Environmental Engineering Reference
In-Depth Information
period from March 1988 and produced over 12 GWh of electricity in the Le Nordais/
Cap Chat wind farm (which also includes 73 NEG-Micon 750 kW HAWTs at the
time of writing) [17].
One of the characteristics of the Darrieus family of turbines is that they have
a limited self-starting capacity because there is often insuffi cient torque to over-
come friction at start-up. This is largely because lift forces on the blades are
small at low rotational speeds and for two-bladed machines in particular the
torque generated is virtually the same for each of the stationary blades at start-up,
irrespective of the rotor azimuth angle relative to the incident wind direction.
Moreover, the blades of a Darrieus rotor are stalled for most azimuth angles at
low tip speed ratios. As a result large commercial machines generally need to be
run up to a suffi ciently high tip speed for the rotor to accelerate in a given wind
velocity. Nevertheless, two-bladed Darrieus machines do have the capacity to
self-start, albeit on a somewhat unpredictable basis, and although this is advan-
tageous in most circumstances it can cause problems. A case in point was in
1978 when the 225 kW Magdelen Islands Darrieus turbine was left for a few
hours overnight without a brake engaged due to maintenance issues and the
belief that such turbines would not self-start. The following morning researchers
returned to fi nd the turbine rotating at high speed with no load. As a result an
energetic resonance developed in one of the rotor guy wires so that the guy came
into catastrophic contact with the rotor which was entirely destroyed [17].
Another scenario where this can be a problem is when the turbine starts and
turns initially at low tip speed ratio in a strong wind. If there is then a sudden
decrease in the wind speed so the tip speed ratio increases the rotor power output
may then be suffi cient to cause rapid acceleration and damage may occur [18].
Self-starting capability may be enhanced through a number of strategies includ-
ing: increasing solidity; using an odd number of blades; providing a form of
blade pitch mechanism; and using blades that are skewed so that the blade azi-
muth angle is a function of axial distance along the rotor. A recent study of the
self-starting characteristics of small Darrieus machines has been reported by
Hill
et al.
[ 19 ].
In terms of noise generation there appears to have been very little experimental
or theoretical analysis reported on Darrieus rotors or other types of VAWTs. Since
Darrieus turbines have relatively low tip speeds compared to HAWTs one might
expect the noise generated to be less problematic, as indicated by the analysis of
Iida
et al.
[ 21 ].
2.2.3 Straight-blade VAWTs
The name Darrieus is usually associated with the curved-blade version of Darrieus'
patent. However, a great deal of work over the past three decades has gone into
the development and analysis of the straight-blade version of his original inven-
tion, which is sometimes known as the H-VAWT from the shape of the blades
and supporting spars. One of the key researchers in the 1970s was Peter
Musgrove who spent over 20 years working on wind turbines at the University
of Reading.
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