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comparing with a single set of blades. In a contra-rotating win turbine, each
set of blades contributes independently to the total power output. Because the
length of each set of blades can be made differently, the wind turbine may
produce electricity at a lower cut-in speed by the set of blades with a shorter
blade length.
Achieving higher power density
With the increase in power output and the
moderate increase in wind turbine volume, the power density becomes higher.
Reducing wind turbine cost per kWh
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Though the total cost of this type of
wind turbines increases due to the addition of extra parts, the cost per kWh will
decrease for the large increase in power output.
Increasing wind turbine operation reliability
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Some types of contra-rotating
wind turbines eliminate gearboxes to simplify the design and increase the tur-
bine operation reliability.
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However, this type of wind turbine also has some drawbacks. The main negative
effect is the wake vortices created by the fi rst set of blades which can substantially
lower the performance of the second set of blades. In fact, the wake vortices will
enhance the wind turbulence intensity to strengthen unbalanced dynamic loads
on the second set of blades, decreasing the mean wind velocity and the power
captured by the second set of blades.
7.7 Drivetrain
In a geared wind turbine, the term “drivetrain” usually encompasses all rotating
parts, from the rotor hub, the main gearbox, to the generator. The main gearbox
is one of the most important components in the wind turbine, for increasing the
slow rotation speed of the blades to the desired high speed of the generator's
rotor. It is also the most expensive component in the turbine and can easily fail
before reaching the intended life. With the increase in the turbine size and capac-
ity in the last decade, the gearbox has been subjected to even greater loads and
stresses. There are signifi cant challenges presented to gearbox designers and
manufacturers.
Wind turbines typically use planetary gears to divide torque along three paths
and reduce individual loads on each gear. However, torsional loads twist gears
out of alignment, and slight dimensional variation in gearbox components, indi-
cate that planetary gears do not equally share the load. Misaligned gears, shock
loads, and uneven forces lead to highly localized stress and eventually fracture
along the gear edges. To solve these problems, an innovative type of gearbox has
been developed (Fig. 15), using the Integrated Flex-pin Bearing (IFB) to equal-
ize gear loads, eliminate misalignment, and dramatically improve wind turbine
reliability. This novel design increases torque capacity of planetary gears up to
50% [ 109 ].
Based on the failure analysis of gearboxes, the Gearbox Reliability Collaborative
initiated at US National Renewable Energy Laboratory (NREL) provides a fresh
approach toward better gearboxes that combines the resources of key members of
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