Discussion on the verification of the overlap ratio influence on performance coefficients of a Savonius wind rotor using computational fluid dynamics

doi:10.1016/j.renene.2011.07.013

Renewable Energy

Volume 38, Issue 1, February 2012, Pages 141-149

https://doi.org/10.1016/j.renene.2011.07.013 Get rights and content

Abstract

This paper discusses the influence of the buckets overlap ratio of a Savonius wind rotor on the averaged moment and power coefficients, over complete cycles of operation. The continuity and Reynolds Averaged Navier–Stokes (RANS) equations, and the Eddy Viscosity Model k–ω SST, on its Low-Reynolds approaches, using hybrid near wall treatment; are numerically solved by the commercial software Star-CCM⁺. This software is based on Finite Volume Method and computes the pressure and velocity fields of the flow and the forces acting on the rotor buckets. The moment and power coefficients are achieved by integrating the forces coming from the effects of pressure and viscosity of the wind on the device. The influence of the buckets overlap ratio on the moment and power coefficients is checked by changing the geometry of the rotor, keeping the Reynolds number, based on rotor diameter, equal to 433,500. The results obtained for the rotor with zero overlap are in agreement with those obtained experimentally by other authors what indicates that the method can be successfully used for such analysis. The values of the moment and power coefficients obtained as a function of tip speed ratio and the buckets overlap of the rotor indicates that the maximum device performance occurs for buckets overlap ratios with values close to 0.15.

Highlights

► Influence of overlap ratio on the Savonius rotor performance is obtained using CFD. ► 15% of buckets overlap provides the best performance. ► The rotor operation is more efficiently at tip speed ratio 1.25. ► The best performance gives an averaged power coefficient equals to 0.3161.

Introduction

The use of unconventional devices, like the Savonius wind rotor, can be a solution for low cost and reduced environmental impacts for decentralized power generation. The wind rotor developed and patented by Sigurd J. Savonius in the 1920s, has: simple construction; high start up and full operation moment; wind acceptance from any direction for the operation; low noise and angular velocity in operation, reducing wear on moving parts; in addition to various rotor configuration options, as the use of multiple stages and different shapes of buckets [1], [2], [3].

A Savonius wind rotor, however, cannot be regarded as a wind machine with performance higher and neither lower than performance of the other wind machines from power generation. Savonius rotors should only be understood as different mechanisms for the utilization of wind energy, and technological alternatives to conventional wind turbines [3], [4]. The device created by Savonius works mainly due to the resulting drag force on their buckets [1]. According to Akwa [5], lift forces are also responsible for part of the power provided by the device.

As it was reported by Akwa [5], the performance of a Savonius rotor is affected by flow parameters and the geometry of the rotor. For a given flow configuration, different arrangements of Savonius rotor provide different performances while converting wind energy into useful energy. Studies related to the influence of the buckets overlap ratio on the rotor performance are among the main research work carried out on such wind device. In most of the previous studies on the influence of the buckets overlap ratio on the performance of Savonius rotor, it was conclude that a small overlap between the rotor buckets provides an improvement in the efficiency of the device. However, there is not yet an accurate indication of the optimum size of this overlap what reinforcing the need for further investigation.

According to Fujisawa [6], the optimum size for the buckets overlap is equal to 15% of the size of the bucket chord. Blackwell et al. [7] conclude that this dimension is equivalent to a value between 10 and 15% that size. Alexander and Holownia [8] and Mojola [9] indicate that values between 20 and 30% of the length chord provide the best results for the performance of Savonius wind turbines.

This research paper contains results of the studies conducted at the Federal University of Rio Grande do Sul (in the Brazil) on wind energy. In it is discussed the influence of the buckets overlap ratio of a Savonius wind rotor for averaged moment and power coefficients over complete cycles of the device operation. The discussed results were obtained by applying the Finite Volume Method, solving the conservation equations of the turbulent air flow on the rotor, which allows the calculation of the performance coefficients of the rotor. Although the geometry of the turbine is relatively simple, the flow under study is complex, which prevents the search for analytic solutions for velocity and pressure fields. The Computational Fluid Dynamics (CFD) has become a useful tool for the study of wind rotors, since it allows obtaining values for the aerodynamic coefficients and the visualization of the flow without the need for instrumentation.

Section snippets

Description of phenomenon

In this research work, the performance of a two-bucket Savonius rotor with semicircular profiles buckets, like the one shown in Fig. 1, was analyzed. A two-dimensional modeling for the geometry of the rotor (Fig. 1(b)) in operation is performed, whereas in such device, with high aspect ratio as can be seen in Fig. 1(a), the changes in the profile of the flow caused by the effects of the bucket tips can be considered negligible.

The operation of the rotor is simulated with the variation in

Results and discussion

The Savonius wind turbines operate due to the pressure drag and lift forces acting on their buckets. These forces vary with attack angle of rotor buckets. Depending on this angle, which the buckets move in their trajectories, they expose their different shapes to the wind. Each shape has its own force coefficients. Thus, the resultant moment of a Savonius rotor varies with this angle, due to the variation of the forces coefficients of the buckets during the device rotation. This value also

Conclusions

This study is developed in order to numerically verify the influence of different buckets overlap ratios in the performance of Savonius wind turbines, aiming at the wind device geometry optimization. Numerical simulations of the air flow on the rotor in 40 different situations are carried out. The conservation equations of mass and momentum are solved using the Finite Volume Method. For treatment of the turbulence it uses the Eddy Viscosity Turbulence Model k–ω SST. It was obtained appropriate

Acknowledgements

The authors thank the financial support from CNPq through scholarship grant to J.V. Akwa.

Nomenclature

A: area, m²
A_r: projected area of the rotor, m²
a: spacing between the rotor buckets (Fig. 1(b)), m
a: face area vector, m²
a₁: a coefficient
arg₁: function defined by the Eq. (15)
arg₂: function defined by the Eq. (18)
${CD}_{k ω_{t}}$: function related to the cross-diffusion term by Eq. (16)
C_P: power coefficient [C_P = C_Tλ]
C_T: moment coefficient given by Eq. (29)
c: bucket chord, m
$D_{ω_{t}}$: cross-derivative term
d: vector defining the axis through point x₀ about which the moment is taken, m
d_f: face area vector, m
d_pe: diameter of the end plates, m
d_r

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There are more references available in the full text version of this article.

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Discussion on the verification of the overlap ratio influence on performance coefficients of a Savonius wind rotor using computational fluid dynamics

Abstract

Highlights

Introduction

Section snippets

Description of phenomenon

Results and discussion

Conclusions

Acknowledgements

Nomenclature

Renew Energy

J Wind Eng Ind Aerodyn

J Wind Eng Ind Aerodyn

J Wind Eng Ind Aerodyn

Energy

Energy Convers Manage

The S-rotor and its applications

Mech Eng

Wind machines