Fundamentals of wind energy - Wind Power Generation and Wind Turbine Design - page 20

is positioned on the top of a wind tower, housing the most turbine components
inside. Three blades (not shown) mounted on the rotor hub, which is connected
via the main shaft to the gearbox. The rotor of the wind generator is connected to
the output shaft of the gearbox. Thus, the slow rotating speed of the rotor hub is
increased to a desired high rotating speed of the generator rotor.
Using the pitch control system, each blade is pitched individually to optimize
the angle of attack of the blade for allowing a higher energy capture in normal
operation and for protecting the turbine components (blade, tower, etc.) from dam-
aging in emergency situations. With the feedback information such as measured
instantaneous wind direction and speed from the wind vane, the yaw control sys-
tem provides the yaw orientation control for ensuring the turbine constantly against
the wind.

5.3 Wind power parameters

5.3.1 Power coeffi cient
The conversion of wind energy to electrical energy involves primarily two stages:
in the fi rst stage, kinetic energy in wind is converted into mechanical energy to
drive the shaft of a wind generator. The critical converting devices in this stage are
wind blades. For maximizing the capture of wind energy, wind blades need to be
carefully designed.
The power coeffi cient C p deals with the converting effi ciency in the fi rst stage,
defi ned as the ratio of the actually captured mechanical power by blades to the
available power in wind:

P P

me,out me,out

C = = (16)

p

3

P (1 / 2) r Au

w

Because there are various aerodynamic losses in wind turbine systems, for
instance, blade-tip, blade-root, profi le, and wake rotation losses, etc., the real
power coeffi cient C p is much lower than its theoretical limit, usually ranging from
30 to 45%.

5.3.2 Total power conversion coeffi cient and effective power output
In the second stage, mechanical energy captured by wind blades is further converted
into electrical energy via wind generators. In this stage, the converting effi ciency is
determined by several parameters

￿ Gearbox effi ciency h gear - The power losses in a gearbox can be classifi ed as

load-dependent and no-load power losses. The load-dependent losses consist of
gear tooth friction and bearing losses and no-load losses consist of oil churning,
windage, and shaft seal losses. The planetary gearboxes, which are widely used
in wind turbines, have higher power transmission effi ciencies over traditional
gearboxes.