Civil Engineering Reference
In-Depth Information
lax on providing the higher side of the wind speed, claiming greater output
from the same design.
To avoid such rating confusion, some European manufacturers refer to
only the rotor diameter. But the confusion continues as to the maximum
power the machine can generate under the highest wind speed the turbine
can continuously operate. Many manufacturers have, therefore, adopted the
combined rating designations — the wind turbine diameter following the
generator peak electrical rating. For example, the 300/30 wind system means
300 kW electrical generator and 30-meter diameter turbine.
The specific rated capacity (SRC) is often used as a comparative index of
the wind turbine designs. It is defined as follows:
Generator electrical capacity
Rotor swept area
SRC
=
(5-1)
For the 300/30 wind turbine, the specific rated capacity is 300/
π
15
=
2
0.42 kW/m
. The specific rated capacity increases with the diameter, giving
a favorable economy of scale to large machine. It ranges from approximately
0.2 kW/m
2
for 40-meter diameter
rotor. Some aggressively rated turbines have SRC of 0.7 kW/m
for 10-meter diameter rotor to 0.5 kW/m
2
2
, and some
2
reaching as high as 1.0 kW/m
. The operating stresses in rotor blades of the
high SRC are high, generally resulting in shorter fatigue life. All stress
concentration regions are carefully identified and eliminated in high SCR
designs. Modern design tools, such as the finite element stress analysis and
the modal vibration analysis can be of great help in the rotor design.
The turbine rating is important as it indicates to the system designer how
to size the induction generator, the plant's transformer, connecting cables to
the substation, and the transmission link interfacing the grid. The power
system must be sized on the peak capacity of the generator, and the generator
is rated in a different manner than the wind turbine. The turbine power
depends on the cube of the wind speed. The system design engineer is,
therefore, required to match the turbine and the generator performance
characteristics. This means selecting the rated speed of the turbine to match
with the generator. Since the gearbox and the generator are manufactured
only in discrete sizes, selecting the turbine rated speed can be complex. The
selection process goes through several iterations, trading the cost with benefit
of the available speeds. Selecting a low rated speed would result in wasting
much energy at high winds. On the other hand, if the rated speed is high,
the rotor efficiency will suffer most of the times.
Figure 5-8
is an example of the summary data sheet of the 550/41 kW/m
wind turbine manufactured by Nordtank Energy Group of Denmark. Such
data is used in the preliminary design of the overall system. The specific
rated capacity of this machine is 0.415. It has the cut-in wind speed of 5 m/s,
the cutout speed of 25 m/s and it reaches the peak power at 15 m/s.
2
