In all new WAsP versions (WAsP8 and on), wind farm and single-turbine productions are based on the so-called emergent distribution (see the definition below), and consequently this distribution is the standard total or all-sector distribution referred to.
This is a change from older versions, where the so-called combined Weibull distribution was the standard all-sector distribution used for production calculations. The use of the emergent all-sector distribution for production calculations has been found to be more accurate – the reason for using the combined distribution in previous WAsP versions was mostly historical (low computer speed).
In most cases, the difference is practically of no importance, but in cases of large sector-wise variations in the wind speed distributions, the difference in calculated production may be non-negligible (up to 5% has been encountered). Please look in Emergent distribution example page to see an illustration of the difference between the different omni-directional or total wind speed distributions.
Definitions of all-sector wind speed distributions
All-sector wind distributions are sometimes also referred to as 'stacked', 'total' or 'omni-directional'.
Fitted Weibull distribution Weibull distribution fitted to an all-sector (stacked) wind speed histogram. Used only in connection with the observed wind climate, i.e. the *.tab and *.owc files.
Emergent distribution The weighted sum of the Weibull distribution functions from all the N direction sectors.
Here fi, Ai and Ki are the wind rose frequency of occurrence, the Weibull scaling and Weibull shape parameters for sector no. i, respectively.
In general, this distribution is NOT a Weibull distribution; e.g. it could be bi-modal in case of two dominant sectors with very different mean speeds. Used in connection with regional (wind atlas) as well as predicted wind climates.
Combined Weibull distribution The Weibull distribution matching the mean speed and power density (1st and 3rd moments) with the weighted sum of the sector-wise mean speeds and power densities, respectively. The combined Weibull distribution has the same mean speed and power density as the emergent distribution. Used in connection with regional as well as predicted wind climates.