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Research Papers

Geometrical Parameters Influencing the Aerodynamic Efficiency of a Small-Scale Self-Pitch High-Solidity VAWT

[+] Author and Article Information
Carlos M. Xisto

Division of Fluid Dynamics,
Department of Applied Mechanics,
Chalmers University of Technology,
Gothenburg SE-41296, Sweden
e-mail: carlos.xisto@chalmers.se

José C. Páscoa

Departamento de Engenharia Electromecânica,
Universidade da Beira Interior,
Covilhã 6200, Portugal
e-mail: pascoa@ubi.pt

Michele Trancossi

Department of Engineering and Mathematics,
Sheffield Hallam University,
Sheffield, South Yorkshire S1 1WB, UK
e-mail: m.trancossi@shu.ac.uk

1Corresponding author.

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING: INCLUDING WIND ENERGY AND BUILDING ENERGY CONSERVATION. Manuscript received April 16, 2015; final manuscript received February 12, 2016; published online March 9, 2016. Assoc. Editor: Yves Gagnon.

J. Sol. Energy Eng 138(3), 031006 (Mar 09, 2016) (8 pages) Paper No: SOL-15-1096; doi: 10.1115/1.4032794 History: Received April 16, 2015; Revised February 12, 2016

In this paper, four key design parameters with a strong influence on the performance of a high-solidity variable pitch vertical axis wind turbine (VAWT) operating at low tip-speed-ratio (TSR) are addressed. To this aim, a numerical approach, based on a finite-volume discretization of two-dimensional (2D) unsteady Reynolds-averaged Navier–Stokes (URANS) equations, on a multiple sliding mesh, is proposed and validated against experimental data. The self-pitch VAWT design is based on a straight-blade Darrieus wind turbine with blades that are allowed to pitch around a feathering axis, which is also parallel to the axis of rotation. The pitch angle amplitude and periodic variation are dynamically controlled by a four-bar linkage system. We only consider the efficiency at low and intermediate TSR; therefore, the pitch amplitude is chosen to be a sinusoidal function with a considerable amplitude. The results of this parametric analysis will contribute to define the guidelines for building a full-size prototype of a small-scale wind turbine of increased efficiency.

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References

Figures

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Fig. 1

(a) Variable pitch VAWT with a four-bar linkage pitching system and (b) comparison between the velocity vectors at low TSR between a variable and fixed pitch VAWT. Vt=Ω R is the tip-speed velocity, and VR=Vwind2+Vt2 is the resultant velocity on the blade. One can observe that, in the backward region of the VAWT (Ψ=270 deg), the angle of attack, α, is higher for the fixed pitch VAWT (α1>α2).

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Fig. 2

Two-dimensional numerical domain used for computing the IAT21-L3 rotor test case, a close-up of the rotor and blade grid is also displayed. The grid is composed by three circular zones, which are separated by sliding-mesh interfaces. The same numerical configuration and boundary conditions are used throughout this paper.

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Fig. 3

The results obtained for the validation test case in  log10−log10 plots: (a) variation of thrust with rotational speed and (b) variation of power with rotational speed

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Fig. 4

The results obtained for the airfoil thickness: (a) power coefficient variation with TSR and (b) power coefficient variation with airfoil thickness for different TSRs

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Fig. 5

The results obtained for the variation of the number of blades: (a) power coefficient variation with TSR and (b) power coefficient variation with the number of blades for different TSRs

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Fig. 6

The results obtained for the variation of blade chord length: (a) power coefficient variation with TSR and (b) power coefficient variation with the blade chord length for different TSRs

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Fig. 7

The results obtained for variation of the pitching axis location: (a) power coefficient variation with TSR and (b) power coefficient variation with the pitching axis location for different TSRs

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Fig. 8

Comparison between the proposed and conventional design: (a) power coefficient variation with TSR (F—fixed pitch and V—variable pitch, for θ0 = 5 deg, 15 deg, and 30 deg) and (b) power coefficient variation with the azimuthal position of one blade for λ = 0.5

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Fig. 9

Instantaneous contour plots of velocity magnitude (λ = 0.5) computed after five revolutions in the (a) proposed design (c = 0.25 m; N = 4; NACA0018; x′/c= 0.35; and θ0 = 30 deg) and (b) conventional rotor with a fixed pitch of 5 deg. Instantaneous contour plots for the nondimensional vorticity, ωz*=ωz c/Uwind, with super imposed streamlines: (c) proposed design and (d) conventional rotor.

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