Time‐accurate blade surface static pressure behaviour on a rotating H‐Darrieus wind turbine

Time‐accurate blade pressure distributions on a rotating H‐Darrieus wind turbine at representative tip speed ratios during start‐up are presented here, which allow blade dynamic stall and laminar separation bubbles to be observed clearly and which provide a rare experimental demonstration of the flow curvature effect inherent in H‐Darrieus turbine operation. The convection of a dynamic stall vortex along the blade surface at high reduced frequency has also been clearly identified. This study provides new information of the complex aerodynamics of the vertical axis wind turbines (VAWTs) and provides unique experimental data to validate the transient blade static surface pressure distribution predicted by CFD models. To the best of the authors' knowledge, this is the first time that the instantaneous pressure variation around the blade has been measured and recorded directly for an H‐Darrieus wind turbine.     

Tests on ducted and bare helical and straight blade Darrieus hydrokinetic turbines

Despite much optimistic language on commercial websites, little data is available on actual performance of hydrokinetic turbines. This paper summarises the findings of a series of tests on several Darrieus type cross flow hydrokinetic turbines (HKTs). Although this type of hydrokinetic turbine (HKT) has some advantages over axial flow turbines, fixed pitch Darrieus HKTs also have some drawbacks, including inability to self-start under load, low efficiency and shaking. Variable pitch has been suggested to increase starting torque and efficiency, ducts to increase power output and helical blades to produce smooth torque. To assess each of these modifications, tests were conducted in Australia and Canada on HKTs with fixed and variable pitch straight blades, fixed helical blades, with and without a slatted diffuser, by mounting each turbine in front of a barge and motoring through still water at speeds ranging from less than 1 m/s up to 5 m/s. The diffuser increased the power output by a factor of 3 in one configuration but considerably less in others. A reason for this finding is suggested. The maximum coefficient of performance Cp of the fixed pitch straight blade and helical turbines without a diffuser ranged from about 0.25 at 1.5 m/s down to less than 0.1 at 5 m/s, while Cp for those with a diffuser ranged from about 0.45 down to about 0.3. Fixed blade turbines, both straight and helical, exhibited low starting torque, while variable pitch turbines started easily. Considerable differences in Cp were observed for the same turbine configuration at different speeds. The turbine with fixed pitch, straight blades was found to shake violently due to cyclical hydrodynamic forces on blades, while the helical and variable pitch turbines did not shake excessively. These findings suggest that variable pitch cross flow HKTs should be further investigated.     

Experimental verifications of numerical predictions for the hydrodynamic performance of horizontal axis marine current turbines

The conversion of the kinetic energy presented by ocean or marine currents offers an exciting proposition as it can provide regular and predictable energy resource. The majority of the proposed designs for converting this type of kinetic energy are based on the concept of the horizontal axis turbines, which has common characteristics to those being used in wind energy. Although a lot can be learnt and transferred from wind turbine technology, there are significant differences. These include the effects of the free surface and the occurrence of cavitation. Consequently, any developed numerical methods need to be verified. This study reports on the development and verification of simulation tools based on blade element momentum theory—a commercial code (GH-Tidal Bladed) and an academic in-house code (SERG-Tidal). Validation is derived from experimental measurements conducted on a model 800 mm diameter turbine in a cavitation tunnel and a towing tank. The experimental data includes measurements of shaft power and thrust generated by the turbine for a series of blade pitch settings and speeds. The results derived from the two codes are compared. These indicate that the two developed codes demonstrate similar trends in the results and provide a satisfactory representation of the experimental turbine performance. Such results give the necessary confidence in the developed codes resulting in appropriate tools that can to be utilised by developers of marine current turbines.     

水平轴潮流能水轮机复合材料叶片自适应性研究

叶片是水轮机中潮流能向机械能转化的核心部件,直接影响水轮机的水动力性能和结构性能。以水平轴潮流能水轮机复合材料叶片为例,基于复合材料弯扭耦合理论,采用双向流固耦合方法研究了叶片的自适应性。结果表明,采用对称铺层方式的复合材料叶片,可提高水轮机的能量利用率,并在较大的速比范围内使水轮机保持较高的效率;降低叶片上的应力集中,可改善结构性能;对称铺层可利用叶片的弯扭耦合特性,使叶片具备自适应性,从而提高水轮机的水动力性能和结构性能。     

On applicability of reciprocating flow turbines developed for wave power to tidal power conversion

Tidal power generation with reciprocating turbines in a simple system is investigated on a performance simulation in order to enlarge the capability of practical use of tidal power with extra-low head and time-varying energy density characteristics. Four reciprocating turbines, which are two types of impulse and a Wells developed for wave power conversion systems, and a cross-flow type of Darrieus for extra-low head hydropower are focused for utilizing extra-low head tidal power. Their turbine characteristics in a unidirectional steady flow obtained by physical test models are compared in non dimensional forms and power plant performance with the turbines are numerically simulated on equivalently scaled turbines based on the low of similitude on turbine performance with the non dimensional characteristics under one of the simplest controls in combination with suitable reservoir ponds area. The output of the power plant depends on tidal difference and a pond inundation area. The results are summarized and discussed on the averaged electric output of the power plant and the optimum scale of pond inundation area.     

Numerical investigations of the effects of different arrays on power extractions of horizontal axis tidal current turbines

As the tidal current industry grows, power extraction from tidal sites has received widespread attention. In this paper, a blade element actuator disk model that is coupled with the blade element method and a three-dimensional Navier–Stokes code is developed to analyse the relationship between power extraction and the layout of turbine arrays. First, a numerical model is constructed to simulate an isolated turbine and the model is validated using experimental data. Then, using this validated model, the power extraction of horizontal axis tidal current turbines using different tidal turbine arrays and rotation directions is predicted. The results of this study demonstrate that staggered grid array turbines can absorb more power from tidal flows than can rectilinear grid array turbines and that staggered grid array turbines are less affected by the rotation of upstream turbines. In addition, for staggered gird arrays, the relationships between power coefficients, lateral distance and longitudinal distance are discussed. The appropriate lateral distance is approximately 2.5 turbine diameters, whereas for the longitudinal distance, the largest value possible should be used. The relative power coefficient can achieve 3.74 when the longitudinal distance is 6 times the turbine diameter. To further increase the power extraction, this study suggests an improved staggered grid array layout. The relative power coefficient of the improved four-row turbine arrays is approximately 3–4% higher than that of the original arrays and will increase as the distance between the second-row and third-row increases. Considering only the first two rows of turbines, the total power extraction can be 11% higher than for an equivalent number of isolated turbines.     

Intracycle RPM control for vertical axis wind turbines

The wind energy market is currently dominated by horizontal axis wind turbines (HAWTs); however, vertical axis wind turbines (VAWTs) are emerging as a design alternative, especially for deep-water offshore siting due to their low center of gravity, ease of access to drivetrain components, and overall simplicity. Due to the absence of a pitch mechanism in large-scale Darrieus VAWTs, stall control has often been used to manage power and loads. Introducing a pitching mechanism in H-type VAWTs has been studied, but this diminishes the mechanical simplicity advantage, and the use of a pitching mechanism in a large-scale Darrieus-type VAWT is not practical. This work examines an innovative, alternative method to control the rotor dynamics of a large-scale 5 MW VAWT to maximize power while constraining loads without introducing any new or complex mechanical elements. This control strategy is termed intracycle revolution per minute (RPM) control, where the rotational speed of the turbine is allowed to vary in an optimal fashion with the azimuthal location of blades as opposed to typical constant RPM operation. An optimization framework is formulated for an open-loop optimal control problem and solved to maximize power subject to constraints on aerodynamic design loads. Results are presented to demonstrate the benefits and the performance limits of intracycle RPM control for large-scale 5 MW Darrieus VAWTs, namely, (1) power production (quantified in terms of AEP) that can be increased subject to baseline load limits and (2) opportunities to significantly increase AEP or decrease loads via intracycle RPM control that are examined for both two-bladed and three-bladed VAWTs.     

基于CFD的二维垂直轴风力机性能计算

采用计算流体力学方法(CFD)针对垂直轴风力发电机,开展简化的二维绕流特性研究。首先,基于开放型转子和增强型转子,研究网格节点数和壁面y+、计算时间步长和湍流模型等的变化对计算结果的影响,对计算模型和方法进行确认。随后,计算分析增强型垂直轴风力机与开放型垂直轴风力机的特性。结果表明,与开放性垂直轴风力发电机相比,增强型垂直轴风力发电机的功率系数和转矩系数有明显增加,且达到最大值的位置向叶尖速比增大的方向移动。然后对增强型垂直轴风力机发电机在不同来流风速下进行计算,发现增强型垂直轴风力发电机的转子转矩随来流风速增加,而转矩系数和功率系数与来流风速无关。最后,针对定子叶片在不同的方向开展计算研究。结果表明,定子叶片在不同方向时,增强型垂直轴风力机的转子转矩不同,且转矩到达峰值的位置也不同;在当前3个方向角中,叶片处于0°方向角时风力机具有最高的转矩系数,即具有最佳的功率系数。     

A computational hydrodynamics method for horizontal axis turbine – Panel method modeling migration from propulsion to turbine energy

A computational hydrodynamics method was formulated and implemented for horizontal axis tidal turbines. This paper presents a comparative analysis between screw propellers and horizontal axis turbines, in terms of geometry and motion parameters, inflow velocity analysis and the implementation methodologies. Comparison and analysis are given for a marine propeller model and a horizontal axis turbine model that have experimental measurements available in literature. Analysis and comparison are presented in terms of thrust coefficients, shaft torque/power coefficients, blade surface pressure distributions, and downstream velocity profiles. The effect of number of blades from 2 to 5, of a tidal turbine on hydrodynamic efficiency is also obtained and presented. The key implementation techniques and methodologies are provided in detail for the propeller based panel method tool to migrate as a prediction tool for tidal turbine. While the method has been proven to be accurate and robust for many propellers tested in the past, this numerical tool could be validated further for turbines. To further refine and validate the panel method for various turbines, it requires substantial additional experimental measurements. These measurements include downstream velocity profile by using LDV and/or SPIV, which are essential for numerical wake vortices descritization.     

水平轴潮流水轮机转轮尾流特性数值分析

针对潮流水轮机转轮尾流对机组之间水力性能的干扰问题,利用CFD分析软件Fluent对单个水平轴潮流水轮机转轮模型和10倍转轮直径间距下的两台机组模型在额定流速条件下进行三维流场的数值模拟。结果表明,在水轮机转轮旋转平面内不同半径位置处的尾流流速恢复情况明显不同,离旋转轴线越远,尾流流速恢复越快,其流速亏损也越少;当水轮机组之间串列布置,且来流方向与旋转平面垂直情况下,下游机组运行受上游机组转轮的尾流影响较大,应尽量避免该布置方式。     

遗传算法在水平轴洋流机叶片优化设计中的应用

水平轴洋流机是捕获洋流能的主要设备,其叶片外形直接影响捕能效率。通过Bezier参数化曲线描述定速定桨距洋流机的叶片弦长和扭角分布规律,采用叶素-动量理论计算其水动特性。以额定流速下能量利用系数系数最大为目标,基于遗传算法建立了叶片外形优化模型。同时,为了避免因汽蚀导致功率输出不稳定的现象,在优化过程中以汽蚀作为约束条件,与经典设计方法Wilson理论设计叶片进行了比较。结果表明:优化叶片在叶根处的扭角更小,具有更佳的抗扭性能;叶根和叶尖处弦长均更小,节省了材料;在设计流速范围内,优化叶片在低流速下效率更高,平均提高了4.6%,具有更好的启动性能。     

Dynamic response analysis of wind turbines under blade pitch system fault,grid loss,and shutdown events

This study focuses on the dynamic responses of land‐based and floating wind turbines under blade pitch system fault and emergency shutdown conditions. The NREL 5 MW turbine is studied. A hydraulic pitch system is considered, and the faults under study are events with a seized blade or a blade running out of control. Emergency shutdown is defined as a fast pitch‐to‐feather maneuver of the blades. Load cases with power production and grid fault with ensuing shutdown are also analysed for comparison. The fault scenarios and the blades' fast pitching activity are simulated using HAWC2 through external Dynamic Link Libraries. On the basis of the time‐domain simulations, the response characteristics of the land‐based and the floating turbines in the four design load cases are compared. The load effects from the fault conditions are compared with the operational cases. Strong system dynamics and resonant responses, such as the tower elastic mode and the yaw resonant response, are elicited during shutdown. If the pitch system has a fault and one blade is hindered from normal pitching, the uneven load distribution of the blades leads to large structural and motion responses. For both turbines, the response maxima vary cyclically with the instantaneous azimuth when the blades start pitching to feather. For the floating wind turbine, the interaction of waves and wind also affects the results. The effect of the pitch rate during shutdown is analysed. The responses of the land‐based turbine in grid loss and shutdown conditions are proportional to the pitch rate, whereas decreased sensitivity is found in the cases with pitch system faults. For the floating turbine, the effect of the pitch rate is small, and reduced pitch and yaw motion extremes are observed as the pitch rate increases. Copyright © 2013 John Wiley & Sons, Ltd.     

固定偏角垂直轴潮流能水轮机叶片安装位置试验研究

叶片安装位置是影响固定偏角垂直轴潮流能水轮机水动力性能的关键参数之一,为了研究其对水轮机性能影响的基本规律,建立了垂直轴水轮机水槽模型试验系统,设计了垂直轴水轮机性能和载荷测试方法。通过试验结果的分析,得到了叶片安装位置对水轮机能量利用率、推力系数、侧向力系数和合力系数的影响规律,为水轮机设计提供了试验依据。     

Modeling,simulation and control of a wind turbine with a hydraulic transmission system

A complete mathematical model of a hydraulic transmission concept for use in wind turbines is presented. The hydraulic system transfers the power from the nacelle to ground level. The main focus has been to develop a model that takes into account the most important dynamics affecting the wind turbine and the hydraulic transmission system involved, such that the model can be used to analyze the dynamic feasibility of a hydraulic transmission concept. Further, dynamic analysis of a hydraulic transmission system for wind turbines is investigated. The nonlinear dynamic model is developed in MATLAB Simulink. Analytical calculation of natural periods of a linearized model corresponds well with simulations of the overall system. A valve control system is proposed to reduce pressure and power fluctuations at operation both below and above the rated wind speed for the wind turbine. Further, a blade pitch control system based on an aerodynamic power estimator is proposed for operation above the rated wind speed. System simulations for one case below and one case above the rated wind speed show that the dynamic response of the overall system is stable and that the wind turbine variables are within typical ranges for conventional variable speed wind turbines with mechanical transmission. Copyright © 2012 John Wiley & Sons, Ltd.     

Numerical optimization of axial turbine with self‐pitch‐controlled blades used for wave energy conversion

Wells turbines are among the most practical wave energy converters despite their low aerodynamic efficiency and power produced. It is proposed to improve the performance of Wells turbines by optimizing the blade pitch angle. Optimization is implemented using a fully automated optimization algorithm. Two different airfoil geometries are numerically investigated: the standard NACA 0021 and an airfoil with an optimized profile. Numerical results show that each airfoil has its own optimum blade pitch angle. The present computational fluid dynamics optimization results show that the optimum blade pitch angle for NACA 0021 is +0.3° while that of the airfoil with an optimized profile equals +0.6°.The performance of the investigated airfoils is substantially improved by setting the blades at the optimum blade pitch angle. Both the turbine efficiency and tangential force coefficient are improved, especially at low flow rate and during turbine startup. Up to 4.3% average increase in turbine efficiency is achieved by optimizing the blade pitch angle. A slight improvement of the tangential force coefficient and decrease of the axial force coefficient are also obtained. A tangible increase of the stall‐free operating range is also achieved by optimizing the blade pitch angle. Copyright © 2013 John Wiley & Sons, Ltd.     

Design of bare and ducted axial marine current turbines

To convert the kinetic energy of marine current into electricity, the most sensible generator is a horizontal axis turbine. The know-how and the tools used for marine propulsion devices find a new range of applications in this field. An academic panel method code developed for the design of bare and ducted marine propellers was applied to design a marine current turbine. The turbine dimension and the tidal current velocity have been taken to fit the conditions in the Race of Alderney. The wing section theory and the optimum rotor theory based on the blade element momentum were used to obtain the design condition and a first geometry approaching the Betz limit for a bare rotor. The panel method was then used to verify the power coefficient obtained in the presence of the 3D effects and if the cavitation constraints are respected. Subsequently, the same panel code was used to verify if the addition of a duct could improve the power output per unit surface.     

Experimental study on blade pitch angle effect on the performance of a three‐bladed vertical‐axis Darrieus hydro turbine

Because of higher concerns about increasing global warming, energy consumption and reduction of conventional energy resources and growing attention are given to renewable energy and to cross flow turbines such as the Darrieus turbine to harness water energy (water currents, reservoirs, rivers, and oceans). The aim of the experimental investigation presented in this paper is to evaluate the effect of hydro Darrieus turbine blades fixation pitch angle “i_g” on its performance. Four main sets of experimental tests were conducted for the same vertical‐axis hydro turbine model (VAHT) with four blade fixation pitch angles (i_g = ?1.75°, ?4.5°, 1.75°, and 4.5°), at various water flow velocities (V = 0.3‐0.64 m/s corresponding to a water free flow Reynolds number of 2.5 × 10⁴ to 4.36 × 10⁴). A comparison between the results of the present work and with those of a previous experimental study for i_g = 0° shows that the best performance of the tested turbine is obtained when the blades are set at a pitch angle of 1.75°. In fact, the corresponding optimum mechanical power and power coefficient relative increases are respectively as much as 82% and 67% with respect to i_g = 0° at V = 0.37 m/s and as much as 65% and 77% at V = 0.46 m/s. The worst performance is obtained for the negative blades fixation pitch angle of ?4.5°; at the water flow velocity of 0.37 m/s, this leads to power, and power coefficient relative decreases respectively about 75% and 81% with respect to the results obtained for i_g = 1.75° and respectively about 54% and 68% of those obtained in the previous work for i_g = 0°.     

Limitations of fixed pitch Darrieus hydrokinetic turbines and the challenge of variable pitch

Small Darrieus hydrokinetic turbines with fixed pitch blades typically suffer from poor starting torque, low efficiency and shaking due to large fluctuations in both radial and tangential force with azimuth angle. Efficiency improves as size increases, since adequate blade chord Reynolds numbers can be maintained with low solidity. Shaking can be eliminated by using helical blades, or reduced by using multiple blades. Starting torque can be marginally improved by the use of cambered blade profiles but may still be inadequate to overcome drive train friction for self-starting. Variable pitch can generate high starting torque, high efficiency and reduced shaking but active pitch control systems add considerably to complexity and cost, while passive systems must have effective pitch control to achieve higher efficiency than fixed pitch systems.     

Power optimization of wind turbines with data mining and evolutionary computation

A data-driven approach for maximization of the power produced by wind turbines is presented. The power optimization objective is accomplished by computing optimal control settings of wind turbines using data mining and evolutionary strategy algorithms. Data mining algorithms identify a functional mapping between the power output and controllable and non-controllable variables of a wind turbine. An evolutionary strategy algorithm is applied to determine control settings maximizing the power output of a turbine based on the identified model. Computational studies have demonstrated meaningful opportunities to improve the turbine power output by optimizing blade pitch and yaw angle. It is shown that the pitch angle is an important variable in maximizing energy captured from the wind. Power output can be increased by optimization of the pitch angle. The concepts proposed in this paper are illustrated with industrial wind farm data.     

Experimental study of the effect of turbulence on horizontal axis wind turbine aerodynamics

Incident flows on wind turbines are often highly turbulent, because these devices operate in the atmospheric boundary layer and often in the wake of other wind turbines. This article presents experimental investigations of the effects of a high turbulence level on wind turbine aerodynamics. Power and thrust are measured on a horizontal axis wind turbine model in the ‘Lucien Malavard’ wind tunnel. A grid is used to generate three turbulence levels (4·4%, 9% and 12%) with integral length scale of the order of magnitude of the chord length. Experiments show little effect of turbulence on the wind turbine model power and thrust. This can be justified by analysis of the aerodynamic loads along the blade. Copyright © 2005 John Wiley & Sons, Ltd.