CFD-Based Performance Analysis and Experimental Investigation of Design Factors of Vertical Axis Wind Turbines under Low Wind Speed Conditions in Thailand

This paper presents effects of design factors on mechanical performance of Vertical Axis Wind Turbines (VAWTs), and an experimental investigation of optimal VAWT performance under low wind speed conditions in Thailand. Design factors include types of wind turbines, number of blades, types of materials, height-to-radius ratios, and design modifications. Potential VAWT models with different design factors are numerically analyzed within a virtual wind tunnel at various wind speeds by utilizing XflowTM?Computational Fluid Dynamics (CFD) software. The performance curves of each VAWT are obtained as plots of power coefficients against tip speed ratios. It is found that the type of wind turbine, number of blades, and height-to-radius ratio have significant effects on mechanical performance whereas types of materials result in shifts of operating speeds of VAWTs. Accordingly, an optimal VAWT prototype is developed to operate under actual low speed wind conditions. The performance curve from experimental results agrees with the CFD results. The proposed methodology can be used in the computer design of VAWTs to improve mechanical performance before physical fabrication.

Performance Assessment of Darrieus Turbine with Modified Trailing Edge Airfoil for Low Wind Speeds

Darrieus wind turbines are simple lift based machines with exceptionally high efficiencies in terms of power coefficient compared to similar drag based vertical axis turbines. However, in low Reynolds numbers, a notable performance loss was reported. As a potential solution, truncated NACA 0018 airfoil (NACA 0018TC-39) has been introduced with baseline cavity modification to achieve better start-up characteristics and to enhance the low wind speed performance. The baseline cavity will provide an additional benefit of reverse drag at low TSR which is obligatory for low wind speed start-up. Numerical optimization has been carried out on the conceived airfoil NACA 0018TC-39 to find out the effective truncation percentage in terms of the chord. The numerical study has been extended to compare NACA 0018 and NACA 0018TC-39 airfoil for their aerodynamic performances in terms of lift, drag coefficients and separation characteristics. The NACA 0018TC-39 airfoil was incorporated within a non-swept straight bladed Darrieus turbine miniature to experimentally evaluate the performance in terms of dynamic power coefficient, dynamic torque coefficient and static torque coefficient and compared with conventional NACA 0018 airfoil at six different Reynolds numbers 178917, 193827, 208737, 223646, 238556 and 268376. The experimental contrast implied that NACA 0018TC-39 airfoil turbine yielded almost double power coefficients at low Reynolds number compared to conventional NACA 0018 airfoil without hampering its performance at higher Reynolds number.

Study on Hydraulic Transmission of Offshore Low-speed Wind Energy Generator

For offshore hydraulic drive wind turbines,the problems of unsatisfactory speed control and low efficiency at low wind speeds are targeted.A low-speed high-torque radial piston pump is designed to replace the traditional fixed pump with a particular focus on its low-speed performance.The pump is characterized by small internal leakage at low wind speeds and high volumetric efficiency,which is beneficial to improve the power generation efficiency of the system.A new linear control method based on the PID algorithm and feedforward compensation was proposed to obtain the constant speed output control of variable motor at low wind speed.With the model for wind turbine and fixed pump-variable motor main drive system,the system was simulated and experimentally proved to verify the feasibility and anti-interference performance of the system control method at low wind speeds.A promising outcome was obtained on the response characteristics of system power and efficiency at low wind speeds.This can be a powerful technical support for the normal ustility of hydraulic drive wind turbines.

Experiment about Drag Reduction of Bionic Non-smooth Surface in Low Speed Wind Tunnel

The body surface of some organisms has non-smooth structure, which is related to drag reduction in moving fluid. To imitate these structures, models with a non-smooth surface were made. In order to find a relationship between drag reduction and the non-smooth surface, an orthogonal design test was employed in a low speed wind tunnel. Six factors likely to influence drag reduction were considered, and each factor tested at three levels. The six factors were the configuration, diameter/bottom width, height/depth, distribution, the arrangement of the rough structures on the experimental model and the wind speed. It was shown that the non-smooth surface causes drag reduction and the distribution of non-smooth structures on the model, and wind speed, are the predominant factors affecting drag reduction. Using analysis of variance, the optimal combination and levels were obtained, which were a wind speed of 44 m/s, distribution of the non-smooth structure on the tail of the experimental model, the configuration of riblets, diameter/bottom width of i mm, height/depth of 0.5 mm, arranged in a rhombic formation. At the optimal combination mentioned above, the 99% confidence interval for drag reduction was 11.13% to 22.30%.

Nocturnal Low-levelWinds and Their Impacts on Particulate Matter over the Beijing Area

Three-month wind profiles, 260 m PM_1 concentrations [i.e., particulate matter(PM) with an aerodynamic diameter ≤1μm], and carrier-to-noise ratio data at two Beijing sites 55 km apart(urban and suburban) were collected to analyze the characteristics of low-level nocturnal wind and PM in autumn and winter. Three mountain-plain wind events with wind shear were selected for analysis. The measurements indicated that the maximum wind speeds of the northerly weak low-level jet(LLJ) below 320 m at the suburban site were weaker than those at the urban site, and the LLJ heights and depths at the suburban site were lower than those at the urban site. The nocturnal 140 m mean vertical velocities and the variations in vertical velocity at the urban site were larger than those at the suburban site. A nocturnal breeze with a weak LLJ of ～3 m s^(-1) noticeably offset nocturnal PM transport due to southerly flow and convergence within the northern urban area of Beijing. Characteristics of the nocturnal LLJ, such as start-up time, structure, intensity, and duration, were important factors in determining the decrease in the nocturnal horizontal range and site-based low-level variations in PM.

Performance analysis of 20 Pole 1.5 KW Three Phase Permanent Magnet Synchronous Generator for low Speed Vertical Axis Wind Turbine

This paper gives performance analysis of a three phase Permanent Magnet Synchronous Generator (PMSG) connected to a Vertical Axis Wind Turbine (VAWT). Low speed wind condition (less than 5 m/s) is taken in consideration and the entire simulation is carried in Matlab/Simulink environment. The rated power for the generator is fixed at 1.5 KW and number of pole at 20. It is observed under low wind speed of6 m/s, a turbine having approximately1 mof radius and2.6 mof height develops 150 Nm mechanical torque that can generate power up to 1.5 KW. The generator is designed using modeling tool and is fabricated. The fabricated generator is tested in the laboratory with the simulation result for the error analysis. The range of error is about 5%-27% for the same output power value. The limitations and possible causes for error are presented and discussed.

Low Frequency Characteristics of Tropical Pacific Wind Stress Anomalies in Observations and Simulations from a Simple and a Comprehensive Models

Low frequency characteristics of tropical Pacific wind stress anomalies in observation and simulations from the CZ simple atmospheric model and COLA R15 AGCM are analyzed.The results show that ENSO event may be a multi-scale process,that is,ENSO time scale has the period longer than three years; biennial oscillation and annual variability.Dynamical characteristics are involved in the evolution process of wind stress anomaly with ENSO time scale: 1) the development and eastward movement of a cyclonic anomaly circulation in subtropical northwestern Pacific and weakening of Southern Oscillation result in the eastward propagation of westerly anomaly along the equator,therefore,interactions between flows in subtropics and in tropics play an important role in the evolution of wind stress anomaly with ENSO time scale; 2) easterly and westerly anomalies with ENSO time scale are one kind of propagating wave,which differs from Barnett's (1991).It is interesting that the evolution of observed and simulated wind stress anomalies with biennial time scale bears a strong resemble to that with ENSO time scale although their period is different.Observed annual variability is weak during 1979-1981 and intensified after 1981,especially it reaches to maximum during 1982-1984,and the spatial structure of the first mode is the ENSO-like pattern.

Piezoelectric Power Harvesting via Acoustic-Pressure Driven by Low-Speed Wind-Force with Resonating-Tube and Wind-Collector

Wind-driven power harvestings attract attentions since their target wind speeds are quite low less than the so-called cut-in wind speed, which is generally recognized as around 3 m/s. The extant power harvestings driven by wind-induced-air-column-resonations (i.e. acoustic-pressures) are still lacking simplicity, scale flexibility and solid strategies for practical applications. Therefore, the piezoelectric power harvesters via acoustic-pressures driven by low-speedwind-forces with resonating-tubes and wind-collectors were invented so as to complement all the lacks. The wind-collector as well as the resonating-tube contributed to upraise the power harvesting density. The champion power harvesting density of 19.5 nW/dm2 could be procured at 2.3 m/s of an artificial wind and the optimal resonating-tube and wind-collector. Power harvesting proofs from the natural wind with low mean speeds down to about 0.6 m/s were successfully obtained. The cut-in wind speed of the prototype piezoelectric power harvester was found to be quite low as about 0.4 m/s, signifying its ubiquity. Finally, a multi-bundle pendant-type piezoelectric power harvester was specifically presented together with professing the solid and multiple strategies for practical applications.

RELATIONSHIPS BETWEEN ZONAL WIND ANOMALIES IN HIGH AND LOW TROPOSPHERE AND ANNUAL FREQUENCY OF NW PACIFIC TROPICAL CYCLONES

Relationships between large-scale zonal wind anomalies and annual frequency of NW Pacific tropical cyclones and possible mechanisms are investigated with the methods of correlation and composition. It is indicated that when A U2oo- A U850 〉0 in the eastern tropical Pacific and A U2oo- A U850 〈0 in western tropical Pacific, the Walker cell is stronger in the Pacific tropical region and the annual frequency of NW Pacific tropical cyclone are above normal. In the years with zonal wind anomalies, the circulation of high and low troposphere and the vertical motions in the troposphere have significant characteristics. In the time scale of short-range climate prediction, zonal wind anomalies in high and low troposphere are useful as a preliminary signal of the annual frequency prediction of NW Pacific tropical cyclones.

STATISTICAL ANALYSIS OF LOW-LEVEL JET STREAMS IN NANJING AREA BASED ON WIND PROFILER DATA

In order to understand the activity characteristics of low-level jets in the Nanjing area,statistical analysis and comparative study are carried out on their monthly and diurnal variations,characteristics of their cores and accompanying weather conditions using wind profile data in 2005-2008 collected by two wind profilers.The results show that low-level jets have significant monthly and diurnal variations.They occur more frequently in spring and summer than in autumn and winter and are more active in early morning and at night,with the maximum wind speed usually occurring at midnight.The central part of the low-level jet occurs mainly at the height of less than 1400 meters,and the enhancement of central speed is beneficial to the appearance of precipitation.Meanwhile,when the low-level jet appears in summer,it helps cause heavy rain.The statistical results of the boundary wind profiler are well consistent with those of the tropospheric wind profiler.Two kinds of wind profilers also have the capability of continuously detecting the development of low-level jets.

计及高风速和低风速影响的海上风电机组两阶段滚动优化维护策略

针对复杂天气变化影响下海上风电机组维护决策问题,提出了考虑高风速与低风速影响的海上风电机组两阶段滚动优化机会维护策略。首先,提出考虑高风速冲击影响的维护役龄回退模型,量化实际部件退化与及维护成本关系。其次,建立了考虑低风速停机事件维护机会影响的海上风电机组长短期两阶段预防性机会维护模型,滚动更新长期预防性机会维护阈值。最后仿真结果表明,本文所提模型能够有效降低海上风电机组停机成本以及故障损失,提高机组寿命周期内的维护效益。

多频率组合波形阵风场模拟与测量研究

基于数值模拟方法选取设定FL-51风洞阵风发生器的装置参数,如发生器叶片的数量、展长、弦长及间距等。发生器采用液压摆动缸独立驱动形式,可模拟正弦波、三角波及随机波等多种波形的多频率组合运动,同时能够减少安装传动件,并提升发生器性能。通过阵风场校测结果表明:叶片能够实现摆动频率为0 Hz~15 Hz,在校测范围内发生器产生的正弦流场较均匀;在来流风速40 m/s、叶片摆动频率为10 Hz、摆角为15°时,最大阵风幅值为8.5 m/s。地面测试与流场校测结果表明:该文所提出的阵风场模拟技术可以实现低速风洞多频率多波形组合运动,为风洞试验提供复杂的阵风流场。

大型低速风洞全模阵风试验支撑装置研制与验证

为适应日益增多的低速风洞全模阵风试验的需要,发展阵风试验技术,在航空工业气动院FL-10风洞研制了一套双自由度支撑装置,该装置放开了模型升沉和俯仰方向的较大运动自由度,实现了飞机刚体运动模态的模拟。装置主要结构为“钢梁+小滑车”,小滑车可在钢梁上自由滑动,避免了采用滑轨形式导致模型运动过程中出现卡滞现象;采用整流翼型与风洞上下壁板连接,减小了对风洞的破坏,降低了对风洞流场的影响;装置升沉运动高度为3 m,俯仰角范围可达±34°;升沉摩擦因数小,机构变形量小,具备模型防护功能。应用该装置成功开展了民机阵风载荷减缓试验,证明了装置设计合理,可以应用于低速全模阵风试验。

海上风电低频输电系统快速母线保护

海上风电低频输电系统母线故障后,换流器型电源等效电势与阻抗因控制的切换与调整过程,不再像传统电网可视为恒定。文中理论分析了母线电流差动保护适应性,明确故障控制策略对差动保护灵敏度的影响方式。利用母线区内外故障所对应的故障等效模型不同,提出基于时域全量模型识别的母线保护。该保护原理反映被保护元件自身拓扑的变化,理论上具有不受电源特性影响的优势,同样适用于其他换流器型电网。同时,所提保护在时域中实现,不存在相量提取问题,动作速度快。最后,在PSCAD/EMTDC中搭建了仿真模型,验证了所提母线保护的可靠性和快速性。

计及低电压穿越及故障全过程动态的双馈风电场等值方法

在分析双馈感应发电机(DFIG)故障各阶段动态的基础上,提出了一种计及低电压穿越控制的双馈风电场等值方法。首先,讨论了低电压穿越控制策略,给出了含双馈风电场的电力系统故障稳态潮流计算方法。在考虑电压暂态过程的条件下,分析了外部系统短路故障后DFIG转子电流的变化机理,指出电压跌落深度及风电功率是影响短路电流的主要因素,可根据动作分界线判断撬棒是否动作。然后,根据初始风速及撬棒状态将风电场中的DFIG分为3群,并对等值DFIG及集电网络进行聚合。为提高等值模型在恢复阶段的精度,根据故障稳态电压对双馈风电场功率恢复曲线进行预估,并在此基础上对等值DFIG恢复速率进行分段修正。最后,对含双馈风电场的算例系统进行了等值计算。仿真结果表明,等值模型较好地保持了原系统在故障各阶段的动态,在保持较高精度的前提下有效简化了系统,提高了计算速度。

Reactive Current Allocation and Control Strategies Improvement of Low Voltage Ride Though for Doubly Fed Induction Wind Turbine Generation System

为满足风电机组低电压穿越（low voltage ride through,LVRT）的测试要求及其电气模型一致性评估需要,提出考虑向电网注入无功电流的双馈风电机组LVRT的控制策略。在阐述风电机组LVRT测试要求及控制原理的基础上,推导双馈发电机（doubly fed induction generator,DFIG）定子侧及网侧变流器输出无功电流极限表达式,研究电网电压跌落深度和发电机总输出有功功率对其无功电流极限值的影响规律,进而提出DFIG在LVRT期间的无功电流分配算法和改进的有功、无功功率控制策略。最后,以某实际2 MW双馈风电机组为例,分别对风速为5和12 m/s、电网电压对称跌落至20%和50%工况下的LVRT运行性能进行仿真比较和样机测试。与传统LVRT控制方法的对比表明,所提改进控制策略能更好地满足风电机组LVRT的测试要求。样机测试结果进一步证明了改进控制策略和仿真模型的有效性。

Wind-Driven Ocean Circulation in Shallow Water Lattice Boltzmann Model

A lattice Boltzmann (LB) model with overall second-order accuracy is applied to the 1.5-layer shallow water equation for a wind-driven double-gyre ocean circulation. By introducing the second-order integral approximation for the collision operator, the model becomes fully explicit. In this case, any iterative technique is not needed. The Coriolis force and other external forces are included in the model with second-order accuracy, which is consistent with the discretized accuracy of the LB equation. The numerical results show correct physics of the ocean circulation driven by the double-gyre wind stress with different Reynolds numbers and different spatial resolutions. An intrinsic low-frequency variability of the shallow water model is also found. The wind-driven ocean circulation exhibits subannual and interannual oscillations, which are comparable to those of models in which the conventional numerical methods are used.

基于D3AR的半球共形阵低空风切变风速估计方法

针对半球共形阵体制下进行低空风切变检测时会受到强地杂波信号的干扰,导致风切变信号难以检测的问题,提出了一种基于空时自回归的直接数据域算法(Space-Time Autoregressive Direct Data Domain,D3AR)的低空风切变风速估计方法。该方法首先将待检测距离单元的数据从空域、时域以及空时域进行信号对消处理;然后将处理后的数据矩阵描述为空时自回归(Autoregression,AR)模型并估计模型参数;再通过构造与杂波子空间正交的空间来实现对杂波的抑制,最后通过提取待检测单元的最大多普勒频率来估计风场速度。根据仿真结果显示,该方法有效地实现了地杂波抑制,并且能够精确估计风速。

卷积神经网络STAP低空风切变风速估计

由于机载气象雷达前视阵下存在非均匀性地杂波,导致难以获得足够的独立同分布样本,影响杂波协方差矩阵准确估计,进而影响风速估计。对此,该文提出一种基于卷积神经网络STAP的低空风切变风速估计方法,通过少量样本就能够实现高分辨杂波空时谱估计。首先,基于卷积神经网络模型训练好高分辨杂波空时谱卷积神经网络,接着计算杂波协方差矩阵,进而计算卷积神经网络STAP最优权矢量进行杂波抑制,达到对低空风切变风速精确估计。该文在小样本情况下,将稀疏恢复问题通过卷积神经网络实现,完成对高分辨杂波空时谱有效估计,仿真实验结果表明该方法可以有效估计空时谱,并完成风速估计。

螺旋桨驱动用低速大扭矩电机扁线绕组应用探索

针对低空航空器螺旋桨用低速大扭矩推进电机的使用需求,从绕组形式选取方面着手开展圆线方案与扁线方案综合对比分析。结果显示,扁线方案的散热性能更佳,但在设计加工、电磁性能、整机质量和效率方面不及圆线方案。分析结果可为低速大扭矩电机设计者在绕组选型、极槽组合等方面的选取提供参考。