ESTIMATING THE RESOURCE AT HUB HEIGHT - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 140

11
ESTIMATING THE RESOURCE
AT HUB HEIGHT
Since the height of the top anemometers on many towers is below the hub height of
modern, large wind turbines, it is often necessary to extrapolate the wind resource data
to the turbine hub height, where the power curve is defined. 1 This is so not only for
wind speeds but also for other information such as air density and turbulence intensity
(TI). At this stage, the resource analyst begins to depart from what is strictly measured
to what must be assumed or modeled. The task requires a careful and often subjective
analysis of information about the site, including the local meteorology, topography,
and land cover, as well as the data themselves. This chapter discusses methods of
carrying out this analysis.
11.1 WIND SPEED
The most widely used method of projecting the wind speed from the height of obser-
vation to the turbine hub height is by means of the power law. This was described
1 Although estimating the wind at hub height remains the main goal of resource assessment, attention is
increasingly being paid to characterizing the resource from the bottom to the top of the turbine rotor. This
is especially important at sites where the shear is highly uncertain or variable. Where such conditions are
suspected, extra-tall towers or ground-based remote sensing may be called for. The principles outlined in
this chapter nonetheless still apply.