Environmental Engineering Reference
In-Depth Information
Structural Dynamics
Because of its relatively long and slender blades, a detailed knowledge of the structural-
dynamic behavior of a Darrieus VAWT is critical to achieving an acceptable fatigue
lifetime. Engineers must be able to accurately predict a VAWT's many modes and
frequencies of vibration, as well as static and dynamic stresses caused by gravity,
centrifugal forces, and the wind. Predicting natural frequencies requires a computer model
of the complete VAWT system, including the supporting cables, that must be validated by
modal testing
of the actual turbine [Lauffer
et al.
1987]. To conduct a modal test,
accelerometers are mounted at strategic points on the structure; a cable is attached to the
VAWT, tensioned, and then quickly released. The resulting dynamic signals from the
accelerometers are analyzed by computer for their frequency content, which identiies the
vibration modes. The modal testing procedures developed at Sandia using the 17-m VAWT
as a test bed are now applied throughout the world on both VAWTs and HAWTs.
Sandia/DOE 34 m VAWT
Between 1984 and 1987 a
34-m Darrieus VAWT
test bed for developing advanced
concepts was designed and fabricated in modules at Sandia and then installed at the U.S.
Department of Agriculture Test Station at Bushland, Texas [Ashwill
et al.
1987]. This
highly-instrumented 625-kW turbine (Fig. 2-2) has
variable-chord blades
for aerodynamic
and structural optimization,
tailored airfoils
designed speciically for VAWTs, and a
variable-speed constant-frequency (VSCF)
generating system. The test program began in
1988, in cooperation with the USDA personnel, and by early 1989 full rated power was
achieved.
Figure 3-22 shows the
operating map
of the control system. The test bed uses a
variable-speed system in which the generator's variable-frequency AC output is converted
to DC and then converted back to AC at the utility line frequency.
Elastomeric couplings
are incorporated in the drive train to attenuate the
torque ripples
associated with VAWTs
(caused by the blades traveling in alternating upwind and downwind paths).
Figure 3-22. Operating map of the 34-m VAWT test bed.
[Ashwill
et al.
1987]
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