LIDAR: Wind Lidar Systems for Wind Energy Deployment

1.0 Introduction he aim of IEA Wind Task 32 is to address the very fast development of wind lidar technologies and their applicability for more accurate measurement of wind characteristics relevant to reliable deployment of wind energy power systems. The purpose is to bring together actors in the research community and industry to create synergies in the many R&D activities already on-going in this new and very promising remote sensing-based measurement technology. The Task has three main drivers. Firstly, no consolidated multi-lateral and international exchange on lidar technology has taken place until today despite several research projects during the last years. Secondly, the spread of several new commercial lidar systems with different specifications makes it very difficult for the community to keep up with the advances of this specific technology. Finally, a large number of new applications only possible with wind lidar systems are being developed. However, their real potential cannot be assessed nor exploited without T strong work between the research community and the industry.The present state of the lidar technology (Figure 1) can be summarized as: follows. Several commercial and research systems are available. At the moment, all but one of these are based on the coherent detection principle. The majority of the commercial systems available today are built on a pulsed (range-gated) measurement technology. There already exists quite a fair amount of verification data, which have been, or are being, measured by high-quality calibrated standard meteorological masts, of heights up to 100+ m. This serves as a basis for comparison of wind lidar system performance. Ground-based lidar systems using the Velocity Azimuth Display (VAD) mode offer high correlation of the measured mean wind speed with conventional cup anemometry in flat terrain. This has been supported by extensive comparison studies (1). Today, there is high confidence in wind lidar measurements performed over flat terrain and fair atmospheric conditions. In complex conditions, however, there are needs for better site characterizations and corresponding mitigation of errors due to non-homogeneities in the flow fields. An outstanding issue is also that lidar and conventional wind anemometry (e.g., cups) measure turbulence differently. This becomes evident when vertical profiles of turbulence measured by lidars are compared with turbulence measured by conventional instrumentation (2, 3). Accurate turbulence measurements are important for assessment of site-specific design conditions and wind turbine loads. Most of the lidar systems at present have been developed for ground operation as replacement for conventional anemometry. However, new applications such as power curve measurement, load estimation, and wind turbine control make use of less standard approaches from the nacelle (4), spinner hub (5), or even blade-integrated. Likewise, floating lidars are under development to replace extremely expensive bottom-mounted offshore met towers. New developments are being tested: • Nacelle-based systems for control of wind turbines (6) and power curve measurements • Systems based on multiple synchronized lidar devices for ‘true’ three dimensional measurement • Lidar measurements inside and in the wakes of wind farms