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Mechanical (kinetic) energy of fl owing or falling water was traditionally converted to rotary motion by a variety of waterwheels, and, starting in the 1880s, by water turbines that have been used to turn generators. Unlike fossil fuels, this form of electricity generation does not produce air pollution directly, but its other environmental impacts have become a matter of considerable controversy.
It is not known by how many generations or centuries the origins of waterwheels predate the first reference to their existence by Antipater of Thessalonica, who wrote during the first century BCE about their use in grain milling. A millennium later such simple devices were common in parts of Europe: in 1086 the Domesday Book listed 5,624 mills in southern and eastern England, one for every 350 people. The usual arrangement was to channel fl owing water through a sloping wooden trough onto wooden paddles, often fitted to a sturdy shaft that was directly attached to a millstone above. Vertical waterwheels, first mentioned by Vitruvius in 27 BCE, were much more efficient. All of them turned the millstones by right-angle gears, but they were propelled in three distinct ways.
Wheels and Turbines
Undershot wheels were driven by kinetic energy of moving water. As doubling the speed boosts the capacity eightfold, they were preferably located on swift-fl owing streams. The best designs could eventually convert 35–45 percent of water’s kinetic energy into useful rotary motion. Breast wheels were powered by a combination of fl owing and falling water and operated with heads between 2 and 5 meters. Overshot wheels were driven primarily by the weight of descending water and hence could be located on streams with placid water fl ows. With heads over 3 meters their conversion efficiencies were commonly in excess of 60 percent with peaks of up to 85 percent. Wheels, as well as shafts and gears, were almost completely wooden until the beginning of the eighteenth century; hubs and shafts were the first iron parts and the first all-iron wheel was built early in the nineteenth century.
Besides the wheels fixed in streams there were also floating wheels on barges and tidal mills and common uses of waterwheels eventually expanded far beyond grain milling to power machines ranging from wood saws and oil presses to furnace bellows and forge hammers, and to mechanize manufacturing processes ranging from wire pulling to tile glazing. Even as their uses widened, capacities of waterwheels remained limited, averaging less than 4 kilowatts in early eighteenth-century Europe. Post-1750 innovations led to a rapid increase of individual capacities as arrays of waterwheels, sometimes rating in excess of 1 megawatt (one megawatt equals one thousand kilowatts), became the leading prime movers of expanded mass manufacturing in Europe and North America. In 1832, Benoit Fourneyron’s invention of the reaction turbine ushered in the era of much more powerful water-driven machines. James B. Francis designed an inward-flow turbine in 1847, Lester A. Pelton patented his jet-driven turbine in 1889, and Viktor Kaplan introduced his axial flow turbines in 1920.
The first water turbines were used merely to replace waterwheels as the prime movers in many industries, but by the late 1880s the machine began to be coupled with generators to produce electricity. The first American hydroelectric plant was built in 1882 in Wisconsin. More than century later, water turns turbines that supply almost 20 percent of the world’s electricity. In dozens of tropical countries, water power is the dominant means of electricity production.
Most of the world’s hydro energy remains to be tapped. Worldwide total of economically feasible hydro generation is over 8 petawatt-hours (PWh, that is 1015—one quadrillion—watt-hours) or roughly three times the currently exploited total. Europe has the highest share of exploited capacity (more than 45 percent), Africa the lowest (below 4 percent). The greatest boom in construction of large dams took place during the 1960s and 1970s, with about five thousand new structures built per decade.
Advantages and Drawbacks
The air pollution advantage of hydropower is the most obvious one: if coal-fired power plants were to generate electricity that is currently produced worldwide by running water, the global emissions of carbon dioxide and sulfur dioxide would be, respectively, 15 percent and 35 percent higher. Hydrogeneration also has low operating costs, and its spinning reserve (zero load synchronized to the system) in particular is an excellent way to cover peak loads created by sudden increases in demand. Moreover, many reservoirs built primarily for hydrogeneration have multiple uses—serving as sources of irrigation and drinking water, as protection against flooding, and as resources for aquaculture and recreation. But during the closing years of the twentieth century, large dams were widely seen as economically dubious, socially disruptive, and environmentally harmful.
Displacement of a large number of usually poor people has been the most contentious matter. Construction of large dams dislocated at least 40 million people during the twentieth century (some estimates go as high as 80 million), and during the early 1990s, when the work began on three hundred new large dams began every year, the annual total reached 4 million people. China and India, the two countries that have built nearly 60 percent of the world’s large dams, had to relocate most people: more than 10 million in China and at least 16 million in India. Large hydro projects also have a multitude of undesirable environmental impacts, and recent studies of these previously ignored changes have helped to weaken the case for hydrogeneration as a clean source of renewable energy and a highly acceptable substitute for fossil fuels.
Perhaps the most surprising finding is that large reservoirs in warm climates are significant sources of greenhouse gases emitted by decaying vegetation. Water storage behind large dams has increased the average age of river runoff and lowered the temperature of downstream fl ows. Several of the world’s largest rivers have reservoir-induced aging of runoff exceeding six months or even one year (Colorado, Rio Grande del Norte, Nile, Volta). Many tropical reservoirs create excellent breeding sites for malaria mosquitoes and for the schistosomiasis-carrying snails, and most dams present insurmountable obstacles to the movement of migratory fish. Multiple dams have caused river channel fragmentation that now affects more than three-quarters of the world’s largest streams.
Other environmental impacts caused by large dams now include massive reduction of aquatic biodiversity both upstream and downstream, increased evaporative losses from large reservoirs in arid climates, invasion of tropical reservoirs by aquatic weeds, reduced dissolved oxygen and hydrogen sulfide toxicity in reservoir waters, and excessive silting. The last problem is particularly noticeable in tropical and monsoonal climates. China’s Huang (Yellow) River, fl owing through the world’s most erodible area, and India’s Himalayan rivers carry enormous silt loads. Silt deposition in reservoirs has effects far downstream as it cuts the global sediment fl ow in rivers by more than 25 percent, and reduces the amount of material, organic matter, and nutrients available for alluvial plains and coastal wetlands downstream, and hence increases coastal erosion.
The ultimate life span of large dams remains unknown. Many have already served well past their designed economic life of fifty years but silting and structural degradation will shorten the useful life of many others. As a significant share of the Western public sentiment has turned against new hydro projects, some governments took action. Sweden has banned further hydrostations on most of its rivers, and Norway has postponed all construction plans. Since 1998, the decommissioning rate for large U.S. dams has overtaken the construction rate. Major hydro projects of the twenty-first century will thus be built only in Asia, Latin America, and Africa.
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