Page 203 - Wind Energy Handbook

CONSTANT ROTATIONAL SPEED OPERATION 177

0.12

0.1

0.08 One blade
C Two blades
T
Three blades
Four blades
0.06
Five blades

0.04

0.02

0
λ

0 5 10 15 20
Figure 4.5 The Effect of Solidity on Thrust

4.2 Constant Rotational Speed Operation

The majority of wind turbines currently installed generate electricity. Whether or
not these turbines are grid connected they need to produce an electricity supply
which is of constant frequency or else many common appliances will not function
properly. Consequently, the most common mode of operation for a wind turbine is
constant rotational speed. Connected to the grid a constant speed turbine is
automatically controlled whereas a stand-alone machine needs to have speed
control and a means of dumping excess power.

4.2.1 The K P –1=º curve

An alternative performance curve can be produced for a turbine controlled at
constant speed. The C P – º curve shows, non-dimensionally, how the power would
vary with rotational speed if the wind speed was held constant. The K P –1=º curve
describes, again non-dimensionally, how the power would change with wind speed
when constant rotational speed is enforced. K P is defined as