INTRODUCTION - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 5

itself with the wrong pitch setting or not pointing directly into the wind. In addition,
turbulence contributes to wear in mechanical components such as pitch actuators and
yaw motors. For this reason, manufacturers may not warrant their turbines at sites
where the turbulence exceeds the design range. Knowledge of turbulence at a site is
thus very important for resource assessment.
Fluctuations in wind speed and direction also occur over periods of minutes to
hours. Unlike true turbulence, however, these variations are readily captured by wind
turbines, resulting in changes in output. This is a time frame of great interest for
electric power system operators, who must respond to the wind fluctuations with
corresponding changes in the output of other plants on their systems to maintain
steady power delivery to their customers. It is consequently a focus of short-term
wind energy forecasting.
On a timescale of 12 - 24 h, we see variations associated with the daily pattern of
solar heating and radiative cooling of the earth's surface. Depending on the height
above ground and the nature of the wind climate, wind speeds at a given location
typically peak either at midafternoon or at night. Which pattern predominates can
have a significant impact on plant revenues in markets that price power according to
the demand or time of day. For example, regions in which air-conditioning loads are
important often see a peak in power demand in the afternoon, and regions in which
there is heavy use of electricity for home heating may experience a peak in the early
evening.
The influence of the seasons begins at timescales of months. In most midlatitude
regions, the better winds usually occur from late fall to spring, while the summer is
less windy. Sites experiencing strong warm-weather mesoscale circulations, such as
the coastal mountain passes mentioned earlier, are often an exception to this rule, and
winds there tend to be strongest from late spring to early fall. Because of seasonal
variations like this, it is difficult to get an accurate fix on the mean wind resource
with a measurement campaign spanning much less than a full year. Furthermore, as
with diurnal variations, seasonal variations can impact plant revenues. Power prices
are usually the highest in summer on a summer-peaking system and in winter on a
winter-peaking system.
At annual and longer timescales, we enter the domain of regional, hemispheric,
and global climate oscillations, such as the famous El Nino. These oscillations, as
well as chaotic processes, account for much of the variability in wind climate from
year to year. They are the main reason why it is usually desirable to correct wind
measurements taken at a site to the long-term historical norm.
1.2.2 The Spatial Dimension
The spatial dimension of wind resource assessment is especially important for wind
plant design. Most wind power plants have more than one wind turbine. To predict
the total power production, it is necessary to understand how the wind resource varies
among the turbines. This is especially challenging in complex, mountainous terrain,
where topographic influences are strong. One approach is to measure the wind at
numerous locations within the wind project area. Even then, it is usually necessary to