Renewable Energy Resources for Irrigation - Practices of Irrigation and On-farm Water Management - page 498

13.5.11 Wind Power Plants or Wind Farms

Wind power can be tapped in a variety of places. Wind can also be tapped on water,
or on farms. Wind turbines, like windmills, are mounted on a tower to capture the
most energy. At 30 m or more aboveground, they can take advantage of the faster
and less turbulent wind. Turbines catch the wind's energy with their propeller-like
blades. Usually, two or three blades are mounted on a shaft to form a rotor.
Wind-power plants or wind farms are clusters of wind machines used to produce
electricity (Fig. 13.5 ) . A wind farm usually has dozens of wind machines scattered
over a large area. The world's largest wind farm, the Horse Hollow Wind Energy
Center in Texas, has 421 wind turbines that generate enough electricity to power
230,000 homes per year.

13.5.11.1 Seasonal Variation of Wind Energy

Operating a wind-power plant is not as simple as just building a windmill in a windy
place. In Tehachapi, California, the wind blows more from April through October
than it does in the winter. This is because of the extreme heating of the Mojave
Desert during the summer months. The hot air over the desert rises, and the cooler,
denser air above the Pacific Ocean rushes through the Tehachapi mountain pass to
take its place. In a state like Montana, on the other hand, the wind blows more
during the winter. Fortunately, these seasonal variations are a good match for the
electricity demands of the regions. In California, people use more electricity during
the summer for air conditioners. In Montana, people use more electricity during the
winter months for heating.

13.5.12 Calculation of Wind Power

Wind turbines convert the kinetic energy in the wind into mechanical power. This
mechanical power can be used for pumping water, or a generator can convert this
mechanical power into electricity.
Consider a segment of air shaped like a horizontal cylinder. The energy in it
depends on its volume, density, and speed. The kinetic energy ( E k )ofthewind
segment is

1
2 MV 2

E k = (13.6)

The mass per unit time for the air volume, M = ρ AV

where

density of air (kg/m 3 )

ρ =

swept area (m 2 ) A =

V = wind speed (m/s)