1. Department of Earth Sciences, M.Sc. Wind Power Project Management
Determination of an Optimum Sector Size for Plan Position Indicator Measurements using a Long Range Coherent Scanning Atmospheric Doppler LiDAR
Elliot I. Simon
Uppsala University
Department of Earth Sciences, M.Sc. Wind Power Project Management
October 02, 2015

2. Background Why do we need LiDARs?
Limitations with in-situ measurements (particularly offshore)
Wind power projects growing in size, complexity and cost
Complex terrain and flow
Wind farm control
Research (e.g. wakes, noise, loads, etc.)
Development of long range WindScanner
Improvements identified for commercial LiDARs
: DTU Development timeline
Goal: Become standardised measurement device in wind energy industry

3. Motivation of Study RUNE project: Two questions need to be answered!
Near shore resource assessment using scanning LiDAR
Coastal zone atmospheric interactions
Improve wind atlases (i.e. NEWA)
Two questions need to be answered!
How many scanning LiDARs are necessary?
In what configuration should they be placed?
Thesis objective: Determine optimal PPI scanning strategies which will be implemented in RUNE and subsequent campaigns

4. Principles of Pulsed LiDAR
Same principles as radar, but using pulsed laser light
Laser beam (spatially and temporally coherent source) is emitted into the atmosphere
After emission, the laser pulse interacts with micron sized aerosols suspended in the atmosphere (Mie scattering)
The Doppler effect causes a shift in the pulse’s wavelength relative to the particle’s LOS velocity

5. Radial velocity sampling
Wind vector consists of 3 components (u, v, w)
Radial velocity is a projection of the true wind speed along the laser’s line of sight
One LiDAR can only measure a portion of the wind vector!

6. Principles of Pulsed LiDAR (contd.)
A small portion of the pulses backscatter and land back on the LiDAR’s lens
The Doppler effect is used to obtain radial wind speeds from the backscattered signal (after FFT and MLE):
On board signal processing includes time gating of the reflected pulses to measure multiple range gates (distances) along a single LOS
Unfortunately it’s not that simple in practise..
Coherent (optical heterodyne detection) which modulates CW LO to obtain beat signal, as opposed to direct detection
Eye safe(r), lower power, higher resolution
Dual-axis beam positioning system (scanner head)

7. Long Range WindScanner System
Two parts:
Coherent Doppler scanning LiDAR (WindScanners)
Master and client software utilising RSComPro, remotely administered
Together represents a time and space synchronised long range coherent scanning multi-LiDAR array capable of complex measurement scenarios
Current hardware modified from Leosphere WindCube 200S pulsed LiDAR

8. WindCube 200S (Long Range WindScanner) Hardware

9. Plan Position Indicator
Fixed elevation angle
Azimuth sweep with constant speed
Volume represents conical section
Sector size is the angular width
Measurements represent a (full/partial) sine wave
Amplitude = wind speed
Phase = wind direction
Offset = vertical velocity
Drawbacks:
Horizontal flow is assumed to be homogeneous
Elevation angle needs to be kept low

10. Dual Doppler
2 LiDARs cross their beam simultaneously at a single point in space
Static or complex (dynamic)
2 independent radial velocity measurements, still no vertical component
Pointing accuracy extremely important! (hard target calibration)

11. Why optimize sector size?
Fixed measurement duration (movement and acquisition)
Trade off between sampled area and rate
Potential benefits with a smaller sector size:
Faster refresh rates over the area sampled, since the angular size is smaller
Improved resolution by incorporating more line of sight measurements within the sector area
Increased measurement distance, since more time could be spent on lengthening the reflected pulse acquisition time
Better averaging (e.g. 10 minute) results due to the larger number of samples included in the average
Better representation of the targeted region, especially at far distances where a large sector size could envelop a vast area

12. SSvsDD Campaign Introduction
Location: Danish National Test Centre for Large Wind Turbines (Høvsøre)
Period: 30 April – 7 May, 2014
Purpose: To test dual Doppler and sector scan performance

13. Høvsøre Site Overview 5 turbine test stands 6 meteorological masts
Simple, flat terrain (-1, 3m) elevation
Westerly winds from the North Sea

14. Experimental Design 3 WindScanners deployed
1x 60 degree sector scan
2x static dual Doppler
All beams intersect atop 116.5m met-mast
Cup anemometer at 116.5m
Wind vane and sonic at 100m
Calibration and deployment procedures outlined in written thesis

15. Methodology Load raw data Apply offsets Filtering Reduce sector size
CNR, radial speed
Partial scans
Low availability of scans in 10min period
Wakes ( ˚ free)
Wind speed (4-25 m/s)
Reduce sector size
Apply iVAP or DD reconstruction
Resample (fast, 1min, 10min)
Compare output between SS, DD, and cup/vane

16. Results: Dual Doppler vs. Reference

17. Results: 60˚ Sector Scan vs. Reference

18. Reduction in Sector Size (Animated)
Wind Speed
Wind Direction

19. Conclusions SSvsDD for commercial purposes Optimum sector size
1 LiDAR in PPI configuration performs well (wind speed, 99.8% accuracy) but with more scatter
Wind direction result is nearly identical, horizontal homogeneity is more applicable than wind speed
Is the improvement using dual Doppler worth the added cost?
Optimum sector size
60 degrees does not perform noticeably better than degree sector size!
We can now divert the saved time to increase distance, sampling rate, LOS density, etc.
RUNE experiment will apply this result

20. Tack så mycket!