Characteristics of the Wind - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 474

frequently come in series and have a periodicity of 4 to 6 days. When a wave passes over
a location, large variations are experienced in both wind speed and wind direction. Despite
this variability, however, most regions of the middle-latitude belts have Westerlies that can
be used for energy conversion, with peaks occurring most commonly in spring and winter
and lulls in summer.
Synoptic-scale motions ( i.e. correlated over a wide geographical region) are associated
with periodic systems, such as traveling waves in the tropical Easterlies or the temperate
Westerlies, or at temperate latitudes. Some parts of these waves have very good wind
potential (Class 5 or better). Typically the area of influence of a travelling wave is of the
order of 1,000 to 1,500 km, with a time scale of about 2 to 4 days.
Mesoscale wind systems can be associated either with traveling disturbances (such as
squall lines) or with topographical features (such as valleys and coastal areas). Squall lines
are generally convective systems that consist of several convective cells of the cumulo-
nimbus type. Squall-line winds can be very violent and destructive, and may not always
be of value for wind energy conversion. Mesoscale winds caused by differential heating
of topographical features are generally referred to as breezes. A breeze is similar to a
monsoon, but it operates on much smaller scales, typically a few hundred kilometers and
a few hours. In many areas breezes are a regular daily occurrence and, therefore, are of
great value as a wind energy resource, especially when they enhance the existing basic
wind.
Convective-scale motion is associated with vertical activity in the lower atmosphere,
especially in connection with cumulus clouds. Since the scales of convective flow are a
few kilometers and minutes to a couple of hours, this motion of the air does not contribute
significantly to the wind energy resource. An exception to this is the condition where
topographic lifting occurs on the windward side of a mountain. The convective activity that
may result could keep local circulations going for several hours. In regions where the
winds are prevailing from one direction ( e.g. , the trades) this phenomenon may repeat itself
from day to day, and the enhanced low-level winds may contain sufficient energy to be
extracted and used.
Time Variations
It has been pointed out that the motions of the atmosphere vary over a wide range of
time scales (seconds to months) and space scales (meters to thousands of kilometers), and
that these time and space scales are related. In this section, time variation of the wind is
discussed in general terms. Statistical methods and data are described later.
Long-Term Variability
The first concern about a site that is under consideration for a wind power station is
with the long-term mean wind speed. Can the winds be counted on to “fuel” cost-effective
power production over many years? What is the year-to-year variability at this site? What
period of wind speed measurement at the site is adequate to establish a reliable estimate of
the long-term mean wind speed? Wind power station operators have indicated that the
ability to estimate the interannual variability at a site is almost as important as estimating
its annual mean wind speed. The complexity of the interactions of the meteorological and
topographical factors that cause the mean wind to vary from one year to the next have ham-
pered the development of a reliable prediction method.
One statistically-developed rule of thumb is that one year of record is generally sufficient
to predict long-term seasonal mean wind speeds to within an accuracy of 10 percent with a
confidence level of 90 percent [Corotis 1977]. A study that compared seven different methods