Final results of the joint project “Development of LIDAR wind sensing for the German offshore test site” Determination of power curves based on wind field measurements using a nacelle-based LiDAR scanner
A. Rettenmeier 1, P. Klausmann 1, O. Bischoff 1, M. Hofsäß 1, D. Schlipf 1
B. Siegmeier 2 , M. Kühn 3
1 Endowed Chair of Wind Energy (SWE), University of Stuttgart, Germany
2 AREVA Wind GmbH, Bremerhaven, Germany
3 Institute of Physics, University Oldenburg, Germany
Rettenmeier et al.
Determination of power curves based on wind field measurements using a nacelle-based LiDAR scanner, EWEA 2011
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Thank you Mr Chairman for the kind introduction. Before I start with my presentation, I want to thank my co-authors P.K. O.B., M.H. and D.S from the Chair of Wind Energy at the University Stuttgart. I also want to thank B.S. from AREVA-Wind and MK from the University of Oldenburg
Table of Contents
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Determination of power curves based on wind field measurements using a nacelle-based LiDAR scanner, EWEA 2011
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The presentation is divided into following topics: first I will talk short and briefly about the motivation, the experiment setup and the used LiDAR system. Followed by different ways of determining the power curve and the results. The presentation will close with the outlook and conclusions
Motivation
Sales argument / economic profitability
Worldwide comparability (IEC 61400-12-1)
Increasing hub heights of wind turbines
Cost expansive determination of p-v- curves offshore/ complex terrain
Nacelle-based LiDAR wind field measurements taking into account
Whole swept rotor disc
Wind direction (slow variation)
Horizontal wind shear, vertical wind shear (fast variation)
More free valid areas Less sectors to exclude Faster measurement campaigns
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Rettenmeier et al.
Determination of power curves based on wind field measurements using a nacelle-based LiDAR scanner, EWEA 2011
The reason we need power curves is to calculate and to estimate the annual energy production which is a major sales argument. If the determination of power curves is carried out according to the IEC standard, the power curves can be compared worldwide. The reason why to develop new techniques is on the one hand based on the permanent increasing hub height of wind turbines onshore and the high costs offshore, especially on floating turbines.
If the LIDAR measurement takes place from the nacelle and the laser beam steers into different points one can measure the incoming wind field over the whole swept rotor disc. It‘s possible to determine the wind direction, horizontal and vertical wind shear. According to the IEC standard less sectors have to be excluded which leads to a faster measurement campaign.
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Met mast (102 m height)
Meteorological sensors
1st Class anemometer
[Fig. SWE]
Rettenmeier et al.
Determination of power curves based on wind field measurements using a nacelle-based LiDAR scanner, EWEA 2011
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The measurements have taken place at the onshore test site of AREVA Winds’ M5000 prototype in Bremerhaven. We operated a data acquisition system for power curve and load measurements and a met mast which is equipped with several meteorological sensors at different heights up to 102m.
LiDAR system installed on the nacelle
[Fig. SWE, EWEC 2010]
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Determination of power curves based on wind field measurements using a nacelle-based LiDAR scanner, EWEA 2011
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The LiDAR we used was a Windcube of Leosphere. Additionally we developed a scanner, which allows us to steer the laser beam in any direction we want. The scanner could be adapted to the Windcube, as you can see in the picture below. Both were installed on the nacelle.
Basic Trajectory
Fastest way to scan(8.4 sec), good temporal and spatial resolution
Adapted to 7x7 grid points
Use of pulsed LiDAR System: 5 focus planes simultaneously
How to scan the incoming wind field
hub
height
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Rettenmeier et al.
Determination of power curves based on wind field measurements using a nacelle-based LiDAR scanner, EWEA 2011
The question now was how to scan the incoming wind field to get as most information as possible. After an investigation of different trajectories we decided for a Lissajous figure, which allows us to scan in a high temporal and spatial resolution as fast as possible. Then we adapted the Lissajous Figure to a 7-7 measurement grid. Due to the pulsed LiDAR technology used, each trajectory point was measured in five different focus planes along the laser beam simultaneously.
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Rettenmeier et al.
Determination of power curves based on wind field measurements using a nacelle-based LiDAR scanner, EWEA 2011
In this movie, the real measured wind fields of the five focus planes are shown. As you can see, the LiDAR detects pretty well the vertical shear of the wind field. The LiDAR system is mounted behind the rotor blades, but even though hitting the blades and loosing points, the wind field can be reconstructed.
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Rettenmeier et al.
Determination of power curves based on wind field measurements using a nacelle-based LiDAR scanner, EWEA 2011
In detail, the laser beam which is coming from the Windcube hits the mirror and is deflected in the way we programmed the trajectory, in our case the Lissajous figure. With the assumption of a plane parallel wind field and no vertical wind speed the wind vector can be determined from the line-of-sight velocity of the laser beam. Within 10 minutes 68 measurement data sets are stored for each trajectory point from one focus plane.
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State of the art of power curve determination
From the scatter plot to a power curve (separating the measurement data into BINs of 0.5 m/s)
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Rettenmeier et al.
Determination of power curves based on wind field measurements using a nacelle-based LiDAR scanner, EWEA 2011
The common way how to deal with measurement points and the power curve is described here. On the left hand side one can see a scatter plot of the power curve in red crosses and the standard deviation in blue circles. After classifying the data into wind speed bins of 0,5m/s, the electrical power of each bin is averaged. This value and the BIN value give us the x and y coordinate. The standard deviation of the electrical power of each BIN is displayed above and below the mean power.
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Rettenmeier et al.
Determination of power curves based on wind field measurements using a nacelle-based LiDAR scanner, EWEA 2011
We investigated different ways which of the 49 measurement points of only one focus plane can be taken into account for the power curve determination. For example we just had a look to these five points, arranged as a big cross. We averaged the data to one wind speed and compared the new power curve in blue with the standard conform power curve. As you can see, both curves look pretty similar, but the standard deviation is much smaller of the big cross curve.
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uTop_Layer
uMiddle_Layer
uLowest_Layer
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Rettenmeier et al.
Determination of power curves based on wind field measurements using a nacelle-based LiDAR scanner, EWEA 2011
In a second way we averaged seven measurement points of three different heights. The equivalent wind speed was calculated with a 2nd order polynomial function. When comparing both curves it‘s obvious that in the region of 0.8 the blue curve differs from the red one. The standard deviation of the blue curve is smaller than the standard deviation of the red one.
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[R.Wagner]
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A2
A3
A4
A5
A6
A7
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Rettenmeier et al.
Determination of power curves based on wind field measurements using a nacelle-based LiDAR scanner, EWEA 2011
In a third step, we used an approach from Risoe-DTU, where the rotor disc is divided in several segments. The equivalent wind speed is calculated according to this equation on the left side. Comparing the two power curves one can see, that the new power curve in blue is shifted to the left and still has less standard deviation.
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Comparison criteria
Summed up (over all bins) the standard deviation of Pel of each BIN
Summed up (over all bins) the difference between Pel (anemometer binning, IEC) and Pel (LiDAR binning) of each BIN
Name
Rettenmeier et al.
Determination of power curves based on wind field measurements using a nacelle-based LiDAR scanner, EWEA 2011
The upcoming question now was how to compare the different determination approaches. First criterion was, to sum up the standard deviation the electrical power of each BIN over all BINs. Here the approach with the area weighted rotor disc has the lowest value what means the highest accuracy. The second criterion was related to the power curve determination according to the IEC standard. Here the difference between the electrical power of the standard procedure and the new approach from each BIN was summed up over all BINs. In this case, the polynomial approach was the closest.
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Investigation of p-v- curves with interpolated measurement points of different focus planes
Use of further quality and comparison criteria for best fit of measurement points
Reduction of measurement points needed (with sufficient accuracy and information)
Best/most optimal beam configuration
Simple LiDAR device without moving parts
Nacelle-based LiDAR power curve determination over the whole rotor disc have less scatter and less standard deviation higher accuracy
Assumptions necessary
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Rettenmeier et al.
Determination of power curves based on wind field measurements using a nacelle-based LiDAR scanner, EWEA 2011
Now I want to conclude my presentation. One can say, that nacelle-based LiDAR power curve determination over the whole rotor disc have less scatter and less standard deviation which means a higher accuracy. Including some assumptions various determination approaches are investigated. But still further studies are necessary: the vertical wind shear and turbulence behaviour has to be investigated. The possibility to interpolate the wind speed between the focus planes hasn’t been included yet. We need further quality and comparison criteria to reduce the measurement points to a smaller number which still has a high accuracy. Then an optimal beam configuration will lead to a simple Lidar device with no moving parts.
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Acknowledgement This project (No. 0327642) is funded by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU).
Thank you for your attention!
Dank U wel voor uw aandacht!
Merci de votre attention !
Rettenmeier et al.
Determination of power curves based on wind field measurements using a nacelle-based LiDAR scanner, EWEA 2011
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