materials. There may be many ways to move beyond current restrictions. For instance, high-temperature forming processes may be able to push the wood veneer into its plastic range and provide forming possibilities that room-temperature molding cannot. Or it may be possible to preprocess the veneer into a form that is much more conformable than the current full veneer sheets but that is still easy and efficient to handle. The fact that the database for wood/epoxy is in good condition for wind turbine blade fatigue design does not mean that further significant advances in this material system are not available, but rather that they may lie in somewhat different areas than for fiberglass.

Finally, both material systems can benefit from explicit feedback into the aerodynamic design process, so that the shapes specified exploit the strengths and cost efficiencies of each to the best overall effect.

Whichever material system is used, long-term fatigue performance will benefit from using simple external shapes with primary load paths that are as straight as possible, particularly in the highly loaded root and root transition areas. Minor power losses due to simplification of the inboard rotor geometry can be offset by a slight increase in rotor length, resulting in a potentially lighter and lower-cost blade of equal energy capture performance and superior long-term fatigue life. New blade designs should explicitly consider this structurally beneficial approach at the aerodynamic design phase.

RECOMMENDATIONS1

Grp

  1. Fatigue data that can resolve whether polyester or vinylester is the appropriate resin for high-cycle wind blade use should be obtained. Common E-glass reinforcing, such as unidirectional fabric and roving, double bias/mat, knit triaxial, and chopped strand mat should be investigated to determine which perform best with these low-cost resins.

  2. Filled bonding material (such as plasterite) appears to have caused fatigue failures of blade shells. Testing to identify and fatigue qualify a low-cost bonding material would be valuable.

  3. Static and fatigue data (including shear) to define the mechanical performance of core mat or other low-cost core material are needed to assure that fatigue design goals will be met.

Wood/Epoxy

  1. Provide proof of the static and fatigue performance of a low-cost field joining technique for use with flow-through rotor concepts and future large or complex shape rotors (such as MOD 5 or Darrieus).

  2. Investigate the potential for low-cost techniques to mold wood/epoxy laminate into double curvature shapes that cannot be made with current methods. Confirm fatigue performance of the resulting laminate.

Generic

Both GRP and wood/epoxy rotors could achieve cost and fatigue benefits from an all-composite one-piece rotor design. The potential life-cycle benefits of such designs should be studied and appropriate composite hub designs identified.

1  

 The recommendations made here are those the committee deemed to be of high priority for research.



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