Rotor Average wind speed for power curve performance Ioannis Antoniou (LAC), Jochen Cleve (LAC), Apostolos Piperas (LAC) March 2, 23
Contents Rotor Average wind speed EU flat terrain wind profiles vs. TI USA Midwest flat wind profiles vs TI Cp vs. TI Using the equivalent wind speed in site calibration campaigns: challenges and solutions Using the equivalent wind speed in power curve measurement campaigns: looking into the future Page 2 March 2, 23
Using a LIDAR: The equivalent wind speed concept H R 3 V 3 v( z)cos( ( z)) da A H R A LIDAR is deployed next to a met mast The LIDAR can measure the wind speed and direction at more heights regularly distributed over the rotor The wind speeds at all heights are normalized by dividing with the LIDAR wind speed at hub height. The LIDAR wind directions at all heights are subtracted from the direction at hub height (wind veer relative to hub height). The normalized LIDAR wind speeds at all heights are multiplied with the cosine of the direction angle relative to hub height Subsequently all wind speeds are multiplied with the cup wind speed at hub height. Page 3 March 2, 23
Wind profiles: Flat coastal location Cosine of wind direction angle relative to HH height 6 4 4 2 2 Lidar wind profiles 6 8 8 6 6 4 4 2 2.6.6.7.7.8.8.9.9 cos(phi) () Difference between cup and LIDAR eqv. wind speed 2 2. TI (%) Cup-Lidar eqv. (.22kg/m3) wind speed (m/s) -. Page 4 Cup HH (.22kg/m3) March 2, 23 2 2 Cup wind speed at hub height (.22kg/m3).
Wind profile deficit vs. TI 6 Lidar wind profiles, TI<.% 6 Lidar wind profiles, 9.%TI<.% 4 4 2 2 8 8 6 6 4 4 2 2 wind speed (m/s) 2 2 wind speed (m/s) Low TI s are correlated with large variations in the wind profile. HIgh TI s are correlated to more uniform profiles The HH wind speed is not representative of the wind profile at low TI s. It is not precise that w/t under produce at low TI s. Page March 2, 23
Cup-Eqv. speed difference vs. TI Cup-Eqv. wind speed Cup-Lidar eqv. (.22kg/m 3 ).8.6.4.2 Cup-Eqv. wind speed vs. TI 4 2 8 -.2 -.4 -.6 -.8 2 Cup HH (.22kg/m 3 ) Large Cup(HH)-Eqv. Wind speed differences occur at low turbulence intensities 6 4 2 Difference between cup and LIDAR eqv. wind speed. -. 2 Cup HH (.22kg/m 3 ) Page 6 March 2, 23
Cp () El. power (kw) High Cup-Eqv. speed difference values relate to low TI s.6..4.3.2. profile deficit vs. TI(%) 2 4 6 8 2 4 6 wind speed (.22kg/m 3 )(m/s).8.6.4.2 El. power for all TI and Sh.exp Sh.x:. Sh.x:. Sh.x:. Sh.x:.2 Sh.x:.2 Sh.x:.3 Sh.x:.4 TI increasing Shear increasing TI increasing.2.4.6.8 wsp (m/s) Low Cp values seem to correlate with low Ti values in the linear part of the power function (flat part of the Cp curve ) 6 4 2 8 6 4 2 In the linear part of the power function, Cp is mostly influenced by the wind speed over the rotor disk and less by variations in turbulence. The use of the equivalent wind speed should be preferred relative to filtering low TI data. In the above example: AEP (TI>%) -AEP (all TI s) =+3% Page 7 March 2, 23
Wind profiles: Flat Midwest USA Cosine of wind direction angle relative to HH height 6 4 4 2 2 Lidar wind profiles 6 8 8 6 6 4 4 2 2.4..6.7.8.9 cos(phi) () Difference between cup and LIDAR eqv. wind speed 4 3. 3 2 TI (%) Cup-Equivalent wind speed (.22kg/m3) wind speed (m/s) 2 -. - Page 8 2 Cup wind speed at hub height (.22kg/m3) March 2, 23 2 Cup wind speed at hub height (.22kg/m 3)
Wind profiles: Flat Midwest USA Cup-Eqv.wind speed 4 Mean (Cup-Eqv.wsp<-.m/s) = -.28m/s, TI<3% 4 Mean (Cup-Eqv.wsp>.m/s) =.38m/s, TI<3% 2 2 8 8 6 6 4 4 2 4 6 8 2 4 6 8 2 wind speed (m/s) TI(%) C(HH)-Eqv.Wsp (m/s) TI<2%.29 2%<TI<3%.2 3%<TI<4%.2 4%<TI<% -.4 %<TI<6% -.4 6%<TI</% -.4 7%<TI<8% 8%<TI<9%.2 9%<TI<%.3 %<TI<%.4 %<TI<2%.3 2%<TI<3%.2 TI can be used as proxy for atmospheric stability, still it is a rather coarse estimator. 2 4 6 8 2 4 6 8 wind speed (m/s). -. Cup HH-Eqv. wind speed vs. TI - 2 4 6 8 2 4 6 Cup HH (.22kg/m 3 ) Wind profiles may form differently under stable conditions; the issue remains: The HH wind speed does not describe accurately the wind profile. 2 2 Page 9 March 2, 23
Ratio () Site calibration and power function verification using a lidar Lidar : comparison. Site calibration. : post-calibration and lidar vs. cup calibration. Next step a combined (?) report: IEC power function vs. HH wind speed. Annex to the report: power function vs. equivalent wind speed (help make the eqv. wind speed better known and more accepted)..99.99.98.98.97.97.96 23 24 2 26 27 28 29 3 3 wind direction ( ) m (s7) Eqv. (s7) Page March 2, 23
Conclusions Low TI allows for wind profiles with irregular shear which are the main reason of the large deviations between the equivalent wind speed and the HH wind speed. The discrepancy can be resolved by either filtering low TI values or by representing the power function vs. the equivalent wind speed. The broader use of remote sensing devices may contribute in reducing both the time, costs and the uncertainties in the wind speed measurements. A frame which will allow the broader use of the equivalent wind speed is urgently needed. Page March 2, 23
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