a study of twin co- and counter-rotating vertical axis wind …€¦ · · 2017-06-19axis wind...

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

The 16th World Wind Energy Conference,

Malmö, Sweden. June 12-15, 2017.

June 14, 2017

Structure of This Presentation

1. Introduction

1

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, 2011; Tescione et al., 2014; Lam and Peng, 2017):

• 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. Introduction

2

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. Numerical Modeling

3

2.1. Wind Turbine & Mesh

Darrieus, H-rotor

Diameter, D = 1.0 m

Blade depth, H = 1.0 m

Blade number, N = 2

Airfoil section, NACA0018

Chord length, c = 0.06 m

Low solidity

Figure 1 Standalone VAWT (Lam and Peng, 2016)

Figure 2 Twin configuration: (a) CR, (b) CFR, and CBR

Figure 3 Typical mesh close to a blade


4

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., 2004) is used to close the URANS equations.

The computational domain spans an area of 12D×20D (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 0.5º rotation of a blade.

ANSYS Fluent is chosen as the solver.


5

2.3. Comparison with Experimental Data

The predicted stream-wise and cross-stream velocities are compared with the experimental results

(Tescione et al., 2014).

-1 -0.5 0 0.5 10

0.5

1

1.5

y/D (x = 0.75D)U

x/U

CFD Test

-1 -0.5 0 0.5 1-0.2

0

0.2

y/D (x = 0.75D)

Uy

/U

-1 -0.5 0 0.5 10

0.5

1

1.5

y/D (x = 1D)

Ux

/U

-1 -0.5 0 0.5 1-0.2

0

0.2

y/D (x = 1D)

Uy

/U

-1 -0.5 0 0.5 10

0.5

1

1.5

y/D (x = 1.5D)

Ux

/U

-1 -0.5 0 0.5 1-0.2

0

0.2

y/D (x = 1.5D)U

y/U

-1 -0.5 0 0.5 10

0.5

1

1.5

y/D (x = 2D)

Ux

/U

-1 -0.5 0 0.5 1-0.2

0

0.2

y/D (x = 2D)

Uy

/U

Figure 4 Comparison between predicted and measured velocities

3. Results and Discussions

6

3.1. Power Performance

The thrust and radial forces are derived as follows:

Figure 4 Schematic of a blade rotation

cos sin

sin cos

T x y

R x y

F F F

F F F

(1)

where Fx and Fy are the forces along the x and y

directions, respectively, FT and FR are the thrust and

radial forces, respectively, and θ is the azimuthal angle.

The power coefficient can thus be calculated:

2

02

P TC C d

(2)

where σ is the solidity ratio of the VAWT, and CT is the

coefficient of the thrust force FT.


7


The power performance of the standalone and twin VAWTs is assessed and presented.

The twin array configuration clearly outperforms the standalone VAWT in terms of power output,

with approximately 14% increase of CP.

The twin CFR system demonstrates the most excellent power performance.

Figure 5 Power coefficients of the standalone VAWT and twin array configurations

Standalone CR CFR CBR0.425

0.45

0.475

0.5

0.525

Array configuration

Po

wer

co

eff

icie

nt,

CP

= 4.5


8


The effect of the shaft-to-shaft distance on CP of the CFR configuration is examined.

The twin CFR unit peaks its power performance at s = 2D, with a CP of 0.5 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.

Figure 6 Relationship between CP and s of the twin CFR VAWTs

1.5 2 2.50.48

0.485

0.49

0.495

0.5

0.505

Shaft-to-shaft distance (s/D)

Po

wer

co

eff

icie

nt,

CP

= 4.5


9

3.2. Wake Aerodynamics

The stream-wise velocities downstream of the CR, CFR, and CBR systems are analyzed.

The wake of the twin CBR VAWTs expand far more substantially than their CR and CFR

counterparts.

Figure 7 Relationship between CP and s of the twin CFR VAWTs

-4 -2 0 2 40

0.5

1

1.5

y/D (x = 1D)

Ux

/U

CR

CFR

CBR

-4 -2 0 2 40

0.5

1

1.5

y/D (x = 2D)U

x/U

CR

CFR

CBR


10

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, 2017)

Figure 9 Triangular array unit (Lam and Peng, 2017)

4. Conclusion

11

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 CP.

The twin CFR array exercises the best power performance and peaks its CP at s = 2D.

4.2. Future Work

Investigation into the wake aerodynamics of the twin array configurations will be conducted.

Acknowledgement

12

The work is supported by the Research Grants Council of the Hong Kong Special Administrative

Region, China (Project No. CityU 11242716). The authors are thankful for the financial assistance.

References

13

1. Lam H.F. and Peng H.Y., 2017, Measurements of the wake characteristics of co- and counter-rotating

twin H-rotor vertical axis wind turbines, Energy, 131, 13-26.

2. Lam H.F., Peng H.Y., 2016, Study of wake characteristics of a vertical axis wind turbine by two- and

three-dimensional computational fluid dynamics simulations, Renewable Energy, 90, 386-398.

3. Peng H.Y., Lam H.F., Lee C.F., 2016, 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, 23-35.

4. Peng H.Y., Lam H.F., 2016, Turbulence effects on the wake characteristics and aerodynamic

performance of a straight-bladed VAWT by wind tunnel tests and large eddy simulations, Energy, 109,

557-568.

5. Tescione G., Ragni D., He C., Simão Ferreira C.J., van Bussel J.W., 2014, Near wake flow analysis of a

vertical axis wind turbine by stereoscopic particle image velocimetry, Renewable Energy, 70, 47-61.

6. Dabiri J.O., 2011, Potential order-of-magnitude enhancement of wind farm power density via counter-

rotating vertical-axis wind turbine arrays, Journal of Renewable and Sustainable Energy, 3, 1-12.

7. Menter F.R., Langtry R.B., Likki S.R., Suzen Y.B., Huang P.G., Volker S., 2004, A correlation based

transition model using local variables, Part 1-Model formulation, Journal of Turbomachinery, ASME,

128, 413-422.

Thank You All

June 14, 2017

The 16th World Wind Energy Conference,

Malmö, Sweden. June 12-15, 2017.

PENG, Hua Yi* and LAM, Heung Fai City University of Hong Kong, Hong Kong

*Speaker, Research Associate

a study of twin co- and counter-rotating vertical axis wind …€¦ · · 2017-06-19axis wind...

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