ISBN 978-3-8439-2373-6

978-3-8439-2373-6, Reihe Energietechnik

Martin Dörenkämper
An investigation of the atmospheric influence on spatial and temporal power fluctuations in offshore wind farms

168 Seiten, Dissertation Carl von Ossietzky Universität Oldenburg (2015), Softcover, B5

Zusammenfassung / Abstract

For a further development of offshore wind energy exploration a profound understanding of the interaction of the marine atmospheric boundary layer with wind energy systems is needed. This study aims at increasing this understanding by investigating spatial and temporal

fluctuations of wind and power in offshore wind farms based on numerical simulations as well as on long-term meteorological measurement and wind farm production data.

A comparison of wind and stability measures derived from meteorological tower data above the North and Baltic Sea demonstrates that the wind conditions are dominated by the influence of an upstream coast if the fetch is in the order of 100 km or less. Consequently, the wind and stability conditions above the Baltic Sea show a distinct diurnal cycle advected from the land, which can be observed to a reduced extent above the North Sea as well. Mesoscale simulations in the coastal zone reveal a strong induced stable stratification when warm inland air flows over the colder sea. Heterogeneities in the land use along the coastline lead to intense horizontal streaks in the wind field which are advected several tens of kilometres onto the open sea.

The investigation of power curves classified by means of atmospheric stability parameters shows a strong dependency on these variables. For the same inflow wind speed power differences between stable and unstable stratification of up to 20 % are observed. By studying the directional variation of the power deficit in an offshore wind farm, the influence of the fetch on the wind farm flow can be measured.

Large-eddy simulations of a coastal discontinuity upstream of a wind farm show the increase of the wind speed with increasing distance to the coast. This increase continues over the extension of a wind farm and thus leads to inhomogeneous inflow conditions at the turbines. An investigation of the impact of the atmospheric stability on wind farm wakes revealed that shape, direction and widening of the wakes are sensitive to this parameter. In non-neutrally stratified situations, the turbine induced mixing leads to a neutralisation of the stratification.

The study shows that a careful modelling of the wind resource for offshore wind farms is needed due to a far offshore extension of the impact of the coast. The methods presented could be utilised for local power forecasting inside wind farms and maintenance planning to reduce downtime costs.