Wind Power Generation - Renewable Energy: Technology, Economics and Environment - page 310

facilitate the general acceptance of wind energy converters by the population
significantly. More than 95 % of the current wind energy converters are three-
blade rotors. Even with regard to the current market development, also includ-
ing the multi-megawatt converters under consideration for offshore installation,
the number of rotor blades is not expected to change in the foreseeable future.
− In comparison to three-blade rotors, two-blade rotors save one blade and thus
material and costs. On the other hand, the rotor hub has to perform more work,
as two blade rotor dynamics are much more difficult to control. Due to the
unfavourable mass distribution additional twisting and bending occurs that may
be transmitted to the entire converter and may result in higher dynamic stress.
Although teetered hubs (see below) are capable of reducing the above disad-
vantages they require a more sophisticated design and thus lead to additional
costs. When compared to three-blade rotors two-blade rotors are characterised
by a marginally higher tip speed ratios, varying between 8 and 14, and thus
present high blade tip speeds; nevertheless the noise at the blade tips can be
kept within certain limits and no longer represents a major problem. Although
two-blade rotors are scarcely available in the marketplace they are only of
minor importance with regard to the entire available converter range. In Ger-
many, for example, probably only a low percentage of converters is equipped
with two rotor blades. Two blade rotors are not expected to be used more
widely in the future.
− Although a minimum of material is consumed for one-blade rotors, they require
an additional counterweight and a robust rotor hub to compensate for rotor ec-
centricity. Due to dynamic effects, rotor eccentricity places particularly high
requirements on the design and thus increases the cost and propensity toward
repairs. However, if the additional technical requirements are fulfilled, the mo-
tion of such a rotor is still comparatively uneasy and impairs the scenery. At
maximum tip speed ratios of 14 to 16 the blade tip speed is very high and cre-
ates high noise emissions. For the described reasons one-blade rotors have not
succeeded in the marketplace up to now. It is expected that one-blade rotors
will continue to be of very low importance due to the high mechanical stress of
the rotor hub and the corresponding requirements placed on the machine de-
sign. One-blade rotors also present marginally lower efficiencies when com-
pared to two or three-blade rotors (Fig. 7.16).
Rotor blades. Rotor blades (Fig. 7.11) are usually made of plastic, in individual
cases also steel or wood are applied. As plastics in general fibre reinforced mate-
rial containing glass, coal or aramide fibres is used /7-3/. Up to now, usually glass
fibre reinforced plastics (FRP) have been applied. Yet, with increasing plant size,
there is a tendency to use coal fibre reinforced plastics. The predominant criterion
for material selection is fatigue strength, but also the specific weight, admissible
stress, modulus of elasticity and breaking strength. Deciding factors are also the
development, material and manufacturing costs resulting from these technical key
factors.