## Generator Size

This method gives a rough approximation because wind turbines with the same size rotors can have different size generators:

where AKWH = annual energy production, kWh/year; CF = capacity factor; and 8,760 = number of hours in a year.

The effect of the wind regime and the rated power for the rated wind speed can be estimated by changing the capacity factor. The capacity factor is the average power divided by the rated power (generator size). The capacity factor is estimated from energy production over a selected time period, and in general, capacity factors are quoted on an annual basis, although some are calculated for a quarter of a year. Capacity factors can also be calculated for wind farms, and they should be close to the same values as capacity factors calculated for individual wind turbines. However, 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.

## Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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