Wind Power Generation - Renewable Energy: Technology, Economics and Environment - page 311

Depending on the installed plant capacity rotor blades of common wind energy
converters used for power generation usually have lengths of about 5 m, in case of
very small wind energy converters, and of about 60 m and above for multi-
megawatt wind power stations, for instance required for potential offshore instal-
lations. The respective rotors thus cover surfaces ranging from 80 to above
10,000 m 2 . In exceptional cases, the surface size may fall below or exceed the
above range.
Rotor hub. The rotor hub connects the rotor blades to the rotor shaft (Fig. 7.11).
For wind energy converters provided with a blade adjustment mechanism the hub
also contains the corresponding mechanics and the blade bearing. For the hub and
the pertaining construction besides welded steel sheet constructions primarily
cast-steel bodies and forged pieces are applied. The following three different hub
designs are distinguished.
− Rigid or hingeless hub. This type of hub is used for three and partly also for
two-blade rotors and represents the typical hub for stall-controlled converters
(see below). Advantages are low manufacturing and maintenance costs as well
as low wear. Disadvantages are relatively high stress of rotor blades and fol-
lowing machine components due to the rigid junction.
− Teetered hub. The teetered hub is a semi-rigid hub design which is partly ap-
plied for two-blade rotors. For this hub type rotor blades are suspended in tee-
tered position (gimbal-mounted); they may thus teeter around the (rigid) rotor
shaft, considerably reducing asymmetrical rotor loads. Especially in case of a
standstill or at very low rotor revolutions, mechanic or hydraulic dampening
prevents the blades from teetering too much; however, dampening elements
must be designed to securely absorb the enormous strain that may occur at cer-
tain wind velocities. This kind of rotor blade suspension also better compen-
sates and reduces the strain resulting from increased twisting and bending.
− Flap and/or lag hinge hub. Rotor blades can also be individually connected to
the hub by means of flap hinges. This kind of design is suitable for high-
capacity one-blade systems and small multiple-blade converters. Rotor blades
are suspended relatively independently from each other and their clamping in
shock direction is free from any bending moments. The hinge only needs to ab-
sorb centrifugal forces and transmit the torque. Disadvantages of flap and lag
hinges are relatively high manufacturing and maintenance costs. This suspen-
sion is principally independent from the rotor blade number. However, due to
the high design costs and the hardly controllable dynamic behaviour, this kind
of hub design is hardly applied in practice.
Blade adjustment mechanism. For common rotor and hub designs, systems pro-
vided with rigid and adjustable blades are distinguished. To ensure power and
revolution control (Fig. 7.11) wind energy converters of capacities of several
100 kW and above are nowadays usually equipped with a blade adjustment
mechanism.