Browsing by Author "Hofsäß, Martin"

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Open Access
Field testing of feedforward collective pitch control on the CART2 using a nacelle-based lidar scanner
(2012) Schlipf, David; Fleming, Paul; Haizmann, Florian; Scholbrock, Andrew; Hofsäß, Martin; Wright, Alan; Cheng, Po Wen
This work presents the first results from a field test to proof the concept of LIDAR assisted collective pitch control using a scanning LIDAR device installed on the nacelle of a research turbine. The purpose of the campaign was to show that a reduction of rotor speed variation is feasible with a feedforward update without changing the feedback controller. Although only a small amount of data could be collected, positive effects can be observed not only on the rotor speed but also on tower, blade and shaft loads in the case that the correlation of the wind preview and the turbine reaction is taken into account.
Open Access
Flying UltraSonic - a new way to measure the wind
(2020) Hofsäß, Martin; Bergmann, Dominique; Denzel, Jan; Clifton, Andrew; Cheng, Po Wen
Measurements of flow conditions with tall meteorological measurement masts at complex sites are expensive and can only be carried out with great effort. Concepts and new measuring methods are needed to assess these sites. This work aims to validate the performance of a measuring system based on UAV in complex terrain using on-site measurement. An unmanned aerial vehicle (UAV), more precisely a helicopter, was equipped with a standard 3-D ultrasonic anemometer. This UAV was positioned closed to a meteorological measuring mast and remained stationary at a constant altitude to measure the wind speed components. The data of the UAV were compared with a sensor installed on the measurement mast. The measurements shows a good agreement with an absolute deviation of 0.004 m/s and a relative deviation of 0.047 % for the horizontal wind speed. In the frequency domain the PSDs of the wind components u, v, w match the theoretical spectrum f^(-5/3) for the inertial subrange very well. With further improvements, this UAV equipped with a 3-D ultrasonic anemometer could be a very effective measurement tool for atmospheric research.
Open Access
Lidars and wind turbine control
(2010) Schlipf, David; Bischoff, Oliver; Hofsäß, Martin; Rettenmeier, Andreas; Trujillo, Juan José; Kühn, Martin
Reducing mechanical loads caused by atmospheric turbulence and energy optimization in the presence of varying wind are the key issue for wind turbine control. In terms of control theory changes in the inflowing wind field as gusts, varying shears and directional changes represent unknown disturbances. However, conventional feedback controllers can compensate such excitations only with a delay since the disturbance has to be detected by its effects to the turbine. This usually results in undesired loads and energy losses of wind turbines. From the control theory point of view disturbance rejection can be improved by a feedforward control if the disturbance is known. Not fully covered by theory, but used in practice is the further advantage of knowing the disturbance in the future, e.g. in chassis suspension or in daily life when vision is used to circumnavigate obstacles with a bicycle. In a similar way wind field measurements with remote sensing technologies such as Light Detection and Ranging (LIDAR) might pave the way for predictive wind turbine control strategies aiming to increase energy yield and reduce excessive loads on turbine components. Remote sensing offers wind speed tracking at various points in space and time in advance of reaching the turbine and before hitting sensors at the blades or nacelle. This provides the control and safety system with sufficient reaction and processing time.
Open Access
Model based wind vector field reconstruction from lidar data
(2012) Schlipf, David; Rettenmeier, Andreas; Haizmann, Florian; Hofsäß, Martin; Courtney, Mike; Cheng, Po Wen
In recent years lidar technology found its way into wind energy for resource assessment and control. For both fields of application it is crucial to reconstruct the wind field from the limited information provided by a lidar system. For lidar assisted wind turbine control model based wind field reconstruction is used to obtain signals from wind characteristics such as wind speed, direction and shears in a high temporal resolution. This work shows how these methods can be used for lidar based wind resource assessment in complex situations, where high accuracy is important, but cannot be archived by conventional technique. The reconstruction is validated for ground based lidar systems with measurement data and for floating lidar systems with detailed simulations.
Open Access
Prospects of optimization of energy production by LIDAR assisted control of wind turbines
(2011) Schlipf, David; Kapp, Stefan; Anger, Jan; Bischoff, Oliver; Hofsäß, Martin; Rettenmeier, Andreas; Kühn, Martin
In the presented work two approaches to increase the energy production of wind turbines are studied assuming the usage of a wind speed measurement provided by a nacelle based LIDAR system: The first approach uses the knowledge of the incoming wind speed to assist variable speed control. The second approach uses the wind direction information measured by a LIDAR system for yaw control. From this first analysis only marginal benefit can be gained by the LIDAR assisted speed control, but an increase of energy production by a couple of percent can be expected by LIDAR assisted yaw control.
Open Access
Reducing the uncertainty of lidar measurements in complex terrain using a linear model approach
(2018) Hofsäß, Martin; Clifton, Andrew; Cheng, Po Wen
In complex terrain, ground-based lidar wind speed measurements sometimes show noticeable differences compared to measurements made with in-situ sensors mounted on meteorological masts. These differences are mostly caused by the inhomogeneities of the ﬂow ﬁeld and the applied reconstruction methods. This study investigates three different methods to optimize the reconstruction algorithm in order to improve the agreement between lidar measurements and data from sensors on meteorological masts. The methods include a typical velocity azimuth display (VAD) method, a leave-one-out cross-validation method, and a linear model which takes into account the gradients of the wind velocity components. In addition, further aspects such as the inﬂuence of the half opening angle of the scanning cone and the scan duration are considered. The measurements were carried out with two different lidar systems, that measured simultaneously. The reference was a 100 m high meteorological mast. The measurements took place in complex terrain characterized by a 150 m high escarpment. The results from the individual methods are quantitatively compared with the measurements of the cup anemometer mounted on the meteorological mast by means of the three parameters of a linear regression (slope, offset, R2) and the width of the 5th–95th quantile. The results show that expanding the half angle of the scanning cone from 20◦ to 55◦ reduces the offset by a factor of 14.9, but reducing the scan duration does not have an observable beneﬁt. The linear method has the lowest uncertainty and the best agreement with the reference data (i.e., lowest offset and scatter) of all of the methods that were investigated.
Open Access
Statistical load estimation using a nacelle-based lidar system
(2010) Bischoff, Oliver; Hofsäß, Martin; Rettenmeier, Andreas; Schlipf, David; Siegmeier, Björn
The paper presents the results of statistical load analyses based on data measured at the 5MW AREVA Wind M5000 onshore prototype. Measurements with standard meteorological measurement devices are analysed and compared to measurements with a pulsed LIDAR system which is enhanced with a multi-purpose scanning device installed on the top of the nacelle of the turbine. Based on these measurements statistical summaries of relevant meteorological parameters have been used for normative procedures to calculate the mechanical loads which occur at the wind energy turbine. It could be verified that LIDAR systems can substitute standard measurement devices for a load estimation of wind energy turbines.
Open Access
Testing of frozen turbulence hypothesis for wind turbine applications with a scanning LIDAR system
(2011) Schlipf, David; Trabucchi, Davide; Bischoff, Oliver; Hofsäß, Martin; Mann, Jakob; Mikkelsen, Torben; Rettenmeier, Andreas; Trujillo, Juan José; Kühn, Martin
Taylor’s frozen turbulence hypothesis is tested in its applicability for wind turbine applications. In this research full field measurements are performed at a test site for multi-megawatt wind turbines by means of a pulsed LIDAR with a scanning device. The system is installed at the top of the nacelle of a 5MW wind turbine. It provides simultaneous wind speed, with a maximum sampling rate of 5 Hz, at different stations parallel to the mean wind. Measurements in a range between 0.4 and 1.6 rotor diameter are performed following several two and three dimensional trajectories. The spectral characteristics of measurements taken simultaneously at different separation distances are studied. The scanning strategy which maximizes the wavenumber region where results are consistent with Taylor’s hypothesis is assessed. The best results are achieved by a horizontal sliding trajectory with valid wavenumbers up to 0.125 rad/m.