Technical Report

NREL/TP-500-49069

August 2010

Wind Turbine Generator System 

Duration Test Report for the 

Gaia-Wind 11 kW Wind Turbine

Arlinda Huskey, Amy Bowen, and David Jager
National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393

303-275-3000 • www.nrel.gov

NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency and Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

Contract No. DE-AC36-08-GO28308

Technical Report

NREL/TP-500-49069

August 2010

Wind Turbine Generator System 

Duration Test Report for the 

Gaia-Wind 11 kW Wind Turbine

Arlinda Huskey, Amy Bowen, and David Jager

Prepared under Task No. WE10.2211
NOTICE

This report was prepared as an account of work sponsored by an agency of the United States government. 

Neither the United States government nor any agency thereof, nor any of their employees, makes any 

warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or 

usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not 

infringe privately owned rights. Reference herein to any specific commercial product, process, or service by 

trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, 

recommendation, or favoring by the United States government or any agency thereof. The views and 

opinions of authors expressed herein do not necessarily state or reflect those of the United States 

government or any agency thereof.

Available electronically at http://www.osti.gov/bridge

Available for a processing fee to U.S. Department of Energy

and its contractors, in paper, from:

U.S. Department of Energy

Office of Scientific and Technical Information

P.O. Box 62

Oak Ridge, TN 37831-0062

phone: 865.576.8401

fax: 865.576.5728

email: mailto:reports@adonis.osti.gov

Available for sale to the public, in paper, from:

U.S. Department of Commerce

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

phone: 800.553.6847

fax: 703.605.6900

email: orders@ntis.fedworld.gov

online ordering: http://www.ntis.gov/ordering.htm

Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste
iii

Notice

This report was prepared by the National Renewable Energy Laboratory (NREL), 

operated for the United States Department of Energy (DOE) by the Alliance for 

Sustainable Energy, LLC (Alliance), as an account of work sponsored by the United 

States government. The test results documented in this report define the characteristics 

of the test article as configured and under the conditions tested.

THIS REPORT IS PROVIDED "AS IS" AND NEITHER THE GOVERNMENT, ALLIANCE, NREL 

NOR ANY OF THEIR EMPLOYEES, MAKES ANY WARRANTY, EXPRESS OR IMPLIED, 

INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 

PURPOSE, OR ASSUMES ANY LEGAL LIABILITY OR RESPONSIBILITY FOR THE 

ACCURACY, COMPLETENESS, OR USEFULNESS OF ANY SUCH INFORMATION 

DISCLOSED IN THE REPORT, OR OF ANY APPARATUS, PRODUCT, OR PROCESS 

DISCLOSED, OR REPRESENTS THAT ITS USE WOULD NOT INFRINGE PRIVATELY 

OWNED RIGHTS. 

Neither Alliance nor the U. S. Government shall be liable for special, consequential or 

incidental damages. Reference herein to any specific commercial product, process, or 

service by trade name, trademark, manufacturer, or otherwise does not necessarily 

constitute or imply its endorsement, recommendation, or favoring by the United States 

government or any agency thereof. The views and opinions of the authors expressed 

herein do not necessarily state or reflect those of the United States government or any 

agency thereof or Alliance. 

NREL is a DOE Laboratory, and as an adjunct of the United States government, cannot 

certify wind turbines. The information in this report is limited to NREL’s knowledge and 

understanding as of this date. 

NREL is accredited by the American Association for Laboratory Accreditation (A2LA) 

and the results shown in this test report have been determined in accordance with the 

NREL’s terms of accreditation unless stated otherwise in the report.

This report shall not be reproduced, except in full, without the written approval of Alliance 

or successor operator of NREL.

Approval By: __________________________________________________________ 

Arlinda Huskey, NREL Test Engineer Date

Approval By: __________________________________________________________ 

Jeroen van Dam, NREL Test Engineer Date
iv

Table of Contents

Table of Contents................................................................................................................................. iv

Table of Tables...................................................................................................................................... v

Table of Figures..................................................................................................................................... v

1. Background................................................................................................................................... 1

2. Test Objective and Requirements..................................................................................................1

3. Description of Test Turbine ........................................................................................................... 2

4. Description of Test Site ................................................................................................................. 6

5. Description of Instrumentation .....................................................................................................7

6. Results .......................................................................................................................................... 9

6.1 Operation Time .................................................................................................................................... 9

6.2 Months of Operation.......................................................................................................................... 10

6.3 Hours of Power Production ................................................................................................................ 10

6.4 Operational Time Fraction ................................................................................................................. 10

6.5 Environmental Conditions.................................................................................................................. 12

6.6 Power Degradation Checks ................................................................................................................ 12

6.7 Dynamic Behavior .............................................................................................................................. 12

6.8 Tear-Down Inspection ........................................................................................................................ 13

7. Uncertainty ................................................................................................................................. 15

8. Deviations and Exceptions........................................................................................................... 16

8.1. Deviations from the Standard ..................................................................................................... 16

8.2. Deviations from Quality Assurance............................................................................................. 16

Appendix A: Instrument Calibration Certificates .................................................................................. 17

Appendix B: Turbine Commissioning Checklist .................................................................................... 31

Appendix C: Post-Test Inspection......................................................................................................... 33

C.1 Blades................................................................................................................................................. 33

C.2 Hub..................................................................................................................................................... 33

C.3 Brakes................................................................................................................................................. 33

C.4 Vibration Sensor................................................................................................................................. 33

C.5 Electrical............................................................................................................................................. 33

C.6 Yaw system......................................................................................................................................... 33

C.7 Tower ................................................................................................................................................. 34

C.8 Gearbox.............................................................................................................................................. 34
v 

Table of Tables

Table 1. Test turbine configuration and operational data ............................................................................. 3

Table 2. Equipment List for Duration Test...................................................................................................... 9

Table 3. Monthly and overall results of the Gaia-Wind 11 kW duration test............................................... 10

Table of Figures

Figure 1. The Gaia-Wind 11 kW wind turbine ................................................................................................ 4

Figure 2. General electrical arrangement....................................................................................................... 5

Figure 3. Map of the test site ......................................................................................................................... 6

Figure 4. Location of the data acquisition sensors......................................................................................... 8

Figure 5. Operational time fraction for each month .................................................................................... 13

Figure 6. Scatter plot of power versus wind speed (10-minute averages)................................................... 14

Figure 7. Power level in several wind speed bins (in m/s) as a function of time.......................................... 14
1 

1. Background

This test was conducted as part of the U.S. Department of Energy’s (DOE) Independent 

Testing project. This project was established to help reduce the barriers of wind energy 

expansion by providing independent testing results for small turbines. In total, five

turbines are being tested at the National Renewable Energy Laboratory’s (NREL’s) 

National Wind Technology Center (NWTC) as a part of this project. Duration testing is 

one of up to five tests that may be performed on the turbines, including power 

performance, safety and function, noise, and power quality tests. The results of the 

testing will provide the manufacturers with reports that may be used for small wind 

turbine certification.

The test equipment includes a Gaia-Wind 11 kW wind turbine mounted on an 18 m 

monopole tower. Gaia-Wind Ltd. manufactured the turbine in Denmark, although the 

company is based in Scotland. The system was installed by the NWTC Site Operations 

group with guidance and assistance from Gaia-Wind. 

2. Test Objective and Requirements

This test was conducted in accordance with Clause 9.4 of the International 

Electrotechnical Commission’s (IEC) standard, Wind turbines - Part 2: Design 

requirements for small wind turbines, IEC 61400-2 Ed. 2.0:2006-03. This test report 

refers to these procedures as the “Standard.” The objective of this test is to assess the 

following aspects of the Gaia-Wind 11 kW turbine:

• Structural integrity and material degradation 

• Quality of environmental protection 

• The dynamic behavior. 

Based on the parameters defined in the Standard for small wind turbine classes, GaiaWind identified the test turbine as a Class III. This corresponds to a Vave of 7.5 m/s.

The wind turbine will pass the duration test when it has achieved reliable operation for:

• 6 months of operation

• 2,500 hours of power production in winds of any velocity

• 250 hours of power production in winds of 1.2Vave (9.0 m/s) and above

• 25 hours of power production in winds of 1.8Vave (13.5 m/s) and above.

Reliable operation means:

• Operational time fraction of at least 90%
2 

• No major failure of the turbine or components in the turbine system

• No significant wear, corrosion, or damage to turbine components 

• No significant degradation of produced power at comparable wind speeds. 

In addition, NREL has conducted this test in accordance with our quality system 

procedures such that this report will meet the full requirements of our accreditation by 

A2LA. Our quality system requires that we meet all applicable requirements specified by 

A2LA and ISO/IEC 17025 or to note any exceptions in the test report and these are 

listed in section 8. 

3. Description of Test Turbine

The Gaia-Wind 11 kW is a two bladed downwind wind turbine rated at 11 kW output at 

9.5 m/s. The Gaia-Wind 11 kW uses an induction generator to produce three-phase, 60 

Hz output at 480-volts. The turbine’s power output is grid compatible and is supplied 

directly to the grid. In wind speeds higher than 25 m/s, the turbine uses a mechanical 

brake to come to a complete stop to protect the turbine from over speeding. The turbine 

blades are made from fiberglass. 

Table 1 lists basic turbine configuration and operational data.

The following components were considered part of the test turbine system:

1. The turbine system includes a tower and foundation that have been designed for 

installation at the NWTC test site 3.3B. 

2. The turbine system is connected to the electrical grid at the test site through a 

subpanel. All wiring and components on the turbine side of this subpanel are 

considered part of the turbine system.

3. The turbine system includes all control components including wiring between the 

up-tower components and the down-tower control panel.
3 

Table 1. Test turbine configuration and operational data

General Configuration:

Make, Model, Serial Number Gaia-Wind Ltd., Gaia-Wind 11 kW, 10711114

Rotation Axis (H / V) Horizontal

Orientation (upwind / downwind) Downwind

Number of Blades 2

Rotor Hub Type Teetering

Rotor Diameter (m) 13.0

Small Wind Turbine Class III

Hub Height (m) 18.2

Performance:

Rated Electrical Power (kW) 11

Rated Wind Speed (m/s) 9.5

Cut-in Wind Speed (m/s) 3.5

Cut-out Wind speed (m/s) 25

Rotor:

Swept Area (m2) 133

Blade Pitch Control None

Direction of Rotation Clockwise viewed from upwind

Rotor Speed 0 – 62

Power Regulation (active or 

passive)

Passive

Tower:

Type Tubular

Height (m) 18.0

Control / Electrical System:

Controller: Make, Type Gaia-Wind IC-1000 

Electrical Output: Voltage 480 VAC, three-phase

Yaw System:

Yaw control Passive
4 

Figure 1. The Gaia-Wind 11 kW wind turbine. PIX # 17673

The test configuration consists of the turbine mounted on a tubular tower, the controller, 

the meteorological tower, associated wiring and junction boxes, and a data shed 

containing the data acquisition instrumentation. The turbine is installed on a standalone 

18 meter, tubular tower. The wire that runs from the base of the tower to the data shed is 

approximately 64 meters of #6 AWG wire. Inside the data shed there is a disconnect 

switch and a breaker panel on the turbine side of the transformer. Figure 2 shows the 

general electrical arrangement. 
5 

5 cond # 6

3L, N, Ground

THHN

Disconnect

Switch

Test Shed

Breaker Panel

480 VAC

3P, 60 Amp

To NREL Grid

Gaia 11kW

Instrumentation 

cable

Turbine

Control 

Panel

Met 

Tower 

Junction

Box

Power

Transducer Box

150-5 AMP CT

DMT 1040E PT

kWh

Meter

To data 

acquisition

Figure 2. General electrical arrangement
6 

4. Description of Test Site

The Gaia-Wind 11 kW wind turbine was located at Test Site 3.3B at the National Wind 

Technology Center (hereafter referred to as the test site), approximately 8 km south of 

Boulder, Colorado. The site is located on level terrain at an approximate elevation of 

1845 m above sea level. Figure 3 shows a plot plan of the test site with topography lines 

listed in meters above sea level.

The meteorological tower is a 16.4 meter Rohn, 25 G lattice tower located 32.8 m (about 

2.5 rotor diameters) from the test turbine at an azimuth of 297° true.

For measurements, where it is important to accurately measure wind speed, NREL used 

data obtained when the wind direction was between 258° to 331° with respect to true 

north, thus including all westerly winds. In this measurement sector, established in 

accordance with IEC 61400-12-1, the influence of terrain and obstructions on the 

anemometer and turbine are small. The closest operating turbine to the test turbine was

a 10 kW Abundant Renewable Energy, ARE 442, turbine located on a 30.5 meter tower. 

It was located approximately 48.0 meters north of the test turbine at site 3.3A. 

Figure 3. Map of the test site
7 

5. Description of Instrumentation

Equipment used for duration testing differs only slightly from that used for power 

performance testing. Normal power performance testing requires measurements of wind 

speed, wind direction, turbine power, air temperature, air pressure, precipitation, and 

overall turbine system availability. For duration testing, NREL added signals to monitor 

the turbine brake and the grid voltage.

Figure 4 gives the location of the met tower instruments and Table 2 gives an equipment 

list that provides the specifications for each of the instruments used. Per the power 

performance standard IEC 61400-12-1, the primary anemometer was sent out for 

recalibration after the test period. The difference between the two calibrations was within 

the tolerances allowed by the power performance standard. 

The data acquisition system ran out of calibration during the test. It was sent out for post 

test calibration and found within specification. The calibration sheet of the post test 

calibrations are in Appendix A.
8 

Figure 4. Location of the data acquisition sensors
9 

Table 2. Equipment List for Duration Test

Instrument Make and Model Serial Number Calibration Due 

Date

Power transducer Ohio Semitronics, DMT 

1040E

06091046 February 15, 2010

Current transformers Ohio Semitronics, 12974 001293045

001235428

001293049

February 15, 2010

Primary anemometer Thies, First Class 0707890 April 7, 2009

Reference 

anemometer

NRG, Max 40 179500049023 In situ

Wind vane Met One, 020C with 

Aluminum Vane

X4357 April 7, 2009

Pressure sensor 

(replaced during test)

Vaisala, PTB101B C1040014

T5030003

October 29, 2008

August, 26 2009

Temperature sensor 

(replaced during test)

Met One, T200 0890084

0789021

October 29, 2008

October 10, 2009

Precipitation sensor Campbell Scientific, 237 None In situ

Data acquisition 

system

Compact DAQ w/LabView 

cDAQ backplane

NI 9229

NI 9217

NI 9205

NI 9229

NI 9217

NI 9205

12E4DA3

12CBC7A

12BFEE2

12E9C99

140A596

140DCB9

140E2BD

August 14, 2008

July 20, 2008

October 8, 2008

Modules post-test 

calibrated on May 5, 

2009, was in 

compliance

February 10, 2010

February 12, 2010

February 10, 2010

6. Results

6.1 Operation Time

The test turbine system was installed during the week of May 12, 2008, and operated 

continuously until May 6, 2010 when it was shut down for removal. The commissioning 

checklist from the installation is in Appendix B. After installation, the turbine experienced 

a ”break-in” period of approximately one month. During this period, several bolts from 

the hub were found on the ground at the base of the turbine. It was assumed the bolts 

were from the hub fork since there were bolts missed from that area. It was determined 

that the originally installed bolts were not long enough to fully engage the nuts and

longer bolts were installed. In early June 2008, a “brake pads worn” warning appeared

on the controller screen. It was determined that the mechanical brake needed to be recalibrated. After the break in period, the duration test was officially started on June 9, 
10

2008. The duration test was completed on March 31, 2009, after enough data was

collected to demonstrate sufficient hours of operation, as required by the standard. 

6.2 Months of Operation

The duration test was conducted over a period of approximately 9.5 months from June 9, 

2008, to March 31, 2009, (6 months were required). The turbine continued to operate 

until May 6, 2010 when it was shut down for removal.

6.3 Hours of Power Production

The hours of power production at any wind speed totaled 2,705 hours (2,500 hours 

required). 

The hours of power production above 1.2*Vave (9 m/s) totaled 711 hours (250 hours 

required). 

The hours of power production above 1.8*Vave (13.5 m/s) totaled 215 hours (25 hours 

required). 

Thus the turbine met the requirements for hours of power production during the test. 

Table 3 shows the overall and month-by-month results of the duration test.

Table 3. Monthly and overall results of the Gaia-Wind 11 kW duration test

6.4 Operational Time Fraction

The operational time fraction is defined as follows:

× %100 −−

−−− =

EUT

EUNT

TTT

TTTT O

where:

TT is the total time period under consideration,

TN is the time during which the turbine is known to be non-operational,

TU is the time during which the turbine status is unknown,

max gust TI @ 15 # Data T T T U T E T N O

Month 0 m/s 9 m/s 13.5 m/s (m/s) m/s (%) points (hours) (hours) (hours) (hours) (%)

Overall 2704.9 710.6 215.0 41.9 19.0 255 7094 172.5 152.0 624.6 90.8

Jun 2008 238.2 36.2 3.8 28.6 18.5 5 518 11.3 7.8 3.3 99.3

Jul 256.0 8.5 0.3 23.9 - 0 744 78.2 2.2 38.8 94.1

Aug 115.8 4.5 0.0 19.2 - 0 744 6.3 20.0 323.0 55.0

Sep 120.5 11.7 1.8 22.4 - 0 720 36.2 30.3 174.7 73.3

Oct 236.0 45.0 12.2 32.8 17.3 10 744 0.7 1.3 0.0 100.0

Nov 348.0 98.7 22.5 37.0 20.9 40 720 22.1 0.0 0.0 100.0

Dec 339.7 160.5 54.8 41.4 17.4 68 744 7.9 27.2 32.8 95.4

Jan 2009 385.0 155.5 56.0 38.8 19.9 76 744 4.9 32.0 36.5 94.8

Feb 333.2 107.3 36.8 41.9 20.0 23 672 3.2 27.0 0.0 100.0

Mar 332.5 82.7 26.8 36.7 18.0 33 744 1.7 4.2 15.5 97.9

Hours of power production above:
11

TE is the time which is excluded in the analysis.

The overall operational time fraction of the combined wind turbine system (wind turbine, 

tower, and controller) in the total test period was 90.8%. Figure 5 and Table 3 show the 

operational time fraction per month. Figure 6 shows a scatter plot of power verses wind 

speed during the test. 

The main reasons for the wind turbine system’s downtime (TN) during the test period 

were electric contactor failures, brake-time errors, and vibration errors. These faults are 

described in more detail below. 

Electric Contactor Failure

The low operational time fraction for August and September of 2008 was caused by the 

failure of two contactors in the controller. Investigations suggest that the 2-pin flat 

connectors used to wire the contactors were poorly connected when installed at GaiaWind’s electrical supplier. Additionally, the Gaia-Wind 11 kW turbine controller was 

originally designed for a 50-Hz grid, and it is possible that the contactors that originally 

were installed in the controller were underrated for the 60-Hz grid at the NWTC. 

Approximately 78% of the total time classified as TN during the test can be attributed to 

the contactor failures. Since September 10, 2008 when both contactors were replaced 

with higher rated models and the pin connectors were replaced, there have been no 

further contactor failures. 

Brake-time Errors

The brake-time errors occurred when the turbine took longer to brake than designed; this 

usually occurred during high winds. Although this error occurred, the turbine successfully 

braked each time. The brake-time error requires permission from Gaia-Wind to manually 

reset the turbine.

Vibration Errors

Based on evidence found in the nacelle, the vibration errors are believed to have 

occurred from birds entering the nacelle and physically brushing against the vibration 

sensor. This activated the vibration error. With permission from Gaia-Wind, NREL 

installed a screen over the opening in the nacelle to prevent birds from entering the 

nacelle. After the installation of the screen on September 19, 2008, the turbine ran 

without any further vibration errors.

Remaining TN

The remaining time classified as TN was due to manufacturer mandated maintenance, 

auto motor start errors, generator over speed errors, and a tachometer defect error. 

The main reasons for excluding time (TE) in the duration test were:

• On site activities that either prevented NREL from performing needed maintenance 

on the turbine or caused an error
12

• The time difference between the NWTC and Gaia-Wind in the United Kingdom 

occasionally prevented Gaia-Wind from responding to error reset requests promptly

• Noise or safety and function testing that required the turbine to be shut down. 

If no reliable measurements were available, the time was classified as TU since the 

turbine’s status was unknown. 

6.5 Environmental Conditions

As an indication of the environmental conditions during the duration test, the standard 

requests reporting of the maximum wind speed gust and the average turbulence 

intensity at 15 m/s. The maximum recorded gust was 41.9 m/s at 12:51 PM on February 

17, 2009. The average turbulence intensity at 15 m/s during the duration test was 

19.0%.

6.6 Power Degradation Checks

A factor of reliable operation is that the turbine should experience no significant power 

degradation. During the power degradation analysis, the average power level for each 

wind speed bin is plotted as a function of time over the whole test period. This plot is 

analyzed for any obvious trends in power production.

Figure 7 shows the power degradation plot, which gives the power level in individual 

wind speed bins for each month. Variations in the power levels from season to season 

are caused by air density variations. The apparent degradation in power for winds above 

13 m/s in December 2008 through March 2009 was caused by the turbine reaching high 

wind cut-out. This resulted in a lower average power at higher wind speeds, but is not 

considered power degradation. 

6.7 Dynamic Behavior

The turbine was observed over a wide range of wind speeds. The turbine did not exhibit 

excessive vibration during any of the recorded observations. The following are specific 

examples of dynamic behavior observations made in the logbook: 

23 February 2009 – “Observed the turbine start up from low winds and run in 8 m/s 

winds. Some slight tower vibration and rattling during motor start pulses. Observed a 

very slight thumping noise as blades passed behind tower. When standing next to the 

tower, some per revolution noise can be heard resonating down the tower, though it is 

very minimal. Blade noise is audible, more so as winds slow. The turbine yaws slightly in 

higher winds (greater than 8 m/s). No excessive tower vibration was noticed. Overall 

observation time 15 minutes in winds from approximately 1 m/s up to 9 m/s.” 

24 February 2009 – “Observed the turbine in 8 – 15 m/s. A slight whooshing noise is 

coming from the blades. The turbine tracks the wind well and yaws back and forth 

slightly in gusty winds. Some slight tower movements. The turbine was producing 12 – 

14 kW instantaneously during the observation. The overall observation time was 10 

minutes.” 
13

24 February 2009 – “Observed turbine operating in winds between 7 – 13 m/s. The 

turbine tracks wind direction well. No big vibrations, slight thumping heard. The overall 

observation time was approximately 2.5 hours.” 

6.8 Tear-Down Inspection

The tear down inspection was performed on May 10, 2010. The results are documented 

in Appendix C. The main finding was a crack along the seam between the two halves of 

both blades, see Figure C.1. 

Figure 5. Operational time fraction for each month

0

10

20

30

40

50

60

70

80

90

100

Operational Time Fraction (%)

Month
14

Figure 6. Scatter plot of power versus wind speed (10-minute averages)

Figure 7. Power level in several wind speed bins (in m/s) as a function of time

-2

0

2

4

6

8

10

12

14

16

0 5 10 15 20 25 30

Power (kW)

Wind Speed (m/s)

0

2

4

6

8

10

12

14

Jun-08 Jul-08 Aug-08 Sep-08 Oct-08 Nov-08 Dec-08 Jan-09 Feb-09 Mar-09

Power (kW)

Month

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26
15

7. Uncertainty

The uncertainty is estimated for the following parameters:

• Hours of power production 

• Operational time fraction

• Highest instantaneous wind speed

No uncertainty analysis was done for the power degradation results. These results were 

used only to find relative trends that might indicate hidden faults in the turbine.

Hours of Power Production

NREL assumes that the turbine is producing power for the entire 10-minute period 

whenever the average power for that period is positive. This method overestimates time 

for power production in wind speeds between 4 and 6 m/s. At these wind speeds, the 

turbine may have been producing power for about half of the time recorded by NREL. At 

higher wind speeds, this method would produce less of an overestimate. NREL 

estimates that the reported time of power production in wind speeds greater than 0 m/s 

may be 20% less than calculated. However, the turbine operated for over a month 

before the duration test was started in June 2008 and it continued to run after the 

duration test was complete until May 6, 2010. Thus, NREL is confident that it has 

achieved the 2,500 hours required by the standard. 

For the hours of power production above 9 and 13.5 m/s, the uncertainty in the wind 

speed is assumed to be the dominant factor. Assuming an uncertainty in wind speed of 

0.3 m/s, there is an approximate variation of 8% in the hours of power production at 

these wind speeds.

Operational Time Fraction

The total test time is 7,094 hours. Even if the classification of TE and TN was wrong by 

5% (which is a very conservative assumption), the operational time fraction would still be 

above 90% at 90.3%. 

Highest Instantaneous Wind Speed

The uncertainties in the wind speed measurements are 0.009 m/s calibration 

uncertainty, 0.052 m/s + 0.52% operational characteristics, 1% mounting effects, and 2% 

terrain effects. For the maximum instantaneous gust of 41.9 m/s, the uncertainty is 0.96 

m/s.
16

8. Deviations and Exceptions

8.1. Deviations from the Standard 

There were no deviations from the standard. 

8.2. Deviations from Quality Assurance 

The data acquisition modules were used beyond the calibration due date. The modules 

were post-test calibrated and found to be in compliance within the specifications. 

Appendix B includes the post-test calibration sheets.
17

Appendix A: Instrument Calibration Certificates

Figures A.1 through A.13 show the calibration sheets for the instruments used during the 

duration test. 
18

Figure A.1. Power transducer calibration sheet
19

Figure A.2. Primary anemometer calibration sheet
20

Figure A.3. Primary anemometer post-test calibration sheet
21

Figure A.4. Wind vane calibration sheet
22

Figure A.5. RTD probe calibration sheet I

 
23

Figure A.6. RTD probe calibration sheet II
24

Figure A.7. Pressure transmitter calibration sheet
25

Figure A.8. NI 9229 data acquisition module calibration sheet
26

Figure A.9. NI 9217 data acquisition module calibration sheet
27

Figure A.10. NI 9205 data acquisition module calibration sheet
28

Figure A.11. NI 9229 data acquisition module post-test calibration sheet
29

Figure A.12. NI 9217 data acquisition module post-test calibration sheet
30

Figure A.13. NI 9205 data acquisition module post-test calibration sheet
31

Appendix B: Turbine Commissioning Checklist
32
33

Appendix C: Post-Test Inspection

The Gaia Wind 11-kW wind turbine was removed from Site 3.3B on May 6, 2010, after 

NREL completed all testing activities in the Independent Testing project. At the end of 

the duration test, a post test inspection is performed on the turbine. This report describes 

the tear down inspection. 

C.1 Blades

Both blades had cracks along the seam between the two halves near the hub, see 

Figure C.1. The putty at the blade ends, where the tip brakes are located, had 

deteriorated slightly, see Figure C.2. One of the blades was chipped, see Figure C.3. 

C.2 Hub

The nacelle cover was in good condition. Some grease was observed around the rotor 

shaft, see Figure C.4. Some slight rust on the rotor plate and rotor assembly was 

discovered, see Figure C.5. 

C.3 Brakes

Part of the brake actuator system was damaged during uninstall. The brake pad 

assembly was removed and the brake pads were measured. The brake pad on the rotor 

side measured 0.72 inches. The brake pad on the generator side measured 0.78 inches. 

The original brake pad measured 0.805 inches. The pins on the brake caliper were 

inspected; no wear was observed. The brake caliper arms appeared free of rust and 

binding. 

C.4 Vibration Sensor

The rubber seal on the vibration sensor was degraded and partially ripped, see Figure 

C.7. It was noted that during uninstall, the vibration sensor was damaged and was no 

longer in a vertical position. However, the degradation of the rubber seal does not seem 

to have been caused by this. 

C.5 Electrical

All wiring in the hub and the control cabinet was inspected. No degradation or evidence 

of overheating was observed. 

C.6 Yaw system

The yaw bearing moves smoothly. Some grease was observed around the yaw bearing. 
34

C.7 Tower

The welds, bolts, and ladder were inspected on the tower. No abnormalities or cracks 

were observed. 

C.8 Gearbox

The gearbox was drained and inspected using a borescope. Some small scratches and 

abrasions were found on the gear teeth. No clunking or noise was heard when rotating 

the shaft. The inspection window on the side of the gearbox still showed an adequate 

amount of oil, even after a complete drain. The oil appeared to be in good shape; no 

particles were found. 
35

Figure C.1 Crack in blade along seam. PIX # 17672.

Figure C.2 Deteriorating putty on tip brake. PIX #17676.
36

Figure C.3 Chip in blade. PIX #17669.

Figure C.4 Grease on shaft. PIX #17670.
37

Figure C.5 Rust on rotor assembly. PIX #17671.

Figure C.6 Broken brake actuator. PIX #17675.
38

Figure C.7 Broken vibration sensor and deteriorating rubber seal. PIX #17674.
F1147-E(10/2008) 

REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, 

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this 

collection of information, including suggestions for reducing the burden, to Department of Defense, Executive Services and Communications Directorate (0704-0188). Respondents 

should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a 

currently valid OMB control number.

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION.

1. REPORT DATE (DD-MM-YYYY)

September 2010

2. REPORT TYPE

Technical Report

3. DATES COVERED (From - To)

 

4. TITLE AND SUBTITLE

Wind Turbine Generator System Duration Test Report for the 

Gaia-Wind 11 kW Wind Turbine

5a. CONTRACT NUMBER

DE-AC36-08-GO28308

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

A. Huskey, A. Bowen, and D. Jager

5d. PROJECT NUMBER

NREL/TP-500-49069

5e. TASK NUMBER

WE102211

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Renewable Energy Laboratory

1617 Cole Blvd.

Golden, CO 80401-3393

8. PERFORMING ORGANIZATION

REPORT NUMBER

NREL/TP-500-49069

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR'S ACRONYM(S)

NREL

11. SPONSORING/MONITORING

AGENCY REPORT NUMBER

12. DISTRIBUTION AVAILABILITY STATEMENT

National Technical Information Service

U.S. Department of Commerce

5285 Port Royal Road

Springfield, VA 22161

13. SUPPLEMENTARY NOTES

14. ABSTRACT (Maximum 200 Words)

This test was conducted as part of the U.S. Department of Energy's (DOE) Independent Testing project. This project 

was established to help reduce the barriers of wind energy expansion by providing independent testing results for 

small turbines. In total, five turbines are being tested at the National Renewable Energy Laboratory's (NRELs) 

National Wind Technology Center (NWTC) as a part of this project. Duration testing is one of up to five tests that may 

be performed on the turbines, including power performance, safety and function, noise, and power quality tests. The 

results of the testing will provide the manufacturers with reports that may be used for small wind turbine certification. 

The test equipment includes a Gaia-Wind 11 kW wind turbine mounted on an 18 m monopole tower. Gaia-Wind Ltd. 

manufactured the turbine in Denmark, although the company is based in Scotland. The system was installed by the 

NWTC Site Operations group with guidance and assistance from Gaia-Wind.

15. SUBJECT TERMS

small wind turbine; wind turbine; independent testing; Gaia; duration test

16. SECURITY CLASSIFICATION OF: 17. LIMITATION 

OF ABSTRACT

UL

18. NUMBER 

OF PAGES

19a. NAME OF RESPONSIBLE PERSON

a. REPORT

Unclassified

b. ABSTRACT

Unclassified

c. THIS PAGE

Unclassified 19b. TELEPHONE NUMBER (Include area code)

Standard Form 298 (Rev. 8/98)

Prescribed by ANSI Std. Z39.18