Offshore wind turbine wake characteristics using scanning doppler lidar

Within an offshore wind park, wind flow characteristics are quite complex and govern both the energy production and the structural wind turbine response. An experimental study focussed on assessing the spatial variability of winds near the German offshore wind energy platform FINO1 was conducted using multiple remote sensing devices. This study focuses on measuring the wind turbine wake characteristics, such as velocity deficit, the extent (length and width) of the wake and wake meandering under various atmospheric conditions using the data collected from a single scanning Doppler Lidar for several months in 2016. A new algorithm based on using a Gaussian model to measure the downwind wake characteristics is developed. The wind turbine downwind wake deficits compared well to previous models at far-wake regions, while at near-wake regions the models deviated due to different instruments & methodologies used in measuring the wake characteristics. It was also observed that the length of the Alpha Ventus wind turbine wake varied from 3 to 15 times the Rotor Diameter (RD), and the maximum velocity deficit varied from 55% to 75% of the free-stream wind speed, depending on mean wind speed and atmospheric stability. Detailed analysis of the Alpha Ventus wind turbine wake characteristics is presented.publishedVersio

Recommendation on use of wind lidars

The 15 Early Stage Researchers (ESRs) in the LIKE project investigate topics in which wind lidar play a significant role. This report provides the ESRs an introductory reading and gives a short introduction into the basic principles, as well as an overview on the practical application of lidar wind measurement technology for a wide range of research fields, including a corresponding literature review. Wherever possible, it will also give the ESRs recommendations on the use of lidars and related best practices and provide corresponding state-of-the-art documents in the attachment.publishedVersio

Recommendation on use of wind lidars

The 15 Early Stage Researchers (ESRs) in the LIKE project investigate topics in which wind lidar play a significant role. This report provides the ESRs an introductory reading and gives a short introduction into the basic principles, as well as an overview on the practical application of lidar wind measurement technology for a wide range of research fields, including a corresponding literature review. Wherever possible, it will also give the ESRs recommendations on the use of lidars and related best practices and provide corresponding state-of-the-art documents in the attachment.publishedVersio

Innovative measurement techniques for atmospheric turbulence and wind energy

The measurement of different atmospheric flow quantities is of utmost importance for a correct understanding of most atmospheric phenomena. Researchers and industry in the fields of meteorology and wind engineering demand extensive and accurate measurements of atmospheric turbulence for a better understanding of its role in a wide range of applications such as weather forecast, wind resource evaluation, wind turbine wake, pollutant transport or urban climate. Quantitative measurements of relevant variables are particularly valuable for the development, testing and validation of turbulence parameterizations used in both analytical and numerical models. This thesis focuses in the development of innovative measurement techniques for atmospheric turbulence, particularly suitable for wind energy applications, and it is divided into four different studies. The first study presents a multirotor UAV-based technique for the measurement of atmospheric turbulence and temperature. The technique is based on the integration of a fast-response multi-hole pressure probe and a thermocouple with an inertial measurement unit (IMU). This technique allows for an accurate measurement of time series of the three components of the velocity vector and temperature at any point in the atmosphere in which the UAV can fly. The technique relies on the correction of the velocity vector measured by the pressure probe on the frame of reference of the UAV -non inertial- with the information provided by the IMU. The study includes a validation of the technique against sonic anemometry and the measurement of the signature of tip vortices shed by the blades of a full-scale wind turbine as an example of its potential. The second study presents a triple-lidar technique developed for the measurement of atmospheric turbulence at a point in space from synchronous measurements of three intersecting Doppler wind lidars. The laser beams must be non-coplanar so that trigonometric relationships allow the reconstruction of the velocity vector. The technique is validated against sonic anemometry in terms of the instantaneous velocity vector, turbulence statistics, Reynolds stresses and the spectra of the three components of the velocity and the turbulent kinetic energy. The third study investigates the theoretical accuracy of the reconstruction of a full-scale wind turbine wake in terms of the average and the standard deviation of the longitudinal velocity component by volumetric scans from lidar measurements. To that end, a series of virtual experiments are performed, where synthetic lidar measurements are obtained from LES simulation results. The methodology described quantifies the errors and allows the optimization of the scan pattern so that it balances the different error sources and minimizes the total error. The fourth study presents a measurement campaign dedicated to the characterization of full-scale wind turbine wakes under different inflow conditions. The measurements are performed with two nacelle-mounted scanning lidars. The first lidar characterizes the inflow while the second performs horizontal planar scans of the wake. The relationships obtained for the growth rate of wake width, velocity recovery and length of the near wake are compared to analytical models and allow to correct the parameters prescribed until now with new, more accurate values directly derived from full-scale experiments

The Role of Atmospheric Stability and Turbulence in Offshore Wind-Farm Wakes in the German Bight

Airborne meteorological in situ measurements as well as stationary measurements at the offshore masts FINO1 and FINO3 in the German Bight are evaluated in order to examine the hypothesis that the wake dissipation downstream of large offshore wind farms depends on atmospheric stability. A long-term study of the mast data for the years 2016 and 2017 demonstrates a clear dependence of stability on the wind direction. Stable conditions are predominantly expected during southerly winds coming from the land. The analysis of various stability and turbulence criteria shows that the lapse rate is the most robust parameter for stability classification in the German Bight, but further implies that stability depends on the measurement height. A near-surface (0 to 30 m), predominantly convective, layer is present and more stable conditions are found aloft (55 to 95 m). Combing the stability data with the airborne measurements of the offshore wind-farm wakes reveals the trend of a correlation between longer wake lengths and an increase in the initial wind-speed deficit downwind of a wind farm with stronger thermal stability. However, the stability correlation criteria with the wake length downstream of the four investigated wind farms, Godewind, Amrumbank West, Meerwind Süd/Ost, and Nordsee Ost, contain large variance. It is assumed that the observed scattering is due to the influence of the wind-farm architecture and temperature inversions around hub height. These, however, are crucial for the classification of stability and illustrate the complexity of a clear stability metric