Page 399 - Wind Energy Handbook

ROTOR POSITION WITH RESPECT TO TOWER 373

just one or two degrees at the low-speed shaft. An interesting variant (Leithead and
Rogers, 1995) is to mount the generator on flexible mounts. This system can be
tuned to absorb energy at the blade-passing frequency through an additional mode
of vibration of the generator casing against its mountings. This mode also affects
the generator slip speed (the difference between rotor and casing speeds) and is
therefore damped by the slip curve. Nevertheless, generator casing displacements
would still need to be of the order of 10–158, which is still not easy to engineer.

6.13 Rotor Position with Respect to Tower

6.13.1 Upwind configuration

The upwind configuration is the one most commonly chosen. The principal
advantage is that the tower shadow effect is much less for the same blade–tower
spacing, reducing both dynamic loads on the blade and rhythmic noise effects. Set
against this is the need to take great care to avoid the risk of blade–tower strikes
with upwind machines, requiring accurate prediction of blade deflections under
turbulent wind loading.
The clearance between the undeflected blade and the tower can be increased by
tilting the low-speed shaft upwards or by increasing the rotor overhang. It is
desirable to keep the rotor overhang small in order to minimize low-speed shaft
and nacelle bedplate bending moments, so the low-speed shaft is normally tilted
upwards by 58 or 68 to provide the necessary blade–tower clearance, at the cost of a
very small reduction in power output.

6.13.2 Downwind configuration

The wind velocity deficit behind a wind-turbine tower is much greater than that in
front of it, to the extent that Powles (1983) has reported a turbulent region with
essentially no forward velocity extending up to four tower diameters downstream
of an octagonal tower. Beyond this distance, recovery is relatively rapid, with the
deficit reduced to about 25 percent at seven tower diameters downstream.
In addition to the mean wind-speed velocity deficit behind the tower, vortex
shedding results in additional wind-speed fluctuations over and above those
already present due to turbulence. The two effects combine to present a harsh
environment to the blades immediately behind the tower. The blades are subjected
to a large negative impulsive load each time they pass the tower, which contributes
significantly to blade fatigue damage, and the audible tower ‘thump’ that results is
liable to be unwelcome. Designers usually mitigate both effects by positioning the
rotor plane well clear of the tower, but this inevitably increases nacelle costs
somewhat.
An important benefit of the downwind configuration is that it allows the use of
very flexible blades without the risk of tower strike. Such blades benefit by being