity to roughness due to the rather aft-located free transition points. The Ct/Cn optimized airfoil was found to have a very similar shape to that of the Cl/Cd airfoil, with a slightly more blunt leading-edge which aided in providing higher levels of lift and moderate insensitivity to roughness. The influence of the chosen aerodynamic evaluator under the specified conditions and constraints in the optimization of wind turbine airfoils is shown to have a direct impact on the airfoil shape and performance.

The end of the line for today's wind turbines

We need to start thinking today about the future of our wind turbines, according to Dr Athanasios Kolios and Maria Martinez-Luengo from Cranfield University.

A continuum damage model to predict the influence of ply waviness on stiffness and strength in ultra-thick unidirectional Fiber-reinforced Plastics

Ply waviness is a commonly observed manufacturing defect of ultra-thick composite materials essentially affecting stiffness, strength, and fatigue behavior of the composite. A specimen's geometry is designed to represent the failure mechanisms in thick wavy laminates, typically observed in spar caps of today's wind turbine blades. A material model is developed to simulate the phenomenological processes in wavy laminates to obtain a strength knock-down. Particular attention is taken on the nonlinear shear behavior and kink band formation. The presented model correlates well with results achieved by experiments. This paper shows a significantly higher influence of compressive compared with tensile loading on the mechanical material behavior of wavy laminates.

A Short-Term Wind Power Forecasting Approach With Adjustment of Numerical Weather Prediction Input by Data Mining

This paper proposes a novel short-term wind power forecasting approach by mining the bad data of numerical weather prediction (NWP). Today's short-term wind power forecast (WPF) highly depends on the NWP, which contributes the most in the WPF error. This paper first introduces a bad data analyzer to fully study the relationship between the WPF error with several new extracted features from the raw NWP. Second, a hierarchical structure is proposed, which is composed of a K -means clustering-based bad data detection module and a neural network (NN)-based forecasting module. In the NN module, the WPF is fully adjusted based on the output of the bad data analyzer. Simulations are performed comparing with two other different methods. It proves that the proposed approach can improve the short-term wind power forecasting by effectively identifying and adjusting the errors from NWP.

Comparative CFD study of the effect of the presence of downstream turbines on upstream ones using a rotational speed control system

The effect of a downstream turbine on the production of a turbine located upstream of the latter is studied in this work. This is done through the use of two CFD simulation codes, namely OpenFOAM and EllipSys3D, which solve the Navier-Stokes equations in their incompressible form using a finite volume approach. In both EllipSys3D and OpenFoam, the LES (Large Eddy Simulation) technique is used for modelling turbulence. The wind turbine rotors are modelled as actuator disks whose loading is determined through the use of tabulated airfoil data by applying the blade-element method. A generator torque controller is used in both simulation methods to ensure that the simulated turbines adapt, in terms of rotational velocity, to the inflow conditions they are submited to. Results from both simulation codes, although they differ slightly, show that the downstream turbine affects the upstream one when the spacing between the turbines is small. This is also suggested to be the case looking at measurements performed at the Lillgrund offshore wind farm, whose turbines are located unusually close to each other. However, for distances used in today's typical wind farms, this effect is shown by our calculations not to be significant.

Study of the influence of imposed turbulence on the asymptotic wake deficit in a very long line of wind turbines

The influence of imposed turbulence on the development of the flow along a long row of wind turbines is studied, in search for an asymptotic wake deficit state. Calculations are performed using EllipSys3D, a CFD code that solves the Navier–Stokes equations in their incompressible form using a finite volume approach. In this code, the Large-Eddy Simulation technique is used for modeling turbulence, and the wind turbine rotors are represented as actuator discs whose loading is determined through the use of tabulated airfoil data by applying the blade-element method.Ten turbines are located along a row and separated from each other by seven rotor diameters, which is representative of the distance used in today's offshore wind farms. Turbulence is pre-generated with the Mann model, with imposed turbulence intensity levels of 4.5% and 8.9%. The aim with this study is to investigate features of the flow that depend solely on imposed turbulence and the presence of wind turbine rotors. For this reason, the turbines are isolated from their environment, and no effect from the presence of the atmospheric boundary layer is modeled, i.e., a non-sheared inflow is used. Analysis of the characteristics of the flow as a function of the position along the row of turbines is performed in terms of standard deviation of the velocity components, turbulence kinetic energy, mean velocity, and power spectra of the axial velocity fluctuations. The mean power production along the row of turbines is also used as an indicator.Calculations are performed below rated power, where a generator torque controller implemented in EllipSys3D renders it possible for the turbines to adapt to the flow conditions in which they operate.The results obtained for the standard deviation of the velocity components, turbulence kinetic energy, power and mean velocity as functions of downstream distance show that an asymptotic wake state seems close to be reached, in the conditions tested, near the end of the 10 turbine row. Significant changes towards this state are seen to happen faster when imposing turbulence in the domain. Power spectra of the axial velocity fluctuations are shown to provide interesting information about the turbulence in the flow, but are found not to be useful in determining if an asymptotic wake state is reached.

Modeling of Wind Energy on Isolated Area

In this paper, a model of the wind turbine (WT) with permanent magnet generator (PMSG) and its associated controllers is presented, The increase of wind power penetration in power systems has meant that conventional power plants are gradually being replaced by wind farms. In fact, today wind farms are required to actively participate in power system operation in the same way as conventional power plants. In fact, power system operators have revised the grid connection requirements for wind turbines and wind farms, and now demand that these installations be able to carry out more or less the same control tasks as conventional power plants. For dynamic power system simulations, the PMSG wind turbine model includes an aerodynamic rotor model, a lumped mass representation of the drive train system and generator model. In this paper we propose a model with an implementation in MATLAB / Simulink, each of the system components off-grid small wind turbines. DOI: http://dx.doi.org/10.11591/ijpeds.v4i2.4859

INVELOX: Description of a new concept in wind power and its performance evaluation

A new concept in wind power harnessing is described which significantly outperforms traditional wind turbines of the same diameter and aerodynamic characteristics under the same wind conditions and it delivers significantly higher output, at reduced cost. Its first innovative feature is the elimination of tower-mounted turbines. These large, mechanically complex turbines, and the enormous towers used to hoist them into the sky, are the hallmark of today's wind power industry. They are also expensive, unwieldy, inefficient, and hazardous to people and wildlife. The second innovative feature of INVELOX is that it captures wind flow through an omnidirectional intake and thereby there is no need for a passive or active yaw control. Third, it accelerates the flow within a shrouded Venturi section which is subsequently expanded and released into the ambient environment through a diffuser. In addition, INVELOX provides solutions to all the major problems that have so far undermined the wind industry, such as low turbine reliability, intermittency issues and adverse environmental and radar impact. Simulating the performance of this wind delivery system is quite challenging because of the complexity of the wind delivery system and its interaction with wind at the front end and with a turbine at the back end. The objectives of the present work are to model and understand the flow field inside the INVELOX where the actual wind turbine is located as well the external flow field which not only provides the intake flow but also has to match the exhaust flow of the system. The present computations involved cases with different incoming wind directions and changes in the intake geometry. The results show that it is possible to capture, accelerate and concentrate the wind. Increased wind velocities result in significant improvement in the power output. These results led to the design of a demonstration facility which has provided actual data which verified the significantly increased power expectations.

Efficient blade production relies on smart release agents

Today's large wind turbine blades need to come out of the mould right the first time, with no defects. An important contributor to this is a mould release system which assists in extracting the blades perfectly while protecting the expensive moulds. George Barton, Senior Business Development Director – Composites, and Michael Woodcock, European Business Director – Composites, at Chem-Trend, discuss the importance of using the correct release system.

Turbulent fluxes, stability and shear in the offshore environment: Mesoscale modelling and field observations at FINO1

This paper is focused on the evaluation of five planetary boundary layer (PBL) schemes in the Weather Research and Forecasting model for offshore wind energy purposes. One first order scheme: Yonsey University and four one-and-a-half order schemes: Mellor-Yamada-Janić, Quasi-Normal Scale Elimination, Mellor-Yamada-Nakanishi-Niino, and Bougeault-Lacarrère, are considered. Turbulent flux measurements from the FINO1 platform in the North Sea are used to estimate the Obukhov length, allowing the sorting of the data into different stability classes. In addition, wind LiDAR measurements are used to analyze wind profiles up to 251.5 m, encompassing the heights where today's wind turbines operate. The ability of the different PBL schemes to forecast turbulent fluxes of heat and momentum and surface stability is evaluated. Obukhov length results show that in general, PBL schemes forecast more moderated stable stratifications and a reinforcement of the instability for neutral and convective conditions, compared to FINO1 observations. The vertical structure of the wind speed profile is thoroughly analyzed for stable, near-neutral, unstable, and very unstable conditions by using total shear stresses, eddy diffusivities, and wind speed shears. The Mellor-Yamada-Nakanishi-Niino scheme presents the best agreement with measurements considering the different atmospheric stabilities analyzed. Stable conditions are the most complicated scenario for the PBL schemes to reproduce due to their overdiffusive formulations, which effect is to lower the vertical wind shear. Under such conditions, Quasi-Normal Scale Elimination and Yonsey University outperform the rest of the PBL schemes, the latest using a revised diffusion formulation.

A Transformer-Less High-Power Converter for Large Permanent Magnet Wind Generator Systems

The power conversion systems for large wind turbines are facing a great challenge as today's wind turbine power outputs approach 5 MW and above. The conventional low voltage power conversion system will suffer from a high transmission current, which significantly increases losses and cost of the cables as well as voltage drop. This paper proposes a modular, medium voltage, high-power converter topology for the large permanent magnet wind generator system, eliminating the grid-side step-up transformer, which is desirable for both onshore and offshore wind turbines. The converter modules are cascaded to achieve medium voltage output. Each converter module is fed by a pair of generator coils with 90 phase shift to get the stable dc-link power. The power factor correction (PFC) circuit enables the generator to achieve unity power factor operation and the generator armature inductance is used as ac-side PFC boost inductance. At the grid-side, H-bridge inverters are connected in series to generate multilevel medium voltage output and the voltage-oriented vector control scheme is adopted to regulate the converter active and reactive power transferred to the grid. Simulation results with a 2-MW wind turbine system and experimental results with a down-scaled 3-kW system validate the proposed topology and control methods. The proposed system can successfully deliver power from the wind generator to the grid.

Influence of a wind turbine service life on the mechanical properties of the material and the blade

ABSTRACTThis paper presents results out of investigations of the DEBRA‐25 wind turbine blades. Almost unique in the history of modern wind energy, these blades were in operation for 18 years next to a weather station and were investigated afterward. Therefore, the loads experienced in the operational life could be post‐processed accurately with the measured data of the weather station and the turbine. The blades are made of materials that are similar with today's wind turbines. Furthermore, intensive laboratory tests and free field tests have been carried out, and all load assumptions and data and results are still available today. The results include experimental investigations on the moisture content of the load‐carrying material, static and fatigue behavior of the material, the relaxation of the coupling joints, the natural frequencies of the blade and a full scale static blade test. It is shown that the structural performance of the DEBRA‐25 service blades is comparable with modern wind turbine blades. Although some damage was found by visual inspection, the service blade of the DEBRA‐25 showed excellent mechanical behavior in the full scale blade test. Only small changes of the edgewise eigenfrequencies were detected. The pre‐tensioning forces of the IKEA bolts, where the two blade parts are connected, were measured and were still adequate. Copyright © 2012 John Wiley & Sons, Ltd.

A socio-political analysis of policies and incentives applicable to community wind in Oregon

As a new type of ownership structure, community owned wind projects are becoming more and more important in today's wind energy generation in the U.S. Unlike traditional large wind farms, community wind features local ownership and small-scale generation capacity. The goal of this paper is to identify policies, incentives, and regulations in place that are applicable to community wind projects in Oregon by interviewing project representatives and governmental officials and to depict the Oregon context from strategic, tactical, and operational perspectives for researchers, farmers, private businesses, government entities, and others who are interested in learning about the community wind in the state.

Soccer ball lift coefficients via trajectory analysis

We performed experiments in which a soccer ball was launched from a machine while two high-speed cameras recorded portions of the trajectory. Using the trajectory data and published drag coefficients, we extracted lift coefficients for a soccer ball. We determined lift coefficients for a wide range of spin parameters, including several spin parameters that have not been obtained by today's wind tunnels. Our trajectory analysis technique is not only a valuable tool for professional sports scientists, it is also accessible to students with a background in undergraduate-level classical mechanics.

Operation of Grid-Connected DFIG Under Unbalanced Grid Voltage Condition

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Doubly fed induction generator (DFIG) still shares a large part in today's wind power market. It provides the benefits of variable speed operation cost-effectively, and can control its active and reactive power independently. Crowbar protection is often adopted to protect the rotor-side voltage source converter (VSC) from transient overcurrent during grid voltage dip. But under unbalanced grid voltage condition, the severe problems are not the transient overcurrent, but the electric torque pulsation and dc voltage ripple in the back-to-back VSCs. This paper develops dynamic models in MATLAB/Simulink, validates it through experiments, investigates the behavior of DFIG during unbalanced grid voltage condition, and proposes new controllers in separated positive and negative sequence. Methods to separate positive and negative sequence components in real time are also developed, and their responses to unsymmetrical voltage dip are compared. Simulation results prove that the separated positive and negative sequence controllers limit the torque pulsation and dc voltage ripple effectively. </para>