Renewable Energy Resources for Irrigation - Practices of Irrigation and On-farm Water Management - page 493

land and water, it absorbs the sun's heat at different rates. During the day, the air
above the land heats up more quickly than the air over water. The warm air over the
land expands and rises, and the heavier, cooler air rushes in to take its place, creating
winds. At night, the winds are reversed because the air cools more rapidly over land
than over water.
In the same way, the large atmospheric winds that circle the earth are created
because the land near the earth's equator is heated more by the sun than the land
near the North and South Poles.

13.5.4 Energy from Wind

Airflows can be used to run wind turbines. Modern wind turbines range from around
600 kW to 5 MW of rated power, although turbines, with rated output of 1.5-3 MW,
have become the most common for commercial use. The power output of a tur-
bine is a function of the cube of the wind speed, so as wind speed increases,
power output increases dramatically. Areas where winds are stronger and more
constant, such as offshore and high altitude sites, are preferred locations for wind
farms.
Since wind speed is not constant, a wind farm's annual energy production is never
as much as the sum of the generator nameplate ratings multiplied by the total hours
in a year. The ratio of actual productivity in a year to this theoretical maximum is
called the capacity factor . Typical capacity factors are 20-40%, with values at the
upper end of the range in particularly favorable sites. For example, a 1 MW turbine
with a capacity factor of 35% will not produce 8,760 MWh in a year, but only
1 3,066 MWh, averaging to 0.35 MW.

Globally, the long-term technical potential of wind energy is believed to be
five times of total current global energy production, or 40 times current electric-
ity demand. This could require large amounts of land to be used for wind turbines,
particularly in areas of higher wind resources. Offshore resources experience mean
wind speeds of ~90% greater than that of land, so offshore resources could con-
tribute substantially more energy. This number could also increase with higher
altitude ground-based or airborne wind turbines.

× 0.35 × 24 × 365 =

13.5.5 Advantages of Wind Energy

Wind energy has many advantages over conventional energies. The significant
advantages are the following:

• A wind farm, when installed on agricultural land, has one of the lowest

environmental impacts of all energy sources.

• It occupies less land area per kilowatt-hour (kWh) of electricity generated than

any other energy conversion system, apart from rooftop solar energy, and is
compatible with grazing and crops.