Presentations

Abstract

As offshore wind turbines continue to grow larger, with blade tips reaching heights of 250+ m, there is a requirement to obtain accurate wind resource and site condition measurements at these higher heights. Such wind data are often obtained using a floating lidar system (FLS) which are typically calibrated to offshore meteorological masts, limited to top heights of 100 m or below.  Previous work by the Offshore Wind Accelerator (OWA) revealed evidence of potential biases that could be introduced at higher heights by buoy motion, lidar characteristics, atmospheric gradients and combinations of these. Any such effects could bias P50 (50% exceedance probability) estimates in energy yield assessments or lead to an increase in uncertainty and hence a reduction in P90 (90% exceedance probability) estimates. Building on these findings, the Tall Floating lidar System Trial for Offshore wind (TaLiSman) project undertook a comprehensive trial to deliver a body of evidence to support using FLS for measurements at high heights. The new knowledge can be used by wind farm developers to drive down uncertainties on their commercial projects. TaLiSman’s objectives include: (1) address the risks of using floating lidar data for measurements obtained at heights well above 100 m, and (2) gather evidence to better understand the accuracy and validity of data from floating lidars.  To achieve this, the project performed a detailed calibration of several FLS.  The measurement campaign took place at Oldbaum Services C-TEST facility using ORE Catapult’s National Offshore Anemometry Hub (NOAH) met mast as a reference. The measurement campaigns, include data collection and analysis from the reference met mast, a continuous wave (ZX) and a pulsed vertical profiling (WindCube) lidar, two dual Doppler scanning lidars, several FLS, as well as a ceilometer and a micro-wave radiometer (MWR) for providing detailed prevailing atmospheric conditions during the test. This comprehensive measurement setup enables a consistent assessment of performance and uncertainty of FLS data up to heights of 300m  To assess the performance of the tested FLS, the two-stage FLS validation methodology developed in previous OWA work [1] is applied to the collected trial data to estimate the uncertainties of the FLS measurement at the high heights of interest. Outcome of the respective analysis informs a recommendation on how to set up the calibration process to optimize the uncertainty estimations results for the final application, which calibration references to consider in the different steps of the methodology and which requirements to push forward for the test to provide safe but not overly conservative uncertainty estimates. Thus, the project not only addresses the previously formulated end-user needs, but also demonstrates novel concepts – such as using scanning lidars as high-accuracy reference measurement over a distance of 7+ km, considering calibrated FLS measurements at heights of 250 m and above for  wind resource and site condition assessments in the industry, and relating the FLS performance to reference measurements for atmospheric conditions by ceilometer and MWR measurements – and shows “world’s first” results for these.  [1] https://www.carbontrust.com/our-work-and-impact/guides-reports-and-tools/two-stage-floating-lidar-validation-methodology-and-worked-example