Environmental Engineering Reference
In-Depth Information
The blades of all wind turbines are comprised of aerofoil sections whose
purpose is to produce lift, which is the primary component of the torque about the
turbine axis in the direction of blade rotation. For steady flow, the product of this
torque and the blade angular velocity, X, gives the power extracted from the wind.
Blade analysis is introduced in
Chap. 3
and the aerodynamics of lift and drag in
Chap. 4
. The calculation of power output is the subject of
Chap. 5
. It is not very
clear from the photographs in Figs.
1.2
and
1.3
that most turbine blades are
twisted, that is they are more ''square on'' near the tips, but it is more obvious that
the blade width, or chord, c, decreases towards the tip. A fuller definition of the
twist and chord, along with the reasons why both decrease with radius, are major
aspects of wind turbine performance and design, covered in
Chap. 5
.
Most large turbines have three blades, partly because this number is held to be
visually more appealing than the main alternative of two blades. Large blades can
be over 60 m long and weigh over 20 tonnes. The small wind turbines in Fig.
1.2
have between two and seven blades. The choice of blade number is a recurrent
theme of this topic and is discussed in terms of both power extraction (
Chaps. 5
and
7
) and starting performance,
Chaps. 6
and
7
. Small blade manufacture and
testing is covered in
Chap. 7
.
1.3 Wind Turbine Noise
In siting a wind turbine, the first and often far from trivial task is to determine the
wind resource, which may vary significantly over short distances because of the
surface roughness, the topography, and proximity to buildings, trees and the like.
These issues are covered in
Sect. 1.5
and
Chap. 12
. There remain at least three
further important issues: noise, visual impact, and possible restrictions on tower
height. The first two are often addressed for large wind farms using sophisticated
software that optimises the layout of the turbines to maximise power extraction
and minimise the visual impact of the turbines.
Well designed wind turbines are extremely quiet: one simple data correlation
for the sound power level, L
P
, gives
L
P
10
7
P
ð
1
:
4
Þ
[
6
]; that is one-ten millionth of the turbine's power is output as noise. For this
reason, a well designed small wind turbine is almost guaranteed to be quiet.
Another correlation that is more accurate in some cases, is
L
P
50 log
10
XR
þ
10 log
10
R
1
ð
1
:
5
Þ
where now L
P
is measured in the more common unit of A-weighted decibels
(dBA) [
7
]. Recall that X is the blade angular velocity in rad/s, so XR is the
circumferential velocity of the blade tip in m/s and R is measured in m. L
P
is the
strength of the source of the sound as a multiple of the standard base level of
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