New small turbine technologies - Wind Power Generation and Wind Turbine Design - page 401

5.3.3 Blade material
Aerodynamic performance of a rotor depends on blade structures and materials.
If rotor stiffness is not enough, deformation of rotor blade will move the opera-
tion point from the designed point, thereby the power performance will decrease.
As long as such an aero-elastic effect cannot be refl ected in the rotor design, stiff
materials shall be chosen.
On the other hand, an advanced design may utilize the aero-elastic effect. A
thought-out design of a fl exible rotor may obtain an ability of regulating power
although it is a fi xed pitch system.

5.4 Structural design

Most important works in the structural design is to verify that all the limit states
are not exceeded for the WT design [1]. Therefore, all the loads that will act on the
WT are determined fi rst and then structural design of the WT and its components
will follow in combination with the WT conditions.
Loads are classifi ed as:

• Vibration, internal and gravitation loads

• Aerodynamic loads

• Operational loads caused by yawing, braking, furling, pitching, etc.

• Other loads such as wave loads, wake loads, transport, etc.

Three different methods for the determination of design loads are described in
IEC Standard 61400-2. Simplifi ed load equation methods are simple but conserva-
tive. The method using aero-elastic modelling is fi ner and more complicated than
the fi rst one. This method has common base with that for general wind turbines.
Design loads can be derived from load measurements.
A designer of SWTs shall refer the IEC standard to understand general design
methodology and conduct design by using equations for loads, table of load cases
and safety factors.

6 Control strategy of SWTs

Decision of the control strategy of a SWT is also important element in the initial
system design. Both of its power output control and yaw control are usually quite
different if compared with those of LWTs, particularly with mini- and micro-WTs.
The reasons are:

• The representative system speed D is proportional to 1/ D as shown in eqn (4),

which means SWTs are higher rotational speed machines than LWTs and the
control rate shall be faster in proportion to 1/ D .
SWTs are more apt to be infl uenced by various kinds of local turbulence than

Ω

•

LWTs.