Environmental Engineering Reference
In-Depth Information
One of the most successful families of blades were (surprisingly) made of wood. They
evolved from the
WEST
laminated wood-epoxy technique developed by
Gougeon Brothers,
Inc.
(GBI) for racing yachts. Under contract to NASA, GBI adapted the process to the
building of wind turbine blades of their own design. Carried further by private develop-
ment, laminated wood blades have been utilized on a signiicant number of turbines up to
43 m in diameter. GBI became a major supplier of wood-epoxy blades which competed
successfully with iberglass blades in the U.S.
A comprehensive review and assessment of the status of materials for wind tur-
bine rotor blades is contained in a report from a special committee of the National Re-
search Council [Dieter
et al.
1991]. This committee reviewed the three related subjects
of structural loading characteristics, materials properties and life prediction, and wind
turbine rotor design, drawing conclusions regarding the following issues:
-- adequacy of existing models to predict dynamic stress patterns;
-- properties of wind turbine materials in dynamic and fatigue failure;
-- understanding of the performance of joints, fasteners, and critical sections in
relation to failure modes and fracture;
-- adequacy of analytical tools such as computer design models and material
databases;
-- need for special laboratory facilities and turbine prototypes to improve the
design and operation of wind energy systems;
-- opportunities for new materials, better manufacturing processes, and advanced
control techniques to improve wind turbine performance and durability.
Aerodynamic Control
Some form of aerodynamic control is generally required for speed and power
regulation, normal startup and shutdown, overspeed protection, and emergency shut-down
situations. In particular cases, some of these functions can be performed mechanically,
electrically, or even with passive aerodynamic techniques. Mechanical brakes (other than
for parking) may be prohibitive in size and cost in the largest turbines because of the
amount of rotational energy contained in the rotor and power-train equipment. Part of the
tradeoff in selecting the method of aerodynamic control depends on whether a ixed- or
variable-speed system is contemplated. In any event, the synchronous speed of the
electrical generator provides the basic speed regulation, but separate overspeed protection
is also normally required in the event of electrical system failure.
Traditionally, many of the smaller systems have been able to use the
ixed-pitch stall-
regulated
approach, albeit with some penalty in energy capture. Medium- and large-scale
systems have utilized
full-span variable-pitch
rotors, much like conventional constant-speed
aircraft propellers. This does, however, involve relatively complex and heavy
pitch change
mechanisms
at the blade root where structural loads are high. A number of rotors have
been built with
tip controls
, wherein the majority of the blade is at a ixed pitch while the
pitch of the remaining tip section may be varied. Examples of this approach include the
2.5-MW Mod-2
and the
3.2-MW Mod-5B HAWTs
in the U.S. and several commercial
Danish, Dutch, and British turbines.
Research has been performed using outboard
lap control surfaces
which show a
number of mechanical and structural advantages, but which have only been used
experimentally in the U.S. and Japan. The
multiple hinge points
of a lap control surface
have intrinsically better structural and safety features (because of
redundancy
) than the
single spindle shaft which typically anchors a tip control surface.
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