Hydropower is the world's largest used renewable energy source for the production of electricity. The use of water power dates back to antiquity, with the use of water wheels turned either by the flow of a river or the weight of water falling from a dam or waterfall. Following the invention of the electric generator and hydraulic turbine, hydropower has been an important part of electricity generation since the 19th century. Indeed, today it supplies 17% of the world's electricity needs. For a number of countries, hydropower is the major electricity source (Fig. 8.2); for example, it generates 50% of electricity in Canada and Sweden, over 80% in Brazil, and nearly 100% in Norway. Hydroelectric power, although still capable of being improved, is a mature technology. New large-scale projects in developed countries, where most suitable sites have already been developed, are however by necessity limited. Current world hydroelectricity production exploits only 18% of the overall potential , and in developing countries and transition economies, the unexploited resources are still vast. Latin America for example, has tapped only 20% of its potential, Asia 11%, and Africa about 4%. Therefore, 90% of future hydroelectric plants are expected to be constructed on these
continents. A rational utilization of hydroelectric resources could help these developing regions to modernize their economies and raise their standard of living. Even if hydropower has lower operation costs and longer life expectancy than most other modes of electricity generation, resulting in generally lower energy prices for the consumer, it is also highly capital intensive. The large initial investment necessary for the construction of significant hydropower plants is an important issue, and many developing nations may find it difficult to finance such large new ventures.
Between the 1930s and the 1970s, the construction of large dams was, in the eyes of many, synonymous with development and economic progress. These dams not only produced electric power, but also provided water for irrigation and helped in flood control. In the United States, the Hoover dam on the Colorado River (Fig. 8.3), constructed during the height of the great depression and completed in 1936, was the largest dam of its time and viewed as a symbol of modernization and man's ability to harness nature. It opened the way to the development of the western United States by powering cities as far as Los Angeles and nearby Las Vegas, which was at that time hardly more than a water refilling stop on the Union Pacific railroad for the steam engine locomotives. In the former Soviet Union, major hydroelectric projects were essential to the industrialization of selected areas. The construction of dams accelerated dramatically after World War II, to peak during the 1970s when numerous large dams were commissioned around the world, not only for hydroelectric power generation but also for flood control and irrigation purposes. Today, there is an estimated 45 000 large dams on our planet, of which 22 000 are located in China alone!
. Large dams are some of the biggest structures built by mankind. The Itaipú hydroelectric plant on the Paraná River in South America, the largest in the world, has an output of 12 600 MW, equivalent to about 12 large commercial nuclear reactors, and provides 25% of the energy supply in Brazil and 78% in Paraguay. In recent times however, large hydro projects have raised many criticisms on environmental and social grounds. They inevitably require large dams and reservoirs which inundate the affected land areas and can disturb local ecosystems, reduce biodiversity, modify water quality, as well as cause major socio-economic damage through the displacement and relocation of local populations. Land inundations are large, some running into thousands of square kilometers. The world's two largest dam reservoirs, Ghana's Akosombo on the Volta River with 8730 km2 and Russia's Kuybyshev on the Volga River with 6 500 km2, approach the size of small countries such as Lebanon or Cyprus. In tropical regions, these waters can also create feeding grounds for malaria-bearing mosquitoes and other water-borne diseases. Until the 1960s, most hydroelectric megaprojects did not involve the massive relocation of population, but as large dams began to be constructed in more densely populated areas, especially in Asia, the population to be resettled grew in size. It is estimated that during the 20th century, between 40 and 80 million people have been displaced because of large dam construction. The Three Gorge Dam (Fig. 8.4), which is under construction on the Yangtze River in China, will alone cause the relocation of more than one million people. When completed in 2009, this 2 km-long and 200 m-high dam will be the world's largest hydroelectric plant, with a production capacity of 18 000 MW, which is much needed by China's booming economy, but with a price tag in excess of $20 billion. To make way for this enormous project, the more than a 500 km-long artificial lake behind the dam will submerge more than 4500 towns and villages, ancient temples, burial grounds, and spectacular scenic canyons that have inspired poets and painters for centuries and attracted tourists from all around the world. Environmentalists argue that the dam will doom migratory fish and wipe out a number of rare species, including the Yangtze River dolphin. There
are also concerns that the lake will trap millions of tonnes of pollutants spewing from Chongqing, one of China's largest industrial cities. In Egypt, the High Aswan dam was completed in 1970, and captures the Nile River in the world's third largest reservoir, Lake Nasser. Before the dam was built, the Nile overflowed its banks once a year, depositing millions of tonnes of nutrient-rich silt on the valley floor, and making the otherwise dry land fertile and productive. In some years however, when the river did not rise, it caused drought and famine. By constructing the dam the floods could be controlled, as could the drought by water release. At the same time, huge amounts of electricity were generated. Unfortunately, the rich silt that normally fertilized the dry desert land during annual floods is now stuck at the bottom of Lake Nasser, forcing the use of about one million tonnes of artificial fertilizer to substitute for natural nutrients that once fertilized the arid floodplains . This exemplifies one of the major problems associated with the construction of dams: the progressive silting of reservoirs over time, especially in erosion-prone regions such as the high mountain ranges of the Himalayas and China's Loess plateau. Possible technical solutions are to de-silt reservoirs by flushing or adapted dredging or heightening of the dam walls.
A significant potential for expanding hydropower lies in small systems that have relatively modest and localized effects on the environment and populations. They have generating capacities in the range of 1 to 30 MW, or even less for mini-and micro-hydro installations, and are especially attractive to supply electricity to remote areas far from any electrical grid. These units are typically "run-of-river" plants which transform the kinetic energy of rivers and streams into electricity, and have little or no storage capacity. This lowers their environmental impact compared to large hydroelectric installations, but it also means that they are vulnerable to seasonal fluctuations and will not be able to produce electricity in dry seasons. Today, "small-hydro" represents only about 5% of the worldwide hydro-electricity generation.
Most concerns associated with the development of hydroelectric power have been addressed, and many were successfully mitigated. The key to future projects will be prior planning not only of the economical feasibility, but also of environmental and social factors. Potential benefits as well as the downsides of each project should be carefully weighed. Adverse publicity and negative media comments on dams should not overshadow the many benefits associated with their construction. Hydropower installations, beside providing electricity at the lowest cost compared to any other energy source, have also provided other important benefits such as flood control, irrigation, drinking water, and improved navigation. The human displacement of thousands of people associated with the construction of a new dam should be compared against the benefit to millions provided with electricity. The further expansion of hydropower will continue to be a key step in the modernization of many developing countries as a reliable and affordable source of energy. The construction of hydroelectric instead of fossil fuel-burning plants also reduces the emission of significant air pollutants, mainly SO2 and NOx as well as CO2, the main greenhouse gas.
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