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outputs corresponding to about 400 W/m 2 of rotor area. A 40 m diameter machine,
marketed in the early 1990s, would therefore have an output of around 500 kW. With
an average wind speed of, say, 7.5 m/s and a typical wind speed distribution pattern,
this meant that maximum output would be achieved for around 8% of the year.
Ratings have increased, however, as the markets have become increasingly compe-
titive, and some of the very largest machines now approach 600 W/m 2 .Windtur-
bines with high ratings are, however, less suited to low wind speed sites. Many
manufacturers now offer machines with large rotors and modest ratings for such
sites. Turbine manufacturer Vestas, for example, offers a 110 m diameter, 2 MW
machine for low wind speed sites, an 80 m diameter machine with the same rating
for high wind speed sites, and various other options.
1.3.3 Energy productivity
The increases in size and specific rotor outputs, coupled with a small contribution
from aerodynamic scale effects and from design improvements, have all combined
to bring about marked increases in energy production. To illustrate this point,
Figure 1.3 shows that annual yields, per unit area, increase by over 50% as rotor
diameters increase from 20 to 80 m. This figure was constructed using actual per-
formance data from manufacturers' specifications and taking a reference wind
speed of 7 m/s at 30 m. It was assumed that wind speed increases with height
according to a 1/7-power law.
Design options
Although energy yield is important, so is price, and there are various ways in which
manufacturers seek to achieve the best balance between high yield and low price. At
one end of the spectrum are the simple and rugged stall-regulated wind turbines,
running at a fixed speed and using induction generators. Since the turn-of-the-century,
however, the number of machines in this category has declined. At the other end of the
Rotor diameter (m)
Figure 1.3
Annual yields from turbine rotors
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