Environmental Engineering Reference
In-Depth Information
height; this is true up to the height of the boundary layer, which is at approximately 1000
metres, but depends on atmospheric conditions. The change of wind speed with height is
known as the wind shear.
It is clear from this that the available resource depends on the hub height of the turbine.
This has increased over recent years, refl ecting the scaling-up of wind turbine technology,
with the hub heights of the multimegawatt machines now being over 100 m.
The European accessible onshore wind resource has been estimated at 4800TWh/year
taking into account typical wind turbine conversion effi ciencies, with the European offshore
resource in the region of 3000TW h/year although this is highly dependent on the assumed
allowable distance from shore. A recent report suggests that by 2030 the EU could be gener-
ating 965 TW h from onshore and offshore wind, amounting to 22.6% of electricity require-
ments [4]. The world onshore resource is approximately 53 000 TW h/year, taking into account
siting constraints. To see these fi gures in context note that the UK annual electricity demand
is in the region of 350 TW h and the USA demand is 3500 TW h. No fi gure is currently avail-
able for the world offshore resource, and this itself will be highly dependent on the allowable
distance from shore.
Of the new renewables wind power is the most developed. On very windy sites wind farms
can produce energy at costs comparable to those of the most economic traditional generators.
Due to advances in technology, the economies of scale, mass production and accumulated
experience, over the next decade wind power is the renewable energy form likely to make
the greatest contribution to electricity production. As a consequence, more work has been
carried out on the integration of this resource than any of the other renewables and, naturally,
this is refl ected in the amount of attention given to wind power integration in this topic.
2.4.2 Wind Variability
The wind speed at a given location is continuously varying. There are changes in the annual
mean wind speed from year to year ( annual ) changes with season ( seasonal ), with passing
weather systems ( synoptic ), on a daily basis ( diurnal ) and from second to second ( turbulence ).
All these changes, on their different timescales, can cause problems in predicting the overall
energy capture from a site (annual and seasonal), and in ensuring that the variability of energy
production does not adversely affect the local electricity network to which the wind turbine
is connected.
In Figure 2.3 each graph shows the wind speed over the time periods indicated. Wind speed
measured continuously over 100 days is shown on the fi rst graph followed by graphs, which
in sequence zoom in on smaller and smaller windows of the series. It is easy to see the much
larger relative variability in the longer time series (synoptic) as compared with the time series
covering hours or less (diurnal, turbulence). This information is summarized in the spectral
density presentation in Figure 2.4. In a spectral density function the height indicates the
contribution to variation (strictly the variance) for the frequency indicated. A logarithmic
scale as used here is the norm, and allows a very wide range of frequencies/timescales to be
represented easily. The y axis is scaled by n to preserve the connection between areas under
any part of the curve and the variance. The area under the entire curve is the total variance.
It can be seen that the largest contribution to variation is the synoptic variation, confi rming
the interpretation of Figure 2.3. Fortunately these variations, characterized by durations of
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