New small turbine technologies - Wind Power Generation and Wind Turbine Design - page 396

4.2 Normal wind conditions and external wind conditions

Wind conditions are usually divided into two wind conditions: normal wind
conditions and extreme wind conditions. The former will occur frequently during
normal operation of a WT through its lifetime and very much related to fatigue,
while the latter will occur very rare after their defi nition of 1-year or 50-year
recurrence period and relate to ultimate loads.

4.3 Models of wind characteristics

Wind speed distribution, wind profi le (wind shear model), turbulence model,
extreme wind speed model, gust model and model of wind direction change are to
be used for calculations of loads.

5 D esign of SWTs

A wind turbine is designed to capture maximum energy from the wind and to ensure
the safety under any external and internal condition. Safety and performance are the
main technical requirements in the design procedure.
Design procedure is given in IEC 16400-2 to attain the engineering and techni-
cal requirements to ensure the safety of the structural, mechanical, electrical and
control systems.

5.1 Conceptual design

5.1.1 HAWT or VAWT
Wind turbines are classifi ed into two types: HAWT and VAWT. They have some
different technical features as shown in Table 3.
It is well known that lift-driven wind turbines have much higher performance
than drag-driven types from the aerodynamic theory.

Table 3: Technical features of HAWT and VAWT.

Item HAWT VAWT

Performance Lift-driven and high Lift-driven and high (Darrieus

rotor), drag-driven and low
(Savonius rotor)

Power/speed
control

Variable pitch control is
possible

Usually, variable pitch control

is not possible

Yaw control Needed Free from yaw control

Structural
arrangement

Nacelle is on the top of the
tower

All the components except

rotor are on the ground level

Support structure Support structure like gay Support structure like gay wire

wire will not induce vibration may induce vibration