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PO008: Comparison of wind speed and TI measurement in induction zone from 3 sensor technologies
Olivier Coupiac, Wind & Solar Analyst, INNERGEX
Wind Speed from nacelle anemometer are unreliable as these values are taken behind the blades and subjected to NTF, which is sensitive to different parameters, such as yaw misalignment or PLC software updates. As the rotor strongly perturbates the wind flow, nacelle anemometers cannot assess correctly incoming wind turbulence intensity either. Long range nacelle-mounted lidars are usually used to measure in front of the rotor, but these devices are sensitive to wakes and rather expensive. We compare here two alternative independent sensors measuring in front of the wind turbine: the Romowind iSpin and the Epsiline WindEagle. These two sensors were installed on a wind turbine and a third one, a sodar was put on ground at 2.5 D for control purpose. The goal was to measure the wind speed and turbulence intensity in front of the rotor before the wind is perturbed by the rotating blades. During approximately one year, these three sensors were measuring simultaneously wind speed and TI on a 120m diameter wind turbine part of an onshore French wind farm. After checking that there is no yaw misalignment on this turbine, INNERGEX performed the calculation of speed and TI correlation. Results: 1) Wind speed The EPSILINE WindEagle direct wind speed measurement is affected by the rotor proximity (induction zone) due to the distance of measurement (at 10m ahead of the LiDAR). By comparing wind speed in induction zone and calibrated free wind speed, a strong linear correlation by part was found, each part being linked to the Cpmax of the wind turbine. The average wind speed measured in the induction zone was 21% below the free wind speed measured. This strong correlation allows to calibrate the WindEagle wind speed for later performance analysis on the studied wind turbine and for other wind turbines of the same model. 2. Turbulence Intensity The TI measurements from WindEagle and iSpin were compared and found to be consistent. We show here that TI measurements in the induction zone from either device can be used to characterize precisely incoming TI at hub height for load investigations, especially in waked situations. Subresult: Additionnally, by analysing standard deviation from the Line-of-Sight measures of each lidar beam, it appears that TI on one beam only is higher that recalculated TI (from the two beams). By investigating the potential causes, an analysis of the high frequency wind direction data from the lidar was done and a link with the rotation of the wind turbine was established: yaw movements are visible on each line-of-sight of the lidar when erased on the recalculated TI. Conclusion: By comparing WindEagle and iSpin, this study shows that both devices can reliably assess incoming wind speed and TI for operational performance and load analysis, especially in wake effect situations.