comparing pulse doppler lidar with sodar and direct measurements for wind assessment, awea wind...

Windpower 2007 – Los Angeles

Comparing Pulse Doppler LIDAR with SODAR and Direct Measurements for

Wind Assessment Neil D. Kelley

Bonnie J. Jonkman George N. Scott

National Wind Technology Center

Yelena L. Pichugina Cooperative Institute for Research in Environmental Sciences/NOAA

University of Colorado at Boulder

Windpower 2007 – Los Angeles Windpower 2007 - Los AngelesWindpower 2007

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Background The 2001-2003 Lamar Low-Level Jet Project

provided an opportunity to simultaneously compare the wind fields measured remotely by pulsed LIDAR and SODAR and directly by tower-mounted sonic anemometers

These measurements were taken by NREL/NWTC and the National Oceanic and Atmospheric Administration (NOAA) during the first two weeks of September 2003 south of Lamar, Colorado which is now the site of the 166 MW Colorado Green Wind Plant


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We acknowledge the support of this study by the NOAA Earth System Research Laboratory (ESRL) and Dr. Robert M. Banta Dr. W. Alan Brewer Scott P. Sandberg Janet L. Machol

in particular without whose professional and scientific dedication the results being presented today would not have been possible.

Acknowledgements


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Presentation Objectives

Present the results of a simultaneous inter-comparison of wind fields measured by two remote sensing technologies and direct tower-based measurements

Present the results of a longer term inter-comparison of simultaneous measurements taken with a SODAR and in-situ instruments


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Continuous emissions of infrared energy

Nominal 200 m range Line-of-sight radial wind

speeds made within a single focused region along the beam

Multiple heights measured by varying position of focal point and/or elevation angle

Very narrow beam diameter Useful for highly detailed

measurements of a limited spatial area

Very short pulses of intense infrared energy

Up to 9 km range Line-of-sight radial wind

speeds made simultaneously at up to 300 positions (range gates) along the beam

A narrow, highly collimated beam whose diameter slowly increases with increasing range

Can perform a wide range of scanning operations for 3D spatial measurements

BASIC ATTRIBUTES OF EYESAFE DOPPLER WINDFINDING LIDARS

Continuous Wave (CW) Pulsed


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The Comparison and Inter-Comparison of Wind Fields Measured by Three Techniques

In-situ measurements using sonic anemometry at heights of 54, 67, 85, & 116 m

Scintec MFAS Medium-Range SODAR (50-500 m)

NOAA High-Resolution Doppler LIDAR (HRDL)

120-m tower & four levels of sonic anemometry

Scintec MFAS

SODAR

NOAA HRDL LIDAR


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120-m Tower & Sonic Anemometry

ATI SAT/3K 3-axis sonic anemometers (7 Hz bandwidth, 0.05 sec time resolution)

Mounted on support arms specifically engineered to damp out vibrations below 10 Hz

Mounted 5 m from edge of 1-m wide, torsionally-stiff, triangular tower

Arms orientated towards 300 degrees w.r.t. true north


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Scintec MFAS Phased Array SODAR

Observed winds between 50 and 500 m

20-min averaging period 10-m vertical resolution Horizontal winds from 8

tilted beams and 10 frequencies over range of 1816-2742 Hz

30-70 m pulse lengths Automatic gain control Very quiet site


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NOAA High Resolution Doppler LIDAR (as configured for Lamar experiment)

Research instrument Solid State Tm:Lu,YAG laser Wavelength 2.02 µm Pulse energy 1.5 mJ Pulse rate 200/s Range resolution 30 m Velocity resolution ~ 0.1 m/s Time resolution 0.25 s Minimum range 0.2 km Maximum range 3 km Beam width range 6 to 28 cm

vertical scan mode

conical scan mode

φ

θ

φ

θ

stare mode


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Inter-comparison of Measured Wind Fields

LIDAR

Sonics

SODAR


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Sources of Flow Distortion Around Triangular Lattice Tower

Instrument mounting arm assemblies

Aircraft warning beacons

Tower composed of circular structural elements: 1.6 cm main vertical legs

0.6 cm cross members

“Star” mount guy wire connections provide torsional stiffness

RESULT: Flow distortion characteristics vary with height and wind approach angle


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Tower – SODAR Positions

North

109.05m

Guy Wires

Fenced Area(Tower and Shed)

AR (includinge panels andc enclosure)

- Guy Wire Anchor Points (x6)

Tower Coordinates:37° 40.099N,102° 39.825W

SODAR Coordinates:37° 40.059N,102° 39.879W

Note: SODAR and Tower Coordinateswere measured on June 25, 2002 usinga Brunton Multinavigator MNS GPS Receiver using Datum WGS84.

guy wires

Fenced Area (data building)

North

LIDAR

109.1 m

SODAR

instrument arms

orientation 120-m tower

210o


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116m

-1

-1

-1 -1

00

0

11

1

2

2

3

-2

-2-2

-3

2

1

Sodar WD (deg)

160 200 240 280 320 360 400 440

Soda

r UH (m

/s)

2

4

6

8

10

12

14

16

18

20

22

-3 -2 -1 0 1 2 3

40 80

-4

-4

-4-2

-2

-2

-4

00

0

0

-4

-4

-4

-2

-2

24

-4

-4

6

-6-8

8

Sodar WD (deg)

160 200 240 280 320 360 400 440

Soda

r UH (m

/s)

2

4

6

8

10

12

14

16

18

20

22

-8 -6 -4 -2 0 2 4 6 8

40 80

Estimate of Local Flow Distortion at 116-m Sonic Anemometer Using High Reliability

SODAR Data As Reference

Horizontal Wind Speed Wind Direction

(deg) (m/s)

instrument arms azimuth location


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Stationary Stare Mode Geometry for Optimal LIDAR-Sonic Inter-comparison

31o

Wind Flow

LIDAR

30-m range gates 6 & 7

plan view elevation view

UH

Uradial

Chosen for minimal flow distortion at the sonic anemometers

North

109.05m

Guy Wires

Fenced Area(Tower and Shed)

AR (includinge panels andc enclosure)

- Guy Wire Anchor Points (x6)

Tower Coordinates:37° 40.099N,102° 39.825W

SODAR Coordinates:37° 40.059N,102° 39.879W

Note: SODAR and Tower Coordinateswere measured on June 25, 2002 usinga Brunton Multinavigator MNS GPS Receiver using Datum WGS84.

North

LIDAR (167 m) 210o


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Results of Stationary Stare Inter-Comparisons Under Optimal Observing Conditions

Sonic full vector velocity is projected on to the LIDAR radial velocity for direct comparison over nominal periods of 10 minutes

The two compare nominally within 0.1 ± 0.3 m/s or ± 2.5% over the observed velocity range of 1.0 to 11.3 m/s

Compares favorably with similar measurements by Hall, et al# using a much earlier CO2 laser version of the HRDL at height of 300 m and an observed velocity range of 1 to 22 m/s

#Hall, et al, 1984, “Wind measurement accuracy of the NOAA pulse infrared Doppler LIDAR.” Applied Optics, 23, No. 15.

Mean Bias

Ulidar – Usonic

Std Dev

RMS

(m/s) (m/s) (m/s)

0.14 0.27 0.31

0.34#


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Obtaining Streamwise LIDAR Wind Profiles Using Vertical Scan Mode Data

By design the majority of available data was collected in this mode

Not optimal for obtaining horizontal wind speeds due to

a potential lack of horizontal homogeneity at low angles

sparse spatial sampling at high angles


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Tower, SODAR, LIDAR Vertical-Scan Mode Inter-Comparison Results

Tower sonics UH (m/s)2 4 6 8 10 12 14 16 18 20

Soda

r UH (m

/s)

2

4

6

8

10

12

14

16

18

20

Sodar UH (m/s)2 4 6 8 10 12 14 16 18

Lida

r ver

tical

sca

n U

H (m

/s)

2

4

6

8

10

12

14

16

18

SODAR UH Referenced

To All Tower Sonics UH

LIDAR Vertical-Scan UH Referenced

To All Tower Sonics UH

LIDAR Vertical-Scan UH Referenced

To SODAR UH

• Small bias, +0.12 ± 0.11 m/s

• Tower higher at higher speeds

• Large slope error, 0.921 ± 0.010

• 1σ variation, 0.65 m/s

• R2 = 0.956

Tower sonics UH (m/s)2 4 6 8 10 12 14 16 18 20

Lida

r ver

tical

sca

n U

H (m

/s)

2

4

6

8

10

12

14

16

18

20

• Large bias, -1.02 ± 0.16 m/s

• LIDAR lower at all wind speeds

• Small slope error, 1.023 ± 0.010


• R2 = 0.918

• Large bias, -1.35 ± 0.12 m/s

• LIDAR lower at all wind speeds

• Small slope error, 0.984 ± 0.011


• R2 = 0.955


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LIDAR Vertical Wind Profiles Derived Using Conical Scanning Mode

More optimal technique, but only short records (~1 min) available

15 deg elevation angle provides 8 m vertical resolution

Used by CW LIDAR profilers but only at 5 heights

φ

θ

φ

θ(1 minute record)


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Long-Term High SNR# SODAR and Tower Sonics UH Inter-Comparison

All sonic heights included

Wind directions of 120 ± 20o excluded

14649 records (585 hours)

Mean bias of -0.5 m/s

Slope error of 1.035 (sonics read higher than SODAR)

R2 = 0.845

1σ variation of 1.5 m/s consistent with estimated local flow distortion magnitudes

# signal-to-noise ratio


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Conclusions The achievable RMS accuracy of the pulsed LIDAR under

optimal sampling conditions appears to be in the vicinity of 0.3 m/s or 2.5%

Tower-induced flow distortion in the vicinity of the sonic anemometers has limited the precision of the inter-comparisons with the remote sensing instruments

The SODAR provided an RMS uncertainty in the range of 0.6 to 0.7 m/s or 5 to 6% under high SNR conditions and is limited by the local flow distortion at the sonic anemometers

The pulsed LIDAR, when used in the conical scanning mode, can provide very detailed vertical wind profiles