Green Sanctuary Program
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Bellingham Unitarian Fellowship
Green Sanctuary Program |
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Union of Concerned Scientists http://www.ucsusa.org/clean_energy/renewable_energy_basics/how-hydroelectric-energy-works.html Hydroelectric power is the largest source of renewable electricity in the world and in the United States. By taking advantage of the water cycle, we have tapped into one of nature's engines to create a useful form of energy. Humans have been using the power of moving water for thousands of years. But by blocking rivers with massive concrete dams, we have created a number of serious environmental problems that affect future development of large dams. Still, if it's done right, small run-of-the-river hydropower can be a sustainable and nonpolluting power source. The Hydropower Resource On earth, water is constantly moved around in various states. Water evaporates from the oceans, forming into clouds, falling out as rain and snow, gathering into streams and rivers, and flowing back to the sea. All this movement provides an enormous opportunity to create useful energy. The United Nations estimates that the total "technically exploitable" potential for hydropower is 15,090 terawatt-hours per year, or 15 trillion kilowatt-hours. Only about 13 percent has been developed so far. Much of the potential for new development is in the former Soviet Union, South Asia, and South America. Hydropower provides one-fifth of the world's electricity, second only to fossil fuels. Worldwide capacity is 650,000 megawatts (MW), with 14 percent of this in the United States, 10 percent in Canada, and 9 percent in the former Soviet Union. China's Three Gorges Dam, currently under construction, will move China into fourth place, ahead of Brazil. Hydro capacity has more than doubled since 1970. In the United States, hydropower has grown steadily, from 56,000 MW in 1970 to over 90,000 MW today. As a portion of the electricity supply, it has fallen to 10 percent, down from 14 percent 20 years ago -- but it still accounts for a greater share than petroleum. In fact, US hydropower plants produce the energy equivalent of 500 million barrels of oil per year. In some parts of the country, hydropower is even more important. It provides 63 percent of power used along the West coast. The Pacific Northwest is the region with the most hydropower, obtaining two-thirds of its electricity from 58 hydroelectric dams. The Grand Coulee dam on the Columbia River is one of the largest dams in the world, with a capacity of 10,000 megawatts. In addition to very large plants in the West, the United States has many smaller hydro plants. In 1940 there were 3,100 hydropower plants across the country, but by 1980 that number had fallen to 1,425. Since then, a number of these small plants have been restored; there are currently 2,500 hydro plants in operation. These plants account for only a tiny fraction of the 70,000 dams that block and divert our rivers. An estimated 4,600 MW of capacity could be added at existing small dams, with another 6,000 MW in improvements at large dams. Worldwide there is a great deal of growth in small hydro projects. Over 100 countries are developing small hydro plants, adding to the current 23,000 megawatts of small hydro capacity. Forty percent of the current capacity is in China, which has a goal of 23,000 MW itself by the turn of the century. An important issue now in the United States is the relicensing of hydropower plants. Hydro plants have very long lives; the Grand Coulee dam, for example, has been in operation since 1942. The federal government issues licenses for all dams for a 30- to 50-year period. In 1993, for instance, over 200 licenses were due for renewal, amounting to 2,000 MW of capacity. Relicensing some of these dams will require dam owners to find ways to reduce environmental impacts. Converting Moving Water to Energy Dams produce power not just from moving water, but from water that is moving with sufficient speed and volume to turn a generator. To increase the force of moving water, dams raise the water level, creating a "hydraulic head," or height differential. Roughly speaking, one gallon of water per second falling one hundred feet can generate one kilowatt of electrical power. When water behind a dam is released, it runs through a pipe called a penstock, and is delivered to the turbine. There are a variety of turbines, and their use depends on the amount of hydraulic head at the plant, but the most common are Kaplan, Francis, and Pelton wheel designs. Some of these designs, called reaction and impulse wheels, use not just the kinetic force of the moving water but also the water pressure. Hydro plants can also work without dams. Fast-flowing rivers can be diverted in part through a turbine set in the river or off to the side. Another design uses a traditional water wheel on a floating platform to capture the kinetic force of the moving river. While this approach is inexpensive and easy to implement, it doesn't produce much power. The entire Amazon river, if harnessed this way, would produce only 650 MW of power. Another use of hydropower, though not a true energy source, is pumped storage. In a pumped storage plant, water is pumped from a lower reservoir to a higher reservoir during off-peak times, using electricity from other types of generators. When the power is needed, it is released back into the lower reservoir through turbines. Inevitably, some power is lost, but pumped storage systems can be about 80 percent efficient. As of 1990, there was 75,000 MW of pumped storage capacity worldwide, with 25 percent of that in the United States. The ability to meet power demand fluctuations is an advantage of hydro plants with reservoirs. Unlike run-of-the-river plants, which produce power around the clock, hydro plants with dams are typically used only when the power is most needed. Utilities save up the water, letting it loose only during peak times. Hydro plants, especially the large older plants built from the 1930s to the 1950s, are the least-expensive source of electricity. The production cost of hydropower is typically less than a third that of coal or nuclear. Environmental Problems Although hydropower is inexpensive and nonpolluting, the environmental impacts of hydropower can be serious. The most obvious effect is that fish are blocked from moving up and down the river, but there are many more problems. When a dam blocks a river, a river habitat is replaced by a lake habitat. While this may not sound so bad -- fish and birds like lakes, too -- it can cause a number of environmental problems. In the Pacific Northwest, large federally owned dams have blocked the migration of coho, chinook, and sockeye salmon from the ocean to their upstream spawning grounds. Some steps are being taken to help the fish around the dams, such as putting them in barges or building fish ladders, but this has only helped a little. Also, when young fish head downriver to the ocean, they can be chewed up in the turbines of the dam. The salmon population in the Northwest currently seems headed for extinction, falling from a population of 16 million to 300,000. Dams can create large reservoirs over what used to be dry land, producing many problems. The Balbina dam in Brazil, for example, flooded 2,750 square kilometers (965 square miles), an area the size of Rhode Island. This land is often composed of wetlands, which are important wildlife habitats, and low-lying flood plains, often the most fertile crop land in the area. Population density is typically higher along rivers, leading to mass dislocation of urban centers. The Three Gorges Dam in China is expected to dislocate 1.4 million people. Wildlife habitats destroyed by reservoirs can be especially valuable. In South America, 80 percent of the hydroelectric potential is located in rain forests. The Rosana dam in Brazil destroyed one of the few remaining habitats of the black-lion tamarin, a rare and beautiful species of long-haired monkey. Another problem can occur when the area behind the dam is flooded without being properly prepared. In Brazil, the Tucurui dam was built creating a reservoir in a rain forest region, without the forest first being cleared. Later, as the plants and trees that were submerged began to rot, they reduced the oxygen content of the water, killing off the plants and fish in the water. Moreover, the rotting plants gave off large quantities of methane, a powerful heat-trapping gas. The utility there is now testing underwater power saws to remove the trees. A similar problem has occurred in Canada, in hydro projects built by Hydro Quebec. The stones and soil in the flooded area contain naturally occurring mercury and other metals. When the land was flooded, the mercury dissolved into the water, and then into the fish in the water. The creatures that eat the fish, from bears and eagles to the native Cree people, are suffering from mercury poisoning. Mercury poisoning can cause brain damage, birth defects, liver disorders, and other ailments. Hydropower can cause many other effects on water quality. Rivers and lakes can be filled with sediment from erosion. Water falling over spillways can force air bubbles into the water, which can be absorbed into fish tissue, ultimately killing the fish. By slowing down rivers, the water can become stratified, with warm water on top and cold water on the bottom. Since the cold water is not exposed to the surface, it loses its oxygen and becomes uninhabitable for fish. And as illustrated by the Colorado River in the Grand Canyon, fast-moving rivers can be filled up with sediment when they are slowed down. The Department of Interior, overseen by Secretary Bruce Babbitt, recently flushed huge amounts of water out of dams in an attempt to clear away the sediment. Another important habitat disruption comes from the operation of the dam to meet electric demand. Water is stored up behind the dam and released through the turbines when power demand is greatest. This causes water levels to fluctuate widely on both sides of the dam, stranding fish in shallow waters and drying out the habitat. There are many competing pressures on dam operators -- to produce power, to provide water for recreational use on the reservoir and downstream, to provide drinking and irrigation water, to allow Native Americans to carry out traditional religious practices, and to preserve habitat for fish and plant species. In most cases, nature loses out to boaters and electric customers. The risk of a dam breaking should not be ignored. The great Johnstown flood in Pennsylvania was the result of a dam break (although not a hydroelectric dam); 2,000 people were killed. In northern India and Nepal, in the Himalayas, huge hydroelectric projects are planned that would create large reservoirs in a geographically unstable region. Frequent earthquakes make the dam a risky venture for heavily populated areas downstream. This is compounded by the fear that large, heavy reservoirs would put additional pressure on the plates in the region, causing even more earthquakes. Another cause of dam breakage has so far never occurred, but came close during the Gulf War. One Iraqi target considered by American bombers in the war was the hydroelectric dam upstream from Bagdad. Since too many civilian casualties would have resulted, the dam was spared. Conclusion The environmental impacts of large hydropower can be immense. But while hydropower has its problems, it can still be a safe and sustainable source of electricity, on a smaller scale. By upgrading and improving the equipment at small plants, by increasing fish-friendly efforts at large dams, and by improving run-of-the-river turbine technology, it may be possible to reduce the environmental effects of hydropower. Nonetheless, remediation may be impossible at some sites, and wild rivers should be unshackled. Either way, the environmental effects of hydropower must be compared with alternatives. The damage to aquatic habitat from dams may be significant, but acid rain, nitrogen deposition, and thermal pollution from coal plants also lead to aquatic damage, as well as to air pollution and global warming emissions. Provided we replace the worst hydro plants with environmentally friendly renewable energy, we will be better off. If the only option is unsustainable fossil and nuclear power, hydropower may still have a role to play. |
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