The problem of urban wind shear, and

how to choose the best type of wind

turbine for wind conditions in cities

Emily Hounslow

dtp09eph@sheffield.ac.uk

Contents

1. Introduction to the problem of urban wind

conditions for wind turbines

2. Methods for measuring turbine performance

3. A comparison of the performance of two

types of turbine in a simulated urban wind

environment

The problem of wind shear in cities

• Efficiently extracting energy from the wind

requires the turbine to be tailored to wind

conditions.

• Urban wind is unpredictable and variable, and

subject to wind shear.

Wind shearUniform flow Building

Building Aerodynamics

(Zhang, Y. Q. et al. 1993)

Why is a wind shear a problem?

• Lift = Lift coefficient * air density * blade area

* (wind velocity)2

Aims of project

How do the performances of Horizontal Axis

Wind Turbines (HAWT) and Vertical Axis Wind

Turbines (VAWT) change in wind shear

conditions?

Vs.

Key terms to describe turbine

efficiency

• Power coefficient Cp=

Power extracted by the turbine

power in the wind

• Tip Speed Ratio = speed of the blade tip

speed of the wind

HAWT and VAWT

VAWT Measurements

HAWT Measurements

•Measuring forces

with spring balances

•Bicycle odometer used to measure rotational speed

Creating wind shear

•All experiments take place in a wind tunnel –

simulated environment.

•Masking tape on a screen used to create 3 different

wind profiles.

Horizontal shear Vertical shear

V V

V

HAWT performance in wind shear

profile at 4m/s

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00

Po

we

r C

oe

ffic

ien

t (C

p)

Tip Speed Ratio

4m/s with wind shear 4m/s

VAWT performance in wind shear

profiles at 5m/s

-90.00%

-80.00%

-70.00%

-60.00%

-50.00%

-40.00%

-30.00%

-20.00%

-10.00%

0.00%

10.00%

0 1 2 3 4 5 6 7

Po

wer

co

eff

icie

nt

Cp

Tip Speed Ratio

5m/s unblocked

5m/s left side blocked

5m/s right side blocked

5m/s vertical shear

VAWT performance in wind shear

profiles at 6m/s

-40.00%

-35.00%

-30.00%

-25.00%

-20.00%

-15.00%

-10.00%

-5.00%

0.00%

5.00%

10.00%

0 1 2 3 4 5 6

Po

wer

co

eff

icie

nt

Cp

Tip Speed Ratio

6m/s unblocked

6m/s left side blocked

6m/s right side blocked

6m/s vertical shear

Comparing VAWT and HAWTCp with

uniform flow

Cp with wind

shear

Difference in Cp

Horizontal axis (4m/s)

Vertical wind shear 42 24 -18Vertical axis (5m/s)

Vertical wind shear -18.88 -16.9 1.98Horizontal wind shear (left side blocked) -18.88 -10.11 8.77

Horizontal wind shear (right side blocked) -18.88 -9.73 9.15

Vertical axis (6m/s)

Vertical wind shear 4.59 -2.27 -6.86Horizontal wind shear (left side blocked) 4.59 -1.13 -5.72Horizontal wind shear (right side blocked) 4.59 5.21 0.62

Main question:

How do the performances of Horizontal

Axis Wind Turbines (HAWT) and Vertical

Axis Wind Turbines (VAWT) change in wind

shear conditions?

• HAWT shows a much larger decrease in Cp

than VAWT with a vertical wind shear.

• VAWT response to wind shear is variable with

shear type and wind speed.

• VAWT Cp is lower than HAWT Cp over all.

• All conclusions apply to these particular

turbines only.

Why do they respond in this way?

Why do they respond in this way?

Conclusions

1. Urban wind is a big problem to existing

turbine designs

2. HAWT performance is affected more than

VAWT performance by vertical wind shear

3. Future work should focus on designing and

improving VAWTs to cope with urban wind

Thank you for listening.

Any questions?

Acknowledgements

Dr Robert Howell

Jonathan Edwards