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 elliot@elliot-simon.com Uppsala University Department of Earth Sciences, M.Sc. Wind Power Project Management October 02, 2015

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 2010-14: DTU Development timeline Goal: Become standardised measurement device in wind energy industry

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

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

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!

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)

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

WindCube 200S (Long Range WindScanner) Hardware

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

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)

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

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

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

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

Methodology Load raw data Apply offsets Filtering Reduce sector size CNR, radial speed Partial scans Low availability of scans in 10min period Wakes (118-270˚ 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

Results: Dual Doppler vs. Reference

Results: 60˚ Sector Scan vs. Reference

Reduction in Sector Size (Animated) Wind Speed Wind Direction

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 30-38 degree sector size! We can now divert the saved time to increase distance, sampling rate, LOS density, etc. RUNE experiment will apply this result

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