INTRODUCTION - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 6

extrapolate the observed wind resource to other locations using some kind of model,
typically a numerical wind flow model.
The spatial scales of interest are related to the size of wind turbines and the dimen-
sions of wind power projects. The rotors of modern, large wind turbines range in
diameter from 70 to 120 m. Wind turbines are typically spaced some 200 - 800 m
apart, and large wind projects can span a region as wide as 10 - 30 km. Within this
overall range, a detailed map of the variations is essential for the optimal placement
of wind turbines and accurate estimates of their energy production.
The vertical dimension is just as important. The variation in speed with height
is known as wind shear . In most places, the shear is positive, meaning the speed
increases with increasing height because of the declining influence of surface drag.
Knowing the shear is important for projecting wind speed measurements from one
height (such as the top of a mast) to another (such as the hub height of a turbine).
Extreme wind shear (either positive or negative) can cause extra wear and tear on
turbine components as well as losses in energy production. The shear is typically
measured either by taking simultaneous speed readings at more than one height on a
mast or with a remote sensing device such as a sodar (sonic detection and ranging)
or lidar (light detection and ranging).
1.2.3 Other Characteristics of the Wind Resource
Although wind speed is the dominant characteristic of the wind resource, there are
other important ones, including wind direction, air density, and icing frequency, all of
which need to be well characterized to produce an accurate energy production estimate.
Knowledge of the frequency distribution of wind directions is key for optimizing the
layout of wind turbines. To reduce wake interference between them (described below),
turbines are generally spaced farther apart along the predominant wind directions than
along other directions.
Air density determines the amount of energy available in the wind at a particular
wind speed: the greater the density, the more energy is available and the more electric
power a turbine can produce. Air density depends mainly on temperature and elevation.
A substantial amount of ice accumulating on turbine blades can significantly reduce
power production, as it disrupts the carefully designed blade airfoil, and can become so
severe that turbines must be shut down. The two main mechanisms of ice accumulation
are freezing precipitation and direct deposition (rime ice). Other conditions potentially
affecting turbine performance include dust, soil, and insects.
1.3 WIND POWER PLANTS
Conceptually, a wind turbine is a simple machine (Fig. 1-2). The motion of the air is
converted by the blades (lifting airfoils very similar to airplane wings) to torque on a
shaft. The torque turns a power generator, and the power flows to the grid.
However, this simple picture disguises many subtle design features. The typi-
cal modern large wind turbine is an immense, complicated machine ranging from