Correction tool for Lidar in complex terrains based on Meteodyn WT outputs
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1 Correction tool for Lidar in complex terrains based on Meteodyn WT outputs Stéphane SANQUER - Meteodyn New measurement tools are needed: To reduce projects development risk and secure the investment . To have reliable measurements in the upper atmosphere as the wind turbines are getting higher. Difficulties of traditional methods: To have high measurements To be easily mobile To get information on the vertical profile One Solution -> Use a Lidar But remote sensing in complex terrain is a challenge: Discrepancies between remote sensors and traditional anemometry Data needs to be corrected Meteodyn developed the “Lidar Correction Module”:
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The use of Lidars in complex terrain allows wind farm developers to assess the wind climate at multiple locations in an easier and cheaper way than by employing multiple masts with anemometers. Unfortunately Lidars give some discrepancies with the reference measurements (cup or sonic anemometers) especially in complex terrain where spatial variations of mean velocity exist along the Lidar measurement circumference. Therefore a tool is required to enable the correction of the Lidar in complex terrain to represent an anemometer at the Lidar location at the measured levels of the Lidar. Computations can be useful in order to correct the data as they are able to predict the flow on sharp and complex relief. As shown in a recent study presented in the 2009 NZWEA Conference, linear models are recommended for use only on simple terrain. Hence, the correction tools should be based on the full CFD software outputs, here with METEODYN WT outputs, namely the three components of the average velocity and wind direction. Comparisons are presented with two reference cases : simple and complex terrain.
Transcript of Correction tool for Lidar in complex terrains based on Meteodyn WT outputs
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Correction tool for Lidar in complex terrains
based on Meteodyn WT outputs
Stéphane SANQUER - Meteodyn
�New measurement tools are needed:
�To reduce projects development risk and secure the investment .
�To have reliable measurements in the upper atmosphere as the wind turbines are getting
higher.
�Difficulties of traditional methods:
�To have high measurements
�To be easily mobile
�To get information on the vertical profile
One Solution -> Use a Lidar
�But remote sensing in complex terrain is a challenge:
�Discrepancies between remote sensors and traditional anemometry
�Data needs to be corrected
�Meteodyn developed the “Lidar Correction Module”:
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�Why need corrections in complex terrains
Velocity vectors aren’t be constant across the Lidar scan disk
We need the 3D vectors of the wind upstream and downstream the Lidar scan disk center
in order to project them on the beams
The method uses only the main direction informations i.e. along a line parallel to wind at
the center of the scan disk
�� =�� ���� − �� ��
2 sin��=
�������. �� − ������. ��
2 sin��
�Analytic formulation of the velocity measured by LIDAR VL
General equation
Hypothesis : horizontal deviation is negligible across the Lidar disk (CFD gives <1°)
�� =1
2 sin����� cos��� cos��� sin�� + �� cos��� sin��� cos��
+ �� cos��� cos��� sin�� − �� cos��� sin��� cos��
�� =1
2 sin����� sin�� + �� + �� sin�� − ��
Where �� and �� are the deviation of the wind in the horizontal plane according to the direction
defined by the upstream and downstream point. And �� and �� are the incidence of the wind in the
vertical plane.
Lidar software assumes that upstream wind and downstream wind are horizontal
�� =1
2 ��� + ��
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�Correction of the velocity measured by LIDAR VL
The correction factor is defined in order to correct the lidar output
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Wind incidence (°)
�� = �� × !
If �� = −�� and �� = �� = �" then �� =�"
sin ���sin�� cos��� −cos��sin���
And ! =sin ��
sin ��cos ��� −cos ��sin ���
Discrepancies reach 10% when the upstream incidence is + 3° and the
downstream incidence is -3°
Corrections are necessary in complex terrains
�Comparisons Lidar vs Mast in Two sites assessed by Meridian Energy
Validation of the method in a flat terrain (one mast 80 m) and in a complex terrain (two masts
20 m and 80 m)
Comparisons of corrected measurements from Meteodyn WT vs Met Mast measurements
Data from Lidar were filtered before in order to reduce the others sources of discrepancies
(rain, low velocity, mast wake on Lidar or on anemometers)
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U(L
ida
r)/U
(ma
st)
Wind direction (°)
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�Comparisons Lidar vs Mast in Two Meridian sites
Flat terrains - Uncorrected data - MAE = 0.37 m/s - RMS = 0.50 m/s
y = 0.97x + 0.05
R² = 0.95
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U (
Lid
ar)
/ U
ref
U (Mast) / Uref
Site n°1 - Mast n°1- H=80 m
�Comparisons Lidar vs Mast in Two Meridian sites
Complex terrain (first point h=20 m) - Uncorrected data - MAE = 0.30 m/s – RMS = 0.42 m/s
y = 0.96x + 0.02
R² = 0.99
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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6
U (L
ida
r) /
Ure
f
U (Mast) / Uref
Site n°2 - Mast n°1 - H=20 m
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�Comparisons Lidar vs Mast in Two Meridian sites
Complex terrain (second point h=80 m) - Uncorrected data - MAE = 0.57 m/s – RMS = 0.75 m/s
y = 0.95x + 0.01
R² = 0.96
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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6
U (L
ida
r) /
Ure
f
U (Mast) / Uref
Site n°2 - Mast n°2- H=80 m
�Correction factor from Meteodyn WT outputs
First step : calculation for each wind directions with a 10° step
Flat terrain (one mast 20 m) Complex terrain (2 masts : 20 m and 80 m)
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�Correction factor from Meteodyn WT outputs
Computation parameters
Computation domains
Flat terrain : square domain 3000 m
Complex terrain : square domain 5000 m
Minimum horizontal and vertical resolution (ground – 50 Lidar points)
Normal 5 m x 5 m or refined 2 m x 2 m
Expansion coefficients (horizontal – vertical)
1.1 – 1.2
Smoothing
No
Stability
Neutral
�Correction factor from Meteodyn WT outputs
First step : calculation of flows for each wind directions with a 10° step
Speed up factor for flat terrain Complex terrain (wind direction 210°)
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�Correction factor from Meteodyn WT outputs
Second step : calculation of speed up factor and wind incidence over the
Lidar disk for each wind directions with a 10° step
Speed up factor
H=20 m ∆∆∆∆SF=0.03
H=80 m ∆∆∆∆SF=0.05
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324331
338346 353 speed up factor at the Lidar scan points
Site n°2 - H=80m
Site n°2 - H=20 m
Main wind direction = 180°
Lidar wind direction at 80 m = 175°
Lidar Wind direction at 20 m = 166°
�Correction factor from Meteodyn WT outputs
Second step : calculation of speed up factor and wind incidence over the
Lidar disk for each wind directions with a 10° step
Wind incidence
H=20 m - ∆ϕ∆ϕ∆ϕ∆ϕ=0.8°
H=80 m - ∆ϕ∆ϕ∆ϕ∆ϕ=1.4°
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338346 353 Wind incidence (°) at the Lidar scan points
Site n°2 - H=80m
Site n°2 - H=20 m
Main wind direction = 180°
Lidar wind direction at 80 m = 175°
Lidar Wind direction at 20 m = 166°
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�Correction factor from Meteodyn WT outputs
Third step : calculation of correction factor for each wind directions with a
10° step according to the following equation
Site 1 - H=80 m : αααα in [1.;1.01]
Site 2 - H=20 m : α α α α in [1;1.05]
Site 2 - H=80 m : α α α α in [1;1.06]
�� =1
2 sin ����� sin�� + �� + �� sin�� − �� and ! = ��
��&
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Co
rre
ctio
n f
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Wind direction (°)
Mesh 5 x 5
Mesh 2 x 2
�Validation of the Lidar Correction Module with the Meridian sites
Third step : correction of the Lidar data at every time step
Complex terrain (first point h=20 m) - corrected data - MAE = 0.40 m/s –RMS = 0.52 m/s
y = 0.95x + 0.01
R² = 0.96
y = 1.00x + 0.01
R² = 0.96
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U (L
ida
r) /
Ure
f
U (Mast) / Uref
Site n°2 - Mast n°1 - H=20 m
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�Validation of the Lidar Correction Module with the Meridian sites
Complex terrain (second point h=80 m) - corrected data - MAE = 0.40 m/s – RMS = 0.65 m/s
y = 0.95x + 0.01
R² = 0.96
y = 1.00x + 0.01
R² = 0.96
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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6
U (Li
da
r) /
Ure
f
U (Mast) / Uref
Site n°2 - Mast n°2- H=80 m
�Analysis:
� Such remote sensing systems need to be corrected in complex
terrains because of the variability of wind magnitude and incidence
throw the scanning circle.
� Without correction : gap Mast vs Lidar about 3 to 5 %
� With correction : gap vanishes less than 1%
� CFD could be used to calculate the correction coefficients
� Needs to check the correction with others stabilities. Temperature
and velocity profiles give the Richardson number
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�The Lidar Correction Module:
� An integrated module within the CFD software Meteodyn WT
� Developed in close cooperation with Lidar developers
� Correction of all measurements heights at the same time with mesh
refinement at the Lidar points. Use several Lidars for the same project
� Consideration of the thermal stability.
� Automatic delivering of a corrected outputs to be used in the
Meteodyn WT synthesis
Thanks to Meridian for delivering all the data and site informations
necessary to compute the flow.
Thanks to Natural Power for the help to define the correction method for
LIDAR Zephyr
www.meteodyn.com
