Determination of an Optimum Sector Size for Plan

Determination of an Optimum Sector Size for Plan Position Indicator Measurements using a Long Range Coherent Scanning Atmospheric Doppler Li. DAR 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 Li. DARs? § 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 Wind. Scanner § Improvements identified for commercial Li. DARs § 2010 -14: DTU Development timeline § Goal: Become standardised measurement device in wind energy industry

Motivation of Study · RUNE project: § Near shore resource assessment using scanning Li. DAR § Coastal zone atmospheric interactions § Improve wind atlases (i. e. NEWA) · Two questions need to be answered! § How many scanning Li. DARs 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 Li. DAR · 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 Li. DAR can only measure a portion of the wind vector!

Principles of Pulsed Li. DAR (contd. ) · A small portion of the pulses backscatter and land back on the Li. DAR’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 Wind. Scanner System · Two parts: § Coherent Doppler scanning Li. DAR (Wind. Scanners) § Master and client software utilising RSCom. Pro, remotely administered · Together represents a time and space synchronised long range coherent scanning multi-Li. DAR array capable of complex measurement scenarios · Current hardware modified from Leosphere Wind. Cube 200 S pulsed Li. DAR

Wind. Cube 200 S (Long Range Wind. Scanner) 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 Li. DARs 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

SSvs. DD 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, 3 m) elevation · Westerly winds from the North Sea

Experimental Design · 3 Wind. Scanners deployed § 1 x 60 degree sector scan § 2 x static dual Doppler · All beams intersect atop 116. 5 m met-mast § Cup anemometer at 116. 5 m § Wind vane and sonic at 100 m · Calibration and deployment procedures outlined in written thesis

Methodology · Load raw data · Apply offsets · Filtering § § § · · CNR, radial speed Partial scans Low availability of scans in 10 min period Wakes (118 -270˚ free) Wind speed (4 -25 m/s) Reduce sector size Apply i. VAP or DD reconstruction Resample (fast, 1 min, 10 min) 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 · SSvs. DD for commercial purposes § 1 Li. DAR 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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