A Study of Twin Co- and Counter-Rotating Vertical Axis Wind Turbines with Computational Fluid Dynamics

Transcription

1 The 16th World Wind Energy Conference, Malmö, Sweden. June 12-15, 217. A Study of Twin Co- and Counter-Rotating Vertical Axis Wind Turbines with Computational Fluid Dynamics PENG, Hua Yi* and LAM, Heung Fai City University of Hong Kong, Hong Kong *Speaker, Research Associate June 14, 217

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3 1. Introduction 1.1. Background In contrast to the carbon-based fuels, which cause air pollution and greenhouse effect, wind energy is clean and renewable. Recent research suggests that VAWTs have some clear advantages compared to their HAWT counterparts (Dabiri, 211; Tescione et al., 214; Lam and Peng, 217): VAWTs are omnidirectional, and have good scalability. VAWTs perform better in built environments and offshore areas. Twin VAWTs have great potential for higher packing density in wind farms. 1

4 1. Introduction 1.2. Objectives To develop a two-dimensional (2-D) CFD model for twin co-/counter-rotating VAWTs. To assess the power performance of twin co-rotating (CR), counter-forward-rotating (CFR), and counter-backward-rotating (CBR) VAWTs. To examine the effect of the shaft-to-shaft distance on the power performance of a twin turbine system. To study the wake aerodynamics of the twin CR, CFR, and CBR VAWTs. 2

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6 2. Numerical Modeling 2.1. Wind Turbine & Mesh Darrieus, H-rotor Diameter, D = 1. m Blade depth, H = 1. m Blade number, N = 2 Airfoil section, NACA18 Chord length, c =.6 m Low solidity Figure 2 Twin configuration: (a) CR, (b) CFR, and CBR 3 Figure 1 Standalone VAWT (Lam and Peng, 216) Figure 3 Typical mesh close to a blade

7 2. Numerical Modeling 2.2. Modeling & Solution The unsteady Reynolds-averaged Navier-Stokes (URANS) equations are used to describe the flow fields. The transitional SST (Menter et al., 24) is used to close the URANS equations. The computational domain spans an area of 12D 2D (width by length). The sliding mesh technique is used in conjunction with the non-conformal interface to simulate the blade motion. The approaching wind speed is U = 9.3 m/s, and the blade speed ratio is λ = 4.5. One step size corresponds to.5ºrotation of a blade. ANSYS Fluent is chosen as the solver. 4

8 Ux/U Uy/U Ux/U Uy/U Ux/U Uy/U Ux/U Uy/U 2. Numerical Modeling 2.3. Comparison with Experimental Data The predicted stream-wise and cross-stream velocities are compared with the experimental results (Tescione et al., 214) y/d (x =.75D) CFD Test y/d (x =.75D) y/d (x = 1D) y/d (x = 1.5D) y/d (x = 2D) y/d (x = 1D) y/d (x = 1.5D) y/d (x = 2D) Figure 4 Comparison between predicted and measured velocities 5

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10 3. Results and Discussions 3.1. Power Performance The thrust and radial forces are derived as follows: FT Fx cos Fy sin FR Fx sin Fy cos (1) where F x and F y are the forces along the x and y directions, respectively, F T and F R are the thrust and radial forces, respectively, and θ is the azimuthal angle. The power coefficient can thus be calculated: C P 2 CTd 2 (2) Figure 4 Schematic of a blade rotation where σ is the solidity ratio of the VAWT, and C T is the coefficient of the thrust force F T. 6

11 3. Results and Discussions 3.1. Power Performance The power performance of the standalone and twin VAWTs is assessed and presented..525 = Power coefficient, C P The twin array configuration clearly outperforms the standalone VAWT in terms of power output, with approximately 14% increase of C P. The twin CFR system demonstrates the most excellent power performance Standalone CR CFR CBR Array configuration Figure 5 Power coefficients of the standalone VAWT and twin array configurations

12 3. Results and Discussions 3.1. Power Performance The effect of the shaft-to-shaft distance on C P of the CFR configuration is examined..55 = Power coefficient, C P Shaft-to-shaft distance (s/d) Figure 6 Relationship between C P and s of the twin CFR VAWTs The twin CFR unit peaks its power performance at s = 2D, with a C P of.5 which is approximately 4% larger than that of others. The change of the array configuration impacts more on the power performance than the change of the distance does. 8

13 Ux/U Ux/U 3. Results and Discussions 3.2. Wake Aerodynamics The stream-wise velocities downstream of the CR, CFR, and CBR systems are analyzed CR CFR CBR y/d (x = 1D) CR CFR CBR y/d (x = 2D) Figure 7 Relationship between C P and s of the twin CFR VAWTs The wake of the twin CBR VAWTs expand far more substantially than their CR and CFR counterparts. 9

14 3. Results and Discussions 3.2. Wake Aerodynamics The proposed twin and triangular array units of VAWTs for wind farm applications are presented. Figure 8 Twin array unit (Lam and Peng, 217) Figure 9 Triangular array unit (Lam and Peng, 217) 1

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16 4. Conclusion 4.1. Summary Two dimensional CFD models for the twin CR, CFR, and CBR array configurations are built. The twin array configurations clearly demonstrate power augmentation compared to their standalone counterpart, with an increase of approximately 14% in C P. The twin CFR array exercises the best power performance and peaks its C P at s = 2D Future Work Investigation into the wake aerodynamics of the twin array configurations will be conducted. 11

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18 Acknowledgement The work is supported by the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. CityU ). The authors are thankful for the financial assistance. 12

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20 References 1. Lam H.F. and Peng H.Y., 217, Measurements of the wake characteristics of co- and counter-rotating twin H-rotor vertical axis wind turbines, Energy, 131, Lam H.F., Peng H.Y., 216, Study of wake characteristics of a vertical axis wind turbine by two- and three-dimensional computational fluid dynamics simulations, Renewable Energy, 9, Peng H.Y., Lam H.F., Lee C.F., 216, Investigation into the wake aerodynamics of a five-straight-bladed vertical axis wind turbine by wind tunnel tests. Journal of Wind Engineering and Industrial Aerodynamics, 155, Peng H.Y., Lam H.F., 216, Turbulence effects on the wake characteristics and aerodynamic performance of a straight-bladed VAWT by wind tunnel tests and large eddy simulations, Energy, 19, Tescione G., Ragni D., He C., Simão Ferreira C.J., van Bussel J.W., 214, Near wake flow analysis of a vertical axis wind turbine by stereoscopic particle image velocimetry, Renewable Energy, 7, Dabiri J.O., 211, Potential order-of-magnitude enhancement of wind farm power density via counterrotating vertical-axis wind turbine arrays, Journal of Renewable and Sustainable Energy, 3, Menter F.R., Langtry R.B., Likki S.R., Suzen Y.B., Huang P.G., Volker S., 24, A correlation based transition model using local variables, Part 1-Model formulation, Journal of Turbomachinery, ASME, 128,

21 The 16th World Wind Energy Conference, Malmö, Sweden. June 12-15, 217. Thank You All PENG, Hua Yi* and LAM, Heung Fai City University of Hong Kong, Hong Kong *Speaker, Research Associate June 14, 217