Remote sensing systems are more and more used during campaigns of measurements for wind resource assessments as they can be moved easily from one location to another and because they can measure at high height compared to cup anemometers on masts. Lidars like the WINDCUBE by Leosphere or the ZEPHIR by Natural Power have a proven accuracy on flat terrains, while in complex terrain, the loss of flow homogeneity can create a sensor bias during the transformation of measured radial wind speed to horizontal wind speed (in some cases up to 10%).Therefore, a tool is needed to enable the correction of Lidar data in complex terrain. It has been shown that CFD’s (computational fluid dynamics) models are recommended instead of linear models in complex terrain in order to get accurate characteristics of the wind flow. Thus, using the Meteodyn WT CFD results such as wind speed up factor and inflow angle, a method to correct two kinds of Lidars data is presented and validated. These data are then directly used for wind resource assessment. The studies on semi-complex terrain underline the quality of the correction which allows the user to obtain climatological files with a small uncertainty which reduces the uncertainties in the assessment of the annual energy production.

1. Correction tool for lidar
PO.141 in complex terrain based on CFD outputs
Céline BEZAULT (1), Stéphane SANQUER (2), Mohamed Nadah (1)
(1) Meteodyn France, (2) Meteodyn New Caledonia
Abstract
Remote sensing systems are more and more used during campaigns of measurements for wind resource assessments as they can be moved easily from one location to
another and because they can measure at high height compared to cup anemometers on masts.
Lidars like the WINDCUBE by Leosphere or the ZEPHIR by Natural Power have a proven accuracy on flat terrains, while in complex terrain, the loss of flow homogeneity
can create a sensor bias during the transformation of measured radial wind speed to horizontal wind speed (in some cases up to 10%).Therefore, a tool is needed to enable
the correction of lidar data in complex terrain.
Objectives
The Lidar is able to measure the radial wind speed component according to several beams.
In order to correctly retrieve the wind velocity vector at a given height at the center of the scanning cone, it
is assumed that the flow remains homogeneous over the sampled volume at a given height in the internal
correction algorithm used in the Lidar interface.
This is why the errors generated need to be estimated and a correction applied to the lidar measurements.
Moreover, it has been shown that CFD’s (computational fluid dynamics) models are recommended instead
of linear models in complex terrain in order to get accurate characteristics of the wind flow.
Thus, using the Meteodyn WT CFD results such as wind speed up factor and inflow angle, a method to
correct two kinds of lidars data is presented and validated. These data are then directly used for wind Methodology to obtain accurate corrected file coming from lidars
resource assessment.
Methods
The idea of the Meteodyn WT method is to find a relationship between the horizontal wind speed, VL measured by the Lidar and the horizontal wind speed VC at the center
of the circle of measurements using upstream Vu and downstream Vd vectors.
The algorithm, thanks to the speed up coefficient, inflow angle and deviation at
the lidar position and obtained by CFD modeling, produces a directional
coefficient correction rose which is going to be applied at the measurement
data.
After this correction, the corrected data can be used for a resource assessment
study in order to compute AEP values on site.
Top view and profile view of the wind speed component Example of directional factor of
correction
Results
The results presented here are a New Zealander test case for the ZephIR lidar and a Spanish site for the Windcube data. The input files are the 10 minutes average wind
speed and direction at the Lidar location and the topographical data. The directional factors of correction have been computed with CFD model Meteodyn WT . The lidar
measurements are corrected and compared to the anemometers data on site.
Site n°2 - Mast n°2- H=80 m
1.6
1.5
1.4 y = 1.00x + 0.01
Blue = with correction
1.3
1.2
R² = 0.96
Red = without correction
1.1
y = 0.95x + 0.01
U (Lidar) / Uref
1
R² = 0.96
0.9
0.8
Before correction, the data between the met mast data and the lidar one at 80 m
0.7
0.6
high had a correlation of 0.95 which becomes close to 1 thanks to CFD modeling.
0.5
0.4
0.3
0.2 The factor of correction varies from 1.003 to 1.053 according to the direction. The
0.1
0 most important corrections are for the S-E and N-W directions where the flow is the
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 most disturbed because of the difference in elevation data.
U (Mast) / Uref
Before correction, the data between the met mast data and the lidar one at 60m high
had a correlation of 0.87 which becomes 0.99 thanks to CFD modeling.
The factor of correction varies from 1,01 for 260° to 1.09 for the North direction. The
high value of correction is due to the important difference in elevation in the South –
North axis. To the contrary, in the SE – NW axis, the terrain is less steep, so the lidar
measurement were less biased.
Conclusions
CFD computations have been used to correct the lidar horizontal wind speeds in complex terrain (difference of elevation and roughness variation). This study underlines the
quality of the correction which allows the user to obtain climatological files with a small uncertainty which reduces the uncertainties in the evaluation of the annual energy
production. Meteodyn would like to express their gratefulness to Acciona Energia and to Meridian Energy for providing the data necessary for this study.
References
[1] : Yamada, T, (1983), Simulations of nocturnal drainage flows by a q2l turbulence closure model, Journal of Atmospheric Sciences, vol. 40, Issue 1, pp.91-106
[2] : D. Foussekis, “Investigating wind flow properties in complex terrain using 3 lidars and a meteorological mast”, EWEC Proceedings (2009)
[3]: Boquet M. et al.: Innovative Solutions for Pulsed Wind LiDAR Accuracy in Complex Terrain, ISARS 2010
EWEA 2012, Copenhagen, Denmark: Europe’s Premier Wind Energy Event