Proof of concept for wind turbine wake investigations with the RPAS SUMO

The Small Unmanned Meteorological Observer (SUMO) has been operated in the vicinity of five research turbines of the Energy Research Centre of the Netherlands (ECN) at the test site Wieringermeer. The intention of the campaign was to proof the capability of the system for wind turbine wake investigations also for situations above rated wind speed. In rather high wind conditions of 15-20 ms−1 on May 10, 2014, the system showed a satisfying in-flight behavior and performed five racetrack flights. The racetrack patterns flown parallel to the row of the five turbines (four flights downstream the turbine row, one upstream) enable the characterization and investigation of the strength, i.e. the reduction in the mean wind, and structure, i.e. the horizontal extension and turbulent kinetic energy (TKE) distribution of single turbine wakes.publishedVersio

Analysis of lidar wind measurements at the Bruck an der Leitha wind park

Auf Grund der stetig größer werdenden Windkraftanlagen wird es messtechnisch schwieriger, Windprofile mit herkömmlichen meteorologischen Masten zu erfassen. Daher geht der Trend zur Datenbestimmung mittels bodengestützter Fernerkundungssysteme wie SODAR oder LIDAR. Für diese Studie stehen LIDAR Daten einer Messkampagne von VERBUND Renewable Power GmbH zur Verfügung. Ein WINDCUBE(TM), hergestellt von Leosphere, wurde über einen Zeitraum von drei Monaten in einem Windpark westlich von Bruck an der Leitha (NÖ) zur Datenaufnahme betrieben. Das Messprinzip dieses Geräts basiert auf der Ermittlung von radialen Windgeschwindigkeiten durch den Dopplereffekt, der bei der Rückstreuung von Lichtimpulsen an Aerosolen zustande kommt. Die gewonnenen Daten werden in neun verschiedenen Höhenniveaus von 40 bis 200 m über Grund, also innerhalb der atmosphärischen Grenzschicht gemessen. Das Windprofil in dieser Schicht ist nahezu logarithmisch und Turbulenz spielt dabei eine tragende Rolle. Aus einer Messreihe mit einer Datendichte von 0,25 Hz an je einem von vier Punkten kann man diese turbulenten Flüsse berechnen und so abgeleitete Größen wie die turbulente kinetische Energie (TKE) zur Analyse heranziehen. Der Untersuchungswerpunkt liegt vor allem auf dem Einfluss einzelner Wetterlagen und Strömungsmuster auf die TKE Verteilung. Kenntnisse darüber würden Leistungsvorhersagen verbessern, sowie durch Optimierung der Konstruktion eine höhere Lebensdauer der Windkraftanlage ermöglichen. Im Zuge dessen werden relevante Größen wie Windgeschwindigkeit oder TKE in Fallstudien untersucht, abhängig von der Wetterlage. Resultate zeigen, dass die Hauptwindrichtungen durch die Topographie der Region gegeben sind und stark im Zusammenhang mit der Umströmung der Alpen stehen. Bei Ostwinden, bedingt durch eine Vb-artige Wetterlage, sind stromabwärts der Windkraftanlage Verwirbelungen im TKE-Profil sowie ein Rückgang der horizontalen Windgeschwindigkeit zu erkennen.Due to the increase in height of the wind turbine masts in recent years, it has become difficult to obtain much needed wind profile measurements via traditional tower measurements. In response to that, there has been an increase in use of remote sensing methods for obtaining wind profiles for wind turbine operation and siting. In this study we analyzed Doppler LIDAR measurements obtained in a field campaign at a wind park operated by VERBUND Renewable Power GmbH and located near Bruck-an-der-Leitha (Lower Austria). A WINDCUBE(TM) Doppler LIDAR, manufactured by Leosphere, was used to collect data over a three-month period. Given that the top measurement height of 200 m is located inside the atmospheric boundary layer, which has closed to logarithmic wind profile and is turbulent, leads to fluctuations of the wind field having great importance. Due to a sufficiently high sampling rate (0.25 Hz at one out of 4 measurement points) the calculations of variances and covariances of wind parameters are possible using this data set and allow an analysis of derived parameters such as turbulent kinetic energy (TKE). The characteristics of certain weather patterns and flow fields and their influence on the wind park are of particular interest. This knowledge could lead to improvement of power curve prediction and would aid engineering designs, aimed at prolonging the lifetime of wind turbines. Therefore, kinetic energy and the meteorological parameters, such as wind speed and its components, are analyzed and compared for several case studies, dependent on the wind direction and weather situation. Results show that the main wind direction is conditioned by topography of the region and is strongly influenced by the flow around the Alps. Dependent on the weather situation different flow characteristics were documented by the measurements, such as wake effects downstream of the wind turbine seen in profiles of horizontal wind speed and TKE. At the site of the Bruck/Leitha wind park, these situations are due to easterly flows corresponding to Vb Cyclone events

A Comparison of LiDAR and Radiosonde Wind Measurements

Doppler LiDAR measurements are already well established in the wind energy research and their accuracy has been tested against met mast data up to 100 m above ground. However, the new generation of scanning LiDAR have a much higher range and thus it is not possible to verify measurements at higher altitudes. Therefore, the LiDAR Measurement Campaign Sola (LIMECS) was conducted at the airport of Stavanger from March to August 2013 to compare LiDAR and radiosonde winds. It was a collaborative test campaign between the University of Bergen, the Norwegian Meteorological Office (MET), Christian Michelsen Research (CMR) and Avinor. With the airports’ location at the Norwegian West Coast, additional motivations were the investigations in characteristics of coastal winds, as well as the validation of the LES turbulence forecast for the airport of Stavanger. We deployed two Windcubes v1 and a scanning Windcube 100S at two different sites in Sola, one next to the runway and the other one near to the autosonde from MET. The Windcube 100S scans several cross-sections of the ambient flow on hourly basis. In combination with wind profiles up to 200 m (Windcubes v1) and 3 km (Windcube 100S) and temporally more frequent radiosonde ascents, we collect a variety of wind information in the coastal atmospheric boundary layer. First results show increasing correlation of 0.95 to 0.99 for increasing measurement heights (125 to 1325 m) between the scanning LiDAR wind profiles and the radiosonde horizontal wind speeds. Though the number of LiDAR measurements decreases with increasing height, the measurements seem to correlate better with the radiosonde data in high altitudes

Characterization of turbulence in wind turbine wakes under different stability conditions from static Doppler LiDAR measurements

Wake characteristics are of great importance for wind park performance and turbine loads. While wind tunnel experiments provided a solid base for the basic understanding of the structure and dynamics of wind turbine wakes, the consequent step forward to characterize wakes is full-scale measurements in real atmospheric boundary layer conditions under different stability regimes. Scanning Doppler LiDAR measurements have proven to be a flexible and useful tool for such measurements. However, their advantage of measuring spatial fluctuation is accompanied by the limited temporal resolution of individual sampling volumes within the scanned area. This study presents results from LiDAR Doppler Beam Swing (DBS) measurements and highlights the potential of information retrieved from a spectral analysis of wake measurements. Data originate from three Windcube v1 and sonic anemometers, collected during the Wind Turbine Wake Experiment–Wieringermeer. Despite the ongoing research on the reliability of turbulence retrievals based on DBS data, our results show wake peak frequencies consistent with sonic anemometer measurements. The energy spectra show rather distinct maxima during stable conditions, which broaden during unstable and neutral conditions. Investigations on the effect of blade pitch on downstream wind speed and turbulence intensity profiles indicate the potential for the development of stability-dependent wind farm control strategies

Wind coherence measurement by a single pulsed Doppler wind lidar

A single pulsed Doppler wind lidar, deployed at the FINO1 platform in the North Sea, has been used to monitor the lateral and vertical coherence of the along-wind component. To maximize the sampling frequency of the monitoring system, a particular configuration based on small sweeping angles around the mean wind direction is used. The set-up provides wind velocity measurement with an increasing cross-flow separation with increasing distances from the lidar. We present hereby preliminary results from the analysis of the Plan Position Indicator scans carried out with a sampling frequency of 0.13 Hz, and the Range Height Indicator scans sampled at 0.19 Hz. The mean velocity and the turbulence intensity indicated a sufficiently uniform flow, which was verified by the calculation of the along and crosswind turbulence length scales. The lateral and vertical coherence was estimated based on multiple 10-minutes samples, for a mean wind velocity ranging from 10 to 14 ms1. For the conditions examined, the measured coherence showed a good agreement with the IEC model for low and medium cross-flow separations, and a fairly good agreement with the Frøya model for large separations.A single pulsed Doppler wind lidar, deployed at the FINO1 platform in the North Sea, has been used to monitor the lateral and vertical coherence of the along-wind component. To maximize the sampling frequency of the monitoring system, a particular configuration based on small sweeping angles around the mean wind direction is used. The set-up provides wind velocity measurement with an increasing cross-flow separation with increasing distances from the lidar. We present hereby preliminary results from the analysis of the Plan Position Indicator scans carried out with a sampling frequency of 0.13 Hz, and the Range Height Indicator scans sampled at 0.19 Hz. The mean velocity and the turbulence intensity indicated a sufficiently uniform flow, which was verified by the calculation of the along and crosswind turbulence length scales. The lateral and vertical coherence was estimated based on multiple 10-minutes samples, for a mean wind velocity ranging from 10 to 14 m s -1. For the conditions examined, the measured coherence showed a good agreement with the IEC model for low and medium cross-flow separations, and a fairly good agreement with the Frøya model for large separations

Characterisation of Single Wind Turbine Wakes with Static and Scanning WINTWEX-W LiDAR Data

With further development of LiDAR technology wake measurements by use of LiDAR became of common interest in the wind energy community. To study new measurement strategies of scanning and nacelle LiDARs, in combination with already existing measurement principles of static LiDARs, Norcowe conducted in collaboration with the Energy research Centre of the Netherlands (ECN) the Wind Turbine Wake Experiment Wieringermeer (WINTWEX-W). In this study we use data from the static Windcubes V1 to illustrate a proof of concept of wake effects at 1.75 and 3.25 rotor diameter downstream distance. After validating Windcube data against sonic anemometers from the met mast, we compare downstream velocity deficits and turbulence intensities between measurements of static and scanning WindCubes. To further characterize single wind turbine wakes and their frequencies of occurrence we analysed the results in terms of atmospheric stability. Wake measurements are of great importance to further developing tools for optimising wind farm layouts and operations

Turbulent kinetic energy estimates from profiling wind LiDAR measurements and their potential for wind energy applications

This study shows that turbulent kinetic energy (TKE) estimates, derived from static LiDARs in Doppler Beam Swing (DBS) mode, permit a qualitative and quantitative characterization and analysis of turbulent structures as wind turbine wakes, and convective or shear generated eddies in the lower atmospheric boundary layer. The analysed data, collected by a WINDCUBE™ v1 in a wind park in Austria, is compared to WINDCUBE™ v1 and sonic data from the WINd Turbine Wake EXperiment Wieringermeer (WINTWEX-W). Although turbulence measurements with a WINDCUBE™ v1 are limited to a specific length scale, processed measurements above this threshold are in a good agreement with sonic anemometer data. In contrast to the commonly used turbulence intensity, the calculation of TKE not only provides an appropriate measure of turbulence intensities but also gives an insight into its origin. The processed data show typical wake characteristics, as flow decelerations, turbulence enhancement and wake rotation. By comparing these turbulence characteristics to other turbulent structures in the atmospheric boundary layer, we found that convection driven eddies in the surface layer have similar turbulence characteristics as turbine wakes, which makes convective weather situations relevant for wind turbine fatigue considerations

Wind coherence measurement by a single pulsed Doppler wind lidar

A single pulsed Doppler wind lidar, deployed at the FINO1 platform in the North Sea, has been used to monitor the lateral and vertical coherence of the along-wind component. To maximize the sampling frequency of the monitoring system, a particular configuration based on small sweeping angles around the mean wind direction is used. The set-up provides wind velocity measurement with an increasing cross-flow separation with increasing distances from the lidar. We present hereby preliminary results from the analysis of the Plan Position Indicator scans carried out with a sampling frequency of 0.13 Hz, and the Range Height Indicator scans sampled at 0.19 Hz. The mean velocity and the turbulence intensity indicated a sufficiently uniform flow, which was verified by the calculation of the along and crosswind turbulence length scales. The lateral and vertical coherence was estimated based on multiple 10-minutes samples, for a mean wind velocity ranging from 10 to 14 ms1. For the conditions examined, the measured coherence showed a good agreement with the IEC model for low and medium cross-flow separations, and a fairly good agreement with the Frøya model for large separations

Proof of concept for wind turbine wake investigations with the RPAS SUMO

The Small Unmanned Meteorological Observer (SUMO) has been operated in the vicinity of five research turbines of the Energy Research Centre of the Netherlands (ECN) at the test site Wieringermeer. The intention of the campaign was to proof the capability of the system for wind turbine wake investigations also for situations above rated wind speed. In rather high wind conditions of 15-20 ms−1 on May 10, 2014, the system showed a satisfying in-flight behavior and performed five racetrack flights. The racetrack patterns flown parallel to the row of the five turbines (four flights downstream the turbine row, one upstream) enable the characterization and investigation of the strength, i.e. the reduction in the mean wind, and structure, i.e. the horizontal extension and turbulent kinetic energy (TKE) distribution of single turbine wakes