March 17, 2009
SODAR and Extrapolated Tower Wind Shear Profile Comparison in Various Topographic Conditions
Elizabeth WallsNiels LaWhite
Second Wind IncEWEC 2009 Marseille
2
Introduction
• SODAR (Sonic Detection and Ranging):– measure wind data by transmitting
acoustic pulses and analyzing the frequency content of the returned signal
• Triton Sonic Wind Profiler:– Low-power, monostatic, phased-array
SODAR commercialized in early 2008
• Several Triton vs. Tower comparisons– Great correlation at anem. height– How do the extrapolated tower shear
profiles compare to the measured Triton data?
– How does the error in extrapolation translate to error in predicted power?
3
Outline
• Site and Data Set Description– 4 sites across the U.S. with varying topography– 2 months of concurrent tower and Triton data
• Triton vs. Tower Data: Validation• Shear Exponent Estimation using Triton Data• Extrapolated Wind Shear Profile Comparison• Theoretical Power Output Comparison
4
Site and Data Set Descriptions
• Cranberry Bog in Massachusetts– Flat site surrounded by trees – 60 m met tower– Data Used for comparison:
• May 15th – July 15th, 2008
• Open Field in Kansas– Flat and open terrain– 60 m met tower– Data Used for comparison:
• Sept. 1st – Nov. 1st, 2008
5
Site and Data Set Descriptions
• Ridgeline in Washington State– Complex, hilly terrain– 50 m met tower– Data Included: August 15th – Oct.
15th, 2008
• Wind Farm in Washington State– Several wind turbines ~300 m
from Triton– 60 m met tower– Data Included: Sept. 1st – Oct.
17th, 2008
6
Triton vs. Tower Data: Filters
• Data Filtering for Correlation Study:– Triton Quality Factor > 90%
• Quality: function of Signal-to-Noise Ratio (SNR) and the number of valid data points over ten-minutes
– Triton Vertical Wind Speed < +/-1.5 m/s– Max Value of Two Anems Used
• Reduces tower shadow effects
• Data Filtering for Average Wind Speed Comparison– Triton Quality Factor > 95%– Triton Vertical Wind Speed < +/-1 m/s– Average Value of Two Anems Used– Ratio of Anems = 0.98 - 1.02– Anem Wind Speed > 2 m/s– Direction Sectors 45º from boom with 30º width
45
30
Anems
Dir. Sectors Included
7
Triton vs. Tower Data: Cranberry Bog, MA
• Data Interval: May 15th to July 15th, 2008
• Triton Operational Uptime = 98.4%Triton vs. Tower Wind Speeds at Cranberry Bog in MA
y = 1.003x - 0.086R = 0.968
0
2
4
6
8
10
12
14
0 2 4 6 8 10 12 14
Tower 60 m Wind Speed (Max of Two), m/s
Tri
ton
60
m W
ind
Sp
ee
d, m
/s
• Corr. Coeff. = 0.968• Valid Triton data (High
Q) @ 60 m = 99.5%• % Diff. In Avg. Wind
Speed = -1.1 %
8
Triton vs. Tower Data: Open Field, KS
• Data Interval: Sept. 1st to Nov. 1st, 2008
• Triton Operational Uptime = 99.3%
• Corr. Coeff. = 0.976• Valid Triton data (High
Q) @ 60 m = 94.5%• % Diff. In Avg. Wind
Speed = -0.55 %
Triton vs. Tower Wind Speeds at Field in KS
y = 0.982x + 0.014R = 0.976
02468
10121416182022
0 2 4 6 8 10 12 14 16 18 20 22
Tower 60 m Wind Speed (Max of Two), m/s
Tri
ton
60
m W
ind
Sp
ee
d, m
/s
9
Triton vs. Tower Data: Ridgeline, WA
• Data Interval: Aug. 15th to Oct. 15th, 2008
• Triton Operational Uptime = 94.9%
• Corr. Coeff. = 0.988• Valid Triton data (High
Q) @ 50 m = 91.1%• % Diff. In Avg. Wind
Speed = -7.6 %– Large diff. due to
terrain and distance from tower
Triton vs. Tower Wind Speeds on Ridgeline in WA
y = 0.982x - 0.501R = 0.988
02
468
10
121416
1820
0 2 4 6 8 10 12 14 16 18 20
Tower 50 m Wind Speed (Max of Two), m/s
Tri
ton
50
m W
ind
Sp
ee
d, m
/s
10
Triton vs. Tower Data: Wind Farm, WA
• Data Interval: Sept. 1st to Oct. 17th, 2008
• Triton Operational Uptime = 99.8%
• Corr. Coeff. = 0.966• Valid Triton data (High
Q) @ 60 m = 97.4%• % Diff. In Avg. Wind
Speed = -0.6 %
Triton vs. Tower Wind Speeds in Wind Farm in WA
y = 0.954x - 0.108R = 0.966
0
2
4
6
8
10
12
14
16
0 2 4 6 8 10 12 14 16
Tower 60 m Wind Speed (Max of Two), m/s
Tri
ton
60
m W
ind
Sp
ee
d, m
/s
11
Shear Exponent Estimation using Triton
Data• Power Law Profile:
RZ
Z
z
z
U
U
R
Finding Alpha
y = 0.2664x + 0.7255
0.1
1.0
10.0
0.1 1 10
ln(z/zr)
ln(U
/Ur)
• Use Triton Data from 40 m to 120 m
• Plot ln(U/Ur) vs ln(z/zr)
• Slope of best-fit = Power Law Exponent, Alpha
Average Wind Speed Profile
0
20
40
60
80
100
120
140
0 2 4 6 8 10 12
Wind Speed, m/s
He
igh
t, m
Measured byTriton
Average Wind Speed Profile
0
20
40
60
80
100
120
140
0 2 4 6 8 10 12
Wind Speed, m/s
He
igh
t, m
Measured by Triton
Power Law Profile,Alpha = 0.26
12
Shear Exponent Estimation using Triton
Data, cont’d• Alpha
found for each Triton data set:
Triton Wind Speed ProfileField in KS
0
20
40
60
80
100
120
140
0 2 4 6 8 10 12
Wind Speed, m/s
He
igh
t, m
Triton Alpha = 0.266
Triton Wind Speed ProfileRidgeline in WA
0
20
40
60
80
100
120
140
0 1 2 3 4 5 6
Wind Speed, m/s
He
igh
t, m
Triton Alpha = 0.061
Triton Wind Speed ProfileCranberry Bog in MA
0
20
40
60
80
100
120
140
0 2 4 6 8
Wind Speed, m/s
He
igh
t, m
Triton Alpha = 0.392
Triton Average Wind Speed ProfileWind Farm in WA
0
20
40
60
80
100
120
140
0 2 4 6 8 10
Wind Speed, m/s
Hei
gh
t, m
Triton Alpha = 0.176
13
Extrapolated Wind Speed ProfileField in KS
0
20
40
60
80
100
120
140
160
4 6 8 10 12
Wind Speed, m/s
He
igh
t, m
Extrapolated Wind Shear Profile Comparison
• For each data set, found:– Triton Alpha (using data from 40 to 120 m)– Tower Alpha (using data from 2 heights)
• Tower data extrapolated using both Triton and Tower Alphas
Extrapolated Wind Speed ProfileField in KS
0
20
40
60
80
100
120
140
160
4 6 8 10 12
Wind Speed, m/s
He
igh
t, m
Tower Alpha = 0.165
Extrapolated Wind Speed ProfileField in KS
0
20
40
60
80
100
120
140
160
4 6 8 10 12
Wind Speed, m/s
He
igh
t, m
Tower Alpha = 0.165
Triton Alpha = 0.266
Extrapolated Wind Speed ProfileCranberry Bog in MA
0
20
40
60
80
100
120
140
160
2 3 4 5 6 7 8
Wind Speed, m/s
He
igh
t, m
Extrapolated Wind Speed ProfileCranberry Bog in MA
0
20
40
60
80
100
120
140
160
2 3 4 5 6 7 8
Wind Speed, m/s
He
igh
t, m
Tower Alpha = 0.443
Extrapolated Wind Speed ProfileCranberry Bog in MA
0
20
40
60
80
100
120
140
160
2 3 4 5 6 7 8
Wind Speed, m/s
He
igh
t, m
Tower Alpha = 0.443
Triton Alpha = 0.392
14
Extrapolated Wind Shear Profile Comparion, cont’d
• Wind speed profile extrapolations from other two sites:
Extrapolated Wind Speed ProfileRidgeline in WA
0
20
40
60
80
100
120
140
160
4 5 6 7 8 9 10
Wind Speed, m/s
He
igh
t, m
Extrapolated Wind Speed ProfileRidgeline in WA
0
20
40
60
80
100
120
140
160
4 5 6 7 8 9 10
Wind Speed, m/s
He
igh
t, m
Tower Alpha = 0.044
Extrapolated Wind Speed ProfileRidgeline in WA
0
20
40
60
80
100
120
140
160
4 5 6 7 8 9 10
Wind Speed, m/s
He
igh
t, m
Tower Alpha = 0.044
Triton Alpha = 0.061
Extrapolated Wind Speed ProfileWind Farm in WA
0
20
40
60
80
100
120
140
160
2 3 4 5 6 7 8
Wind Speed, m/s
He
igh
t, m
Extrapolated Wind Speed ProfileWind Farm in WA
0
20
40
60
80
100
120
140
160
2 3 4 5 6 7 8
Wind Speed, m/s
He
igh
t, m
Tower Alpha = 0.148
Extrapolated Wind Speed ProfileWind Farm in WA
0
20
40
60
80
100
120
140
160
2 3 4 5 6 7 8
Wind Speed, m/s
He
igh
t, m
Tower Alpha = 0.148
Triton Alpha = 0.176
15
Theoretical Power and Equivalent Wind Speed
• How do varying wind shear profiles translate into theoretical power available in wind?
32
2
1URCP P
dhAUA
U eq 1
• Power Produced:
• Equivalent Hub Height Wind Speed:
16
Theoretical Power Output Comparison
• Assuming ideal turbine operation: Cp = 16/27 and 100% efficiency
• % Difference = 100
AlphaTritononBased
AlphaTritononBasedAlphaToweronBased
Power
PowerPower
Open Field in KS
Ridgeline in WA
Cranberry Bog in MA
Wind Farm in WA
-11.0% -2.8% 6.0% -3.2%
• With hub height = 80 m and rotor radius = 40 m, % difference in predicted power:
17
Power as function of Rotor Radius and Hub Height
• Error increases with both rotor radius and hub height• +ve % diff. : Tower data leads to overprediction• -ve % diff. : Tower data leads to underprediction
% Difference in Predicted Power as function of Rotor Radius
-14%-12%-10%
-8%-6%-4%-2%0%2%4%6%8%
0 20 40 60 80Rotor Radius, m
% D
iff.
in
Po
we
r
Field Ridge Bog Wind Farm
Hub Height = 80 m
% Difference in Predicted Power as function of Hub Height
-20%
-15%
-10%
-5%
0%
5%
10%
15%
0 20 40 60 80 100 120Hub Height, m
% D
iff.
in
Po
we
r
Field Ridge Bog Wind Farm
Radius = 40 m
• With hub height of 100 m and a radius of 40 m, the percent difference ranged from -16.4% to 9.3%
Range of Uncertainty
18
Summary
• Analyzed two months of concurrent Triton and tower data from 4 different sites across the U.S.
• At each site, showed excellent agreement between the tower and Triton data in terms of correlation (Ravg = 0.975) and average wind speed
• Estimated alpha (power law exponent) using both the Triton and tower data
• Used both alphas to generate extrapolated wind shear profiles
• Calculated the theoretical power production with each wind shear profile and found the percent difference
19
Conclusions
• Extrapolating wind shear profiles, based on tower data, can lead to under or over estimation of wind speeds
• Error in theoretical power increases with rotor radius and, more drastically, with hub height
• SODARs (and other remote sensing devices) measure wind speed across the rotor diameter which reduces uncertainty in shear exponent estimation.