Extent to which international wind turbine design standard, IEC61400-2 is valid for a rooftop wind installation

Abstract

The use of small grid-connected wind turbines is increasing within the built environment, yet atmospheric turbulence in this environment is more complex than the open terrain sites on which the turbine design standards are based. The current IEC61400-2 design standard uses stochastic turbulence models adapted from the von Karman and Kaimal power spectra in order to simulate flow fields that are used by designers to predict structural loading on small wind turbines. Both spectra are based on observations in the atmospheric surface layer developed over flat, smooth and uniform terrain yet IEC61400-2 does not offer any modifications of the spectra for more complex terrain such as that which exists in the built environment. This paper investigates the extent to which the von Karman and Kaimal models, as presented in IEC61400-2, are appropriate for use in the design of SWTs installed on the rooftop of a warehouse in the built environment. In particular the paper attempts to gauge how different the turbulence spectra currently used for turbine design are from the actual inflow conditions experienced by the turbines on the roof.The power spectra of all three wind components in neutral and slightly unstable atmospheric conditions at four heights above the rooftop are considered. A degree of misfit function was used to compare von Karman, Kaimal and measured power spectra, as an indicator of model suitability. A sensitivity study was carried out to assess the influence of turbulence length scale and wind direction on the results. The Kaimal spectral function was the better of the existing models in predicting the trends of all wind components and was used as a starting point in developing an approach to modelling turbulence power spectra for a rooftop site in the built environment by incorporating typical length scales at the site.