Page 360 - Wind Energy Handbook

334 CONCEPTUAL DESIGN OF HORIZONTAL-AXIS TURBINES

(2) Hub weight: it is assumed that this is proportional to the blade out-of-plane
bending moments in fatigue, as in the case of the blade itself.

(3) Low speed shaft weight: this is assumed to be governed by the shaft bending
moment due to the cantilevered rotor and hub weights, which are taken as
proportional to rated wind speed

(4) Gearbox and brake: gearbox and brake design are governed by the rated torque,
P=Ù. The rated power is proportional to the cube of the rated wind speed, and
the rotational speed is proportional to the rated wind speed, so the torque
varies as the rated wind speed squared. The weights of the gearbox and brake
are therefore taken to be proportional to the rated speed squared.

(5) Generator: generator design is governed by rated power. The weight of the
generator is therefore assumed to be proportional to the cube of the rated wind
speed.

(6) Nacelle structure and yaw system: it is assumed that the design of these are
governed by the fluctuating moment on the nacelle due to differential blade
out-of-plane root bending moments, which depend on blade out-of-plane
bending moment fluctuations. The weights are therefore taken to be propor-
tional to rated wind speed.

(7) Tower weight: the tower design may be governed either by fatigue loading
during operation or by extreme loads with the turbine shutdown, so both
possibilities will be considered. In the latter case the tower loading will be
independent of rated wind speed, while in the former it will be mainly
governed by rotor thrust fluctuations, which are assumed to be proportional to
rated wind speed.
(8) Foundation: the foundation design is governed by extreme loads rather than by
fatigue, so it is independent of rated speed.

(9) Grid connection: the weight of cables, switchgear and transformers are assumed
to be proportional to rated power, and hence proportional to rated speed
cubed.

(10) Controller, assembly and transport: these are assumed to be fixed.

The various components listed above are classified into different categories
according to the way in which their weights vary with rated wind speed in Table
6.2. Also tabulated are the component costs as a percentage of the total for the
baseline machine, together with the sum for each category.
As noted in Section 6.2.1, the relationship between the cost of a component and
its mass can be approximated by a linear relationship of the form of Equation (6.1).
As before, ì is assumed to take a value of 0.9 for all components. When the cost
functions for all the components are added together, the following expression is