Wind Turbines - Wind Energy: Renewable Energy and the Environment - page 85

if the wind farm is composed of different wind turbines, it should be noted. For example, the Green
Mountain Wind Farm at the Brazos near Fluvana, Texas, has 160 1 MW wind turbines; however,
100 have rotor diameters of 61.4 m and 60 have rotor diameters of 56 m. Therefore, the capacity
factor will be larger for the units with the larger rotor. Notice that capacity factor is like an average
efficiency. In general, the generator size method gives reasonable estimates if the rated power of the
wind turbine is around 10-13 m/s. If the rated power is above that range, or for wind regimes below
class 3, then the capacity factor should be reduced accordingly.

EXAMPLE 5.1

Wind turbine has the following specifications:
Rated power 25 kW at 10 m/s
Rotor diameter 10 m
Estimated capacity factor 0.25

AKWH 0.25 * 25 kW * 8,760 h/year 55,000 kWh/year

For a poor wind regime, AKWH would be closer to 30,000 kWh/year.

A capacity factor of 0.25 would suffice for a generator rated at a wind speed of 10 m/s and
the wind turbine is in a medium wind regime. Wind farms are located in good to excellent wind
regimes, and capacity factors should be 32-40%. There have been reported capacity factors up to
50% for a wind farm located in the Isthmus of Mexico.

5.7.2 R OTOR A REA AND W IND M AP
The amount of energy produced by a wind turbine primarily depends on the rotor area, also referred
to as cross-sectional area, swept area, or intercept area. The swept area for different types of wind
turbines can be calculated from the dimensions of the rotor (see Figure 1.7 ).

HAWT area π r 2 , where r radius.
VAWT, where H height and D diameter of rotor:
Giromill area H*D
Savonius area H*D
Darrieus area 0.65 H * D

The annual average power/area can be obtained from a wind map, and then the energy produced
by the rotor can be calculated from

AKWH CF*Ar*WM*8.76 (5.3)

where Ar is the area of the rotor, m 2 ; WM power/area from a wind map, W/m 2 ; and 8.76 gives the
answer in kWh/year, the conversion W to kW.
Again, the capacity factor reflects the annual average efficiency of the wind turbine, around 0.20
to 0.35.

EXAMPLE 5.2

Use the wind turbine in Example 5.1, and from wind map:

WM 200 W/m 2

Area π r 2 3.14 * 25 m 2 78.5 m 2

AKWH 0.25 * 78.5 m 2 *200 W/m 2 * 8.76 kWh/year 34,000 kWh/year