Page 368 - Wind Energy Handbook

342 CONCEPTUAL DESIGN OF HORIZONTAL-AXIS TURBINES

600
Pitch-regulated machine with
TR blades rotating at 33 r.p.m.
500 (Blade pitch below rated
increased 1.4 degrees compared
with stall-regulated machine) Stall-regulated machine with TR
blades rotating at 30 r.p.m.
400
Power (kW) 300

Rotor diameter = 40 m
200

100

0
0 5 10 15 20 25 30
Wind speed (m/s)
Figure 6.4 Comparison of Power Curves for 500 kW Stall-regulated and Pitch-regulated
Machines with the Same Planform and Twist Distribution
to increase with rotational speed, but no account is taken of the increased loads on
these components for a fixed-hub, two-bladed machine. Tower design is assumed
to be governed by fatigue in the first instance, so tower weight is taken as
proportional to rotational speed. The cyclic thrust loads on the rotor due to
turbulence are virtually the same for two- and three-bladed machines rotating at
the same speeds if the blade planforms are the same, so the tower cost element at
the baseline rotational speed is left unchanged.
The weights of the gearbox and brake are taken to be proportional to the rated
torque, P R =Ù, while those of the generator and of the cables and equipment forming
the grid connection are taken as proportional to rated power, P R . The foundation
cost element is reduced by a quarter for the two-bladed machine in recognition of
the reduced extreme tower base overturning moment.
The various components are classified into different categories according to the
way in which their weights vary with rotational speed and rated power in Table 6.4.
Also tabulated are the two-bladed machine component costs as a percentage of the
total for the baseline three-bladed machine, together with the sum for each category.
Adopting Equation (6.1) with ì ¼ 0:9 once more for the relationship between the
cost of a component and its mass, the following expression is obtained for machine
cost as a function of rotational speed and rated power:

C T ¼ C TB (0:114 þ 0:514f0:1 þ 0:9(Ù=Ù B )gþ 0:142f0:1 þ 0:9(P R =P RB )(Ù B =Ù)g
þ 0:158f0:1 þ 0:9(P R =P RB )g) (6:11)

¼ C TB 0:1954 þ 0:4626(Ù=Ù B ) þ 0:1278(P R =P RB )(Ù B =Ù RB ) þ 0:1422(P R =P RB ))ð