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Assessment of Research Needs for Wind Turbine Rotor Materials Technology
and wood/epoxy composites, as it provides control right at the outer aerodynamic surface, which determines ultimate performance. Both material systems are able to provide the complete range of outboard airfoil shapes currently of interest.
While the outboard portion of the rotor changes little with material choice, the inboard region is a different matter. Fiberglass rotor blades often incorporate a large amount of inboard planform area and twist, and may carry the maximum chord quite far in toward the root (Figure 4-3) (Stoddard, 1989). For a given rotor diameter, this will produce the most power, albeit at the cost of a significant increase in total blade surface area and materials. The flat sheet nature of the veneers used in wood/epoxy construction does not lend itself well to large inboard planform and the twist and the double curvature surfaces that result. Instead, a gradual transition from the inboard airfoil shape to an oval root is performed over the inner third of the blade (Figure 4-4). To regain the minor power loss due to lessened inboard planform area, a slight increase in blade length is provided. Since the turbine rotor designer is free to sweep energy out of the flow at whatever radius provides the least rotor cost, this is an effective solution because the cost of the slight extra length is small compared to the large reduction in inboard planform area. Low wind start-up torque is reduced, which could be a limitation for some turbine designs, but the reduction of planform area also reduces storm wind loading on the turbine as a whole, so the cost trade-off at the system level may or may not be favorable depending on start-up requirements.
AERODYNAMIC TIP BRAKES
Turbine designs, which incorporate full span blade pitch control, can use that system to provide aerodynamic shutdown. Many current turbine designs, which use a mechanical brake for normal shutdown, do not have full span pitch control and must therefore depend on some other method of aerodynamic braking for safety. A great many methods have been tried, but most machines have used either a pivoting outboard tip section or a rotating tip plate.
The pivoting tip design has been dominant for the Danish fiberglass blades. The usual arrangement employs a centrifugal latch that releases the tip when an overspeed condition occurs. The tip is then allowed to move outboard, and a cam rotates it (in pitch) until it is perpendicular to the plane of rotation, thereby providing the drag needed to slow the machine aerodynamically. The typical arrangement to accomplish this uses steel parts for the mechanism, including cam, follower, attachment feet, and the structural tubes, which carry the tip loads back into the inboard blade. The feet that form the attachment to the blade are typically glassed (i.e., embedded in resin/fiber) onto one of the blade shells to secure the mechanism to the inner blade and to secure the tip to the pivoting tube.
Tip failures have been observed with this system, typically near the cut that separates the tip from the inner blade. That is where the bending moment in the tube is at the maximum, and indenting or scuffing from a loosely fitting bearing or collar can help initiate a failure. Damage in this area due to saw cuts, welding, or other manufacturing operations has also been known to lead to tube failure. These problems are not generally due to insufficient fatigue knowledge of the steels used, but rather to manufacturing and quality control issues, and the fact that a small geometric cross section for the embedded tube leads to rather high working stresses. The bonded retention of the assembly to the blade has generally been quite satisfactory (Poore and Patterson, 1990; Stoddard, 1989; Faddoul, 1981).
Composites can be considered as a replacement for steel in this high-stress fatigue application. Fiberglass, while strong enough, has a low modulus that leads to large angular deflections and consequent problems with bearing alignment. Carbon fiber has the stiffness, strength, and fatigue endurance needed for the tip tube application, but it is considerably more costly than steel. However, as the cost of carbon fiber drops, the weight