Posters

Abstract

Although the use of nacelle-mounted lidars is becoming more widespread, their deployment remains constrained by relatively long calibration times and high uncertainty, in part due to the use of cup anemometers as reference instruments. Building on our previous work and innovations we now introduce and demonstrate a refined method for lowering the uncertainties in nacelle lidar calibration, while reducing calibration time. We previously developed and demonstrated a more accurate way of calibrating line-of-sight speeds during the Windcal2.0 project. A new reference sensor was built – a 2-D lidic system consisting of two short range, continuous wave lidars focused at 7m. This concept has now been further simplified and improved by switching to a 1-D lidic system, consisting on only a single short range continuous wave lidar installed on one metmast where installation and alignment is easier. To demonstrate that the new system provides reliable calibration results and lower uncertainties, two measurement campaigns were conducted in 2024 and 2025 at DTU test center for large wind turbines in Høvsøre. The same 4-beam nacelle lidar was calibrated in two different ways. First, against a cup anemometer, following the procedure outlined in the IEC 61400-50-3 standard. Second, against the 1-D lidic system. Both methods (the one described in the standard and the Windcal method) are based on the white-box concept – the reference instrument is installed on a short metmast with additional instruments for wind direction measurements and monitoring of environmental conditions. The nacelle lidar is installed on the ground, a few hundred meters away and each beam is calibrated individually against the reference.  Besides the reference instrument, there exists a few other differences in the measurement setup. Nominal range, inclination angle, calculation of reference wind speed, alignment method, and averaging periods are to some degree different. The Windcal method includes a motion device – a pan and tilt unit (PTU). It is used to rotate the nacelle lidar head so that every two minutes the beams are aligned in turn to the reference, thus achieving calibration of multi-beam lidars much faster. The results from the campaigns show very good agreement between the calibration factors obtained with the cup anemometer and the 1-D lidic. A much lower uncertainty is achieved with a 1-D lidic system – approximately two times lower than the result we obtained with a cup anemometer. The fundamental reason for this being that a lidic has much lower calibration uncertainty than a cup – by a factor of 10. This work presents recent improvements and newly acquired results that verify the proposed calibration method and demonstrate its ability to reduce lidar uncertainty.  This concept could be used, potentially, to calibrate any type of wind speed sensor.