Environmental Engineering Reference
In-Depth Information
horizontal axis machines. Another key advantage is that the mechanical load may
be connected directly to the VAWT rotor shaft and located at ground level. This
removes the need for a substantial tower to support the weight of equipment such
as the gearbox, generator and yaw mechanism. There is also no need for slip rings
or fl exible cables to connect the generator to the load, which can be an important
point for small-scale turbines.
From the 1970s to the 1990s a number of research groups and companies devel-
oped and built hundreds of VAWTs and a great deal has been learnt from that
experience. But despite the inherent advantages of VAWTs they have fallen sig-
nifi cantly behind HAWTs in recent years in terms of technical development and in
the size and number of units manufactured. This has occurred for a number of
reasons, not least because of some inherent disadvantages of VAWTs.
As the VAWT blades rotate about the main rotor shaft the velocity of the air
relative to the blade is constantly changing in respect of both magnitude and direc-
tion. In addition, each blade will interact with the wakes of other blades, and pos-
sibly its own wake, when it passes through the downstream half of its path about
the turbine axis. Both these effects result in fl uctuating aerodynamic forces on the
blades, which in turn lead to a potentially signifi cant fatigue issue for the design of
the blades and overall turbine structure. The fl uctuating blade loads also lead to a
varying torque transferred to the mechanical load.
Many designs of VAWTs produce very low torque when they are stationary and
may produce negative torque at low tip speed ratios, so they must be powered up to
a speed at which the aerodynamic torque is suffi cient to accelerate the rotor to nor-
mal operational speeds. A further disadvantage is that parasitic drag losses may be
high for a given VAWT design. This situation can arise when the VAWT blades need
to be mounted on structures (spars, beams, cables, etc.) that rotate with the blades or
are located upstream of the blades. The drag forces on these passive components can
lead to signifi cant parasitic losses in respect to rotor torque and power output. This
has inhibited the successful development of a number of VAWT designs.
Nevertheless there continues to be widespread interest in VAWTs as a means of
generating electrical and mechanical energy from the wind. Novel VAWT turbine
designs appear relatively frequently at the time of writing and a number of small
companies appear to be undertaking development of VAWTs for small-scale
application, particularly in respect to domestic dwellings.
2 H istorical development of VAWTs
2.1 Early VAWT designs
VAWTs appear to have been developed long before their horizontal axis cousins.
One of the reasons for this is that the VAWT has a number of inherent advantages
including the fact that a drive shaft may be connected directly from the rotor to
a mechanical load at ground level, eliminating the need for a gearbox. The early
pioneers involved in the development of wind turbines many centuries ago applied
VAWTs to the milling of grain, an application where the vertical axis of the mill-
stone could be easily connected to the VAWT rotor. Quite a number of excellent
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