Environmental Engineering Reference
In-Depth Information
Number of rotor blades
Modern horizontal axis wind generators can have one, two or three rotor
blades. More than three blades are usually not used. The lower the number of
rotor blades, the less material is needed during manufacturing.
Single-bladed rotors
must have a counterweight on the opposite side of the
rotor. Single-bladed rotors do not have a smooth motion and therefore exhibit
a high material stress. Only very few prototypes with one rotor blade exist and
it is not expected that this will change soon.
The optimal power coefficient of three-bladed rotors is slightly above that
of two-bladed rotors. Three-bladed rotors have an optically smoother
operation and hence visually integrate better into the landscape. The
mechanical strain is also lower for three-bladed rotors. The advantages of
three-bladed rotors compensate for the disadvantage of the higher material
demand so that today mainly
three-bladed rotors
are built.
Wind speed ranges
The design tip speed ratio depends closely on the number of rotor blades. The
maximum power coefficient of a three-bladed rotor is reached at a tip speed
ratio of between 7 and 8, whereas for two-bladed rotors the figure is 10 and
for a one-bladed rotor about 15. However, these values can vary depending on
the system design. The optimum tip speed ratio
λ
opt
also defines the
design
wind speed
along with the rotor radius
r
and the rotational speed of the rotor
n
in min
-1
.
(5.46)
For example, the optimum tip speed ratio of a three-blade wind turbine with a
rotor radius of
r
= 22 m and a rotational speed of
n
= 28 min
-1
= 0.467
s
-1
is
λ
opt
= 7.5, and therefore the design wind speed is
v
D
= 8.6 m/s. The
wind turbine has its maximum efficiency at this wind speed. The design wind
speed is very important for wind turbines with constant speed. Systems with
variable speed can obtain the optimum efficiency at other wind speeds also. In
this case, the design wind speed
range
should replace the design wind speed.
At very low wind speeds the operation of the wind turbine makes little
sense. No power or only a very little power can be taken from the wind and
the wind generator can even become a power consumer. Therefore, the rotor
brake should stop the wind turbine below a predefined starting wind speed, or
cut-in wind speed v
cut-in
.
The design wind speed
v
D
was explained above; the rated or
nominal wind
speed v
N
of a wind turbine is usually different. At the nominal wind speed, the
wind turbine generates the rated power. The nominal wind speed is usually
higher than the design wind speed. Above the nominal wind speed, the power
of the wind turbine must be limited. If the wind speed becomes too high, the
wind power plant can be overloaded and damaged. Therefore, wind turbines
cut out at high wind speeds,
v
cut-out
: the rotor brakes stop the wind turbine
