On the energy output estimation of wind turbines

During the operation of the German test field for small Wind Energy Conversion Systems (WECS) on the island of Pellworm five wind turbines were tested following recommendations of the International Energy Agency (IEA) expert group. Possible errors in the estimation of a tested wind turbine's total energy output at a potential installation site are investigated. Different wind speed frequency distributions (the measured one, the Rayleigh and the two-parameter Weibull distribution) are used to calculate the total energy output. The differences between the various distributions are mostly below 10 per cent. An improvement of the energy output estimate by a Weibull-instead of a Rayleigh distribution was not found. It is also shown that the use of the recommended 10 min averages or any other average overestimates the WECS' efficiency, up to 14 per cent on average depending on turbulence intensity. Wind power instead of wind speed is the appropriate parameter for power performance testing. Spectra of wind power and electrical power output show three areas of different correlation. A resistance length for wind turbines is shown to be dependent on the WECS operation status.     

Operating experience from the ehecatl wind turbines

This paper presents a summary of the mechanical features and the medium term operating experience from three small horizontal axis wind energy conversion systems covering a range of 500 to 5000 Watt, intented for rural and isolated communities. These turbines have a new regulator which reduces gyroscopic loads, is easy to adjust and could be manufactured stronger and in smaller sizes than the conventional tail vanes. It is concluded that is feasible to build cheaps and reliable wind turbine generators with an acceptable efficiency employing common-use elements, but is needed a careful and optimized design.     

Upgrading conventional wind turbines

When approaching a conventional wind turbine, the air flow is slowed down and widened. This effect causes a loss in the efficiency of the turbine. By creating a field of low pressure behind the turbine, this effect and the corresponding loss in efficiency can be avoided. In order to maintain this low pressure field, the air passing near, but not through the turbine needs to do work.Based on these considerations we have made a model of a wind turbine with a wing profiled ring around it. We present various fluidodynamical calculations in order to study the resulting increase in power and in order to estimate what the geometrical size of such an apparatus would need to be and whether it could be of advantage compared to conventional devices from an economic point of view.     

Research and development for shrouded wind turbines

In order to exploit wind power as economically as possible, it was suggested that the wind turbine should be enclosed inside a specially designed shroud. Various geometries are discussed; it is shown that with a fairly compact shroud a significant power augmentation can be achieved. For improving the shroud performance, the use of a ring-shaped flap or boundary layer control technique is introduced. It is shown that up to 80% improvement in the shroud power augmentation can be obtained by the use of an appropriate ring-shaped flap while proper bleeding of the shroud's external flow into its inner rear part will increase its power augmentation by about 25%. The present review also discussed the design and performance of an axial flow turbine which is the most suitable for the proposed shrouds. It is shown that such a turbine produces a fairly stable output for varying wind speeds while exhibiting a fairly high efficiency. Based on the reported research with shrouds, a pilot plant producing 1 hp at 5 m/s with a 3 m dia. turbine was built. Its design and preliminary field test results are also included in the review.     

New high profitable wind turbines

To generate more quantities of electric energy from wind it is necessary to use a new type of wind turbine built in the regulable mantle's nozzle. This wind turbine type replaces the free air stream from wind by a programmed, i.e. regulated, and partially concentrated stream of air. The nozzle shell is designed as an aerodynamically shaped ring with wings with its lower pressure side pointed towards the centre so that the lift force on each part of the wing is directed radially towards the centre. This induces centrifugal reaction force in the airflow that causes the stream field to expand strongly downstream of the rotor and includes a greater number of streamlines in the active stream in front of the rotor (upstream). Thus the nozzle forces a higher mass flow rate of air through the turbine. The higher mass flow and higher velocity reduction behind the rotor result in a higher energy output from the wind turbine in the nozzle. In this way the wind turbine efficiency is multiplied. New turbines induce more power from weaker and medium winds and their lasting time, because of the relation P=f(v³) (i.e. the power corresponds to wind velocity raised to third power). Wind turbine nozzle produces three times more energy than conventional wind turbine. Short economic analysis for conditions of the island of Lastovo indicates that profit gained by new turbines is up to five times higher than by conventional turbines. The new wind turbine nozzle should generate interest and demand on an international market, even for regions with weaker winds.     

Individual blade pitch control of floating offshore wind turbines

Floating wind turbines offer a feasible solution for going further offshore into deeper waters. However, using a floating platform introduces additional motions that must be taken into account in the design stage. Therefore, the control system becomes an important component in controlling these motions. Several controllers have been developed specifically for floating wind turbines. Some controllers were designed to avoid structural resonance, while others were used to regulate rotor speed and platform pitching. The development of a periodic state space controller that utilizes individual blade pitching to improve power output and reduce platform motions in above rated wind speed region is presented. Individual blade pitching creates asymmetric aerodynamic loads in addition to the symmetric loads created by collective blade pitching to increase the platform restoring moments. Simulation results using a high‐fidelity non‐linear turbine model show that the individual blade pitch controller reduces power fluctuations, platform rolling rate and platform pitching rate by 44%, 39% and 43%, respectively, relative to a baseline controller (gain scheduled proportional–integral blade pitch controller) developed specifically for floating wind turbine systems. Turbine fatigue loads were also reduced; tower side–side fatigue loads were reduced by 39%. Copyright © 2009 John Wiley & Sons, Ltd.     

A cascade model of blade element interaction for wind turbines with unequal blades

Horizontal-axis wind turbines often operate with unequally performing blades. A simple extension of blade element analysis for unequal blades is developed using the two-dimensional cascade analogue of wind turbines. The vortex strengths of the blade elements can vary with blade number. For three-bladed rotors, the unequal strengths induce an extra velocity at each blade, but for two blades there is no additional velocity. For both blade numbers, there is a modification to the rotational inflow factor. To determine the significance of blade differences, test calculations are presented for two- and three-bladed turbines with different blade pitch angles. The modifications proposed here do not substantially alter the calculations of turbine power and thrust near the point of maximum performance. However, some substantial differences were found at higher thrust. Furthermore, the new method predicts much larger variations in the blade element torque between the blades in the hub region for most operating conditions.     

大型水平轴式风电叶片的结构设计

风电叶片是风力发电设备的关键部件之一,其制造成本占总成本的20%～30%.叶片结构是叶片捕获风能的保证,并直接影响风力发电设备的运行寿命.因此,叶片结构设计的好坏在很大程度上决定了风力发电设备的可靠性和利用风能的成本.文章从材料、结构形式、铺层设计、结构分析等4个方面详细地阐述了风电叶片结构的设计技术.     

大型风力机叶片在不同平均风速作用下的挠度及应力分析

利用有限元分析软件对风力机叶片在不同平均风速作用下的挠度和应力进行了分析。采用Davenport脉动风速谱模拟出不同平均风速,并计算出叶片各部位的挠度及其应力分布。计算结果表明,叶片的振动主要以挥舞振动为主,随着工作风速的增大,叶片挥舞振幅增大,叶片承受的mises应力主要集中在叶片迎风面的中部位置。该研究为风力机叶片的安全设计提供了技术参考。     

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

This 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.     

Power,structural and noise performance tests on different wind turbine rotor blade designs

Most blades available for commercial-grade wind turbines incorporate a straight, span-wise profile and airfoil-shaped cross-sections. These blades are found to be very efficient at low and medium wind speeds compared with the potential energy that can be extracted. This paper explores the possibility of increasing the efficiency of the blades by modifying the blade design to incorporate a swept edge. The design intends to maintain efficiency at low to medium wind speeds by selecting the appropriate orientation and size of the airfoil cross-sections based on an oncoming wind speed and given constant rotation rate. The torque generated from a blade with straight-edge geometry is compared with that generated from a blade with a swept edge as predicted by CFD simulations. To validate the simulations, the experimental curve of the NTK500/41 turbine using LM19.1 blades is reproduced using the same computational conditions. In addition, structural deformations, stress distributions and structural vibration modes are compared between these two different turbine blade surfaces.     

Aero-elastic behavior of a flexible blade for wind turbine application: A 2D computational study

This paper presents a computational study into the static aeroelastic response of a 2D wind turbine airfoil under varying wind conditions. An efficient and accurate code that couples the X-Foil software for computation of airfoil aerodynamics and the MATLAB PDE toolbox for computation of the airfoil deformation is developed for the aero-elastic computations. The code is validated qualitatively against computational results in literature. The impact of a flexibility of the airfoil is studied for a range of design parameters including the free stream velocity, pitch angle, airfoil thickness, and airfoil camber. Static aero-elastic effects have the potential to improve lift and the lift over drag ratio at off-design wind speed conditions. Flexibility delays stall to a large pitch angle, increasing the operating range of a flexible blade airfoil. With increased thickness the airfoil deformation decrease only linearly.     

Multivariable control strategy for variable speed, variable pitch wind turbines

Reliable and powerful control strategies are needed for wind energy conversion systems to achieve maximum performance. A new control strategy for a variable speed, variable pitch wind turbine is proposed in this paper for the above-rated power operating condition. This multivariable control strategy is realized by combining a nonlinear dynamic state feedback torque control strategy with a linear control strategy for blade pitch angle. A comparison with existing strategies, PID and LQG controllers, is performed. The proposed approach results in better power regulation. The new control strategy has been validated using an aeroelastic wind turbine simulator developed by NREL for a high turbulence wind condition.     

模态应变能理论及其在风力机叶片损伤识别中的应用

文章给出了风力机叶片的动力特性计算模型、结构体模态应变能的概念及其计算模型,定义了结构体损伤状态下的模态应变能变化率概念并给出其计算模型。在此基础上,以15 kW风力机叶片为研究对象,在ANSYS中建立有限元分析模型,计算该叶片在不同损伤位置与不同损伤程度下的频率以及模态应变能变化率,并以模态应变能变化率作为表征结构损伤的标识量,对含损伤的风力机叶片结构进行损伤辨识仿真。通过神经网络建立起损伤标识量和损伤状态之间的映射模型,为实现叶片损伤的诊断提供理论依据。     

Experimental validation of model‐based blade pitch controller design for floating wind turbines: system identification approach

This paper discusses the model‐based design of a blade pitch controller for a floating offshore wind turbine (FOWT) scale model. A mathematical model of the FOWT is constructed from an input–output measurement in an experiment using system identification. The blade pitch controller is designed by an control method, and the effectiveness of the controller is evaluated by means of a basin experiment using the FOWT scale model. The results show that the blade pitch controller is effective in reducing platform pitch motion and rotor speed fluctuation. Copyright © 2017 John Wiley & Sons, Ltd.     

百千瓦中型变速变桨风电机组叶片设计研究

百千瓦级叶片一般采用定桨方式运行,依靠叶片失速进行功率控制,机组运行过程中无法维持较高的效率。基于100 kW变速变桨机组的运行特征,提出了一种100 kW级中型叶片的设计方法。气动设计采用了BEM方法,利用Harp_opt中的优化算法获得较高的气动性能;结构及载荷设计参考IEC标准进行,采用Focus进行铺层设计及结构特性分析。所设计叶片的长度为10.029 m左右,极限及疲劳载荷特性满足GL IIA类风场的运行要求。     

Low wind speed turbines and wind power potential in Minnesota, USA

Wind power development in Minnesota largely has been focused in the “windy” southwestern part of the state. This research evaluates the additional power that potentially could be generated via low wind speed turbines, particularly for areas of the state where there has been comparatively little wind energy investment. Data consist of 3 years (2002–2004) of wind speed measurements at 70–75 m above ground level, at four sites representing the range of wind speed regimes (Classes 2–5) found in Minnesota. Power estimates use three configurations of the General Electric 1.5-MW series turbine that vary in rotor diameter and in cut-in, cut-out, and rated speeds. Results show that lower cut-in, cut-out, and rated speeds, and especially the larger rotor diameters, yield increases of 15–30% in wind power potential at these sites. Gains are largest at low wind speed (Class 2) sites and during the summer months at all four sites. Total annual wind power at each site shows some year-to-year variability, with peaks at some sites partially compensating for lulls at others. Such compensation does not occur equally in all years: when large-scale atmospheric circulation patterns are strong (e.g., 2002), the four sites show similar patterns of above- and below-average wind power, somewhat reducing the ability of geographic dispersion to mitigate the effects of wind speed variability.     

Multi-objective optimisation of horizontal axis wind turbine structure and energy production using aerofoil and blade properties as design variables

The design of wind turbine blades is a true multi-objective engineering task. The aerodynamic effectiveness of the turbine needs to be balanced with the system loads introduced by the rotor. Moreover the problem is not dependent on a single geometric property, but besides other parameters on a combination of aerofoil family and various blade functions. The aim of this paper is therefore to present a tool which can help designers to get a deeper insight into the complexity of the design space and to find a blade design which is likely to have a low cost of energy. For the research we use a Computational Blade Optimisation and Load Deflation Tool (CoBOLDT) to investigate the three extreme point designs obtained from a multi-objective optimisation of turbine thrust, annual energy production as well as mass for a horizontal axis wind turbine blade. The optimisation algorithm utilised is based on Multi-Objective Tabu Search which constitutes the core of CoBOLDT. The methodology is capable to parametrise the spanning aerofoils with two-dimensional Free Form Deformation and blade functions with two tangentially connected cubic splines. After geometry generation we use a panel code to create aerofoil polars and a stationary Blade Element Momentum code to evaluate turbine performance. Finally, the obtained loads are fed into a structural layout module to estimate the mass and stiffness of the current blade by means of a fully stressed design. For the presented test case we chose post optimisation analysis with parallel coordinates to reveal geometrical features of the extreme point designs and to select a compromise design from the Pareto set. The research revealed that a blade with a feasible laminate layout can be obtained, that can increase the energy capture and lower steady state systems loads. The reduced aerofoil camber and an increased L/D-ratio could be identified as the main drivers. This statement could not be made with other tools of the research community before.     

Performance and viability analysis of small wind turbines in the European Union

Nowadays, wind energy plays a key role as a sustainable source of energy and wind turbines are a relevant source of power for many countries world-wide. In such a context, this paper investigates the technical and economic feasibility of small wind turbines for five of the main European Union countries (France, Germany, Italy, Spain and The Netherlands). Ten commercial turbines with rated power from 2.5 kW to 200 kW are evaluated considering their installation and operative conditions. Several parameters most affecting wind turbine performances are evaluated and the estimation of the annual cash flows during the expected plant life-time are determined as a function of both the installation location (wind speed probability distribution, national incentive scheme and tax level) and the wind turbine characteristics (rated power curve, maintenance, installation and shipping costs). The obtained data are presented and discussed through a parametric analysis based on the Net Present Value capital budget approach, showing the conditions making these systems profitable or non-profitable and explaining the relative motivations. Moreover, the analysis outcomes are further investigated highlighting the dependence of the turbine profitability from the considered parameters, including a comparative analysis among the five analyzed European countries.     

风力发电机多任务液压测试系统技术研究

介绍了利用多任务控制方式来设计风力发电机液压测试系统。该系统根据不同的要求来模拟风力机实际运行中遇到的各种情况，并对其液压系统进行测试，以确保其安全性。