Morten Nielsen

Can Satellite Sampling of Offshore Wind Speeds Realistically Represent Wind Speed Distributions? Part II: Quantifying Uncertainties Associated with Distribution Fitting Methods

Abstract Remote sensing tools represent an attractive proposition for measuring wind speeds over the oceans because, in principle, they also offer a mechanism for determining the spatial variability of flow. Presented here is the continuation of research focused on the uncertainties and biases currently present in these data and quantification of the number of independent observations (scenes) required to characterize various parameters of the probability distribution of wind speeds. Theoretical and empirical estimates are derived of the critical number of independent observations (wind speeds derived from analysis of remotely sensed scenes) required to obtain probability distribution parameters with an uncertainty of ±10% and a confidence level of 90% under the assumption of independent samples, and it is found that approximately 250 independent observations are required to fit the Weibull distribution parameters. Also presented is an evaluation of Weibull fitting methods and determination of the fitting...

Remote Sensing Observation Used in Offshore Wind Energy

Remote sensing observations used in offshore wind energy are described in three parts: ground-based techniques and applications, airborne techniques and applications, and satellite-based techniques and applications. Ground-based remote sensing of winds is relevant, in particular, for new large wind turbines where meteorological masts do not enable observations across the rotor-plane, i.e., at 100 to 200 m above ground level. Light detection and ranging (LiDAR) and sound detection and ranging (SoDAR) offer capabilities to observe winds at high heights. Airborne synthetic aperture radar (SAR) used for ocean wind mapping provides the basis for detailed offshore wind farm wake studies and is highly useful for development of new wind retrieval algorithms from C-, L-, and X-band data. Satellite observations from SAR and scatterometer are used in offshore wind resource estimation. SAR has the advantage of covering the coastal zone where most offshore wind farms are located. The number of samples from scatterometer is relatively high and the scatterometer-based estimate on wind resources appears to agree well with coastal offshore meteorological observations in the North Sea. Finally, passive microwave ocean winds have been used to index the potential offshore wind power production, and the results compare well with observed power production (mainly land-based) covering nearly two decades for the Danish area.

Wind Class Sampling of Satellite SAR Imagery for Offshore Wind Resource Mapping

Abstract High-resolution wind fields retrieved from satellite synthetic aperture radar (SAR) imagery are combined for mapping of wind resources offshore where site measurements are costly and sparse. A new sampling strategy for the SAR scenes is introduced, based on a method for statistical–dynamical downscaling of large-scale wind conditions using a set of wind classes that describe representative wind situations. One or more SAR scenes are then selected to represent each wind class and the classes are weighted according to their frequency of occurrence. The wind class methodology was originally developed for mesoscale modeling of wind resources. Its performance in connection with sampling of SAR scenes is tested against two sets of random SAR samples and meteorological observations at three sites in the North Sea during 2005–08. Predictions of the mean wind speed and the Weibull scale parameter are within 5% from the mast observations whereas the deviation on power density and the Weibull shape paramete...

Validation of ERS-2 SAR offshore wind-speed maps in the North Sea

Wind maps are retrieved from ERS-2 Synthetic Aperture Radar (SAR) scenes by the CMOD-IFR2 and CMOD4 algorithms for 61 cases at the Horns Rev site in the North Sea and compared to meteorological in situ observations from a mast located 14u2009km offshore. The in situ data are corrected for flow distortion and sea-level changes prior to validating the SAR wind maps. The SAR wind maps are area-averaged by a simple footprint method assuming neutral stability and with three nonlinear weighting footprint methods including correction for stability. From a physical point of view, the latter is more correct. However, between in situ and SAR-derived wind-speed estimates comparison results of the nonlinear footprint values are statistically less correlated (R 2=0.73–0.77) and the standard error (SE) is larger (>1.5u2009mu2009s−1) than results from the simple footprint (R 2=0.78–0.80 and SE=1.3u2009mu2009s−1). The results are found with wind direction determined from wind streaks in the SAR images by Fast Fourier Transform. Using in situ wind direction as input to the CMOD-IFR2 and CMOD4 algorithms yields even better linear regression results, e.g. for the simple footprint method R 2=0.88 and SE=0.9u2009mu2009s−1. SAR wind maps may be useful for mapping of future offshore wind resources.

Field experiments with dispersion of pressure liquefied ammonia

Abstract This paper presents dispersion experiments with continuous releases of liquefied ammonia carried out during the CEC ENVIRONMENT project “Fladis Field Experiments”. The source was a horizontal flash boiling jet with release rates of 0.25–0.5 kg s−1. The objective was to study the dispersion in all its stages at the source, in the heavy jet with aerosols, the slightly stabilized plume, and further downstream into the regime of passive dispersion. The concentration field is analyzed in a fixed frame of reference as well as a frame of reference moving with the instantaneous plume centre-line. Empirical probability functions and a spatial correlation of the concentration fluctuations are found, and the enthalpy balance of the cold heavy jet and the aerosol composition are evaluated.

An evaluation of validation procedures and test parameters for dense gas dispersion models

Abstract This paper reports the evaluation of dispersion models with respect to their ability to predict the dispersion of continuous dense gas plumes over flat terrain. The exercise has been carried out in order to derive guidelines for statistical model evaluation exercises. Therefore, the models selected for the evaluation are simple: the Britter and McQuaid model and three variations of the Gaussian Plume Model. Two different evaluation procedures have been used: one using short time-averaged concentrations paired in time and space and another using observed maximum concentrations at certain distances together with a measure of the plume width. The performance numbers and ranking of the models depend substantially on the data sets used and the evaluation procedure and performance number selected. The two evaluation procedures have both advantages and disadvantages. An evaluation based on maximum concentration should not be performed without taking into account the plume dimensions. The evaluation using data paired in time and space suffers from the inability of the models and the data analysis techniques to match the predicted plume path with the actual plume path during the experiment. We suggest that two newly defined performance measures be used, the Mean Relative Bias and the Mean Relative Square Error, together with the well-known ‘Factor-of-2’ statistic in future statistical evaluation exercises. One should use at least two performance measures in an evaluation exercise in order to obtain a measure of ‘bias’ and ‘variance’. Furthermore, we advise that graphical presentations to obtain an understanding of the model behaviour be used.

Heat transfer in large-scale heavy-gas dispersion.

Heavy-gas dispersion of practical interest is usually cold gas dispersion with the enthalpy deficit as the main cause of the density effect. New analysis of existing field experiment data suggests that heat transfer from the ground sometimes reduces this thermally induced density effect considerably. The limited heat capacity of the ground implies that heat transfer to a gas plume must disappear eventually, and our interpretation of Desert Tortoise measurements indicates that the surface heat flux decreased by 38% during a 3-min long release period.

A method for spatial interpolation of wind climatologies

The local wind climate is described by probability distributions for the wind direction and for the wind speed given a certain direction. As an alternative to the latter, the wind speed may be described by its statistical moments which are easily integrated over all directions. This allows representation of the wind climatology as a set of directionally independent statistics and the corresponding normalized directional distributions often called wind roses. The methodology is to spatially interpolate wind climatologies in an irregular mesh by two-dimensioanl third-order Bezier polynomials for the directionally independent statistics and linear interpolation for the wind roses. Local Bezier surfaces are set up in a patchwork of triangles and adjusted for approximate first-order geometric continuity (GC1) and minimal surface curvature. The wind rose interpolation is calculated relative to a typical local wind direction by means of Fourier transformation. A complementary surface extrapolation method is suggested which retains continuity on the grid boundary and asymptotically linear variation away from this. Copyright

Dense gas field experiments with obstacles

Abstract A recent dense gas dispersion project is presented, with emphasis on the field experiments. During these experiments we have studied the effect of obstacles on continuous releases of liquefied propane. A solid wall perpendicular to the wind direction was observed to have a general diluting effect on the dense gas cloud, but at some positions above ground level the concentration was intensified in the presence of the obstacle as a result of increased cloud height. Propane concentrations can be derived from sonic anemometer measurements using a new technique, and the paper has a brief discussion of turbulence inside the gas cloud, including gas flux and concentration fluctuations. Two gas sources were used — a nozzle and a cyclone producing releases with and without initial momentum. The jet from the nozzle was found to have an important effect on the dispersion, producing a lot of turbulence and increasing the vertical gas flux.

Research on continuous and instantaneous gas clouds. Part 2

Abstract Interim results are presented on the dense gas field experiment supported under the Major Technological Hazards Programme of the Commission of the Euro