Design of Wind Turbines - Wind Energy: Renewable Energy and the Environment - page 112

Other parameters, for example, are the design point, wind speed for the rated power (which pri-
marily determines rotor area), and tip speed ratio, which is determined by the solidity of the rotor.
In general, the tip speed of the blades is limited to roughly 70 m/s, as the blade tips cause excessive
acoustical noise at higher tip speeds. For offshore wind turbines, noise is not an important issue.
Besides the rotor design then, there are the rest of the components: hub, which may include compo-
nents for adjusting pitch of the blades; drive train and gearbox in most cases; generator; yaw control;
tower; and the control system.
6.7 MEASURED POWER AND POWER COEFFICIENT
A common specification is the power output of the wind turbine versus wind speed, a power curve.
The power curve generally includes all efficiencies from wind to electrical output, not just the rotor
efficiency. Since all wind turbines must control power output at high wind speeds, at some point the
efficiency is lower. Control can be implemented by changing blade pitch or by operating fixed-pitch
blades at constant angular speed. Operating at fixed pitch is also called stall control. Power curves
are obtained by the method of bins, so in reality, a power curve is not a line but a band of values.
The experimental power and power coefficient curves (Figure 6.13) are for a wind turbine that
has an induction generator, operation at constant angular speed, and fixed pitch, which means it
is stall controlled. Therefore, it reaches maximum power coefficient at only one point, and the
decreased aerodynamic efficiency at wind speeds above this point make the power coefficient also
decrease. The increased power in the wind and the decreased aerodynamic efficiency combine to
give a constant power output above 12 m/s. The high efficiency, which includes drive train and gen-
erator, is because this unit has an almost optimal blade; taper, twist, and thickness.
Besides the tip and hub losses of the blades, there will be a further reduction of the power coef-
ficient due to the inefficiencies of the mechanical system (drive train, coupling) and the generator.
Under the optimum design conditions, the modern two- or three-bladed rotors at tip speed ratios in
the range of approximately 4-10 will have power coefficients of about 0.4 to 0.5 ( Figure 6.14 ) . The
power coefficients for the farm windmill and the Savonius rotors are essentially the same, with a
maximum just over 0.3. The maximum power coefficients for the vertical-axis wind turbines are just
over 0.4, which makes them less than those for the horizontal-axis wind turbines. This is one of the
reasons that in 2008, vertical-axis wind turbines are not commercially available for wind farms.
FIGURE 6.13 Experimental power and power coefficient for a Carter 25, rated 25 kW, 10 m diameter. Notice
at 3 m/s the turbine uses power (energy for the field coils of the induction generator).