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wind energy |
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The "BETZ" LIMIT |
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"Power coefficient curve of some common rotors are
indeed limited to Betz limit of 59% as shown in the figure above. The drag type
rotors, such as Savonius rotor and American multi-blade, have lower tip to speed
ratio and power coefficient compare to the lift type propeller HAWT and
Darrieus. The flat curve from propeller type rotor indicates that the rotor is
able to maintain high efficiency over a long range of rotor rpm while the sharp
curve from Darrieus rotor experiences efficiency drop drastically when the rotor
rpm moves away from the narrow optimum range. The Darrieus rotor with a low
power coefficient at low tsr range indicates a weak self-starting ability." |
