Environmental Engineering Reference
In-Depth Information
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Figure 2.10
UK onshore wind power correlation by distance between sites based on UK long term
averages. (Reproduced from Sinden, G.E., 2007, DPhil Thesis with permission of Environmental
Change Institute, Oxford University Centre for the Environment)
the 72 hour recorded period. Variability now becomes signifi cant and effective integration
of wind power in an electrical power network would have been problematic if the
total
resource were to fl uctuate in sympathy with the fl uctuations in one site. Fortunately this is
not the case due to the geographic diversity of the sites. The benefi ts of geographic diversity
are clearly illustrated by the bold line in Figure 2.9, which represents the average wind speed
from all sites. This average is substantially smoother (i.e. exhibits much reduced variability)
than the wind at the individual wind farm sites.
With large scale exploitation of the wind resource, wind farms are installed inevitably
across a variety of geographically dispersed sites. As suggested above, this has a major ben-
efi cial effect in terms of overall variability. Just as the output from a wind farm has less short
term variability than a single wind turbine due to their dispersion across the site, so the
aggregate output from several geographically dispersed wind farms has less longer term vari-
ability than the output from a single wind farm. This refl ects the fact that distant localities
experience variations in wind due to shifting weather patterns that are time shifted in relation
to one another, and also to an extent distinct. Figure 2.10 presents the correlation between
pairs of onshore wind sites in the UK as a function of the distance between the sites, and
demonstrates that sites very far apart exhibit low cross-correlation. The data from Reference
[11] were recorded over a period of 15-20 years.
Seasonal Variability
Seasonal and monthly average wind speeds vary signifi cantly over most of the world. Figure
2.11 shows the seasonal changes of monthly averaged wind speeds for Billings, Montana in
the USA. The trend of higher wind speeds during winter compared to summer is typical of
the Northern hemisphere. The fi gure also indicates that there is variability from year to
year.
