Page 363 - Wind Energy Handbook

ROTATIONAL SPEED 337

Table 6.3 Variation of Optimum Rated Wind Speed with Annual Mean for Pitch-regulated
Machines
Annual mean Optimum Ratio Optimum Specific power, Cost index, with
wind speed, rated wind U Ro =U ave rated power defined as rated cost of energy
U ave (m=s) speed, (kW) power per unit for a.m.w.s. of
U Ro (m=s) swept area 7:5m=s taken
2
(kW=m ) as100
7 12.4 1.77 1012 358 114
7.5 13.1 1.74 1187 420 100
8 13.7 1.72 1376 487 89
8.5 14.4 1.69 1579 558 80
9 15.0 1.67 1797 635 72

1600

1400

1200

1000
Rated power (kW) 800

600
Mean value of
power/swept area ratio
= 405 W/m 2
400
200

0
0 500 1000 1500 2000 2500 3000 3500 4000
Swept area (m ) 2
Figure 6.3 Rated Power versus Swept Area for Turbines in Production in 1997

have been designed for different annual mean wind speeds, the degree of scatter is
not large, and a clear trend is apparent, with the line of best fit being close to a
straight line passing through the origin. The mean specific power, defined as rated
2
power divided by swept area, is 405 W/m for the 75 machines – close to the
optimum value in Table 6.3 for an annual wind speed of 7.5 m/s.

6.4 Rotational Speed

The aim of the wind turbine designer is the production of energy at minimum cost,
subject to constraints imposed by environmental impact considerations. However,