Wind Power Generation - Renewable Energy: Technology, Economics and Environment - page 324

power coefficient that may amount to almost 50 % at the turbine design point
has a favourable effect on high-speed wind energy converters. Thanks to the
flatter curve - in comparison to slow wind energy converters - of the character-
istic lines c p ( λ ), the relatively high power coefficient is furthermore maintained
within a relatively wide range of the tip speed ratio. For converters equipped
with only a few rotor blades, deviations from the maximum tip speed ratio only
slightly reduce the power coefficient. All in all, according to state-of-the-art
technology, modern two or three blade rotors yield the highest power coeffi-
cients of all rotors built to date.

0.7

Maximum power coefficient according to Betz' law
Maximum power coefficient of an ideal wind rotor

0.6

0.5

Two blade rotor

Three blade rotor

0.4

American

One blade rotor

wind mill

0.3

Darrieus rotor

0.2

Dutch

wind mill

0.1

Savonius
rotor

0

0 2 4 6 8 10 12 14 16 18

Fig. 7.16 Power coefficient-blade tip speed/wind speed ( c p ( λ )) curve of different wind en-
ergy converter designs (see /7-3/)

Blade tip speed / wind speed

− Mechanical losses are mainly due to friction losses and the resulting heat crea-
tion inside the rotor shaft bearings and, if a gearbox is provided, the rotating
speed conversion which is prone to losses.
− Electrical losses comprise conversion losses inside the generator, power losses
of the grid and, as the case may be, losses incurred during power conversion by
the direct current intermediate circuit (losses attributable to semi-conductors,
throttle valves, etc.) or the direct converter. Depending on converter dimen-
sions, additional losses may result from electrical energy conversion by the
transformer.

Characteristic power curve. The power yield of wind energy converters, for in-
stance on the basis of the average power generated within 10 minutes, can be de-
termined by means of a converter-specific characteristic power curve. It reveals
the dependency of the average electrical power from the respective average wind
velocity and thus shows the operational characteristics of the converter. With re-
gard to operational characteristics, four different phases (Fig. 7.17) can be distin-
guished.