WAsP Engineering extreme winds

WAsP Engineering predicts extreme wind climates at turbine sites by observed extreme wind climates (OEWC) based on measurements at reference masts. This is done by the deflection and speedup factors calculated by the WEng flow model. In the absence of local reference measurements of sufficient duration, a remote reference site is used. The OEWC from the reference site is converted to a regional extreme wind climates (REWC) which is  representative for a larger region. This REWC contains extreme wind speeds for standard conditions, i.e. 10 m above uniform flat terrain with a uniform surface roughness. The conversion is done by the Geostrophic drag law – similar to the way WAsP converts mean wind climates to wind atlas (regional wind climate) files. The REWC is stored in a file and used as input in a second WEng project modelling wind conditions at the planned wind farm.

The OEWC is calculated by the WAsP Climate analyst shipped with both WAsP and WEng. Sometimes it is hard to find reference data, even for a large region. As a substitute, and only for some parts of the world, Risø has calculated REWC files by NCEP/NCAR reanalysis data.

The WEng extreme wind model is based on the Gumbel distribution and fitted by annual 10-minute maxima using probability-weighted moments. This allows extrapolation to longer return periods (blue line in below plot) with uncertainty estimates (grey lines). The flow model predicts site-specific annual maxima at the selected sites, heights and wind direction sectors, and then the Gumbel fit is derived. The uncertainties will be large if the Gumbel slope parameter (alpha) is large and if the number of observation years are too few. The IEC 61400-1 standard recommends at least seven years.

Extreme wind predictions for a local site.

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