I grew up on a farm in Kansas. One of my fondest memories is climbing up the rickety ladder on our windmill tower to the platform just below the gearbox and wind vane to view the fields spreading out like a huge checkerboard in every direc­tion. The wind pumped water from underground to fill the stock tank to water the dairy cows and sheep that we raised. Indeed, the wind was critical for providing water so that Kansas and other states could be settled and farmed. Wind has been harnessed to do work for thousands of years. Who has not been charmed by the old Dutch windmills harnessed to grind wheat into flour and do other work? Can wind now provide the clean electricity to drive a modern economy?

Once or twice a year, my wife and I drive from Colorado back to Kansas to see relatives and to visit her farm. Sprouting out of the prairie are enormous towers for wind generators that put the old windmills to shame (Figure 4.4). The most prominent and largest of these is the Smoky Hills Wind Farm about 20 miles west of Salina just north of Interstate 70. This wind farm has 155 turbines spread over an area of 26,000 acres and can produce 250 MW at peak capacity (36). The enormous blades on wind turbines are typically between 125 and 150 feet in length and the towers are 225 to 350 feet high, for a total height approaching 500 feet (37).

Wind energy ultimately comes from the sun because it depends on the tem­perature and pressure differences created by the sun shining on the oceans and land. The wind turns the blades that drive a gearbox to run a generator that pro­duces electricity. The gearbox and generator are in a bus-sized nacelle perched atop the tower. The energy of the wind varies by the cubed power of wind speed, so the electrical power output increases dramatically as the wind speed increases to the turbine’s rated wind speed, typically 11 to 14 meters per second (25 to 31 mph), and then remains flat until the cut-out speed. For example, if the wind speed doubled from 10 to 20 mph, the electrical power output would increase by eight times! Of course, the output also drops off rapidly as the wind falls below the rated wind speed. This makes wind power fluctuate rapidly as wind speeds constantly vary. Wind can also blow too hard. When wind speeds exceed about 60 mph (the cut-out speed), the turbines shut down to avoid permanent damage. The rated output of most wind turbines is 1.5-2.5 MW, which is specified as the rated wind speed (38).

As with solar energy, wind energy is not equally distributed across the United states or the world. Wind is less variable and stronger as you get higher from the earth’s surface, so wind maps are usually based on the wind speed at 50 meters (160 ft.) from the ground. Even better wind resources are available at 75 or 100 meters above the earth, a height that wind blades reach, especially on newer tow­ers. Wind power is classified in a range from 1 to 7 by the NREL, based on the range of wind speeds. Areas that have a wind power classification of 4 or higher are areas where it might make sense to use wind energy to produce electricity. The best wind resources are in a large band across the central plains of the United States and areas of Wyoming and Montana (Figure 4.5). As you might expect, 9 of the top 10 states in terms of wind energy as a percentage of total electricity portfo­lio are in this band, the sole exception being Oregon (39). Ridges of mountains are also good locations for wind power, especially in the Northeast. The West and the East have poor wind resources for onshore wind. Both coasts of the United States have excellent offshore wind power resources. The West Coast resources are not very amenable to development, however, because of the rapid drop-off of the con­tinental shelf, making the water too deep for wind turbines. Since the best onshore wind resources are where the population density is low (compare the wind map to the map of night lights), the problem of long-distance transmission rears its ugly head, as it does for solar power. The plentiful wind resources offshore have their own issues that will be discussed later.

Installed capacity of wind energy has been growing rapidly in both the United States and the world in recent years. About 10 GW was installed in the United States in 2009, but that dropped by over half in 2010 and picked up again in 2012. The total installed capacity by mid-2012 was 49.8 GW in the United States com­pared to 67.8 GW in China—the world leader in wind energy—and 30 GW in third-place Germany (40). Even so, the fraction of electricity generated from wind was only 3.6% in 2012 in the United States (see Chapter 2) and 9.2% in Germany by mid-2012 (11). Denmark has the highest percentage of its electricity generated by wind power, currently about 19% but wind actually satisfies only about 10% of its actual electrical demand (41).