PLANT DESIGN AND ENERGY PRODUCTION ESTIMATION - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 238

IIB” or “Class IIIA” depending on their wind resource. In this table, V ref is the largest
value of the 10-minute average wind speed expected at hub height over a 50-year
period, and I ref is the expected average turbulence intensity at a hub-height speed
of 15 m/s. Both can be estimated from on-site measurements. V ref is defined for a
standard sea-level air density of 1.225 kg/m3. While it may be possible to adjust it
for sites with a much different air density, the manufacturer should be consulted to
determine whether and how this should be done.
Class I turbines are designed to withstand the greatest wind loads. They are often,
though not always, designed with smaller rotor diameters relative to their nominal
power rating than turbines in other classes. Classes II and III turbines are intended to
produce more energy at lower wind speeds than their Class I counterparts. Class III
turbines, in particular, often have a lower high-wind-speed cut-out threshold than the
others, meaning the turbines give up generating some energy at higher wind speeds.
Many Class III turbines also employ larger rotors and smaller generators relative to
their power rating. These differences increase their capacity factor (the average output
divided by maximum output), making them more cost-effective at moderate-resource
sites.
The turbine class for a wind project is usually determined by considering all the
likely locations where turbines might be deployed and finding the lowest appropriate
suitability class. If the windiest and most turbulent points in the project area require a
Class IIA turbine, then the other turbines will usually be Class IIA as well. Occasion-
ally, a wind project may employ two different turbine models, each corresponding to
a different IEC class, to take full advantage of variations in the resource.
In addition to the suitability class, factors to consider in choosing a turbine model
include price, warranty and support options, technology maturity and track record,
proximity of operating and maintenance facilities, and expected mean output. The
analyst might start by listing all available turbines within the site's suitability class.
He or she could then contact the manufacturers to obtain pricing, availability, warranty,
and other pertinent information, along with a turbine power curve. Using the observed
speed frequency distribution from one of the site's monitoring masts, the analyst could
then quickly and easily estimate the mean output of each turbine and compare the
capital and operating costs per unit of output.
Aside from the hub height, rotor diameter, and rated capacity, the most important
turbine characteristics are the power and thrust (force against the wind) produced over
a range of wind speeds and air densities. An example of a set of power curves for a
range of air densities is shown in Figure 16-5. The software interpolates the power
from these data, given the estimated mean air density at the turbine's location and the
appropriate speed bin.
If the power curve is available for only one value of air density (such as the standard
sea-level density), the output can be estimated by adjusting the speed in proportion to
the cube root of the air density, as in the following equation:

v site ρ site
ρ 0

1 / 3

v adj = (16.1)