Numerical study of two-bucket Savonius wind turbine cluster

Abstract

The performance of a single Savonius turbine depends on the interaction between the buckets of its rotor. Recent studies also show that interactions between Savonius turbines in a cluster enhance the output power of individual rotors. The purpose of this study is the development of a multi-turbine cluster for construction of efficient patterned vertical axis wind turbine farms. Numerical solutions are performed for a single Savonius turbine, clusters of two turbines in parallel and oblique positions, and triangular clusters of three turbines facing the wind forward and backward. The commercial CFD software FLUENT 14.5 is used for the numerical simulation. The numerical results of the single Savonius turbine are validated against experimental data. Parametric studies are performed to determine the solution domain size, the grid density, and the turbulence model. The two parallel and oblique turbine clusters are simulated to determine the optimum gap distances and setting angles between adjacent turbines for the maximum power coefficient. The numerical results of the two turbine oblique clusters are used to develop an efficient triangular shaped three turbine cluster. The developed three turbine cluster has an average power coefficient up to 34% higher than that of an isolated turbine.

Introduction

Recently wind attracted more investment globally than any other non-fossil fuel based technology, including hydro and nuclear power (United Nations June, 2008). Although horizontal-axis wind turbines (HAWTs) are most commonly used in wind farms today, a large amount of land is required to separate each wind turbine from the adjacent turbines׳ wakes. This limits the power extracted from HAWT farms. In contrast, vertical-axis wind turbines (VAWTs) efficiency increases when they are placed close together (Dabiri, 2011).

Two types of VAWTs are mainly used, Savonius and Darrieus wind turbines. The Savonius turbine is one of the simplest turbines. It produces the lowest noise level compared to all other wind turbines as it works at low tip speed ratio (λ) (Shigetomi et al., 2011). Aerodynamically, it is a drag type device, consisting of two or three buckets. Because of the curvature, the buckets experience less drag when moving against the wind than when moving with the wind (Wikipedia). The differential drag causes the Savonius turbine to spin even at low wind speeds. In the symmetric drag position the flow attachment to the convex surface of the advancing blade produces a low pressure region above it to pull the blade in torque adding direction. Savonius turbines extract much less of the wind power (15–20%) compared to other similarly sized lift type VAWTs (Darrieus wind turbine rotor efficiency is 35%) (Shigetomi et al., 2011, Sheldahl et al., 1977) and that is only one third of Betz׳s limit of 59.3%.

Experimental studies have been carried out to improve the performance of the Savonius wind turbine rotor by changing the number of blades. The two bucket configurations have better aerodynamic performance than the three bucket configurations shown in Fig. 1. The three bucket configuration has a higher minimum static torque than a two bucket configuration. Vertical arrangement of two sets of Savonius rotors (two buckets), one above the other on the same shaft, with a relative phase angle 90^o has higher power output, higher stability of the auto start-up characteristics, and lower cyclic torsion during rotation (Sheldahl et al., 1977). Performance improvement was also done by investigating bucket overlap, gap width, shape of the bucket and reduction of the anti-rotation torque of the rotor using curtains or ducts (Sheldahl et al., 1977, Altan et al., 2008, Kirke, 2006). The static torque performance is improved by increasing the overlap ratio. The torque and the power performance of the rotor reaches a maximum at an overlap ratio of 0.1–0.15 (Fujisawa, 1992).

Combining multiple Savonius turbines in the horizontal plane enhances their performance in particular configurations. Power improvement interactions occur due to the Magnus effect that bends the main stream behind the upstream turbine providing additional rotation to the downstream turbine, and the periodic coupling of local flow between the two turbines associated with vortex shedding and cyclic pressure fluctuations (Zhou and Rempfer, 2013). Previous studies on closely arranged Savonius rotors showed that the coupling effect is a function of the gap distance, relative direction of rotation and relative phase angle between the rotors. Counter rotating rotors with relative phase angle (φ) equal to 90° and a gap distance of 0.2–0.4 of the rotor diameter have the highest power coefficient (Xiaojing et al., 2012). However for φ equal to 0° the power coefficient is drastically lowered. Two rotors rotating in the same direction have an enhanced average power coefficient at reduced gap distances and the angle φ has no significant effect. For two oblique rotors the effect of the relative phase angle is not significant in all layouts (Xiaojing et al., 2012).

For a wind farm to produce the highest possible power output, the number of turbines that can be installed in the available plot of land has to be maximized, this means that the gap distance between turbines has to be reduced. In this study, in order to be independent on the relative phase angle (which is not possible to be kept without many expenses on additional electro-mechanical control systems) clusters of Savonius wind turbines having the same direction of rotation are adopted. The minimum gap distances corresponding to the highest average power coefficient for closely positioned turbines are investigated. Any effect of (φ) is not considered and the two rotors are set at (φ) equal to 0°. The flow field around a single rotor and clusters of two oblique rotors is analyzed to determine the best angular position and gap distance of the downstream rotors in parallel and oblique arrangements. A triangular shaped efficient three turbine cluster is developed in order to construct efficient Savonius wind farms.