Environmental Engineering Reference
In-Depth Information
5.3 Selection of aerofoil sections
In general,
C
P
is a function of several parameters:
CC
=
(, ,,
lbe
e
)
(16 )
P
P
where
l
,
b
,
e
and
Re
tip-speed ratio, pitch angle, drag-lift ratio and Reynolds num-
ber, respectively. All these parameters relate to the fl ow condition on the rotor
blade. Typically,
l
and
b
defi ne the fl ow angle at every location along the blade and
e
and
Re
directly relate to the performance of the aerofoil section at the location. Thus,
selection of aerofoil section is very important.
5.3.1 Mechanism of performance reduction
Wind turbines of high performance are driven by lift.
According to BEM theory, local power produced by a blade element has a
expression:
Δ
(17 )
Local power:
Δ∞ −
P
(1
ej
cot
)
r
where
j
is fl ow angle on a blade element of length
Δ
r
and
e
is drag lift ratio of
the blade element. At an ideal fl ow angle,
e
0.01-0.02, but if a stall occurs on
the blade element
e
> 1.0, which suggests both selection of aerofoil sections and
control of relative low condition between a blade and air fl ow are keen to the rotor
aerodynamic performance.
Thin aerofoil sections have usually smaller drag coeffi cients than thick sec-
tions. However, they will soon fall into stall if the angle of attack varies from the
ideal condition. At such laminar separations with SWTs, the performance reduc-
tion is considerably large. Thick aerofoil sections have high structural stiffness
than thin aerofoil sections, but usually have higher values of
e
which induce
higher energy loss.
≈
5.3.2 High lift devices
To avoid the laminar separations, and also to regulate the power output and rotor
speed, several techniques are used.
Pitch regulation is the most effective active method, but is sometimes diffi cult to
apply to SWTs with less space or capacity for control power.
Vortex generator (VG) is simple and useful high lift device. Even a large WT
benefi ts from this device. Longitudinal vortexes generated by VGs prevent the
boundary layer from inducing large scale laminar separations, but drag coeffi cient
will usually slightly increase.
Start-up assist method is a smart technology to improve the fl ow condition on
rotor blades by accelerating the rotor. When a rotor condition is far from design
point, such as rotor speed is very low or wind is very low, for example. An instan-
taneous acceleration of the rotor by additional power source for certain seconds
will bring the fl ow angle into regular conditions capable of generation and the
operation becomes more ideal condition. Although this technology does not work
as a brake device, it works as an alternate method with pitch regulation.
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