Environmental Engineering Reference
In-Depth Information
Mod-0 Research Tests and Coniguration Changes
Over the next decade, more testing to investigate new ideas and new conigurations
(Fig. 3-11) was accomplished with the Mod-0 HAWT than with probably any other wind
turbine before or since. Initially designed as a downwind, two-bladed coniguration with
full-blade pitch, it was later tested with one (counterbalanced) blade and with two blades
in both upwind and downwind conigurations. New materials were developed for wind tur-
bine blades and irst tested on the Mod-0. These included
laminated wood-epoxy
(Spera
et al.
1990], a material now used on many small and intermediate wind turbines, and
iberglass composite blades
fabricated with
transverse-ilament-tape
[Weingart 1981].
One detrimental result from the original truss tower with its central stairway was high
impulsive pressure loading on the blades, from the excessive wind speed reduction in the
wake behind this high-drag tower. The staircase was soon removed, which reduced cyclic
loads to a tolerable level. However,
tower wake-induced fatigue loads
had been identiied
as a major design driver for downwind rotors. First-generation machines used rigid hubs
in an attempt to overcome the effects of dynamic loads by a simple brute force technique.
This approach was never wholly satisfactory, so the Mod-0 test coniguration expanded to
include a
teetered hub
(Fig. 3-12).
As structural dynamic knowledge increased, the Mod-0 truss tower was placed on a
new base composed of lexural steel beams (Fig. 3-11(a)). This allowed the natural
vibration frequency of the turbine to be lowered and “tuned” in order to simulate “soft”
tower structural concepts. Such concepts would have the potential for lower tower weight
and cost, but structural dynamic loads could be lower or much higher, depending on
resonances and instabilities that were not well-understood with the available analysis tools.
Later, the Mod-0's truss tower was replaced by a slender shell tower (Fig. 3-11(b)) to prove
out those tools and the effectiveness of soft structural systems.
Another area which received considerable emphasis was
variable-speed constant-
frequency (VSCF)
operation. Prior to the need for interconnection with the utility grid,
many small machines operated at variable rotor speed depending on electric load and wind
speed, but this had rarely been attempted on any turbine much over 15 m in diameter. The
desire to operate at variable rpm is engendered by the potential for higher energy capture
(always operating near peak rotor eficiency) and the potential for gust load alleviation.
Solutions to the electrical problems associated with a variable generator speed could
now be envisioned that were not available in a practical way two decades earlier. The
structural dynamics issues associated with the need to preclude harmonic vibrations over
a range of rotor speeds were not viewed sanguinely. It was enough of a problem in the
1970s to accomplish this at one speed. In spite of this, the Mod-0 was operated as a
variable-speed machine, testing several generator and power-conditioning components.
More importantly, the Mod-0 tests probed the structural dynamics “envelope” on a relatively
large, lexible system and thereby provided the data to develop and validate complex
computer models needed to predict natural frequencies and loads under such conditions.
Also tested were rotors with
partial-span pitch control
,
laps
(often less-correctly
termed
ailerons
),
ixed-pitch stall regulation
, and
free-yaw
response to changing wind
directions. Tests at Plum Brook determined the effects of precipitation, various airfoils,
blade root and tip design innovations, and auxiliary aerodynamic devices such as
vortex
generators
. The validation of computer models and control algorithms, though less visible
than hardware changes, was probably one of the most valuable contributions of the test bed
program. After a useful life of over a dozen years, the Mod-0 experimental HAWT was
dismantled in February 1987, leaving as its legacy an extensive set of documentation that
forms a principal basis of modern wind turbine technology.
Search WWH ::
Custom Search