Introduction - Wind Power Integration: Connection and System Operational Aspects - page 8

design spectrum are the variable-speed, direct-drive machines with power conditioning
equipment that are able to operate with either a leading or lagging power factor.
Numerous permutations of the design options are possible. There has been
some reduction in the types of machine on the market, but no real move towards
uniformity. The majority of the world's wind turbines now have three blades. Most
of the machinery is mounted in the nacelle, which is yawed into the wind under
power and mounted on a steel tower.
1.4.1 Blades
A wide range of materials have been used for blade manufacture, including alu-
minium, steel (for the spar, with a light fairing), wood epoxy, glass reinforced
plastic and Carbon Fibre-Reinforced Plastic (CFRP). The two latter materials are
now most common as they have the best combination of strength, weight and cost.
It is essential to keep weight to the minimum, as the weight of a wind turbine has a
strong influence on its overall cost. The cost of wind turbines usually accounts for
65-75% of the total cost of a wind farm and capital repayments typically account
for around 75% of electricity-generating costs.
The preference for three blades arises from distinct advantages. The moment of
inertia about the yaw axis, defined by the wind direction, does not vary sub-
stantially, whatever the disposition of the blades; this reduces the cyclic gyroscopic
forces when yawing compared with those encountered with two-blade machines.
Second, three-blade machines rotate more slowly, which is important for reducing
noise generation. Lastly, the visual impression of the rotation of a three-blade rotor
is easier on the eye - a consideration which is important to planners. Perhaps
contrary to intuition, three-blade rotors are only slightly heavier (about 15%) than
two-blade rotors. There has been speculation that the emerging offshore market
might reawaken interest in two-blade rotors, as two of the onshore constraints -
noise and visual - are less important. There is little sign of this happening as yet.
Wind turbines are large structures and so weight is important. Blade weight is
especially important, as savings in rotor weights allow related reductions in the
weight of the hub, nacelle and tower structure. Fairly simple reasoning suggests that
blade weight increases with the cube of the rotor diameter and this is borne out by an
examination of rotor weights in the blade size range from 20 to 100 m. However, a
better understanding of rotor aerodynamics and blade loads, which has been acquired
over the years, means that substantial reductions in weight have been achieved.
1.4.2 Control and the power train
At the turn of the century, roughly half of the world's wind turbines had fixed
blades and were of the stall-regulated type; the remainder had variable-pitch blades
to limit the power in high winds. Stall-regulated machines can dispense with
potentially troublesome controls for changing the pitch of the blades, but still need
to have some form of movable surface to regulate rotational speed before
synchronisation and in the event of disconnection. It must be emphasised, however,
that the reliability of both these types of machine is now very high.