Wind power is the second-largest source of renewable energy that does not produce greenhouse gases, surpassed only by hydroelectric power. Many countries are rapidly constructing wind power facilities, and within a few decades, wind power should provide about 4% of global electricity. As much as 72,000 gigawatts (GW) of wind power might be commercially feasible worldwide, many times the current total energy demand. 
Winds derive from temperature differences between the Tropics and the Poles, between land and sea, and between Earth’s surface and upper atmosphere that produce pressure differentials among air masses. Electric power generated from wind is proportional to its speed (velocity) to the third power. Most potential sites for wind turbines are located just offshore, where land/sea temperature differentials drive winds and where open water presents few impediments.
Modern wind turbines fall into two major categories. Horizontal turbine designs position the main turbine rotor shaft and electrical generator at the top of a support tower. They are more efficient in slower winds than are vertical designs, but require stiff, precisely aligned blades (oriented into the wind) to be efficient and to avoid interference from the support tower itself as well as uneven stresses on the blades and the bearings. To minimize stress on the rotor shaft, modern horizontal turbines have an odd number of blades, most commonly three (Figure 8.32).
The other category of wind turbine has a vertical main rotor shaft. Typical designs, such as the Darrieus “eggbeater," allow ground placement of the generator and gearbox for easy access. These turbines operate without needing to be oriented into the wind but suffer from low efficiency, particularly at slow wind speeds, and sometimes require electric motors to start their rotation at low wind velocities.
The intermittent nature of wind makes it an unpredictable energy source. Worse yet, demand for electricity may not coincide with periods of high winds. For instance, in the United States, the state of Texas generates the most wind power; the hottest days in Texas, which have the peak demands for electricity to power air conditioning, tend to be those with the least wind. Thus the success of wind power depends on integration into a large energy grid with a high capacity for energy storage such as hydroelectric power.
On the positive side, the cost of wind power is competitive with other energy sources. Wind power has negligible fuel costs and relatively low maintenance costs; most costs are for construction and transmission. European power companies, which have extensive experience in wind turbines,have found them so profitable that they are financing two-thirds of the wind projects under construction in Texas. 
Ecological concerns about wind turbines center largely on injuries to animals that fly into turbine blades. 
However, wind turbines are responsible for a minute fraction— less than 0.003%—of bird deaths from collisions with human structures. Data is sparse, but fatal encounters between bats and turbines appear to occur at rates similar to those for birds in most places in the United States with the exception of the Appalachian Mountain region, where the rates for bats are several times higher.
Wind farms also face local opposition if their turbines obstruct scenic views. To improve their aesthetics, new installations have more widely spaced turbines, which look less cluttered than old installations, and offshore installations are located farther out to sea.
 Archer, C. L. and M. Z. Jacobson (2005) Evaluation of global wind power. Journal of Geophysical Research-Atmospheres 110:- doi:D12110 Doi 10.1029/2004jd005462.
 Krauss, C. (2008) Move over, oil, there's money in Texas wind. The New York Times, New York, February 23, 2008.
 National Research Council (2007) Environmental Impacts of Wind-Energy Projects, National Academies of Sciences, Washington, D.C.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
©2010 Sinauer Associates and UC Regents