Shigetomi et al. [1] studied the interactive flow field around two Savonius wind rotors by experimental investigation using particle image velocimetry.
They found that there exist power-improvement interactions between two rotating Savonius rotors in appropriate arrangements. The interactions are caused by the Magnus effect to provide the additional rotation of the downstream rotor and the periodic coupling of local flow between two wind rotors. However, the interactions of two wind rotors are sensitive to the direction of wind losing one of the advantages of VAWTs.
Antheaume et al. [2] applied a CFD software, Fluent, to investigate the performances of vertical-axis Darrieus wind rotors in different working fluids by using k-ε turbulent model under steady-state condition. They also discussed the average efficiency of several wind rotors connected in parallel. The results showed that increasing the number of wind rotors or decreasing the distance between wind rotors can make the efficiency higher due to the velocity streamlines straightening effect by the configuration. In addition, the performances working in water are much higher than those in air.
Fujisawa [3] studied the performances of two-bladed Savonius wind rotors with different overlap ratios ranged from 0 to 0.5 by experimental investigations.
The results showed that the performance of a Savonius wind rotor reaches a maximum at overlap ratio 0.15 because the advancing blade is strengthened by the flow through the overlap. When the overlap ratio becomes larger, the recirculation zone grows causing a deterioration of the performance.
Blackwell et al. [4] investigated the performances of fifteen configurations of Savonius wind rotors by testing in a low speed wind tunnel. What they
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investigated included parameters, such as number of blades, wind velocity, wind rotor height, and blade overlap ratio. The results showed that first of all, the two-bladed configurations have better performance than the three-bladed ones, except starting torque. Second, the performance increases with aspect ratio slightly. Last, the optimum overlap ratio is 0.1 to 0.15.
Pope et al. [5] applied Fluent to investigate the performances of one VAWT and compared the predictions with experimental data. By the reason that a free spinning turbine cannot be fully simulated, they used constant rotational speeds of the VAWT in simulations and changed the specification of parameters to reveal freely moving turbine blades in experiments. They indicated that determining the performance at constant rotational speed is valuable since any power generation connected to the electricity grid needs to operate at constant speed.
Howell et al. [6] applied Fluent to investigate the performances of one VAWT in 2-D and 3-D simulations and compared the predictions with experimental data. The turbulence model used was RNG k-ε model, by which the applicability in flow fields involves large flow separations. The error bars on experimental data were fixed at ±20% of measured values. The results showed that the performances predicted by 2-D simulations are apparently higher than those by 3-D simulations and experimental data due to the effect of the over tip vortices.
Hu and Tong [7] used Fluent to analyze the performances in VAWT with windshield for decreasing the counter torque as shown in Fig. 1.5. They used k-ε RNG turbulent model and SIMPLE algorithm in 2-D simulations. The results showed that about 15° of inclination angle between the bottom of windshield and x-axis gives the highest value of torque.
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Kamoji et al. [8] compared the performance differences between conventional Savonius wind rotors with and without central shaft between the end plates in an open jet wind tunnel. Investigation was undertaken to study the effects of overlap ratio, blade arc angle, aspect ratio and Reynolds number. The results showed that the performance of the Savonius wind rotor without central shaft is higher than that with central shaft.
Altan et al. [9] introduced a curtaining arrangement to improve the performance and increase the efficiency of a tow-bladed Savonius wind rotor without changing its basic structure. They placed two wind-deflecting plates in front of the wind rotor to prevent the negative torque opposite the wind rotor rotation. The experimental results showed that cp is increased to about 38% with the optimum curtain arrangement and is much higher than 16% obtained without curtain.
Mohamed et al. [10] used a CFD software to investigate the performances of two-bladed and three-bladed Savonius wind rotors with and without putting an obstacle to prevent the influence of wind on the returning blade. They concluded that an appropriate arrangement of the obstacle can increases cp by 27.3% for two-bladed Savonius wind rotors and 27.5% for three-bladed ones.
Therefore, the overall effect of the obstacle is extremely positive for both designs. Furthermore, the two-bladed wind rotors are better than three-bladed configurations by considering the resultant cp and the cost and complexity of the wind rotor.
Saha et al. [11] used a wind tunnel to test and investigate the performances by different number of blades and stages, different geometries of blade and inserting valves on the concave side of blade or not. The results were as follows.
First, with an increases of the number of blades, the performance of wind rotor
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decreases. Second, twisted geometry blade profile has a better performance than the semicircular blade geometry. Third, the cp of a two-stage Savonius wind rotor is higher than those of single-stage and three-stage wind rotors. Last, the valve-aided Savonius wind rotor with three blades shows a better performance than the conventional wind rotor.
Zhao et al. [12] applied computational fluid dynamics software to investigate the performance of new helical Savonius wind rotors. They analyzed the differences of the wind rotors with different aspects ratio, number of blades, overlap distance and helical angle. The results showed that three-blade helical wind rotor has lower cp compared with two-bladed helical wind rotor. And the best overlap ratio, aspect ratio, and helical angle are 0.3, 6.0 and 180°, respectively.
Gupta et al. [13] studied the performances of a Savonius wind rotor and a Savonius-Darrieus machine with overlap variation by experimental investigations. For the Savonius-Darrieus machine, there was a two-bladed Savonius wind rotor in the upper part and a Darrieus machine in the lower side.
The result showed that cp with 20% overlap is higher than 16.2% without overlap. They also concluded that the improvement of cp can be achieved for the Savonius-Darrieus wind machine compared with the general Savonius rotor.
Irabu and Roy [14] introduced a guide-box tunnel to improve the cp of Savonius wind rotors and prevent the damage by strong wind disaster. The guide-box tunnel was like a rectangular box as wind passage and the test wind rotor was included. It was able to adjust the inlet mass flow rate by its variable area ratio between the inlet and outlet. The experimental results showed that the maximum cp of the two-bladed wind rotor using the guide-box tunnel is about 1.23 times of cp of the wind rotor without the guide-box tunnel and 1.5 times of
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a three-bladed wind rotor. Besides, the two-bladed wind rotor is better than the three-bladed in converting wind power through guide-box tunnel effectively, except the starting rotation.
Chinchilla et al. [15] studied the need for further researches and developments on improved airfoil or blade characteristics on straight bladed VAWT. They indicated that the asymmetric airfoils would enable VAWTs to self start and could be utilized in the regions of low or turbulent air.
Chang et al. [16] analyzed wind characteristics and wind turbine characteristics in Taiwan by mathematical formulations using the measured data of hourly mean wind speed at 25 weather stations from 1961 to 1999. They indicated that in the west coast, Hengchun Peninsula and some small surrounding islands, there are outstanding wind sources being suitable for wind power generation.