wind turbine performance: issues and evidence

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Name of Presenter Title of Presenter

Date

Wind Turbine Performance: Issues and Evidence

Dr. Michael C. Brower Chief Technical Officer



• Established in 1983

• Over 100 professional staff

• Activity

– Resource mapping

– Site assessment

– Plant design and energy estimates

– Operational plant assessment

– Real-time forecasting

– Grid impact studies

Company Overview

New York Barcelona

Bangalore



• The wind industry continues to suffer from an “underperformance gap”

• This sounds better than an “overestimation gap”

• The gap was ~10% in 2008, reduced to 0%-5% by 2011 - better, still not perfect

• What are the main factors causing the gap, and how much does the performance of individual turbines contribute to it?

The Underperformance Gap



AWS Truepower “Backcast” Database

Parameter Database

Wind Plants 24

Total Plant Years 106

Average Years per Plant 4.4

Min-Max Years per Plant 1 - 11

Average Plant Capacity 82 MW

Min-Max Plant Capacity 10 – 210 MW



AWS Truepower “Backcast” Database

Regions (all in North America)

• Texas & Southern Plains

• Upper Midwest

• Northeast

• Inter Mountain West

• California

Clipper

Gamesa

GE

MHI

Micon/Vestas

REpower

Siemens

Suzlon



Production Ratio Histogram

0

5

10

15

20

25

0.75 0.8 0.85 0.9 0.95 1.0 1.05 1.1 1.15 1.2 1.25

Win

d P

lan

t Ye

ars

Actual/Predicted Production Ratio

Farm Years Fitted Distribution

3.6%



• Availability • Assumptions align with experience

• Resource assessment and wind flow modeling • “Smart” resource assessment practices remove most

sources of bias

• Wake losses • AWST “deep-array” wake model (DAWM) aligns with

available data (onshore and offshore)

• Wakes in thermally stable conditions may still be a problem

• Performance of turbines…?

What Could Be Contributing to the Gap?



• Performance under nominal (IEC-compliant) conditions

• “Warranted” v. “advertised” output

• Turbine operation and maintenance practices

• Impacts of non-ideal conditions

• High/low shear

• High/low turbulence

• Non-horizontal flow

Turbine Performance Is a Problem



• Typical performance gap of 1-4% in power curve tests

• Supported by AWST and other industry test data

• IEC compliant, so not caused by site conditions

• Usually contractually allowed

• The results exceed the minimum guaranteed production minus the uncertainty

• Really?

Performance Under Nominal Conditions Falls Short



24 Turbine Power Performance Tests

0

1

2

3

4

5

0.85 0.87 0.89 0.91 0.93 0.95 0.97 0.99 1.01 1.03 1.05 1.07 1.09 1.11 1.13 1.15

Turb

ine

Bin

Co

un

t

Measured/Advertised AEP Ratio

Measured/Advertised AEP Ratio - 24 Turbines, 8 Wind Farms

AWS Frequency Fitted

2.4%

Older turbines



• Software settings, e.g., • Anemometer calibration constants

• Pitch, yaw errors

• Physical condition, e.g., • Wear and tear of moving parts

• Blade degradation

• Icing, soiling

• Plant operators must have the incentive to address such problems

• Operators must be able to measure the benefits of fixing the problems

Operation and Maintenance Practices



At a given site, shear and turbulence are usually closely linked, making their effects on output hard to separate

Site Conditions: Shear and Turbulence

0.00

0.05

0.10

0.15

0.20

0.25

0.00 0.10 0.20 0.30 0.40 0.50

Turb

ule

nce

In

ten

sity

Shear Exponent

Turbulence Intensity v. Shear Exponent At 8 mps

Stable

Neutral/ unstable



• Stability is not the root problem…

• Power curves are specified for “normal” shear (0.2)

• High shear means more power is theoretically available (~½ ρv3), so that should boost output…

• But turbines cannot use the extra power as efficiently as possible because the blade pitch is not optimal for speeds encountered in the top half of the rotor plane

• Individually pitched blades would address this problem

Shear



Effect of Shear on Aerodynamic Efficiency

40

50

60

70

80

90

100

110

120

5 7 9 11

He

igh

t (m

)

Speed (m/s)

0.60

0.15

0.30

Hub-height speed



Effect of Shear in a Typical Power Curve Test

86%

88%

90%

92%

94%

96%

98%

100%

102%

104%

0.0 - 0.1 0.1 - 0.2 0.2 - 0.3 0.3 - 0.4 0.4 - 0.5 0.5 - 0.6 0.6 - 0.7

No

rmal

ize

d P

ow

er

Shear Exponent Range

Power Output v. Shear Exponent At 8 mps



Effect of Shear in a Typical Power Curve Test

95%

96%

97%

98%

99%

100%

101%

102%

103%

0.0 - 0.1 0.1 - 0.2 0.2 - 0.3 0.3 - 0.4 0.4 - 0.5 0.5 - 0.6 0.6 - 0.7

No

rmal

ize

d P

ow

er

Shear Exponent Range

Power Output v. Shear Exponent At 9 mps



• Turbulence = rapid fluctuations in speed

• If not too fast, such fluctuations make a turbine move up and down its power curve

• Whether it means a net gain or loss depends on where the turbine is on the curve and the size of the speed deviations

• A portion of turbulent kinetic energy cannot be converted to power

• Turbulent changes in direction create more losses, hard to track and respond to

Turbulence



Turbulence

0

500

1000

1500

2000

2500

3000

0 2 4 6 8 10 12 14 16 18 20 22 24

Po

we

r (k

W)

Speed (mps)

Effect of Turbulence on Power Output



Turbulence

0.0E+00

2.0E+05

4.0E+05

6.0E+05

8.0E+05

1.0E+06

1.2E+06

1.4E+06

1.6E+06

2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5 10.5 11.5 12.5 13.5 14.5 15.5 16.5 17.5

Wat

ts

Speed (m/s)

Measured Power Curve for Different TI

<8% TI 8-12% TI >12% TI Advertised Curve



Turbulence

86%

88%

90%

92%

94%

96%

98%

100%

102%

104%

<0.050 0.050 - 0.0750.075 - 0.100 0.10 - 0.125 0.125 - 0.1500.150 - 0.1750.175 - 0.200

No

rmal

ize

d P

ow

er

Turbulence Intensity

Power Output v. Turbulence Intensity 5 mps

8 mps

11 mps



• Power curve tests assume level ground

• But in complex terrain and under certain weather conditions, significant vertical speeds can occur

• Placement of turbines along ridgelines can create a persistent bias

Inflow Angle



Inflow Angle

Tilt Angle Inflow Angle

Output = Nominal Output x

COS(angle relative to rotor plane)

Angle relative to rotor plane = Tilt Angle + Inflow Angle



Effect of Inflow Angle on Power Output

Downslope Upslope

86%

88%

90%

92%

94%

96%

98%

100%

102%

-50% -40% -30% -20% -10% 0% 10% 20% 30% 40% 50%

Flow Angle (%)

Normalized Output v. Inflow Angle

Typical upwind edge of ridge Typical downwind edge of ridge



• Turbine underperformance is a big part of general plant underperformance

• Turbines typically fall 1%-3% short of advertised power curves under IEC-compliant conditions • Gap will be incorporated into energy yield estimates to align

with experience – OEMs are “chasing their tail” • Better to be able to rely on turbine-supplied power curves

• Deviations of shear, turbulence, inflow angle from normal conditions can cause additional power deficits • Time-varying, not just average, conditions are important • Where there is a question, use appropriate tools (lidar,

advanced wind flow modeling) to gauge problem • Forget OEM warranted “site power curves”: Use

appropriate methods for energy production estimates

Observations and Conclusions

wind turbine performance: issues and evidence

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