Development and analysis of vertical-axis wind turbines - Wind Power Generation and Wind Turbine Design - page 286

Musgrove recognised that one of the key challenges facing VAWTs was the
need to control the power output of the device at high wind speeds and that active
pitch control of blades would result in an unnecessarily complex mechanical sys-
tem for large devices. His research team developed a furling system whereby the
straight blades could be hinged at their mid-point so that the angle of the blades
relative to the axis of the rotor could be adjusted by mechanical actuators. A num-
ber of geometries were developed (including the V-VAWT machine and tested at
small scales (e.g. diameter of order 3 m). However, it should be mentioned that the
furling method described above can potentially lead to high transient vertical lift
forces due to the effects of turbulence, which may in turn lead to high loads or
failure of the supporting radial arms [18].
In the mid-1980s the UK Department of Energy supported the development of
several VAWTs based on Musgrove's design. These were developed by a UK com-
pany VAWT Ltd. and several prototypes were built at the Carmarthen Bay test site
of the Central Electricity Generating Board [22]. The fi rst machine, the VAWT-
450, was commissioned in 1986 and it had a 25-m diameter rotor with blades 18 m
in length which provided a rated output of 130 kW at a wind speed of 11 m/s.
Subsequently a much larger version of this device was designed and built by
VAWT Ltd. and also installed at Carmarthen Bay. The VAWT-850 had a 45-m
diameter rotor and a rated output of 0.5 MW. The design did not include a furling
system for the blades as previous experience with the VAWT-450 had demon-
strated that this was unnecessary due to the inherent ability of the straight-bladed
VAWT to avoid overspeed and excessive power generation through stall of the
blades at high wind speeds. Although the VAWT-850 was a successful demonstra-
tion of the straight-bladed VAWT technology it suffered a catastrophic failure of
one of the blades in 1991, apparently due to a manufacturing fault [22].
2.2.4 Giromills
One of the consequences of adopting a vertical axis for any wind turbine is that the
apparent velocity of the wind at a particular location on a blade will change through-
out each revolution of the rotor (Fig. 7). For example, when the blade is travelling
upstream (i.e. when the azimuth angle 0° < b < 180°) the resultant air velocity on the
blade is greater than the tangential velocity of the blade relative to a stationary frame
of reference, whereas, when the blade is travelling downstream (180° < b < 360°)
the resultant wind speed is generally less than the tangential blade speed. This in turn
means that the angle of attack on the blade is continually changing and is generally
not optimal throughout its rotation about the axis of the turbine.
To improve this situation various means have been devised to optimise the blade
pitch angle (i.e. the chord angle relative to a tangent to the path of the blade) as a
function of azimuth angle, b . Systems have been devised to achieve this in a num-
ber of ways, including mechanical mechanisms with levers and/or pushrods con-
nected between the blades and the main rotor shaft (as shown in Darrieus' original
patent, Fig. 5b) or by means of aerodynamic mechanisms. Such turbines which
seek to optimise the blade pitch angle are often known as Giromills [24], although
some authors also refer to these systems as cycloturbines.