Presentations

Abstract

Two synchronized scanning lidars (dual-Doppler) have several advantages compared to conventional measurement techniques such as profiling lidars or masts, especially offshore, where a mast or floating lidar buoy would be very cost intensive. Nacelle-mounted lidars (NML) are currently best practice for offshore power curve measurements. The use of a dual-Doppler lidar setup offers an interesting alternative, especially if several turbines are to be measured simultaneously. The high degree of flexibility from scanning lidars allows to measure in front of multiple turbines. In this work the following questions are to be answered: * Is a simultaneous power curve measurement of multiple turbines possible? * What is the alignment uncertainty of the scanning lidar through wind and wave induced turbine tilt and how to correct for that in post-processing? This study presents a dual-Doppler measurement campaign conducted in an operational offshore wind farm in the German Bight using two scanning lidars positioned on the transition pieces of turbines with monopile foundations. The lidars intersect their beams about 2.5 diameters in front of eight turbines in the first row when wind originates from the main wind direction (west). The dataset for this includes a two-month period during the summer/fall of the year 2023. A NML lidar is placed on one of the turbines and is used as reference. Furthermore, the study explores the impact of wave and wind-induced turbine tilt, which can lead to systematic misalignment of the lidars. In order to continuously monitor the tilt of the two turbines where the scanning lidars are positioned on, high-frequency inclinometers (1s) are installed within the towers. This setup enables continuous monitoring of the inclination for uncertainty quantification and allows for data correction in post-processing. The correction method for scanning lidars placed on monopile-founded turbines is tested using data from this measurement campaign. The dual-Doppler system successfully captured the inflow characteristics at all wind speed bins that are required for power curve measurements. The NML deployed on one of the turbines demonstrates a high level of agreement with the dual-Doppler measurements having an R² > 0.99. Given the good agreement, the results of the fit between dual-lidar and NML data can only be improved marginally through the correction made with inclination data. In conclusion, the combination of dual-Doppler lidar measurements and inclinometer data provides an in-depth analysis perspective on the great potential of dual-Doppler lidars to perform multiple power curve measurements simultaneously. The presentation also showcases the challenges and derives solutions for such a measurement setup.