Page 400 - Wind Energy Handbook

374 CONCEPTUAL DESIGN OF HORIZONTAL-AXIS TURBINES

less severely unloaded by the tower shadow, because wind loading deflects them
further from the tower in the first place.

6.14 Tower Stiffness

A key consideration in wind turbine design is the avoidance of resonant tower
oscillations excited by rotor thrust fluctuations at rotational or blade-passing
frequency. The damping ratio may be only 2–3 percent for tower fore-aft oscilla-
tions and an order of magnitude less for side-to-side motion, so unacceptably large
stresses and deflections could develop if the blade-passing frequency and tower
natural frequency were to coincide. Rotational frequency is less of a concern,
because cyclic loadings at this frequency only arise if there are geometrical
differences between blades.
Wind-turbine towers are customarily categorized according to the relationship
between the tower natural frequency and the exciting frequencies. Towers with a
natural frequency greater than the blade-passing frequency are said to be stiff,
while those with a natural frequency lying between rotational frequency and blade-
passing frequency are said to be soft. If the natural frequency is less than rotational
frequency, the tower is described as soft–soft.
If the tower is designed to meet strength requirements and no more, its frequency
category is primarily determined by the ratio of tower height to turbine diameter,
with the higher ratios producing the softer towers. The principal benefits of stiff
towers are modest – they allow the turbine to run up to speed without passing
through resonance, and tend to radiate less sound. However, since stiff towers
usually require the provision of extra material not otherwise required for strength,
soft towers are generally preferred.

6.15 Personnel Safety and Access Issues

An integral part of wind-turbine design is the inclusion of the necessary safety
provisions for operation and maintenance staff. Minimum requirements include the
following:

• ladder access to the nacelle – this needs to be fitted with a fall-arrest device,
unless ladder runs are short and protected by intermediate landings; careful
attention needs to be paid to the route between the tower top and nacelle to avoid
hazards arising from sudden yawing movements;

• an alternative means of egress from the nacelle, for use in case of fire in the tower
– this can take the form of an inertia-reel device, enabling personnel to lower
themselves through a hatch in the nacelle floor;

• locking devices for immobilizing the rotor and the yawing mechanism – rotor
brakes and yaw brakes are not considered sufficient, because of the risk of