Introduction

For the current situation of our modern world, it is obviously seen that mankind has been continuously seeking for energy solutions to meets the not only the growing global demand, but also the environment conservation. Wind turbine, which has a significant long history of development, is still interested as a simple and reliable solution for such demands.

Of all known designs, vertical axis wind turbine (VAWT) has a notable feature, that is the advantage of being capable of catching the wind from all directions without a need to orient the blades.⁽¹⁾ Besides, another advantage is that the blade usually take the shape of simple geometry. Thus, the turbine operates in almost pure tension, it becomes relatively light and inexpensive to construct. The major disadvantage of VAWT is the relatively low power efficiency compared to horizontal designs. This is due to the fact that vertical orientation unwillingly divides the rotor blades into two groups. First group, the so-called “positive component”, consists of part of the blades which move in respect of wind direction, the second one consists of negative blade elements which move against the wind direction, consequently resist or limit the overall power output.

For decades, there are several methods invented for the reduction of negative effect in VAWTs. This paper introduce the design of a 5kW scale VAWT enhanced with stationary guide vanes which help orient the wind direction in a positive way to increase power output. The use of guide vane is not an innovative idea, however, with the combination of blade shape the VAWT design in this study shows promising impoved performance while maintains the economic fator in terms of structure and construction.

The VAWT presented here is of Savonius-based style with several modifications (Fig. 1). Firsly, the blades are mounted at the periphery of the rotor, leaving a big empty space at the center (traditional design has the rotor interior mostly filled). Secondly, a large number of blades is used, and for this scale 24 blades are fitted. The purpose of this design is due to the fact that the most effective region for generating torque usually lies at far distance from the center. Thus, such arrangement is more beneficial for power generating while reduces the effect of resistance and blockage. The final modification is the use of omni-directional guide vane. As mentioned before, the main purpose of these vanes is to prevent turbine’s rotor from generating negative effect. Moreover, the installation of guide vanes also enable seft-starting ability for the VAWT. In this design, 42 guide vanes are fitted.

Fig. 1 
Design of Omni-Directional VAWT

For 5kW scale, device’s sizing was calculated and given in Table 1. Since the guide vanes play flow orientation role, the blades are designed to match the direction of the air stream entering rotor. This is the most importance which directly influences the power output. In this work, three types of blade are analyzed. Fig. 2 shows the shapes of these blade designs, in turn named “V1”, “V2” and “V3”.

Fig. 2 
Blade Designs

Due to the orientation of guide vanes, the air is directed into the rotating rotor with a tilted angle in most of the blades. Thus, the wind flow takes part in two aerodynamic motions expressed as direct inflow and rotating components. The magnitudes of wind speed and rotor revolution shall decide the combined velocity vector of an air particle passing through the turbine. The 3 blade designs are proposed aiming at effectively taking the air and absorbing its kinetic energy based on pre-described principle. Thus, flanging is applied for all three types but with different methods. Computational simulation is carried out for each design in order to understand the flow patterns and find out the most effective one.

The application of guide vane and flanging blade also appeared in other’s work, i.e. Chong et al. in 2014, where a similar VAWT was introduced. The authors uses guide vane to form a so-called power-augmented shroud and state that such design can increase the coefficient of torque and the coefficient of power.^(2,3) However, those researchers use traditional airfoil style blade and applied flanging for the guide vanes in stead.

Computational Fluid Dynamics (CFD) has been being a reliable solution to solve fluid flow problem, especially for wind turbines. The solver used in this work is ANSYS CFX commercial code which is able to generate accurate simulation results for correctly defined problems. In order to do this, the turbine was modeled carefully using CAD tools and precisely meshed. The computational mesh is done with hexa only approach to assure the robustness of the domain. The mesh has high quality, dimensionless wall funtion value is within reliable range where maximum yplus is under 7.67 for the whole computational domain, while at near wall (blade, guide vane) region, this value is only about 0.05 (Fig. 3).

Fig. 3 
Meshing and Mesh Quality of Computational Domain

The computational domain is setup with one inlet and one outlet in rectangular shape in order to simulate the far field flow direction. Turbulence flow is solved by Shear Stress Transport (SST) model which is stable and has low rate of encountering computational errors. Most simulations are done in steady state analysis but some transient cases are also carried out to confirm the results’ accuracy. To estimate turbine’s performance at different conditions, the dimensionless speed called Tip-Speed Ratio (TRS, λ) is given as an input parameter.

For these simulations, TSR is changed by changing rotor’s rotational speed and free stream wind speed is maintained. The coresponding rotor revolutions for different TSR conditions are given in Table 2.

This paper introduces a computational evaluation of a VAWT enhanced with omni-directional guide vane which focus on finding possible solutions to increase power efficiency. In summary, the following conclusions are given.

• 1) The proposed design of VAWT generally has high power efficiency for its type. Especially, the V2 blade design show promising performance with maximum CP up to 25%.
• 2) The application of omni-directional guide vane plays the key role in the improvement of power performance by means of effectivel distribution of wind stream..
• 3) The sizing and structure of the analyzed VAWT satisfied the power requirement while maintaining the advantage of simplicity in both construction, design and maintenance..

The VAWTin this paper is computational model and its capability has not been fully comprehended yet since various factors were excluded, i.e. number of blades, number of guide vanes, tilted angle, blade angle of attack, sizing.... These problems should be investigated and taken into account in future work to discover the optimal design for Savonius-based VAWT. The results of this study help contributing to the present development of wind turbine as the potential method of tidal energy conversion to serve the future demand of renewable energy.