Environmental Engineering Reference
In-Depth Information
system. However, when wind turbines are clustered in wind farms, there is physical spacing
between them and the turbulence seen by each wind turbine is different and to a great extent
uncorrelated. The electrical output from wind farms therefore exhibits substantially lower
relative variability than that from a single wind turbine. At the planning stage, appropriate
analytical studies are carried out to ensure that the variability expected from a wind farm at
a particular site will not adversely affect the power system.
Variability from Minute to Minute
Figures 2.3 and 2.4 indicate that the character of wind is such that if the second to second
turbulence is removed, the average wind speed from 10 minute period to 10 minute period
remains effectively constant. In Chapter 3 will be shown that this 'persistence' nature of such
averaged wind speeds is particularly important in integrating wind generated electricity in
power networks. In practice the output of turbines can be regarded as uncorrelated on the
timescale of minutes and as with the faster variations the affect of aggregation is to smooth
out variations at these higher frequencies.
Variability from Hour to Hour and from Day to Day
Figure 2.9 shows the actual records of wind speed at 13 geographically dispersed wind sites
in the UK. As expected from Figure 2.4, there is substantial variability at each location over
Figure 2.9
UK site specifi c and average hourly wind speed over 72 hours, with 50 or more records
per hour. (Reproduced from Sinden, G.E., 2007, DPhil Thesis with permission of Environmental
Change Institute, Oxford University Centre for the Environment)
