Fundamentals of wind energy - Wind Power Generation and Wind Turbine Design - page 12

4.2 Wind characteristics
Wind varies with the geographical locations, time of day, season, and height above
the earth's surface, weather, and local landforms. The understanding of the wind
characteristics will help optimize wind turbine design, develop wind measuring
techniques, and select wind farm sites.
4.2.1 Wind speed
Wind speed is one of the most critical characteristics in wind power generation.
In fact, wind speed varies in both time and space, determined by many factors
such as geographic and weather conditions. Because wind speed is a random
parameter, measured wind speed data are usually dealt with using statistical
methods.
The diurnal variations of average wind speeds are often described by sine waves.
As an example, the diurnal variations of hourly wind speed values, which are the
average values calculated based on the data between 1970 and 1984, at Dhahran,
Saudi Arabia have shown the wavy pattern [18]. The wind speeds are higher in
daytime and the maximum speed occurs at about 3 p.m., indicating that the day-
time wind speed is proportional to the strength of sunlight. George et al. [ 19 ]
reported that wind speed at Lubbock, TX is near constant during dark hours, and
follows a curvilinear pattern during daylight hours. Later, George et al. [ 20 ] have
demonstrated that diurnal wind patterns at fi ve locations in the Great Plains follow
a pattern similar to that observed in [19].
Based on the wind speed data for the period 1970-2003 from up to 66 onshore
sites around UK, Sinden [21] has concluded that monthly average wind speed is
inversely propositional to the monthly average temperature, i.e. it is higher in the
winter and lower in the summer. The maximum wind speed occurs in January and
the minimum in August. Hassanm and Hill have reported that the month-to-month
variation of mean wind speed values over the period of 1970-1984 at Dhahran,
Saudi Arabia has shown the wavy pattern [13]. However, because the variation in
temperature at Dhahran is small over the whole year, there is no a clear correlation
between wind speed and temperatures.
The year-to-year variation of yearly mean wind speeds depends highly
on selected locations and thus there is no common correlation to predict it.
For instance, except for several years, the annual mean wind speeds decrease
all the way from 1970 to 1983 at Dhahran, Saudi Arabia [18]. In UK, this
variation displays in a more fl uctuated matter for the period 1970-2003 [21].
Similarly, a signifi cant variation in the annual mean wind speed over 20-year
period (1978-1998) is reported in [22], with maximum and minimum values
ranging from less than 7.8 to nearly 9.2 m/s. The long-term wind data (1978-
2007) obtained from automated synoptic observation system of meteorologi-
cal observatories were analyzed and reported by Ko et al. [ 23 ]. The results
show that fl uctuation in yearly average wind speed occurs at the observed sites;
it tends to slightly decrease at Jeju Island, while the other two sites have
random trends.