Environmental Engineering Reference
In-Depth Information
Several approaches have been undertaken to develop
passive pitch control
techniques
that automatically adjust the blade pitch angle without a need for hydraulic or electro-
mechanical actuators and their power supplies and controls. The design of the
Jacobs Wind
Electric
turbines of the 1940s allowed the rotor blades to slide in and out of the hub for a
short distance, balancing centrifugal force against springs. During the sliding action, cams
changed the blade pitch angle, and a simple passive pitch control was obtained that
responded to rotor speed. A number of modern technology equivalents have been
developed in the 1980s. One concept is the
self-twisting blade
, in which the
blade spar
in
the vicinity of the hub is fabricated from a composite material carefully tailored so that
increasing thrust and centrifugal loads cause the blade to twist toward the feathered position.
Utilized in several small wind turbines, this concept may ind broader application as experi-
ence is gained.
A unique method of power regulation for a modern large-scale wind turbine is
embodied in the Italian
Gamma 60
1.5-MW machine (Fig. 10-1) which employs an active
yaw control system for the purpose. The blades of this turbine are ixed in pitch, and peak
power is controlled by yawing the rotor out of the wind.
Considerable research and testing have been undertaken to develop
aerodynamic yaw
control
systems for HAWTs that would face the rotor into the wind without active control,
but these have not yet been wholly successful. This has been partly due to erratic, unstable
performance and also to low cost savings. Often,
high-wind safety
and “parking” under
storm conditions can be more easily satisied by an active yaw control system. The old
techniques of
tail vanes
, or auxiliary
side rotors
and
fantails
have given way to
active yaw
control
using wind-direction sensors and electric or hydraulic drive motors.
Power Train Coniguration
As illustrated in Figure 3-10, a typical irst-generation wind turbine utilized a heavy
steel bedplate
on which to mount mechanical and electrical equipment, with the
turbine
shaft
supported by separate bearings and attached to a conventional
parallel-shaft gearbox
.
Medium- and large-scale systems rapidly moved to lighter-weight
planetary gearboxes
.
Some also introduced a
duplex turbine shaft
, composed of two concentric shafts: An inner
lexible
quill shaft
transmits only rotor torque, while a stiff outer shaft supports the weight
and thrust of the rotor. Various forms of gearbox
shock mountings
have been used to
reduce and dampen dynamic torsional loads entering the power train, for both structural and
electrical reasons. Increased understanding of power-train dynamics and variable-speed
generators have reduced the dependence on such devices in recent years.
Two other design features have appeared on wind turbines and have the possibility of
future development. One is the use of the
gearbox case as primary structure
, thus reducing
the need for a bedplate, or even the
nacelle
itself. The dificulty with this approach is the
need for extra strength and stiffness in the gearbox, which would now have to be a custom-
designed structure.
A second innovation is to omit the gearbox entirely and use a direct-drive to a very low
speed, multiple-pole generator. This could result in a weight increase (such generators are
relatively large), but this may not be critical for a VAWT generator on the ground.
Recently, the large-scale Canadian
Eolé 64-m VAWT
was constructed with a direct drive
between its rotor and a hydroelectric-type generator, with power electronics that permit
operation at sub-synchronous speed. Several very small HAWTs use direct-drive generators
or alternators, and a German manufacturer,
Enercon
, developed a 500-kW HAWT with a
direct drive to its ring-type generator.
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