Environmental Engineering Reference
In-Depth Information
The Turbine Rotor Subsystem
As shown in Figure 2-11, the main components of a Darrieus VAWT rotor are its
curved
blades
with ends fastened to rigid
upper and lower hubs
separated by the
rotor column
. To
minimize internal bending stresses during rotation, blades are shaped to approximate a
tro-
poskien
(from the Greek for “turning rope”), a shape with zero bending stress. VAWT rotors
contain two or three ixed-pitch blades, usually symmetrical in cross-section and without
twist or taper. As with a HAWT, the
swept area
of a VAWT is deined by the projection on
a vertical plane of the surface generated by the moving blades.
Rotor diameter
is the width
of the swept area at its equator.
Rotor height
is the distance between upper and lower hubs
and is usually 15 percent to 30 percent larger than the diameter.
Darrieus rotors are stall-controlled, because pitch-change mechanisms have not been
found to be cost-effective. Motoring of the generator is the usual method for starting Dar-
rieus rotors, since the blades develop lift and torque only through a superposition of rotational
(forward) speed and the wind speed and, therefore, are not normally self-starting. VAWT
rotors are usually stopped by applying a
rotor brake
in the power train, although trailing-edge
laps have also been used for this purpose.
The most common material for Darrieus blades is extruded aluminum alloy. Blades are
bolted to the upper and lower hubs, each of which is rigidly connected to the rotor column.
Thus, the rotor column collects torque from the two hubs and transmits it to the power train.
Buckling strength is the principal structural requirement on the rotor column, since this col-
umn must react the relatively high downward loads produced by the supporting cables.
The Power Train Subsystem
Comparison of Figures 2-11(b) and 2-8 shows that there are three major differences be-
tween HAWT and VAWT power trains. First, VAWT power-train components are located
at or near the ground, which provides for easier maintenance and requires a relatively low
support stand. Second, the VAWT turbine shaft assembly carries axial and torque loads only,
with no bending loads like those on a HAWT turbine shaft. Third, the VAWT rotor brake
is much larger than the parking brake typical of a HAWT, because it must be able to stop a
Darrieus rotor operating at top speed. It may even be located on the turbine shaft for added
reliability, so that the braking torque does not have to be transmitted through the gearbox.
VAWT gearboxes, generator-drive shafts, and generators have the same general conigu-
rations and functions as described previously for HAWT power-train components.
The Support Structure Subsystem
The VAWT support structure consists of
upper and lower rotor bearings
,
structural
cables with tensioning devices
, and a
support stand
. Darrieus rotors require three or four
cables (or sets of cables) to support the upper end of the rotor in a horizontal plane. These
cables stretch from the upper rotor bearing to ground anchors at an elevation angle of about
30 to 40 degrees. Cable tension causes a downward thrust load on the upper rotor bearing
equal to one-half or more of the tensile loads in all cables. This thrust load passes downward
through the rotor column, to the lower rotor bearing, the support stand, and inally to the
foundation. Depending on the design, the upper and lower hubs may also be in the compres-
sive load path.
The fundamental system frequency of a VAWT is determined by the size and tension of
the cables, because they are the elastic springs which restrain motions of the center of mass of
the VAWT rotor. Like a HAWT tower, cables can provide stiff, soft, or soft-soft support to
Search WWH ::
Custom Search