# Wind Energy

#### Wind Energy

An estimated 1% to 3% of energy from the Sun that hits the earth is converted into wind energy. This is about 50 to 100 times more energy than is converted into biomass by all the plants on Earth through photosynthesis. Most of this wind energy can be found at high altitudes where continuous wind speeds of over 160 km/h (100 mph) occur. Eventually, the wind energy is converted through friction into diffuse heat throughout the Earth’s surface and the atmosphere. Wind power is the conversion of wind energy into more useful forms, usually electricity, using wind turbines. Globally, wind power generation more than quadrupled between 2000 and 2006.

The power in the wind can be extracted by allowing it to blow past moving wings that exert torque on a rotor. The amount of power transferred is directly proportional to the density of the air, the area swept out by the rotor, and the cube of the wind speed.

The power P available in the wind is given by: where:

P = power [watts]
ρ = air density [kg/m^3]
A = area swept out by rotors [m^2]
u = wind velocity [m/s^2]

The mass flow of air that travels through the swept area of a wind turbine varies with the wind speed and air density. As an example, on a cool 15°C (59°F) day at sea level, air density is 1.225 kilograms per cubic metre. An 8 m/s breeze blowing through a 100 meter diameter rotor would move almost 77,000 kilograms of air per second through the swept area. Amounting to about 2.5MW of power.