Implementation of the ‘smart’ rotor concept - Wind Power Generation and Wind Turbine Design - page 468

Figure 1: The increase of turbine state of the art size in time. Figure adapted from
van Kuik in [1].
and to upscale existing blade designs. However, the boundaries of current tech-
nologies are being approached.
The increase in size is driven by the fact that the power conversion by a wind
turbine increases with the square of the rotor blade's diameter. However, compo-
nent costs will also increase. For instance, historically the mass of blades has
increased with the radius to the power 2.4 [2] to 2.65 [3], depending on how the
trend line is fi tted. Thus the mass and with it the costs regarding materials and
installation of a blade increase faster than the power it captures. This is more
acceptable for offshore turbines than for their onshore counterparts because a large
part of the costs, such as foundations, are related to the amount of turbines, rather
than the installed power. Increasing the yield per turbine will therefore decrease
the costs per kilowatthour.
Mass and size increase has several negative effects, beside higher material costs.
First of all, installation cost increase because the heavy rotor has to be lifted to hub
height. Secondly, at a certain point in time gravitational loading becomes a serious
design drive. This fl uctuating load comes on top of the already high fatigue loads
that a turbine has to suffer because of fl uctuating aerodynamic loading. These are
caused by changes in the wind fi eld and by the rotation of the blade through this
wind fi eld. Examples of these are wind shear, yaw misalignment, tower shadow
and fl uctuations in infl ow. These loads will also increase with increasing size, for
instance through the more severe wind shear experienced by larger machines. In
fact, fatigue is already one of the biggest design drives and current blades are
dimensioned for at least 10 8 cycles. Mitigating the amplitude of the fl uctuations
could lead to a longer service life of blades, but also to lighter blades and a reduced
down time due to fatigue load induced component failure. In general mitigating
fatigue loads will lead to a large reduction in the price per kilowatthour.
1.1 Current load control on wind turbines
Wind turbines have always had some system to control their aerodynamic loading.
With old wind mills sails could be adjusted and with modern turbines a number