COMPOSITEMATERIALS FORWIND POWER TURBINE BLADES

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Abstract

Renewable energy resources, of which wind energy is prominent, are
part of the solution to the global energy problem. Wind turbine and the rotorblade
concepts are reviewed, and loadings by wind and gravity as important factors for
the fatigue performance of the materials are considered. Wood and composites are
discussed as candidates for rotorblades. The fibers and matrices for composites are
described, and their high stiffness, low density, and good fatigue performance are emphasized.
Manufacturing technologies for composites are presented and evaluated with
respect to advantages, problems, and industrial potential.



INTRODUCTION
The international wind energy market showed a new record in 2003 with a growth
rate of 15%. Globally, a total power of 8.3 GW was installed. The total installed
wind energy power has now reached more than 40 GW, and the average growth in
the market during the past five years has been 26% per year. These figures were
reported in the annual report on the status for wind energy in March 2004 from the
consulting company BTM (1). This illustrates how during the past 25 to 30 years
the use of wind turbines for electricity generation has grown from a grass-root
initiative to an efficient alternative energy resource.



WIND ENERGY
For wind energy a converter is needed to turn the kinetic wind energy into operational
energy, e.g., electricity and/or heat. The converter is based on a rotor driven
by the wind, thereby extracting a power of

where α is an aerodynamic efficiency constant, ρ the density of air, A the area of
rotor-plane, and v the wind velocity. The rotor needs some sort of aerodynamic
device, e.g., a wing or rotorblade with an aerodynamic shape, to be able to rotate.
The rotor is typically placed on a tower, and this converter is usually called a wind
turbine (in the past, a wind mill). In the early years the United States used the designation
wind energy conversion system (WECS).



ROTORBLADES
The present review is focused on the rotorblades, which probably present the most
challenging materials, design, and engineering problems. The rotor and its three
rotorblades constitute a rather flimsy structure, consisting of cantilever-mounted
blades on a central hub. The basic design aspects for a rotorblade are the selection
of material and shape. The material should be stiff, strong, and light. The shape
should be aerodynamic, similar to that of an airplane wing.



LOADS ON ROTORBLADES
The rotor and the rotorblades are exposed to external loads. These originate from
the wind and from gravity. The general rotorblade geometry shown in Figure 2
has the blades arranged with their flat dimension in the plane of the rotor. This is
so because the linear velocity of the outer part of the blade is high with blade tip
speeds of 75–85 m/s, which are much higher than the wind speeds, even at storm
conditions (∼25 m/s). Therefore the relative wind direction, as seen by the blade,
is nearly in the plane of the rotor, although the real wind direction is at a right
angle to the rotor plane.