Wind is caused by the uneven heating of the earth by the sun. During the day, the earth's land masses heat up, and the air above them rises. The earth's water heats up more slowly, and the cooler air above the water is drawn across the land by the rising land air, creating wind. At night, the winds change because the land cools down more quickly than the water. Winds are also created by the overheating of the equator relative to the poles.

All moving objects, including invisible air particles, have kinetic energy. In order to collect this energy, we can use wind turbines, which are essentially backwards fans--fans turn electricity into wind, turbines turn wind into electricity.

The energy available in wind is proportional to the area it is passing through, and the cube of its velocity. For a given wind speed, if you double the radius of the turbine (quadruple the area), you quadruple the wind speed. For a given turbine size, if you double the wind speed, you increase the power generated by a factor of eight.

A wind turbine collects energy from the wind with asymmetrical blade surfaces, which cause different air velocities on the two sides, creating a pressure differential, and the aerodynamic lift force that spins the turbine. Air drag on the blade surfaces slows the spinning of the blades. Usually you want high lift and low drag, but depending on the wind speeds you're working with, you can control the ratio between the two to optimize generator efficiency.

References: Basic Aerodynamic Operating Principles of Wind Turbines

• 1st Century AD: Hero of Alexandria constructs wind-powered organ.
• 9th Century AD: Persians design first windmill that would be recognizable today.
• 1891: An extremely forward thinking Dane named Poul la Cour first uses wind turbine to generate electricity to electrolyze water into hydrogen to burn in the gas lamps at Folk High School. He is way ahead of our time.

References: Illustrated History of Wind Power Development

• Horizontal Axis Turbines


• Vertical Axis Turbines

Vertical axis turbines comprise a very small share of the wind market.

Advantages: They function well no matter which way the wind is blowing.

Disadvantages: They don't always start themselves, and they are hard to shut down in high winds.

The Darrieus rotor, pictured below, is the most popular type of vertical axis wind turbine.


(Source: Wikimedia Commons)

Each year, over the earth's continents, 1.7 million TWh (~200 TW) of wind energy is generated by the sun's heat.⁽¹⁾

Of that, 72 TW has been shown to be technically feasible, which could satisfy 5 times the total primary energy demand of the world, or more than 40 times the world electricity demand.⁽²⁾

Avian Collisions

Anthropogenic Causes of Bird Deaths from A Summary and Comparison of Bird Mortality from Anthropogenic Causes with an Emphasis on Collisions (Erickson et. al. 2005)

The effects of a wind farm on avian life depends largely on where the farm is located. Altamont Pass has notoriously high raptor death rates because it is badly sited in a raptor migration zone. Proper siting of wind turbines can mitigate most, if not all concerns of avian mortality.