Experimental investigation of helical Savonius rotor with a twist of 180°

Conventional Savonius rotors have low performance such as low coefficient of power and low coefficient of torque. In order to increase this performance, a helical Savonius rotor with a twist of 180° is proposed. In this paper, we are interested in studying the aerodynamic behavior of the helical Savonius rotors installed in an open jet wind tunnel. Particularly we are interested in studying the influence of variation of Reynolds number and the overlap ratio on the performance of a modified Savonius rotor with aspect ratio of 1.57 at a Reynolds numbers equal to Re = 79,794, Re = 99,578, Re = 116,064 and Re = 147,059. Results conclude that the variation of Reynolds number and overlap ratio has an effect on the global characteristics of the helical Savonius rotor. A comparison between the helical one and the conventional one shows that the maximum power coefficient of the Savonius wind rotor is higher. This work is developed at Laboratory of Electro-Mechanical System (LASEM) of the National School of Engineers of Sfax (ENIS).     

Comparative study of a three-bucket Savonius rotor with a combined three-bucket Savonius–three-bladed Darrieus rotor

The vertical axis wind turbines are simple in construction, self-starting, inexpensive and can accept wind from any direction without orientation. A combined Savonius–Darrieus type vertical axis wind rotor has got many advantages over individual Savonius or individual Darrieus wind rotor, such as better efficiency than Savonius rotor and high starting torque than Darrieus rotor. But works on the combined Savonius–Darrieus wind rotor are very scare. In view of the above, two types of models, one simple Savonius and the other combined Savonius–Darrieus wind rotors were designed and fabricated. The Savonius rotor was a three-bucket system having provisions for overlap variations. The Savonius–Darrieus rotor was a combination of three-bucket Savonius and three-bladed Darrieus rotors with the Savonius placed on top of the Darrieus rotor. The overlap variation was made in the upper part, i.e. the Savonius rotor only. These were tested in a subsonic wind tunnel available in the department. The various parameters namely, power coefficients and torque coefficients were calculated for both overlap and without overlap conditions. From the present investigation, it is seen that with the increase of overlap, the power coefficients start decreasing. The maximum power coefficient of 51% is obtained at no overlap condition. However, while comparing the power coefficients (C_p) for simple Savonius-rotor with that of the combined configuration of Savonius–Darrieus rotor, it is observed that there is a definite improvement in the power coefficient for the combined Savonius–Darrieus rotor without overlap condition. Combined rotor without overlap condition provided an efficiency of 0.51, which is higher than the efficiency of the Savonius rotor at any overlap positions under the same test conditions.     

Experimental investigations on single stage,two stage and three stage conventional Savonius rotor

The performance of single stage (rotor aspect ratio of 1.0), two stage Savonius rotor with rotor aspect ratios of 1.0 and 2.0 (stage aspect ratios of 0.50 and 1.0) and three stage Savonius rotor with rotor aspect ratios of 1.0 and 3.0 (stage aspect ratios of 0.33 and 1.0) are studied at different Reynolds numbers and compared at the same Reynolds number. The results show that the coefficient of power and the coefficient of torque increase with the increase in the Reynolds numbers for all the rotors tested. The coefficient of static torque is independent of the Reynolds number for all the rotors tested. The performance of two stage and three stage rotors remains the same even after increasing the stage aspect ratio and the rotor aspect ratio by a factor of two and three, respectively. For the same rotor aspect ratio of 1.0, by increasing the number of stages (stage aspect ratio decreases), the performance deteriorates in terms Cp and Ct. However, at the same stage aspect ratio of 1.0 and same Reynolds number, two and three stage rotors show the same performance in terms of coefficient of power and coefficient of torque. The variation in coefficient of static torque is lower for a three stage rotor when compared with the variation of coefficient of static torque for two stage or single stage rotor. Copyright © 2008 John Wiley & Sons, Ltd.     

Investigation of 3D printed Savonius rotor performance

In this study increasing the performance of Conventional Savonius wind rotor has been investigated by a 3D (three dimensional) printer which is one of the rapid prototyping techniques. For this purpose, some design changes have been introduced to increase the performance of conventional Savonius wind rotor. Here, 3D digital designing of Savonius wind rotors have been easily manufactured tangible as a physical model by a 3D printer. Experimental data concerning produced Savonius wind rotors have been acquired by using a wind tunnel. Some numerical data have been obtained from the CFD (Computational Fluid Dynamics) analysis carried out under the same conditions. The effects of the additional blade end design have been examined to obtain more torque increase on improved classical Savonius wind rotor. Furthermore, by means of introducing straight blade, the effects of the flow compression inside the blade have been reduced and rotor performance increased. Based on such optimizations, optimum additional design parameters have been designated as that (1/r) ratio is 0.3, (s/r) is 1, and (α) additional straight blade angle is 135°. It has been determined that power coefficient is increased at a ratio of around 20% together with all these design changes.     

The use of a curtain design to increase the performance level of a Savonius wind rotors

In this study, a curtain design has been arranged so as to improve the low performance levels of the Savonius wind rotors. Designed to prevent the negative torque on the convex blade of the rotor, this curtain has been placed in front of the rotor, and performance experiments have been carried out when the rotor is with and without curtain. It has been determined from here that a significant increase can be achieved in the rotor performance by means of the curtain design. Experiments of the curtain design have been conducted in three different dimensions when the Savonius wind rotor is static, and the highest values have been obtained with the curtain 1. Therefore, the curtain designs and curtain angles in which the highest values obtained have been analyzed numerically with Fluent 6.0 program and the results obtained experimentally have been supported with numerical analysis. Moreover, performance experiments have been made for the curtain 1 with which the best performance values have been obtained when the rotor is in its dynamic position, and the results obtained have been given in figures.     

Influence of the deflector plate on the performance of modified Savonius water turbine

Savonius rotor is simple in design and easy to fabricate at a lower cost. The basic driving force of Savonius rotor is drag. The drag coefficient of a concave surface is more than the convex surface. Hence, the advancing blade with concave side facing the water flow would experience more drag force than the returning blade, thus forcing the rotor to rotate. Net driving force can be increased by reducing the reverse force on the returning blade. This can be realized by providing flow obstacle to the returning blade. The objective of the present work is to find out the optimal position of the deflector plate upstream to the flow which would result in maximum power generated by the rotor. Experimental investigations are carried out to study the influence of the location of the deflector plate on the performance of a modified Savonius rotor with water as the working medium at a Reynolds number of 1.32 × 10⁵. Eight different positions of the deflector plate are attempted in this study. Results conclude that deflector plate placed at its optimal position increases the coefficient of power by 50%. Maximum coefficient of power is found to be 0.21 at a tip speed ratio of 0.82 in the presence of deflector plate. Two stage and three stage modified Savonius rotors are tested to study the influence of deflector plate at the optimal position. Maximum coefficient of power improves by 42%, 31% and 17% with deflector plate for two stage 0° phase shift, 90° phase shift and three stage modified Savonius rotor respectively.     

Influence of overlap ratio and aspect ratio on the performance of Savonius hydrokinetic turbine

A strong contender for the utilization of the hydropower sources is Savonius turbine because of its good starting characteristics and simplicity in fabrication. The aim of the present work is to study the effect of overlap ratio and aspect ratio on the performance of a Savonius turbine for hydrodynamic application. The findings would be useful to decide design parameters of a Savonius hydrokinetic turbine. This study is conducted in three open channels namely small laboratory channel with depth of water 270 mm, large laboratory channel with depth of water 480 mm and a real life irrigation canal with depth of water 900 mm. Influence of endplates is studied to establish the significance of their presence on rotor performance. Maximum coefficient of power is observed for an overlap ratio of around 0.11 for Savonius turbines with aspect ratio less than 0.6. For a given overlap ratio, the increasing trend of the coefficient of power is observed with the increase in the aspect ratio. However, for aspect ratios greater than 1.8, the coefficient of power becomes nearly stagnant. Copyright © 2016 John Wiley & Sons, Ltd.     

叶片转角对小型Savonius风机气动性能的影响

Savonius风机是一种典型的垂直轴风力发电机,通过对其进行流固耦合分析,研究叶片转角对风机气动性能的影响。利用ANSYS的CFX流体模块,流体湍流模型选择基于RANS的标准k-ε湍流模型,对风轮进行流固耦合分析,从而获得叶片产生的力矩情况,并计算了风机的功率特性。利用求解结果,得到了力矩系数与叶片转角之间的关系。分析了风机叶片在旋转一周中所产生的最大扭矩以及负扭矩所处的位置和范围。通过分析转角对风机性能的影响,可为今后的Savonius风机叶片形状优化和效率提升提供参考。     

Performance investigation of a small Savonius-Darrius counter-rotating vertical-axis wind turbine

This paper describes the study of a small vertical-axis wind turbine (VAWT) with a combined design of Darrius and Savonius counter-rotating rotors. The main purpose of this study is to improve the extraction capabilities of a single-rotor VAWT by using two distinct rotor designs while adopting the counter-rotating technique. Given that the conversion capabilities and operational speed of the existing wind turbines are still limited, the current technique is used to enhance the efficiency and expand the operating wind speed range of the VAWT. The Darrius and Savonius counter-rotating rotors were exposed to a similar upstream wind speed using a centrifugal blower. It was found that the Savonius-Darrius counter-rotating rotor was able to operate effectively, particularly at the low-speed wind. By looking at the individual performance of the rotors, it was observed that the conversion efficiency of the H-type rotor increases as the wind speed increases. However, in the case of the S-type rotor, it is higher at lower wind speed and tends to decrease as the operating speed increases. Thus, the maximum efficiency of the S-type rotor was achieved at low speed, whereas the H-type rotor has achieved its maximum efficiency at the highest operating wind speed. The average efficiency of the present Savonius-Darrius counter-rotating rotor has been improved to reach almost 42% and 30% more efficiency in terms of torque and power, respectively.     

阻力型垂直轴风力机导风帘的优化设计

以输出功率100W的阻力型垂直轴型风力机为例,通过优化设计找寻最佳的叶片数目、高径比和折叠率等几何参量,然后加入导风帘装置,给出一个最佳结构方案。运用Fluent求解器中的有限体积法对无导风帘和三种不同长度导风帘设计的叶片周围的流场进行模拟。模拟与最佳优化方式的性能参数的计算结果表明,采用长导风帘设计的阻力型垂直轴风力机具有最好的动力学性能,可以提高运行效率。     

Innovative improvement of a drag wind turbine performance

Drag type wind turbines have strong potential in small and medium power applications due to their simple design. However, a major disadvantage of this design is the noticeable low conversion efficiency. Therefore, more research is required to improve the efficiency of this design. The present work introduces a novel design of a three-rotor Savonius turbine with rotors arranged in a triangular pattern. The performance of the new design is assessed by computational modeling of the flow around the three rotors. The 2D computational model is firstly applied to investigate the performance of a single rotor design to validate the model by comparison with experimental measurements. The model introduced an acceptable accuracy compared to the experimental measurements. The performance of the new design is then investigated using the same model. The results indicated that the new design performance has higher power coefficient compared with single rotor design. The peak power coefficient of the three rotor turbine is 44% higher than that of the single rotor design (relative increase). The improved performance is attributed to the favorable interaction between the rotors which accelerates the flow approaching the downstream rotors and generates higher turning moment in the direction of rotation of each rotor.     

小型Savonius风机的创新设计与仿真分析

Savonius风机是一种典型的垂直轴风力机。针对传统风机的发电机部分采用转子、定子一动一静的设计布局,提出了一种新型的发电机结构,从而提高风机发电效率。通过有限元分析软件ANSYS／CFX,对风力机模型进行流体分析,计算Savonius风力机的效率,验证本设计分析方法的正确性。     

重叠比对三叶片Savonius风力机启动性能影响研究

为获得相对平稳且非负的启动力矩,针对三叶片Savonius风力机开展研究。首先对比研究两叶片和三叶片Savonius风力机的启动性能和输出功率特性。在此基础上,针对其主要的结构参数重叠比开展研究。针对三叶片的结构特点,提出重叠比和净重叠比的定义方式,设置9组不同净重叠比,范围在0～0.36（重叠比范围0.14～0.50）之间。利用数值模拟和风洞试验相结合的方法,研究在不同风速下重叠比对Savonius风力机启动力矩以及输出功率性能的影响。结果表明：净重叠比可消除反向启动力矩,并提升三叶片Savonius风力机的启动性能,平均启动力矩系数最高提升147.06%。净重叠比在0.06～0.11范围内时,对风力机的输出功率有提高作用。     

On the performance analysis of Savonius rotor with twisted blades

The present investigation is aimed at exploring the feasibility of twisted bladed Savonius rotor for power generation. The twisted blade in a three-bladed rotor system has been tested in a low speed wind tunnel, and its performance has been compared with conventional semicircular blades (with twist angle of 0°). Performance analysis has been made on the basis of starting characteristics, static torque and rotational speed. Experimental evidence shows the potential of the twisted bladed rotor in terms of smooth running, higher efficiency and self-starting capability as compared to that of the conventional bladed rotor. Further experiments have been conducted in the same setup to optimize the twist angle.     

Multiple streamtube approximation of flow‐induced forces on a Savonius wind turbine

This paper develops a new approximate model to predict the pressure and momentum forces on a Savonius‐style vertical axis wind turbine. Flow distributions through and around the turbine are examined for analytical predictions of the torque and power output, at all rotor angles. A new approximate streamtube method is developed to predict the momentum, lift, and drag forces on the rotor surfaces by the air stream on the basis of an integral force balance on the turbine blades. Unlike other past analytical methods, the technique predicts both momentum and pressure forces imposed on the rotor surface during operation. The calculated results are validated against experimental data and numerical predictions from computational fluid dynamics. Copyright © 2012 John Wiley & Sons, Ltd.     

Studies of some high solidity symmetrical and unsymmetrical blade H-Darrieus rotors with respect to starting characteristics,dynamic performances and flow physics in low wind streams

Vertical axis wind turbines can be successfully installed in low wind speed conditions but its detailed starting characteristics in terms of starting torque, starting time and dynamic performances have not been investigated thoroughly which is important for increasing the energy yield of such turbines. Amongst their designs, H-Darrieus rotor, in spite of having good power coefficient, possesses poor self-starting features as symmetrical blade profiles are used most of the times. Instead of using symmetrical blades if unsymmetrical or cambered blades are used with high solidity, then starting performance of H-Darrieus rotor along with its power coefficients can be improved. Though this performance improvement measures are correlated with improvement in the starting characteristics, a detailed work in this direction would be useful and for this reason the present work has been carried out. Three types of blade designs have been considered; two unsymmetrical blades namely S815 and EN0005 and one conventional symmetrical NACA 0018 blade, and experiments are conducted using a centrifugal blower test rig for three-bladed H-Darrieus rotors using these three considered blades at low wind streams (4 m/s, 6 m/s and 8 m/s). Considering reality, the effects of flow non-uniformity and turbulence intensity on the rotor performance at optimum condition as well as flow physics have also been studied. It has been found that unsymmetrical S815 blade rotor has higher dynamic torque and higher power coefficient than unsymmetrical EN0005 and symmetrical NACA 0018 blade H-Darrieus rotors.     

Measurements and modelling of the power performance of a model floating wind turbine under controlled conditions

This paper describes power performance measurements undertaken on a model floating wind turbine installed on a tension‐leg platform (TLP) in a wind/wave generator facility. Initially, the surge of the platform was measured under different rotor and wave conditions. The surge behaviour depended considerably on the rotor tip speed ratio and the wave frequency and amplitude. High‐frequency data sampling techniques were then used to derive the instantaneous power coefficient and tip speed ratio directly from the measurements, together with the surge velocity of the floating system. The power measurements were compared with those predicted by three independent numerical models, two of which are based on the blade element momentum approach and the third involving a lifting‐line free‐wake vortex model. The fluctuations of the power coefficient with time predicted by the three models were in close agreement; however, these were all significantly larger than those derived from the rotor shaft torque measurements. This was found to be due to the limitations of the torque measurement technique. Although being accurate in measuring the time‐averaged torque, the sensor was incapable of measuring the high‐frequency low‐amplitude fluctuations in the rotor shaft torque induced by the TLP surge. This was confirmed using an alternative experimental technique involving hot‐wire near‐wake measurements. The study also investigated the influence of the platform surge motion on the time‐averaged power coefficients. Both the measurements and the free‐wake vortex model revealed marginal deviations in the time‐averaged power coefficients when compared with those obtained for a fixed, non‐surging rotor. Copyright © 2014 John Wiley & Sons, Ltd.     

Effects of eccentricities on shaft signals studied throughwindingless rotors

Shaft signals of electric rotating machines offer potentials for defect detection. The signals are affected by many factors. This study specifically focuses on how eccentricities affect shaft signals through theoretical predictions and tests conducted on windingless rotors. Windingless rotors imply currentless rotors. For symmetrical synchronous machines running at steady synchronous speeds, the induced rotor currents are zero, while harmonics are neglected. One advantage in experiments for this study is that air gaps can be shimmed accurately at standstill without end brackets; certain tests can be conducted at standstill without facing unmanageable locked-rotor currents for the test machines. Shaft signals decrease when the rotor of a machine closely situates at the center of stator bore. Shaft signals increase under greater eccentricities when stator and rotor axes are parallel. Tilted rotors reduce shaft signals. Even when there are no rotor windings, inherent positional characteristics exists. This characteristic produces cyclic shaft-signal components that are related to rotor revolutions     

A double-step Savonius rotor for local production of electricity: a design study

This paper presents a study, carried out with the help of the French Agency of Innovation (ANVAR). It deals with the conception of a small Savonius rotor (i.e. of low power) for local production of electricity. Our challenge was to design, develop and ultimately build a prototype of such a rotor, which was considered as a complete electromechanical system. An optimised configuration was chosen for the geometry of the present prototype. The building data were calculated on the basis of the nominal wind velocity V=10 m/s. Particular care was necessary to choose the appropriate generator, which was finally a rewound conventional car alternator. The whole design of the prototype has confirmed the high efficiency and the low technicality of the Savonius rotors for local production of electricity. The present prototype is to be tested in situ.