Page 98 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

Offshore Wind Chapter | 4 91

FIG. 4.8 The power curve and power coefficient of a 6 MW turbine.

Wind turbines cannot generate electricity for very low (insufficient torque
to overcome friction) or extremely high velocities (they will be damaged).
Therefore, cut-in (around 3.5 m/s) and cut-out (around 25 m/s) speeds are
specified on power curves. As the wind exceeds the cut-in speed, the power
output increases rapidly. However, around certain speeds, known as rated wind
speed (12–14 m/s), the power output reaches a limit that is called the rated power
output. This is the maximum level that the electrical generator can still work.
When wind speed is higher than rated output wind speed, the power is kept
almost constant by a method, for instance by adjusting the blade angles (pitch
system).
Using a power curve, it is also possible to plot the power coefficient as a
function of wind speed. Fig. 4.8 shows the power curve and power coefficient
of a 6 MW turbine with a rated output wind speed of 12.5 m/s. As this figure
shows, the maximum efficiency of the turbine (i.e. around 47%) is achieved at
wind speeds between 7 and 8 m/s.

4.3 ASSESSMENT OF WIND ENERGY AT A SITE
As discussed in the previous sections, the power generated by a wind turbine,
and consequently the capacity factor of a wind farm, depends on the wind
speed, which varies in time and space. Therefore, to select a suitable site,