Loads Measurement and Validation of a 2.1-kW HAWT on a Fiberglass Composite TowerScott Dana, Rick Damiani, Jeroen van Dam

Small Wind Conference 2016

June 13th – June 15th

2

Motivation

Ongoing efforts to improve system dynamic modeling and prediction

Assess common industry practice loads derivation methods

Evaluate IEC 61400-2 Simplified Loads Approach

Address fatigue loads more comprehensively

3

Design Load Derivation Methods

1. Full-Scale Loads Measurements (Field)

• A 2.1-kW, free yaw, variable-speed, downwind machine

• An AnemErgonics, LLC 18-m multisection fiberglass composite tower

• Meteorological data recorded at hub height

• Tower strain measured at 0.8 m above tower hinge plate

• Full bending bridges in two orthogonal directions

2. Aeroelastic Modeling (FAST)

• FAST v 7.02 utilized

• Turbulent wind files, 3 seeds, with reference wind speeds of 2, 4…,24, 26 m/s

• Turbulence intensity of 18%

• Air density of 1.0 kg/m3 to match site conditions

• Tower drag not modeled

3. Simplified Loads Approach (SLA)

• IEC 61400-2 design load cases (DLCs) A (fatigue), H, and I (ultimate loads)

• Input parameters derived from the FAST model, preliminary test results, and turbine dimensions

• SLA load components used to determine the tower-base bending moment

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Full-Scale Loads Measurements Approach

X (approximate prevailing wind direction)

Y

285°

15° (from true North)

105°

195°

X Bending Bridge

Y Bending Bridge

My

Mx

Tower-Base Bending

Bridge Location

Primary

Anemometer

Reference

Anemometer

Temperature

and Pressure

Wind Vane

• Measurement sector of 223° to 333 ° of true north following IEC 61400-12-1

• Measurement methods and loads processing following IEC 61400-13

• Coordinate transformation applied to loads

Test article used for full-scale loads measurement with identification of tower strain gage locations

(Photo by Rick Damiani, NREL)Met tower and instrumentation used for field test campaign (Photo by Scott Dana, NREL)

Tower Strain full bridge orientation and loads configuration

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Measurement Data Acquisition System

DAS Enclosure

• NI cDAQ Chassis and Input Modules

• Sampled at 35 kHz• Stored at 100 Hz

• 1,332 10-Minute data files within measurement sector for Loads Calculations

• 3,482 10-Minute data files for Fatigue Calculations (all operating data)

Communication one-line for tower loads DAS

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Simplified Loads Approach – Maximum Bending Moments

0

5

10

15

20

25

30

35

40

45

Load Case H Load Case IB

en

din

g M

om

en

t [k

Nm

]

Rotor Drag

Nacelle Drag

Tower Drag

Component-drag contributions to the total combined tower-base bending moment for DLCs H and I

SLA DLCs for tower analysis:

• IEC 61400-2 DLC H

• Extreme wind speed of 59.5 m/s• A normally parked turbine

• IEC 61400-2 DLC I

• 42.5 m/s wind speed• Maximum exposure (yaw mechanism

failure)• A ‘no-yaw-error’ configuration in

this case

80 kNm

47 kNm

• Total Moment = 80 kNm

• Total Moment = 47 kNm

DLC H develops the greatest bending moment Tower Design Driver

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FAST and Field Bending Moments Results

0 5 10 15 20 25 30-10

-5

0

5

10

15

20

25

30

35

40

Bendin

g M

om

ent

[kN

-m]

0 5 10 15 20 25 30-25

-20

-15

-10

-5

0

5

10

15

Wind Speed [m/s]

Bendin

g M

om

ent

[kN

-m]

Loads extrapolation of the field-measured loads is required for a proper comparison

with SLA results — a future work item of this study.

• Deviation between Field and FAST is likelya result of the actual turbine controlsystem mitigating loads at high windspeeds (18 m/s and above)

Ten-minute statistical comparison of the Field and FAST tower-base ultimate loads in the fore-aft (top) and side-to-side (bottom) directions

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Wind Speed [m/s]

Mom

ent

[Nm

]

Perpendicular to Wind Moments, Mx: 10 Minute Statistics

Field Max

Field Mean

Field Min

FAST Max

FAST Mean

FAST Min

• For the captured wind speeds FASTand field loads are within the loadenvelope of DLCs H and I

• Less than 47 kNm (DLC I)

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Tower-base 1-Hz lifetime (20 years); DELs at zero mean for Field, FAST, and SLA with S-N curves with inverse exponent values (m) of 3, 6, and 10

Fatigue Loads Comparison – Damage Equivalent Loads

0

2

4

6

8

10

12

14

Field Fore-Aft FAST Fore-Aft SLA Fore-Aft

DEL

Be

nd

ing

Mo

me

nt

[kN

m]

m = 3 m = 6 m = 10

m = 3

m = 10

Cycles

Load

Ran

ge

NSLANeqNFAST

SSLA

SFAST

Illustration of the effect of a chosen S-N curve slope on the DEL for a high load range (SFAST) at a low number of cycles (NFAST) and vice versa (SSLA, NSLA)

• Three different S-N curves

• Rayleigh distribution of wind speed

• 20-year design life

• SLA DELs by IEC 61400-2 DLC A

• Peak-to-peak thrust load

• 20-yr # of cycles @ rated rotor speed

Significant effect of material properties S-N curve exponent!

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Fatigue Loads Comparison Continued

0 5 10 15 20 25 300

5

10

15

20

25

30

10 m

in D

EL [

kN

m]

Wind Speed [m/s]

Fore-aft Field

Fore-aft FAST

104

106

108

1010

1012

1014

0

5

10

15

20

25

30

35

40

Load R

ange [

kN

m]

Cumulative Cycles

Fore-aft Field

Fore-aft FAST

SLA Value

Load range = 5.183 kNm

Cycle Count = 10.4e9

Short-term (10 minute) fore-aft tower-base bending moment DELs versus the mean wind speed for an S-N curve with m = 10

Fore-aft tower-base bending moment cumulative fatigue spectra comparison of field-measured loads and FAST predictions

• The SLA DLC A yields a single fatigue load range value of 5.183 kNm, with a cycle count of 10.4e9.

• Although similar, FAST’s over-prediction of high-range loads compared to the Field spectra are demonstrated.

• This outcome is consistent with the ultimate loads comparison.

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This study focused on tower-base bending moments of a horizontal-axis windturbine to provide an indication of the overall performance of the loadderivation methods. Notable outcomes include the following:

• The SLA DELs are different than those calculated from the Field and FASTdata;

o High dependence on the assumed S-N curve slope and the number of cycles.

o SLA DEL more conservative for steel (m = 3 to 6) and less conservative forcomposites (m =10).

• Better agreement between the field and FAST predictions for side-to-sidethan for fore-aft bending at high wind speeds;

o Likely caused by a lower fidelity turbine control model used in the FASTsimulations.

• The fatigue spectra reveals FAST’s over-prediction of large cycles.

• SLA does not comprehensively address fatigue;

o Conservative and coarse → one load range at one cycle count.

• We recommend that this study be repeated with wind turbines of differentconfigurations and sizes, and that field load extrapolation be conducted.

Final Remarks

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• The Authors of this work would like to sincerely thank Paul Migliore of AnemErgonics for allowing his tower and turbine to be used for the field measurement campaign of this study.

• The Presenter would like to thank Rick and Jeroen for their excellent mentorship and guidance throughout the project.

Acknowledgements

Thank you!

Questions?