Design and development of megawatt wind turbines - Wind Power Generation and Wind Turbine Design - page 219

to the full range harsh environmental conditions, and these need to be accounted
to ensure reliable operation and maximum revenue.

4.3.5 Dynamic systems analysis
Turbines and components, like all physical objects, have natural frequencies.
External load sources, operational scenarios and moving parts are potential
stimulus and dynamic reinforcement sources. Relative defl ections and stiff-
nesses for large structures is an extremely acute consideration in the design of
robust MW WTs.
WT tower stiffness is a key consideration to the overall structural design of
WTs. To avoid resonance, the tower's natural frequency ( f 0 ) must not coincide
with the frequency of the cyclical loading resulting from the rotor frequency ( f r )
and the frequency for the blades passing in front of the tower ( f b ). Towers and
turbine systems are classifi ed as soft stiff, depending if the

tower's natural frequency is less than the rotor frequency ( f 0 < f r ), greater than the
rotor frequency but less than the blade passing frequency ( f r < f 0 < f b ), or greater
than the blade passing frequency ( f 0 > f b ), respectively [25]. This is best viewed
relative to a Campbell diagram for the tower and stimulating frequencies, where to
avoid resonance the turbine power producing speed region is defi ned using mar-
gins no less than

− soft, soft − stiff, or stiff −

± 5% [23]. The majority of large WTs are designed for the

soft − stiff regime. Soft − soft or soft − stiff are preferred over stiff − stiff because much

more material and cost are required for this later approach.
Sensors and controls are also used to avoid or mitigate resonance vibration, and
to ensure safe operation. This is a rapidly evolving development area for large
WTs, and holds promise for becoming a key technology for enabling larger “smart”
structures. These designs should be capable of withstanding higher loads with
superior performance while using considerably less material for a lower cost.

4.4 Mechanical

Materials for structures must be inexpensive, readily available, and easy to fabri-
cate, require minimum maintenance over the turbine's 20-year or longer life, and
in the best scenario, be recyclable. When a conceptual design shows promise and
feasibility, the advanced mechanical design (AMD) function works out enough of
the details to warrant advancing further in the process.

4.4.1 Blades
Blades are one of the most important components in MW WT design. They directly
affect AEP and the loads imparted to the entire turbine structure. Blades endure
a large number of cycles for wind speed and direction, extreme gusts, and, with
every revolution, load reversals from their own weight. To be economically viable,
the cost of the material and manufacturing needs to be a fraction of the cost of
blades for aircraft or aerospace counterparts.
Aerofoil sections have max thickness to chord ratio (i.e. aspect ratio) of around

2 − 3 near the root and 5 − 7 near the tip. By comparison a wooden 2 × 4 has a fi nished