Anemometer Calibration Requirements for
Wind Energy Applications
Presented By:
Rachael V Ishaya
Bryza Wind Lab, Inc., Fairfield, California
American Meteorological Society
17th Symposium on Meteorological Observations and Instrumentation
June 11, 2014 Westminster, Colorado
AMS 17
th
Symposium on Meteorological
Observation and Instrumentation
June 11, 2014 Westminster, CO
Rachael V Ishaya, President
rvishaya@bryzawindlab.com
Outline
Importance of Wind Sensors in Wind Energy
Wind Sensors Used in Wind Power
Basic Anemometer Calibration
Applicable Test Standards
Test Facility Requirements
Facility Performance Evaluation
Calibration Uncertainty
Summary
AMS 17
th
Symposium on Meteorological
Observation and Instrumentation
June 11, 2014 Westminster, CO
Rachael V Ishaya, President
rvishaya@bryzawindlab.com
Importance of Wind Sensors in Wind Energy
Wind Plant Operations
• Validate wind turbine power output
• Control start-up and shut-down
Wind Turbine Performance Evaluations
• Power curve (wind turbine power output as a function of wind speed)
Wind Energy Site Assessments
• Use power curves and wind distributions to estimate annual energy
production for power purchase agreements
0
200
400
600
800
1000
1200
1400
1600
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
Wind Speed (m/s)
Turbine Output Power (kW)
Rated Power
Turbine Power
1.5 MW rated power reached
at ~12 m/s
Power estimated at lower
wind speeds can be as much as
30% error depending on curve
Sample Turbine Power Curve
AMS 17
th
Symposium on Meteorological
Observation and Instrumentation
June 11, 2014 Westminster, CO
Rachael V Ishaya, President
rvishaya@bryzawindlab.com
Wind Sensors Used in Wind Power
Wind turbines are designed to
generate power from direct
incoming flow
Key measure from a wind sensor
is the magnitude of the horizontal
wind speed component
AMS 17
th
Symposium on Meteorological
Observation and Instrumentation
June 11, 2014 Westminster, CO
Rachael V Ishaya, President
rvishaya@bryzawindlab.com
Basic Anemometer Calibration
Anemometer Output ⇔
⇔⇔
⇔ Controlled Reference Speed
Rotation rate
(i.e., Hz or rpm)
Wind generated from a
controlled wind tunnel
Analog voltage
or conditioned
digital signal
0
5
10
15
20
25
30
0 200 400 600 800 1000
Anemometer Frequency, f [Hz]
Reference Speed, U [m/s]
Perform a Least Squares Fit
⇒
⇒⇒
⇒ Linear Transfer Function
Anemometers are designed
to be linear instruments
Reference
Pitot-static
tubes
NCSL International
Aug 1, 2012 Anaheim, CA
Rachael Coquilla, President
rvcoquilla@bryzawindlab.com
Applicable Test Standards
• ASTM D5096-02, “Standard test method for determining the
performance of a cup anemometer or propeller anemometer”
• ASTM D6011-96, “Standard test method for determining the
performance of a sonic anemometer/thermometer”
• ISO 17713-1, “Meteorology – Wind measurements Part 1: Wind
tunnel test methods for rotating anemometer performance”
• ISO 16622, “Meteorology – Sonic anemometers/thermometers –
Acceptance test methods for mean wind measurements”
• IEC 61400-12-1, “Wind turbines – Part 12-1: Power performance
measurements of electricity producing wind turbines”
• IEC 61400-12-2, “Wind turbines – Part 12-2: Power performance
of electricity producing wind turbines using nacelle anemometry”
General requirement is to perform anemometer
calibrations in a uniform-flow, low-turbulence wind tunnel
AMS 17
th
Symposium on Meteorological
Observation and Instrumentation
June 11, 2014 Westminster, CO
Rachael V Ishaya, President
rvishaya@bryzawindlab.com
Test Facility Requirements
Wind Tunnel
Characteristic
Standards Requirements
Speed Range IEC 61400-12-1 (4-16 m/s); Others based on % of application speed
Flow Uniformity IEC 61400-12-1 (<0.2%); Others (<1%)
Wind Gradient IEC 61400-12-1 (<0.2%)
Turbulence Intensity IEC 61400-12-1 (<2%); Others (<1%)
Density Uniformity ASTM D5096-2, ISO 17713-1 (<3%)
Data Acquisition Resolution 0.02 m/s, minimum sampling 10 Hz, duration 30-100 sec
Model Blockage 10% max for open test sections, 5% max for closed test sections
Repeatability IEC 61400-12-1 (<0.5% at 10 m/s test speed)
Interlaboratory
Comparison
IEC 61400-12-1 (within 1% in 4-16 m/s test speed range)
AMS 17
th
Symposium on Meteorological
Observation and Instrumentation
June 11, 2014 Westminster, CO
Rachael V Ishaya, President
rvishaya@bryzawindlab.com
Test Facility Requirements
Test
Section
DiffuserContraction
Fan
Motor
Open-circuit, suction or Eiffel-type
Test
Section
Settling
Chamber
Contraction
Blower
Motor
Open-circuit, blower-type
Common Wind Tunnel Configurations
AMS 17
th
Symposium on Meteorological
Observation and Instrumentation
June 11, 2014 Westminster, CO
Rachael V Ishaya, President
rvishaya@bryzawindlab.com
Facility Performance Evaluation
AIAA R-093-2003, “Calibration of Subsonic and Transonic Wind Tunnels”
P
1
P
2
Contraction
Section
Test
Section
Diffuser Section
Inlet
Fan
Motor
General concept is to define dynamic pressure in test section according to
the pressure drop generated by the wind tunnel contraction section.
AMS 17
th
Symposium on Meteorological
Observation and Instrumentation
June 11, 2014 Westminster, CO
Rachael V Ishaya, President
rvishaya@bryzawindlab.com
Facility Performance Evaluation
2.5 ft
2.5 ft
5 ft
Airflow
50 hp fan-motor
VFD
Reference speed measurement:
Pitot-static tube system
(as defined by IEC 61400-12-1)
Four Pitot tubes with sensing tips
positioned at test section inlet where
total and reference ports connected
to an MKS 120AD transducer.
AMS 17
th
Symposium on Meteorological
Observation and Instrumentation
June 11, 2014 Westminster, CO
Rachael V Ishaya, President
rvishaya@bryzawindlab.com
Facility Performance Evaluation
ρ
p
V
∆
=
2
( )
[
]
wair
T
air
MMePM
T
R
−×−=
− 0631846.07
1005.2
*
1
φρ
Pitot-Static Tube System
General Velocity Equation
Differential pressure
from Pitot-static tube
Density of humid air
Relative humidity
Ambient temperature
Ambient pressure
AMS 17
th
Symposium on Meteorological
Observation and Instrumentation
June 11, 2014 Westminster, CO
Rachael V Ishaya, President
rvishaya@bryzawindlab.com
Facility Performance Evaluation
1) Profiles mean speed settings from 4 to 26 m/s showed an
average test section uniformity within +/-0.2%.
2) Preliminary indication of less than 0.2% turbulence.
3) Difference in wind speed between center of test section to
reference Pitot-static tubes at inlet averages to +0.014%.
Velocity profiles at center of test section from traversed Pitot tube.
AMS 17
th
Symposium on Meteorological
Observation and Instrumentation
June 11, 2014 Westminster, CO
Rachael V Ishaya, President
rvishaya@bryzawindlab.com
Facility Performance Evaluation
Blockage
Ratio
%2
3
≤
TS
C
A
A
Empirical
Blockage
Correction
005.1
4
1
1
3
=+=
TS
C
b
A
A
k
Test performance requirements according to IEC 61400-12-1
Calibration test speeds: 4 to 16 m/s at 1 m/s increments
Repeatability (<0.5% at 10 m/s) ⇒
⇒⇒
⇒ within 0.2% for 5 repeated tests
Interlaboratory Comparison (+/-1% at 4-16 m/s) ⇒
⇒⇒
⇒ 1% average variation
in comparison to an accredited wind tunnel laboratory in Denmark
Second Wind C3
1/2” diameter
mounting stand
AMS 17
th
Symposium on Meteorological
Observation and Instrumentation
June 11, 2014 Westminster, CO
Rachael V Ishaya, President
rvishaya@bryzawindlab.com
( )
( )
( )
222
LRIUTVcal
UUUU ++=
Calibration Uncertainty
Reference wind speed
Test sensor output
Calibration linearity
Anemometer calibration uncertainty consists of the
propagation of errors from three general areas
0
5
10
15
20
0 5 10 15 20 25
Reference Speed [m/s]
IUT Output [Hz]
Uncertainty in each area includes systematic or
Type B errors (B
i
) and random or Type A errors (S
i
)
( )
2
2
iii
tSBU +=
Coverage factor at 95% confidence
AMS 17
th
Symposium on Meteorological
Observation and Instrumentation
June 11, 2014 Westminster, CO
Rachael V Ishaya, President
rvishaya@bryzawindlab.com
Calibration Uncertainty
Uncertainty in reference wind speed
( )
2
2
VVV
tSBU +=
Analyzed from Pitot-static tube velocity equation
ρ
p
V
∆
=
2
ρ
p
kkCV
cbh
∆
=
2
( )
[
]
wair
T
air
MMePM
T
R
−×−=
− 0631846.07
1005.2
*
1
φρ
V= f (M
air
, M
w
, k
b
, k
c
, C
h
, R*, P , T
K
,
∆
∆∆
∆
p ,
φ
φφ
φ
)
AMS 17
th
Symposium on Meteorological
Observation and Instrumentation
June 11, 2014 Westminster, CO
Rachael V Ishaya, President
rvishaya@bryzawindlab.com
Calibration Uncertainty
Uncertainty in reference wind speed
( )
2
2
VVV
tSBU +=
2
2
22
2
*
2
22
*
+
∆
+
+
+
+
+
+
=
∆
φ
δφ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
B
V
B
p
V
B
T
V
B
P
V
B
R
V
B
C
V
B
k
V
B
k
V
B
pTP
RC
h
k
c
k
b
V
hcb
2
2
22
+
∆
+
+
=
∆
φ
δφ
δ
δ
δ
δ
δ
δ
δ
S
V
S
p
V
S
T
V
S
P
V
S
pTPV
INDEPENDENT VARIABLES
(exact values defined by NIST)
M
air
and M
w
Measured
P, T,
φ
,
∆
p
Analyzed
k
b
, k
c
, C
h
, R*
DEPENDENT VARIABLES
AMS 17
th
Symposium on Meteorological
Observation and Instrumentation
June 11, 2014 Westminster, CO
Rachael V Ishaya, President
rvishaya@bryzawindlab.com
Calibration Uncertainty
Uncertainty in test sensor output
( )
2
2
IUTIUTIUT
tSBU +=
Uncertainty in calibration linearity
( )
2
resLR
tVU =
Type B errors are acquired from data acquisition system.
Type A errors are quantified by the standard deviations
in the test sensor reading.
0
5
10
15
20
0 5 10 15 20 25
Reference Speed [m/s]
IUT Output [Hz]
-0.1
-0.05
0
0.05
0.1
0 5 10 15 20 25
Speed Residual [m/s]
IUT Output [Hz]
AMS 17
th
Symposium on Meteorological
Observation and Instrumentation
June 11, 2014 Westminster, CO
Rachael V Ishaya, President
rvishaya@bryzawindlab.com
Calibration Uncertainty
0.00
0.05
0.10
0.15
0.20
0.25
4 5 6 7 8 9 10 11 12 13 14 15 16
Uncertainty (m/s)
Reference Speed (m/s)
Reference Speed
C3 Output
Transfer Function
Overall
Calibration Uncertainty for
the C3 Anemometer
0.07 m/s
0.21 m/s
AMS 17
th
Symposium on Meteorological
Observation and Instrumentation
June 11, 2014 Westminster, CO
Rachael V Ishaya, President
rvishaya@bryzawindlab.com
Summary
1) Perform tests in a controlled, uniform-flow, low-
turbulence wind tunnel as required by test standards
2) Verify wind tunnel performance through velocity surveys
3) Qualify ability to perform wind sensor calibrations
Blockage evaluation
Test repeatability
Interlaboratory comparison
4) Document the uncertainty analysis
Key Considerations for an
Anemometer Calibration Program
For more information:
rvishaya@bryzawindlab.com
