tip speed ratio
In reference to a wind
energy conversion device's blades, the ratio between the rotational
speed of the tip of the blade and the
actual velocity of the wind. High efficiency 3-blade-turbines have tip speed
ratios of 6–7. On the whole, a high tip speed ratio is better, but
not to the point where the machine becomes noisy and highly stressed. The
tip speed ratio determines how fast the wind turbine will want to turn and
so has implications for the alternator
that can be used.
|Four rotors designed to run at different
tip speed ratios.
Image courtesy Hugh Scoraig
Modern wind turbines are designed to spin at varying speeds. Use of aluminum
and composites in their blades has contributed to low rotational inertia,
which means that newer wind turbines can accelerate quickly if the winds
pick up, keeping the tip speed ratio more nearly constant. Operating closer
to their optimal tip speed ratio during energetic gusts of wind allows wind
turbines to improve energy capture from sudden gusts that are typical in
In contrast, older style wind turbines were designed with heavier steel
blades, which have higher inertia, and rotated at speeds governed by the
AC frequency of the power lines. The high inertia buffered the changes in
rotation speed and thus made power output more stable.
The speed and torque at which a wind turbine rotates must be controlled
for several reasons:
Overspeed control is exerted in two main ways: aerodynamic stalling or furling,
and mechanical braking. Furling is the preferred method of slowing wind
- To optimize the aerodynamic efficiency of the rotor in light winds.
- To keep the generator within its speed and torque limits.
- To keep the rotor and hub within their centripetal force limits. The
centripetal force from the spinning rotors increases as the square of
the rotation speed, which makes this structure sensitive to overspeed.
- To keep the rotor and tower within their strength limits. Because
the power of the wind increases as the cube of the wind speed, turbines
have to be built to survive much higher wind loads (such as gusts of
wind) than those from which they can practically generate power. Since
the blades generate more downwind force (and thus put far greater stress
on the tower) when they are producing torque, most wind turbines have
ways of reducing torque in high winds.
- To enable maintenance; because it is dangerous to have people working
on a wind turbine while it is active, it is sometimes necessary to bring
a turbine to a full stop.
- To reduce noise; As a rule of thumb, the noise from a wind turbine
increases with the fifth power of the relative wind speed (as seen from
the moving tip of the blades). In noise-sensitive environments, the
tip speed can be limited to approximately 60 m/s.