WIND ENERGY SYSTEMS - Cd3wd

Chapter 2—Wind Characteristics 2–62
year and 100 year curves are distinctly different, however. The once a century extreme wind
in Western Kansas, Oklahoma, and Texas is about 40 m/s, a figure exceeded in large areas of
the coastal regions. The Gulf Coast and Atlantic Coast, as far north as Southern Maine, have
experienced extreme winds from tropical cyclones, or hurricanes. The effect of these storms
often extends inland from 100 to 200 miles. The remainder of the United States experiences
extreme winds largely from thunderstorms. This type of storm accounts for over one third of
the extreme-wind situations in the contiguous United States.
Water areas have a marked effect on extreme wind speeds. Where a location has unobstructed
access to a large body of water, extreme winds may be 15 m/s or more greater than
a short distance inland. High winds in cyclones and near water tend to remain steady over
longer periods than for thunderstorms. They also tend to be much more widespread in a given
situation and hence cause widespread damage, although damage to individual structures may
not be greater than from thunderstorm winds.
The speed of the greatest gust experienced by a wind turbine will be somewhat greater
than the speed of the fastest mile because of the averaging which occurs during the period
of measurement. This will vary with the period chosen for the gust measurement and on the
speed of the fastest mile. It appears that a good estimate of a three second gust speed is 1.25
times the speed of the fastest mile[15]. That is, if the fastest mile is measured at 40 m/s, the
fastest gust will probably be about 1.25(40) = 50 m/s. This is the speed which should be
used in talking about survivability of wind turbines.
Standard civil engineering practice calls for ordinary buildings to be designed for a 50
year recurrence interval, and for structures whose collapse do not threaten human safety to
be designed for a 25 year recurrence interval. The civil engineers then typically add safety
factors to their designs which cause the structures to actually withstand higher winds. A
similar approach would seem appropriate for wind turbines[15]. A 50 year recurrence interval
would imply that a design for a maximum gust speed of 50 m/s would be adequate over
most of the United States, with perhaps 65 m/s being desirable in hurricane prone areas.
Wind turbine costs tend to increase rapidly as the design speed is increased, so rather careful
cost studies are required to insure that the turbine is not over designed. Wind turbines are
different from public buildings and bridges in that they would normally fail in high winds
without people getting hurt. It may be less expensive to replace an occasional wind turbine
with a design speed of 50 m/s than to build all wind turbines to withstand 65 m/s.
11 PROBLEMS
1. What is the density of dry air in kg/m 3 at standard pressure (101.3 kPa) and
(a) T =35 o C
(b) T =-25 o C?
Wind Energy Systems by Dr. Gary L. Johnson November 20, 2001