Environmental Engineering Reference
In-Depth Information
Structural simplicity is reliability as well as beauty. From this point of view, no
pitch control and/or no yaw control is desirable. However, power control/rotational
speed control is vital with such a rotating machine in the free fi eld under all weather
conditions. This results in that power/speed control system including brake system
is very important.
As shown in Table 3, in general, a HAWT can easily be equipped with both
aerodynamic regulation system (pitch control or stall regulation) and mechanical
brake. However, with SWTs, especially with micro-WTs, pitch regulation is pref-
erably not engaged in order to avoid the structural complexity. This point is very
special with SWTs and a designer must fi rst decide the method how to control the
power/speed of the WT together with the type of WT.
5.1.2 Wind characteristics
Design wind turbine class is decided according to Table 2 depending on the wind
characteristics of the site where the wind turbine is to be installed.
The wind characteristics of the site are expressed in a series of mathematical
models in IEC standard so that all important load cases that act on the wind turbine
can be evaluated for the structural design. A full combination of aerodynamic design,
structural design and design of control system will give the engineering integrity.
5.2 Aerodynamic design
5.2.1 Annual energy production
Design methods for SWTs are basically same as those for LWTs. However, vari-
able pitch control system is often avoided with mini- and micro-WTs. Yaw system
is also passive to avoid additional power sources for control. Therefore, simplifi ed
systems in structures or in control will bring some special problems to SWTs.
With a SWT with fi xed pitch control system, aerodynamic design must be
completed in combination with power/speed control system and brake system.
Once technical solutions to start or stop the turbine as designed are found, the
aerodynamic design is proceed.
Let
f
(
V
) and
P
(
V
) be a probability density function of wind speed at a site and
a power curve of a wind turbine, respectively. Then the expected annual energy
production (AEP) is:
V
∞
out
∫
∫
3
AEP
=
fVPV V
(
)
(
)d
=
fV
(
)(0.5
r
V AC
)d
V
(6 )
P
0
V
in
where
V
in
and
V
out
are cut-in and cut-out wind speed respectively, and
A
and
C
P
are
the rotor swept area and power coeffi cient.
f
(
V
) is usually well fi tted by Weibull distribution function expressed in the
following formula:
k
−
1
⎛
k
⎞
kV
V
⎛⎞
⎛⎞
fV
()
=
exp
−
(7 )
⎜
⎟
⎜⎟
⎜⎟
⎝⎠
⎝⎠
CC
C
⎝
⎠
where
k
and
C
are shape and scale parameters of Weibull distribution, respectively.
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