The problem of urban wind shear, and
how to choose the best type of wind
turbine for wind conditions in cities
Emily Hounslow
[email protected]
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.
Uniform flow
Building
Wind shear
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
Vs.
How do the performances of Horizontal Axis
Wind Turbines (HAWT) and Vertical Axis Wind
Turbines (VAWT) change in wind shear
conditions?
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
45%
40%
Power Coefficient (Cp)
35%
30%
25%
20%
15%
10%
5%
0%
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
Tip Speed Ratio
4m/s with wind shear
4m/s
8.00
9.00
10.00
VAWT performance in wind shear
profiles at 5m/s
10.00%
0.00%
Power coefficient Cp
-10.00%
0
1
2
3
4
5
6
7
5m/s unblocked
-20.00%
-30.00%
5m/s left side
blocked
-40.00%
-50.00%
-60.00%
5m/s right side
blocked
-70.00%
-80.00%
5m/s vertical
shear
-90.00%
Tip Speed Ratio
VAWT performance in wind shear
profiles at 6m/s
10.00%
5.00%
6m/s unblocked
Power coefficient Cp
0.00%
-5.00%
0
1
2
3
4
5
6
6m/s left side
blocked
-10.00%
-15.00%
6m/s right side
blocked
-20.00%
-25.00%
6m/s vertical
shear
-30.00%
-35.00%
-40.00%
Tip Speed Ratio
Comparing VAWT and HAWT
Cp with
Cp with wind Difference in Cp
uniform flow
shear
Horizontal axis (4m/s)
Vertical wind shear
42
24
-18
Vertical wind shear
-18.88
-16.9
1.98
Horizontal wind shear
(left side blocked)
-18.88
-10.11
8.77
Horizontal wind shear
(right side blocked)
-18.88
-9.73
9.15
4.59
4.59
-2.27
-1.13
-6.86
-5.72
4.59
5.21
0.62
Vertical axis (5m/s)
Vertical axis (6m/s)
Vertical wind shear
Horizontal wind shear
(left side blocked)
Horizontal wind shear
(right side blocked)
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