Modeling of twin-turbine systems with vertical axis tidal current turbine: Part II—torque fluctuation

We recently showed the advantage of using a numerical system to extract energy from tidal currents by developing a new twin-turbine model (Li and Calisal, 2010a). Encouraged by this result, we decided to use this model to study another important characteristic of the turbine system, torque fluctuation. This effort is summarized in this paper. The torque fluctuation is expected to reduce the fatigue life of tidal current turbines, though potentially it also may deteriorate the power quality of tidal current turbines. In this paper, after reviewing the twin-turbine model, we use it to predict the torque fluctuation of the system with the same configurations as we used to study the power output in Li and Calisal (2010a). Specifically, we investigate the torque fluctuation of twin-turbine systems with various turbine parameters (e.g., relative distance between two turbines and incoming flow angle) and operational condition (e.g., tip speed ratio). The results suggest that the torque of an optimally configured twin-turbine system fluctuates much less than that of the corresponding stand-alone turbine, under the same operating conditions. We then extensively compare the hydrodynamic interaction’s impact on the torque fluctuation and the power output of the system. We conclude that the hydrodynamic interactions pose more constructive impacts on the torque fluctuation than on the power output. The findings indicate that the optimally configured counter-rotating system should be a side-by-side system, and that the optimally configured co-rotating system should have the downstream turbine partially in the wake of the upstream turbine depending on the detailed configuration of the turbines. Furthermore, one must balance the optimal torque fluctuation against the optimal power output.     

Design and Test of 1/5th Scale Horizontal Axis Tidal Current Turbine

Tidal current energy is prominent and renewable. Great progress has been made in the exploitation technology of tidal current energy all over the world in recent years, and the large scale device has become the trend of tidal current turbine (TCT) for its economies. Instead of the similarity to the wind turbine, the tidal turbine has the characteristics of high hydrodynamic efficiency, big thrust, reliable sealing system, tight power transmission structure, etc. In this paper, a 1/5th scale horizontal axis tidal current turbine has been designed, manufactured and tested before the full scale device design. Firstly, the three-blade horizontal axis rotor was designed based on traditional blade element momentum theory and its hydrodynamic performance was predicted in numerical model. Then the power train system and stand-alone electrical control unit of tidal current turbine, whose performances were accessed through the bench test carried out in workshop, were designed and presented. Finally, offshore tests were carried out and the power performance of the rotor was obtained and compared with the published literatures, and the results showed that the power coefficient was satisfactory, which agrees with the theoretical predictions.     

The effects of blade twist and nacelle shape on the performance of horizontal axis tidal current turbines

The paper presents the effects of blade twist and nacelle shape on the performance of horizontal axis tidal current turbines using both analytical and numerical methods. Firstly, in the hydrodynamic design procedure, the optimal profiles of untwisted and twisted blades and their predicted theoretical turbine performance are obtained using the genetic algorithm method. Although both blade profiles produce desired rated rotational speed, the twisted blade achieves higher power and thrust performance. Secondly, numerical simulation is performed using sliding mesh technique to mimic rotating turbine in ANSYS FLUENT to validate the analytical results. The Reynolds-Averaged Navier-Stokes (RANS) approximation of the turbulence parameters is applied to obtain the flow field around the turbine. It is found that power and axial thrust force from BEMT (Blade Element Momentum Theory) method are under-predicted by 2% and 8% respectively, compared with numerical results. Afterwards, the downstream wake field of the turbine is investigated with two different nacelle shapes. It is found that the rotor performance is not significantly affected by the different nacelle shapes. However, the structural turbulence caused by the conventional nacelle is stronger than that by the NACA-profiled shape, and the former can cause detrimental effect on the performance of the downstream turbines in tidal farms.     

Experimentally validated numerical method for the hydrodynamic design of horizontal axis tidal turbines

Although a lot can be learnt from technology transfer from wind turbines and ship propellers, there have been a few experiments investigating marine current turbines. As a result, a study has been carried out on the power, thrust and cavitation characteristics of 1/20th scale model of a possible 16 m diameter horizontal axis tidal turbine. Cavitation tunnel experiments for different blade pitch settings have been compared with simulations based on a developed blade element-momentum theory. This theory has been shown to provide a satisfactory representation of the experimental turbine performance characteristics. As an example application, the developed theory has been used to design possible horizontal axis tidal turbines for the tidal flows around Portland Bill. The results show that there is a clear balance between design loads and optimisation of energy yields.     

A Simulation Model for the Evaluation of the Electrical Power Potential Harnessed by a Marine Current Turbine

This paper deals with the development of a Matlab-Simulink model of a marine current turbine system through the modeling of the resource and the rotor. The simulation model has two purposes: performances and dynamic loads evaluation in different operating conditions and control system development for turbine operation based on pitch and speed control. In this case, it is necessary to find a compromise between the simulation model accuracy and the control-loop computational speed. The blade element momentum (BEM) approach is then used for the turbine modeling. As the developed simulation model is intended to be used as a sizing and site evaluation tool for current turbine installations, it has been applied to evaluate the extractable power from the Raz de Sein (Brittany, France). Indeed, tidal current data from the Raz de Sein are used to run the simulation model over various flow regimes and yield the power capture with time.     

Experimental Study on Aerodynamic Characteristics of the Model Wind Rotor System and on Characterization of A Wind Generation System

In order to investigate the aerodynamic characteristics of 6-MW wind turbine, experimental study on the aerodynamic characteristics of the model rotor system and on characterization of a wind generation system is carried out. In the test, a thrust-matched rotor system and a geometry-matched rotor system, which utilize redesigned thrustmatched and original geometry-matched blades, respectively, are applied. The 6-MW wind turbine system is introduced briefly. The proper scaling laws for model tests are established in the paper, which are then implemented in the construction of a model wind turbine with optimally designed blades. And the parameters of the model are provided. The aerodynamic characteristics of the proposed 6-MW wind rotor system are explored by testing a 1:65.3 scale model at the State Key Laboratory of Ocean Engineering at Shanghai Jiao Tong University. Before carrying out the wind rotor system test, the turbulence intensity and spatial uniformity of the wind generation system are tested and results demonstrate that the characterization of the wind generation system is satisfied and the average turbulence intensity of less than 10% within the wind rotor plane is proved in the test. And then, the aerodynamic characteristics of 6-MW wind rotor system are investigated. The response characteristic differences between the thrust-matched rotor system and the geometry-matched rotor system are presented. Results indicate that the aerodynamic characteristics of 6-MW wind rotor with the thrust-matched rotor system are satisfied. The conclusion is that the thrust-matched rotor system can better reflect the characteristics of the prototype wind turbine. A set of model test method is proposed in the work and preparations for further model basin test of the 6-MW SPAR-type floating offshore wind turbine system are made.     

Verification studies for a z-coordinate primitive-equation model: Tidal conversion at a mid-ocean ridge

We document the accuracy and convergence of solutions for a z-coordinate primitive-equation model of internal tide generation and propagation. The model, which is based on MOM3 numerics, is linearized around a state of rest to facilitate comparison with analytic estimates of baroclinic generation at finite-amplitude topography in a channel forced by barotropic tidal flow at its boundaries. Unlike the analytical model, the numerical model includes mixing of both buoyancy and momentum, and several definitions of “baroclinic conversion” are possible. These are clarified by writing out the energetics of the linearized equations in terms of barotropic kinetic energy, baroclinic kinetic energy, and available potential energy. The tidal conversion computed from the model, defined as the rate of conversion of barotropic kinetic energy into available potential energy, agrees well with analytical predictions. A comparison of different treatments of bottom topography (full-cells, partial-cells, and ghost-cells) indicates that the partial-cell treatment is the most accurate in this application. Convergence studies of flow over a smooth supercritical ridge show that the dissipation along tidal characteristics is, apparently, an integrable singularity. When the ocean bottom is not smooth, the accuracy and convergence of the model depend on the power spectrum of the topography. A numerical experiment suggests that the power spectrum of the resolved topography must roll off faster than k⁻² to obtain convergent results from a linear numerical model of this type.     

Exploring the implications of tidal farms deployment for wetland-birds habitats in a highly protected estuary

Next generation turbines can be installed to exploit tidal energy resource in estuaries. However, the ecological implications of such deployments remain unknown. Accordingly, this study is focused on the impact of tidal farms on intertidal areas and associated waterbird habitats in estuaries. A numerical model simulates the hydrodynamic conditions of the Solway Firth, comprising one of the most extensive intertidal zones in the UK. The effects of tidal farms with different turbine densities have been evaluated in terms of changes to the areas of habitat for wetland bird species of conservation importance. The results suggest that any habitat loss may be minor and that in some cases there may be an increase (also small) in the area of important habitat. The percentage change in intertidal areas is also considerably lower than that expected to result from tidal barrages of similar capacities, implying the potential for tidal farms to present an alternative solution for exploiting tidal energy in highly protected areas. The changes in the extent of intertidal habitat are not proportional to the energy dissipated by the tidal farm. Therefore a range of tidal farm designs should be evaluated in order to optimise installed capacity while minimising habitat loss. However, the site-dependency of the impacts and the complex responses to change of estuarine ecosystems reinforce the requirement for detailed studies to be undertaken for each specific location and array configuration. The methodology presented here is appropriate for such assessments as it can be applied to different locations and turbine types.     

人工神经网络在潮汐数值预报中的应用

潮汐数值预报经过了几十年的发展,但是其预报精度并不能让人十分满意,本文试图将传统的潮汐数值预报模式与近年来发展迅速的人工神经网络相结合并改进潮汐数值预报的精度。文章建立了一个神经网络系统,采用潮汐数值模式的输出结果作为网络输入,潮位观测资料作为输出,用建立的神经网络进行训练,结果表明人工神经网络可以明显地改进潮汐数值预报的精度。     

Model testing and performance comparison of plastic and metal tidal turbine rotors

Experimental model tests were conducted to predict the performance of two sets of metal and plastic bi-directional tidal turbine rotors. This test programme aims to provide reliable and accurate measurement data as references for developers, designers and researchers on both model and full scale. The data set presented in this paper makes available the detailed geometry and motion parameters that are valuable for numerical tools validation. A rotor testing apparatus that was built using an off-the-shelf K&R propeller dynamometer, its configuration, testing set-up, calibration of the apparatus and data acquisition are described. Comparison analysis between the metal and plastic rotors hydrodynamic performance in terms of torque, drag and derived power and drag coefficients are also presented. The results show a substantial decrease in maximum power performance for the plastic rotors – about 40% decrease at a tip speed ratio of around 3.0, compared with rigid metal rotors. The plastic rotors have also a much higher cut-in speed. It showed that materials for rotor models with low rigidity such as polyamide (nylon) produced by selective laser sintering (SLS) systems may substantially under-predict power generation capacity. As a result, they are considered unsuitable for rotor model performance evaluation.     

潮流能水轮机仿真及尾流场分析

为了研究潮流能水轮机尾流场的变化规律,本文在水平轴水轮机现有理论基础上,采用计算流体力学技术建立了数值模型,计算了水轮机的功率特性,进行了网格独立性验证,比较了模型中旋转区域不同对结果的影响。仿真结果与试验结果进行了对比分析,验证了数值模型的有效性。研究了全工况下尾流场速度衰减随尖速比和距离变化的规律,结果显示在小尖速比下尾流场速度恢复情况相对大尖速比下较好;靠近水轮机轮毂中心线位置处,衰减相对较大。研究建议水轮机在同等功率表现下尽量选择在小尖速比下工作,水轮机组在实际排列时避免采用成列布置,水轮机设计阶段应综合考虑功率特性和尾流场衰减。本文研究内容可以为潮流能水轮机的研究以及阵列式应用提供参考。     

中国东部边缘海潮波系统形成机制的模拟研究

以三维高精度潮波数值模拟为基础,动用系统分析方法和地理信息系统技术,对影响中国东部边缘海潮波系统的因素,包括入射潮波、科氏力、海区地形、岸线形状及底摩擦等,进行了模拟试验。结果表明,它们对潮波系统的模式有不同程度的作用,其中科氏力的有无、岸线形态的变化和水下地形的巨变化对潮波系统有重要影响。研究认为,中国东部边缘海潮波系统是一个海－潮相互作用的整体;在一定的入射潮波条件下,该区的海区条件、特别是岸     

Numerical modeling of a wave energy converter based on U-shaped interior oscillating water column

The paper presents a concept of a wave energy converter and the numerical model to calculate the hydrodynamic responses in waves and the power produced by the power take off system. The system consists of an asymmetric floater with an interior U-tank partially filled with water and two lateral air chambers connected by a duct. The motion of the U-shaped oscillating water column, mainly induced by the rolling of the floater, forces the air through the duct where a Wells turbine is installed to absorb the wave energy.The wave-floater hydrodynamics is calculated with a Green's function panel method, while the oscillating water column motions hydro-mechanics are derived from the one-dimensional Euler's equation. The dynamics of the Wells turbine is realistically represented by one additional differential equation on the unknown air pressure fluctuation. This equation is derived assuming small amplitude motions of the water column and assuming the linear isentropic relation is valid for the air thermodynamics in the air chambers. The Wells turbine is characterized by a drastic drop of efficiency above a critical pressure value due to stalling on the blades. The effect of a by-pass valve to prevent stalling is introduced in the numerical model in a simplistic way. The numerical model is implemented and tested for a wave energy converter with a displacement of 1150 t, including 490 t for the interior water column, and an installed turbine with 2.3 m of diameter. An analysis of the influence of changing different design parameters on the system efficiency is also presented.     

带偏航角串列式两风机复杂尾流场数值模拟

在多风机风电场中,通过主动调节上游风机的偏航角度,抑制上游风机尾流对下游风机的影响,减少风力机机组之间的尾流相互干扰,以达到提高整个风电场效率的目的。采用基于开源平台Open FOAM自主开发的FOWT-UALM-SJTU求解器中风电场求解模块ALMWindFarmFoam,将致动线模型与CFD方法相结合,利用大涡模拟(LES)计算研究当上游风机处于不同偏航角度时,两风机之间的复杂尾流干扰效应。对比分析偏航角度改变时,上下游风机气动功率的输出特性,尾流速度变化以及风机的尾涡结构。数值模拟结果表明:在上下游风机沿流向方向距离保持不变的情况下,随着上游风机偏航角度的变化,上下游风机的尾流干扰现象以及下游风机的入流条件会发生明显改变,并会对下游风机的气动功率输出以及两风机风电场的整体流场产生显著影响。     

Numerical and Experimental Study of the 3D Effect on Connecting Arm of Vertical Axis Tidal Current Turbine

Vertical axis tidal current turbine is a promising device to extract energy from ocean current. One of the important components of the turbine is the connecting arm, which can bring about a significant effect on the pressure distribution along the span of the turbine blade, herein we call it 3D effect. However, so far the effect is rarely reported in the research, moreover, in numerical simulation. In the present study, a 3D numerical model of the turbine with the connecting arm was developed by using FLUENT software compiling the UDF (User Defined Function) command. The simulation results show that the pressure distribution along the span of blade with the connecting arm model is significantly different from those without the connecting arm. To facilitate the validation of numerical model, the laboratory experiment has been carried out by using three different types of NACA aerofoil connecting arm and circle section connecting arm. And results show that the turbine with NACA0012 connecting arm has the best start-up performance which is 0.346 m/s and the peak point of power conversion coefficient is around 0.33. A further study has been performed and a conclusion is drawn that the aerofoil and thickness of connecting arm are the most important factors on the power conversion coefficient of the vertical axis tidal current turbine.     

近海测波多传感器优化配置模拟与设计研究

研究了测波浮标中GPS传感器和加速度传感器共同作用下对近海测波的影响。介绍了测波浮标数据采集系统的基本框架和硬件设计;为提高测波浮标系统的测量精度,对多传感器进行了优化配置和信息的融合处理,并通过仿真验证了该设计性能的优越性。     

The effects of yawing motion with different frequencies on the hydrodynamic performance of floating vertical-axis tidal current turbines

Under real sea conditions, the hydrodynamic performance of floating vertical-axis tidal current turbines is affected by waves and currents. The wave circular frequency is a significant factor in determining the frequencies of the wave-induced motion responses of turbines. In this study, the ANSYS-CFX software (manufacturer: ANSYS Inc., Pittsburgh, Pennsylvania, United States) is used to analyse the hydrodynamic performance of a vertical-axis turbine for different yawing frequencies and to study how the yawing frequencies affect the main hydrodynamic coefficients of the turbine, including the power coefficient, thrust coefficient, lateral force coefficient, and yawing moment coefficient. The time-varying curves obtained from the CFX software are fitted using the least-squares method; the damping and added mass coefficients are then calculated to analyse the influence of different yawing frequencies. The simulation results demonstrate that when analysing non-yawing turbines rotating under constant inflow, the main hydrodynamic coefficient time-varying curves of yawing turbines exhibit an additional fluctuation. Furthermore, the amplitude is positively correlated with the yawing frequency, and the oscillation amplitudes also increase with increasing yawing frequency; however, the average values of the hydrodynamic coefficients (except the power coefficient) are only weakly influenced by yawing motion. The power coefficient under yawing motion is lower than that under non-yawing motion, which means that yawing motion will cause the annual energy production of a turbine to decrease. The fitting results show that the damping term and the added mass term exert effects of the same level on the loads and moments of vertical-axis turbines under yawing motion. The results of this study can facilitate the study of the motion response of floating vertical-axis tidal current turbine systems in waves.     

Research on Pitch Control Strategies of Horizontal Axis Tidal Current Turbine

Based on blade element momentum theory and generator characteristic test, a dynamic simulation model of 150 kW horizontal-axis tidal current turbine was established. The matching of the dynamic characteristics between the turbine and generator under various current velocities is studied, and the influence of the pitch angle on the matching is analyzed. For the problem of maximum power output in case of low current speed and limiting power in high current speed, the relation between optimal pitch angle and output power is analyzed. On the basis of dynamic characteristic analysis, the variable pitch control strategy is developed. The performance of the turbine under various tidal conditions is simulated. The research results show that the designed controller enables the turbine to operate efficiently under the condition of low current speed, and achieve the goal of limited power at high current speed.     

用于振荡水柱波能系统的径流式空气透平数值模拟研究

径流式空气透平是振荡水柱式波能发电系统的二级能量转换装置,因其具有结构简单、输出扭矩大、轴向推力小的优势,得到了越来越多的关注。采用计算流体力学软件Ansys-Fluent 12.0,通过数值模拟方法考察了不同转子叶片稠度对径流式空气透平定常无量纲评价参数的影响规律。研究结果表明:虽然径流式透平可在双向气流下实现单向旋转,但当气流通过方向不同时,透平的定常工作性能存在明显差异,这与传统轴流式透平完全不同;此外,稠度对透平效率的影响在不同流量系数范围内也不同。因此,综合考虑气流流向、透平整个工作范围内输出效率,推荐采用的转子叶片稠度为2.34。     

基于致动线模型的错列式两风机尾流场数值模拟

风电场中风机之间存在十分复杂尾流相互干扰现象,尾流相互干扰效应对风机的功率输出、叶片载荷等产生十分显著的影响。采用致动线模型以及计算流体力学方法,研究两风机之间的复杂尾流干扰效应。在保持两风机纵向间距一样的情况下,考虑两风机在不同横向间距下,数值模拟两风机的部分尾流相互干扰现象,分析两风机的气动功率输出特性、尾流速度变化特性、风轮平面附近轴向诱导因子分布特性,尾涡结构以及尾流干扰效应。数值模拟结果表明:在上下游风机在沿着流向方向距离保持不变的情况下,随着横向间距的变化,上下游风机的尾流存在十分复杂的尾流相互干扰效应,对下游风机的气动功率输出以及两风机风电场的整体流场产生了显著的影响。