Far offshore wind conditions in scope of wind energy

Far offshore atmospheric conditions are favourable for wind energy purposes since mean wind speeds are relatively high (i.e., high power production) while turbulence levels are relatively low (i.e., less fatigue loads) compared to onshore conditions. Offshore wind energy, however, is still expensive compared to onshore wind energy. There is little known about exact offshore wind conditions due to the costs involved in far offshore atmospheric measurement campaigns. This causes unnecessary conservatism in wind turbine design and suboptimal wind turbine performance, which subsequently results in an increase in the cost of energy of offshore wind energy. The aim of this PhD research is twofold. First far offshore atmospheric conditions relevant for wind energy are studied from a meteorological point of view, which should result in a comprehensive, accurate and implementable description of offshore atmospheric conditions for wind energy purposes. Second, the influence of the specified atmospheric description on wind turbine performance is studied in more detail. This should result in fundamental insight how offshore wind turbines are influenced by atmospheric conditions. Results can then aid in reducing the cost of offshore wind energy if they are implemented correctly. Atmospheric conditions relevant for wind energy have been studied, and known theoretical relations have been validated, based on measurements from a recently constructed meteorological mast sited 85 km offshore in the Dutch North Sea. In this thesis it is decided to emphasize on frequently occurring conditions that are relevant for wind energy, namely wind shear (the change in wind speed with height) and turbulence (the change in wind speed in time). It is found that a general framework can be used in which wind shear and turbulence characteristics are coupled as a function of atmospheric stability, which is a measure of the vertical temperature gradient. The resulting wind profile however, is only valid relatively close to the surface, which poses problems given the size of state of the art wind turbines. A new wind profile that is valid for the entire atmospheric boundary layer has therefore been derived based on theoretical grounds and it has been validated with observation data. The proposed wind profile is found to provide a better representation of offshore wind shear, especially if the atmosphere is stably stratified. It is recognised that the majority of shear profiles used in wind energy are not capable of describing situations with a local wind maximum. Such phenomena, called low-level jets, are known to occur often onshore at night, but the offshore occurrence is not studied frequently in absence of detailed offshore observation data. The occurrence and characteristics of low-level jets offshore have therefore been studied with aid of the new offshore meteorological mast. It is found that low-level jets occur frequently and with characteristics relevant for wind energy purposes (with respect to the maximum wind speed and height of the wind maximum). The atmospheric knowledge obtained is used for wind energy applications with the aid of numerical simulation software and a reference wind turbine frequently used in research. Adopting the general framework including atmospheric stability in wind turbine fatigue load assessment shows that guidelines, which typically do not consider atmospheric stability, cause an overestimation of simulated wind turbine fatigue loads. This is not necessarily caused by conservatism in either wind shear or the amount of turbulence, but by the lack of a coupling between both conditions. This coupling, which occurs in reality, should be adopted in wind turbine design as well. In wind energy resource assessment one thus also has to observe atmospheric stability conditions accurately. Next, the impact of wind shear and turbulence on wind turbine performance is assessed separately. Although this approach is not representative for actual offshore conditions, in which wind shear and turbulence are coupled, it does provide fundamental insight in wind turbine performance as a function of specific atmospheric conditions. With respect to wind shear, it is found that the validated boundary layer wind shear profile substantially influences wind turbine power production as well as fatigue loads experienced by wind turbine blades. Especially for stable conditions, where the boundary layer profiles deviate most from simple diabatic wind profiles, fatigue loads are reduced by more than 10%. At the same time power production is reduced by up to 2% for stable conditions. In absence of a simple model that can be used to describe low-level jets, it is decided to first formulate a new empirical low-level jet wind model that can easily be implemented for wind energy purposes. Subsequently the influence of low-level jets on wind turbine performance is assessed, where it is found that low-level jets can have a substantial influence depending on the jet characteristics. If the jet occurs exactly at hub height, both power production as well as blade root bending moments reduce substantially. Finally, the influence of specific turbulence scales is studied in detail with aid of numerical simulations. In these simulations turbulence is filtered in such a way that only specific scales remain present. This allows a detailed study in which turbulence scales are relevant for wind energy performance. The results provide insight in modelling of wind conditions and show which turbulence scales have to be incorporated into wind turbine simulations. It is found that the reference wind turbine is hardly influenced by turbulence kinetic energy present in very large and very small scales. The structures with a similar order of magnitude as the turbulence length scale contain a lot of turbulence kinetic energy, and the reference wind turbine is able to convert part of the turbulence kinetic energy into power. Out of all turbulence scales, those structures with a similar order of magnitude to the turbulence length scale also contribute most to wind turbine fatigue loads. The results obtained in this research are primarily theoretical, and they provide more fundamental insight into offshore atmospheric conditions and the subsequent performance of a wind turbine. If properly implemented however, it is possible that wind turbines can be designed with less conservatism, which directly reduces the cost of offshore wind energy. Besides, numerous results are useful to improve the accuracy of resource assessment and wind power forecasting, both contributing to the cost of energy as well. It shows the need to approach offshore wind energy as an interdisciplinary field in which meteorologists and engineers collaborate to optimize wind turbine performance.