Environmental Engineering Reference
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Figure 7: Blade tip speed for the industry study set.
when projecting forward to the 7
10 MW ranges. This projection is probably
on the conservative side of what can be expected, assuming similar technology
progress over the last decade projected to larger machines of the next decade. It
is also reasonable and highly desirable to beat this trending with an acceleration
of breakthrough technology. This scenario is becoming more likely due to the
massive increase in public and venture capital funding that is beginning to be
injected into the wind industry, not to mention the recent exponential response by
academia to shift research to the renewables sector.
Noise emission is one of the fi rst considerations when setting out to design a
new turbine. Blade tip speed is considered one of the primary drivers of noise
with sensitivity proportional to the tip speed raised to the 5th power [10]. Fig-
ure 7 shows the range of blade tip speeds for today's industry study set as a func-
tion of turbine power rating. The curve fi t shown is not very good, in part because
of the wide range of blade lengths for machines with the same generator rating,
but the correlation is better than other types of fi ts. Its character is also consistent
with other related parameters such as the main shaft rotational speed. Tip speeds
of 70
−
85 m/s are being used, with the higher end of the range tending to be for
larger machines. The trend for this data suggests that tip speeds much beyond
90 m/s are unlikely for machines in the 7
−
10 MW sizes. Other noise reducing
considerations would need to be developed if advantages for even higher tip
speeds proved desirable.
As shown in Fig. 8, there is a much better correlation for values of rated rotor
speed with turbine size. Today's machines have rated rotor speeds in the low 20s
to the high teens, but speeds are not projected to fall below about 10 RPM for
7
−
10 MW sizes. This is also consistent with the practical limit of torque that
increases with lower speeds.
−
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