Optimum design configuration of Savonius rotor through wind tunnel experiments

https://doi.org/10.1016/j.jweia.2008.03.005Get rights and content

Abstract

Wind tunnel tests were conducted to assess the aerodynamic performance of single-, two- and three-stage Savonius rotor systems. Both semicircular and twisted blades have been used in either case. A family of rotor systems has been manufactured with identical stage aspect ratio keeping the identical projected area of each rotor. Experiments were carried out to optimize the different parameters like number of stages, number of blades (two and three) and geometry of the blade (semicircular and twisted). A further attempt was made to investigate the performance of two-stage rotor system by inserting valves on the concave side of blade.

Introduction

In recent years, an interest in wind energy has been growing and many researchers have attempted the development to introduce cost-effective, reliable wind energy conversion systems all over the world. In practice, however, there are many difficulties, to introduce the wind turbine into the community because of less wind energy source, profitability, noise emission, etc. Therefore, the decentralization or local clusterization of renewable energy plant made it attractive not only to developing area, where a lot of people do not yet have access to conventional electricity service, but also to an urban area where one can make better living space for future generation (Shikha et al., 2003; Grinspan et al., 2004; Menet, 2004).

This project was undertaken to optimize the design configuration of Savonius rotors with the expectation that this inherently simple vertical axis machines could be manufactured at low cost, leading to their widespread use. The research proposal noted that small units could be manufactured for distributed generation of electricity in residential and commercial locations. The units would be grid connected to take advantage of net metering and would provide pollution-free generation of electricity using a renewable resource at a cost competitive with power supplied by the grid. The operation of Savonius wind turbine rotor is based on the difference of the drag of its semicircular vanes, depending on whether the wind is striking the convex or the concave part of the vane. The advantage of this type of rotor is that it is self-starting and relatively independent of the wind direction. It is simple to design and has relatively low construction cost. However, it has a low efficiency.

It is a known fact that accessories like end plates, shielding and guide vanes (flat, curved) usually increase the Savonius rotor performance; however, all of these increase the complexity of the rotor (Huda et al., 1992, Rajkumar, 2004). The rotor can develop a relatively high torque at low rotational speeds and is cheap to build, but it harnesses only a small fraction of the wind energy incident upon it. An attractive proposition for augmenting its harnessing effectiveness is to keep non-return valves placed inside the concave side of the blades. The valve opens automatically as a result of wind pressure when the blade advances towards the wind thereby experiencing lower flow-resistance. The centrifugal force automatically closes this valve during the power-harnessing part of the cycle. Valve-aided rotor is the mechanism to make direction independent and is the effective way of increasing power capability without unduly affecting the simplicity of rotor (Rajkumar and Saha, 2006). In addition to this, damages to turbine at higher velocities will be reduced with the valve mechanism.

Section snippets

Project objective

In recent times, a double-step or two-stage Savonius rotor has been investigated to find its feasibility for local production of electricity (Menet, 2002, Menet, 2004). The challenge was to design, develop and ultimately build a prototype of such a rotor, which was considered as a complete electromechanical system. An optimum configuration was chosen for the geometry of the prototype. The building data were calculated on the basis of the nominal wind velocity of 10 m/s. The whole design of the

Energy in the wind

For an airstream flowing through an area A, the mass flow rate is ρAV, and therefore the powerP=ρAV12V2=12ρAV3where ρ is the air density (kg/m3), V is the wind speed (m/s) and P is the power (watts). The power is also known as the energy flux or power density of the air (Walker and Jenkins, 1997; Bansal et al., 2002; Menet, 2004). The ratio of shaft power (Ps) to the power available in the wind (P) is known as the power coefficient (Cp), and this indicates the efficiency of conversion. ThusCp=Ps

Blade design and fabrication

Savonius rotor made out of half cylinders (nominal diameter d, height H) is a very simple concept where the whole rotor turns around a vertical axis. There are a number of geometrical parameters that affects the efficiency of Savonius rotor (Alexander and Holownia, 1978; Mojola, 1985; Ushiyama and Nagai, 1988; Modi and Fernando, 1989; Islam et al., 1993; Coton et al., 1996). Among those parameters, the aspect ratio (AR) plays an important role in the aerodynamic performances of a Savonius

End plate design and fabrication

It is known that end plates lead to better aerodynamic performances. The influence of diameter Do of these end plates relatively to the diameter D of the rotor has been experimentally studied (Fujisawa, 1992). The higher value of the power coefficient Cp is obtained for a value of Do around 10% more than D, whatever be the velocity coefficient. In the present investigations, three end plates are used in a two-stage system, one at the top, one at the middle and one at the bottom of the rotor.

Valve design and fabrication

In this mechanism, a small raxine-type cover is pasted in the concave side of the blade, which is purposely holed (Thotla, 2006). When wind is facing the concave side, the raxine cover will be attached to the blade; else it would allow air to flow from convex side to concave side thereby reducing the pressure deference on both sides, as it is the form drag that contributes to the power mechanism of rotor. The static torque performance of the rotor, especially of the returning blade, can be

Test facility

To study the performance of Savonius wind turbine a low-speed wind tunnel with an open test section facility has been designed, developed, fabricated (Grinspan, 2002; Grinspan et al., 2003) and shown in Fig. 6. The rotor axis is placed at a distance of 205 mm from the tunnel exit having a cross-section area of 375 mm×375 mm. By changing the input voltage with the help of variac, the wind tunnel exit air velocity can be changed. The entire tests have been conducted in the range of air velocity of

Discussion of results

The performance of the Savonius rotor depends on the different parameters like number of blades, number of stages and geometry of the blades. Till now, there is no exact theoretical procedure to assess the performance the Savonius rotor. The best way of optimizing the various parameters is to carryout number of experiments on the different types of rotors in a low-speed open test section wind tunnel. Here, experiments have been conducted with different types of rotors by varying number of

Conclusions

Due to slow rotational speed and low power production, the Savonius rotor is unsuitable for electricity generation. Therefore, not enough work has been progressed in the area of this vertical axis wind turbine as opposed to its horizontal axis counterpart. However, for a small-scale power requirement, Savonius rotors are quite useful. Therefore, it has become necessary to go through its various prospects so that its performance can be improved to a greater extent. In the present investigation,

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