Wind Energy

Botkin, Daniel B. Environmental Science: Earth as a Living Planet, 9th Edition. Wiley, 2013-12-23. VitalBook file.

16.4 Water power

Water power is a form of stored solar energy that has been successfully harnessed since at least the time of the Roman Empire. Waterwheels that convert water power to mechanical energy were turning in western Europe in the Middle Ages. During the 18th and 19th centuries, large waterwheels provided energy to pow- er grain mills, sawmills, and other machinery in the United States.

Today, hydroelectric power plants use the water stored behind dams. In the United States, hydroelectric plants generate about 80,000 MW of electricity—about 10% of the total electricity produced in the nation. In some coun- tries, such as Norway and Canada, hydroelectric power plants produce most of the electricity used. Figure 16.12a shows the major components of a hydroelectric power station.

Hydropower can also be used to store energy pro- duced by other means, through the process of pump stor- age (Figure 16.12b and c). During times when demand for power is low, excess electricity produced from oil, coal, or nuclear plants is used to pump water uphill to a higher reservoir (high pool). When demand for electricity is high (on hot summer days, for instance), the stored water flow back down to a low pool through generators to help pro- vide energy. The advantage of pump storage lies in the timing of energy production and use. However, pump storage facilities are generally considered ugly, especially at low water times.

Small-Scale Hydropower Systems

In the coming years, the total amount of electrical power produced by running water from large dams will prob- ably not increase in the United States, where most of the acceptable dam sites are already in use. It may decrease because some dams are being dismantled. They are being removed either because they have reached their useful life- time or because of concerns that a dam interferes with the migration of fish, such as salmon and shad, that lay their eggs in freshwater but spend most of their adult life in the ocean and migrate up and down rivers (see Chapter 18). Because small dams can damage stream environments by blocking fish passage and changing downstream flow, careful consideration must be given to their construction. A few small dams cause little environmental degradation beyond the specific sites, but a large number of dams can have an appreciable impact on a region. (This principle applies to many forms of technology and development. The impact of a single development may be nearly neg- ligible over a broad region; but as the number of such developments increases, the total impact may become significant.)

Hydroturbines are an interesting development and are in use in early tests. These work like the large turbines that operate in hydroelectric dams, but are smaller and lie horizontally within a river. They are fixed in place but are not part of a dam, so they allow the normal river flow and migration of fish (Figure 16.13).

Water Power and the Environment

Water power is clean power in that it requires no burn- ing of fuel, does not pollute the atmosphere, produces no radioactive or other waste, and is efficient. However, there are environmental prices to pay:

• Large dams and reservoirs flood large tracts of land that could have had other uses. For example, towns and agricultural lands may be lost.

• As we noted earlier, dams block the migration of some fish, such as salmon, and the dams and reservoirs greatly alter habitat for many kinds of fish.

• Dams trap sediment that would otherwise reach the sea and eventually replenish the sand on beaches.

• Reservoirs with large surface areas increase evaporation of water compared to pre-dam conditions. In arid re- gions, evaporative loss of water from reservoirs is more significant than in more humid regions.

• For a variety of reasons, many people do not want to turn wild rivers into a series of lakes.

For all these reasons, and because many good sites already have a dam, the growth of large-scale water power in the future (with the exception of a few areas, including Africa, South America, and China) appears limited. Indeed, in the United States there is an emerg- ing social movement to remove dams. Hundreds of dams, especially those with few useful functions, are be- ing considered for removal, and a few have already been dismantled (see Chapters 18 and 19). The U.S. Depart- ment of Energy forecasts that electrical generation from large hydropower dams will decrease significantly.

16.5 Ocean energy

The motion of waves, currents, and tides in oceans involves a lot of energy. Many have dreamed of harnessing this en- ergy, but it’s not easy, for the obvious reasons that ocean storms are destructive and ocean waters are corrosive. The most successful development of energy from the ocean has been tidal power. Use of the power of ocean tides can be traced back to the Roman occupation of Great Britain around Julius Caesar’s time, when the Romans built a dam that captured tidal water and let it flow out through a wa- terwheel. By the 10th century, tides were used once again to power coastal mills in Britain.8 However, only in a few places with favorable topography—such as the north coast of France, the Bay of Fundy in Canada, and the north- eastern United States—are the tides strong enough to pro- duce commercial electricity. The tides in the Bay of Fundy have a maximum range of about 15 m (49 ft). A minimum range of about 8 m (26 ft) appears necessary with present technology for development of tidal power.

Traditionally, to harness tidal power, a dam is built across the entrance to a bay or an estuary, creating a res- ervoir. As the tide rises (flood tide), water is initially pre- vented from entering the bay landward of the dam. Then, when there is sufficient water (from the oceanside high tide) to run the turbines, the dam is opened, and water flows through it into the reservoir (the bay), turning the blades of the turbines and generating electricity. When the bay is filled, the dam is closed, stopping the flow and hold- ing the water in that reservoir. When the tide falls (ebb tide), the water level in the reservoir is higher than that in the ocean. The dam is then opened to run the turbines (which are reversible), and electric power is produced as the water is let out of the reservoir. Figure 16.14 shows the Rance tidal power plant on the north coast of France.

Constructed in the 1960s, it is the first and largest modern tidal power plant and has remained in operation since that time. The plant at capacity produces about 240,000 kW from 24 power units spread out across the dam. At the Rance power plant, most electricity is produced from the ebb tide, which is easier to control.

Tidal power, too, has environmental impacts. The dam changes the hydrology of a bay or an estuary, which can adversely affect the vegetation and wildlife. The dam restricts upstream and downstream passage of fish, and the periodic rapid filling and emptying of the bay as the dam opens and closes with the tides rapidly changes habi- tats for birds and other organisms.

New Ocean Energy Technologies

In September, 2012, an ocean turbine similar in prin- ciple to the horizontal turbine installed in New York City’s East River began delivering electricity to the Ban- gor Hydro Electric Company of Maine. This horizontal turbine, designed and built by the Ocean Renewable En- ergy Project, generated electricity from the movement of tidal waters in Cobscook Bay, an estuary in Washington County, Maine, which opens into the Bay of Fundy. The Bay of Fundy has one of the largest tidal flows in the world. This marked the first power from any ocean en- ergy project to provide electricity to a U.S. electrical grid (Figure 16.15).9

Another new approach is the use of ocean waves to power ocean-going vessels. One of the first is the Sun- tory Mermaid I. In 2008 its inventor, Yutaka Terao, of the Department of Naval Architecture and Ocean En- gineering at the Tokai University School of Marine Sci- ence and Technology, sailed the Suntory Mermaid I from Honolulu, Hawaii, 7,000 km (3,700 nautical miles) to the western shore of Japan (Figure 16.16). The trip had been done before, of course, but this boat’s propulsion system was new—the Suntory Mermaid has two horizon- tal fins that move up and down with the waves and gen- erate the power to push the boat forward. Solar energy provides electricity.10

Another example of imaginative ocean engineering is a robotic boat called the Wave Glider. Using wave and solar energy, it sailed 10,000 miles from San Francisco to Australia in the fall of 2012. This boat makes use of wave motion in a somewhat different way than does the Suntory Mermaid. It is made of two units—a floater and a sub. The waves raise and lower the floater, and this motion is trans- ferred to the sub in a way that moves the two forward. Us- ing a small computer and GPS, the Wave Glider was able to make its way past sharks and the Great Barrier Reef of Australia. These two vessels demonstrate that there are op- portunities for imaginative engineering designs to change the way boats are powered, independent of fossil fuels.

16.6 Wind power

Wind power, like solar power, has evolved over a long time. From early Chinese and Persian civilizations to the present, wind has propelled ships and has driven wind- mills to grind grain and pump water. In the past, thou- sands of windmills in the western United States were used to pump water for ranches. More recently, wind has been used to generate electricity. The trouble is that wind tends to be highly variable in time, place, and intensity.

Basics of Wind Power

Winds are produced when differential heating of Earth’s surface creates air masses with differing heat contents and densities. The potential for energy from the wind is large, and thus wind “prospecting” has become an important en- deavor. On a national scale, regions with the greatest po- tential are the Pacific Northwest coastal area, the coastal region of the northeastern United States, and a belt within the Great Plains extending from northern Texas through the Rocky Mountain states and the Dakotas (Figure 16.17). Other windy sites include mountain areas in North Carolina and the northern Coachella Valley in Southern California. A site with average wind velocity of about 18 km/h (11 mph) or greater is considered a good prospect for de- velopment of wind energy, although starting speeds for modern wind turbines can be considerably lower.

In any location, the wind’s direction, velocity, and dura- tion may be quite variable, depending on local topography and temperature differences in the atmosphere. For example, wind velocity often increases over hilltops and when wind is funneled through a mountain pass (Figure 16.18). The increase in wind velocity over a mountain is due to a verti- cal convergence of wind, whereas in a pass the increase is partly due to a horizontal convergence. Because the shape of a mountain or a pass is often related to the local or regional geology, prospecting for wind energy is a geologic as well as a geographic and meteorological task. The wind energy po- tential of a region or site is determined by instruments that measure and monitor over time the strength, direction, and duration of the wind.

378 C h A p t e r 1 6 Alternative Energy and the Environment

Significant improvements in the size of windmills and the amount of power they produce occurred from the late 1800s to the present, when many European countries and the United States became interested in large-scale genera- tors driven by wind. In the United States in the first half of the 20th century, thousands of small, wind-driven gen- erators were used on farms. Most of these small windmills generated approximately 1 kW of power, which is much too little to be considered for central power-generation needs. Interest in wind power declined for several decades prior to the 1970s because of the abundance of cheap fossil fuels. Since the 1980s, interest in building wind- mills has revived greatly. Modern wind turbines are big, as much as 70 m (230 ft) high, as tall as a 23-story building, and have a generating capacity of more than 1 million watts—enough electricity for 500 modern U.S. homes (Figure 16.19).11

Much of the electricity produced from these large tur- bines is connected to the electrical grid, and many of them are installed in so-called wind farms, which are provid- ing large amounts of electricity. One of the biggest in the

United States is the Horse Hollow Wind Energy Center near Abilene, Texas, owned and operated by Florida Power & Light. It has 421 wind turbines with a total generating capacity of 735 megawatts, enough to meet the electricity needs of approximately 220,000 homes and enough for all domestic use for a city the size of Austin, Texas. (To put this in perspective, one large fossil fuel plant or nuclear power plant produces about 1,000 MW.) The Horse Hol- low wind turbines are spread widely across approximately 47,000 acres, and the land is used for both ranching and energy production.8

Today, wind energy is the cheapest form of alterna- tive energy. Electricity produced from wind often costs less than that from natural gas and coal. Worldwide, wind energy generated 4,154 trillion kilowatt-hours in 2011, an increase of 18% from 2007.11 In the United States, wind electricity generated increased 27% between 2010 and 2011. Meanwhile, electricity generated from coal de- clined 6.2%. However, wind still provided only a small percentage of U.S. electricity—1.4% in 2011 compared to 21% provided by coal.11

Wind Power and the Environment

Wind energy does have a few disadvantages:

• Wind turbines can kill birds. (Birds of prey, such as hawks and falcons, are particularly vulnerable.)

• Wind turbines and wind farms may degrade an area’s scenery, as we saw in the chapter’s opening case study.

However, although wind farms must often compete with other land uses, in many cases wind turbines can share land used for farms, military bases, and other facili- ties. Everything considered, wind energy has a relatively low environmental impact.

The Future of Wind Power

Wind power’s continued use is certain. As noted earlier, the use of wind energy has been growing fast. It is believed that there is sufficient wind energy in Texas, South Dakota, and North Dakota to satisfy the electricity needs of the entire United States. Consider the implications for nations such as China. China burns tremendous amounts of coal at a heavy environmental cost that includes exposing mil- lions of people to hazardous air pollution. In rural China, exposure to the smoke from burning coal in homes has in- creased the rate of lung cancer by a factor of nine or more. Estimates are that China could probably double its current capacity to generate electricity with wind alone!

One scenario suggests that wind power could supply 10% of the world’s electricity in the coming decades and, in the long run, could provide more energy than hydro- power, which today supplies approximately 20% of the electricity in the world. The wind energy industry has cre- ated thousands of jobs in recent years. Worldwide, more than half a million people are employed in wind energy, and according to the World Wind Energy Association, wind energy “creates many more jobs than centralized, non-renewable energy sources.”11 Technology, mean- while, is producing more efficient wind turbines, thereby reducing the price of wind power. All told, wind power is becoming a major investment opportunity.

16.7 Biofuels

Biofuel is energy recovered from biomass (organic matter). We can divide biofuels into five groups:

1. unmanaged growth harvested by people, including firewood, grasses, and peat;

2. organic wastes used directly, including cooking oil, which can fuel diesel engines, and methane, emitted from bacterial decomposition of waste;

3. agrifuels, which are crops grown to be converted into liquid fuels;

4. ethanol produced by some algae as a by-product of photosynthesis;

16.7 Biofuels 379 5. ethanol produced by bacteria as a by-product of

bacteria’s decomposition of organic wastes.

Biofuels and Human History

Biomass is the oldest fuel used by people. Our Pleistocene ancestors burned wood in caves to keep warm and cook food. Biofuels remained a major source of energy through- out most of the history of civilization. When North Ameri- ca was first settled, there was more wood fuel than could be used. Forests often were cleared for agriculture by girdling trees (cutting through the bark all the way around the base of a tree) to kill them and then burning the forests.

Until the end of the 19th century, wood was the ma- jor fuel in the United States. During the mid-20th cen- tury, when coal, oil, and gas were plentiful, burning wood became old-fashioned and quaint, done just for pleasure in an open fireplace, even though most of the heat went up the chimney. Now, with other fuels reaching a limit in abundance and production, there is renewed interest in using natural organic materials for fuel.

More than 1 billion people in the world today still use wood as their primary source of energy for heat and cook- ing.12 Although firewood is the most familiar and most widely used biomass fuel, there are many others. In India and other countries, cattle dung is burned for cooking.

Peat, a form of compressed dead vegetation, provides heating and cooking fuel in northern countries, such as Scotland, where it is abundant. Renewed interest in bio- fuels grew during the last decades of the 20th century and continues today. For example, on December 10, 2012, a ship carrying nearly 20,000 metric tons of wood pellets produced in eastern Canada arrived at the Wismar seaport in Germany. The pellets will be used in steam-generating electric power plants.13 Why are wood pellets becoming a new energy source? Because electricity produced from burningthemreleaseslesscarbondioxideintotheatmo- sphere than coal and oil. Nations that want to show that they are reducing carbon emissions are therefore turning to wood pellets. The problem is that the total carbon release involved in growing and harvesting trees and energy used to produce the pellets and shipping the pellets are not ac- counted for by the nations burning the pellets. It may be that in the end more carbon dioxide is released in total using wood pellets to fuel power plants than is produced by fossil fuels. This carbon accounting has been the subject of only a few studies. One, issued by the Massachusetts Institute of Technology, stated that burning wood pellets releases a large amount of CO2, creating a carbon debt.13

In recent years, biofuels have become controversial. Do biofuels offer a net benefit or disbenefit? (See Table 16.1.) In brief:

• Using wastes as a fuel is a good way to dispose of them. Making them takes more energy than they yield; on the other hand, they reduce the amount of energy we must obtain from other sources. Firewood that regen- erates naturally or in plantations that require little en- ergy input will remain an important energy source in developing nations and locally in industrialized nations. Despite pressure from some agricultural corporations and some governments to promote crops grown sole- ly for conversion into liquid fuels (called agrifuels), at present these are poor sources of energy. Most scientific research shows that producing agrifuels takes more en- ergy than they yield. In some cases, there appears to be a net benefit, but the energy produced per unit of land area is low, much lower than can be obtained from solar and wind.

What it boils down to is that photosynthesis, though a remarkable natural process, is less efficient than mod- ern photovoltaic cells in converting sunlight to electricity. Some algae and bacteria appear to provide a net energy benefit and can yield ethanol directly, but production of ethanol from these sources is just beginning and is experimental.8

Multimedia

1. energynownews. (2011, December 16). Energy 101: Wind power [Video clip]. Retrieved from http://www.youtube.com/watch?v=niZ_cvu9Fts

· Transcript

2. U.S. Department of Energy. (2011, February 8). Energy 101: Wind turbines [Video clip]. Retrieved from http://www.youtube.com/watch?v=tsZITSeQFR0

· Captions are available by clicking the “CC” icon in the video window.

3. U.S. Department of Energy. (2013, April 19). Energy 101: Hydropower [Video clip]. Retrieved from http://www.youtube.com/watch?v=tpigNNTQix8

· Captions are available by clicking the “CC” icon in the video window.