axis wind turbine (HAWT) and the vertical axis
wind turbine (VAWT). HAWTs are very effective in
generating electricity from the wind [1] but they
are not without their problems, such as the need
for yaw mechanisms, regular maintenance and
repair for the transmission, additional expenses
in reinforcing the tower structure supporting the
heavy nacelle, maximising the diameter of the
rotor and the number of rotor blades, posing a
danger to surrounding animals and birds, a high
degree of noise pollution and the rotor having
to face the wind direction for effective power
extraction [2].
On the other hand, the VAWT is deemed more
suitable to be used in urban areas [3]. The VAWT
can be scaled down easily and still harness
wind energy efficiently in urban areas due to its
gearbox and generator being situated in its lower
portion (lowering its center of gravity) and its rotor
size can be increased or decreased horizontally
without affecting its height. A yawing mechanism
is not required by the VAWT as it can harness wind
energy from all directions. This results in lower
manufacturing and maintenance costs because
of the simpler structure due to the elimination of
the yaw mechanism. The VAWT might harness less
wind energy than the HAWT in steady wind but it is
fairly efficient in capturing rapidly changing winds
such as air gusts. The lower operating rotational
speed of a VAWT ensures safety to birds and also
produces lower levels of noise.
Despite the VAWTs general superiority in
comparison with the HAWTs, the VAWT also has
its disadvantages such as the relatively lower
efficiency (e.g. Savonius rotor) because the wind
strikes on both sides of the rotor blade, i.e. one
following the wind direction and the other which
counters it, thereby neutralizing part of the
available wind force. The other disadvantage of
the VAWT is the inability of the rotor to start by
itself (e.g. Darrieus rotor) [4].
Due to various disadvantages of both the
VAWT and the HAWT, the main objective of the
project is to overcome these drawbacks by
introducing a cross axis wind turbine (CAWT)
capable of maximum exploitation of wind power
irrespective of the direction of the wind, without
necessitating any type of orientation mechanism
and providing better self-starting capability. The
CAWT takes up the same amount of space as a
VAWT but consists of more lift force generating
surfaces. The disadvantages in terms of oncoming wind directions for the HAWT and VAWT
are illustrated in Figure 1. The HAWT shown in
Figure 1 relies on one horizontal wind direction,
therefore requiring a yaw mechanism to rotate
the wind turbine. Although the VAWT shown
in Figure 1 is an omni-directional wind energy
device, it is limited to rotating on one axis only,
reacting to winds coming along the horizontal
axis. As mentioned, the wind conditions in urban
areas require specially designed wind turbines to
maximise the potential wind energy, hence the
novel cross axis wind turbine has been proposed.
Figure 1. Comparison between wind direction for HAWT and VAWT
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