HYDROELECTRIC POWER

electricity produced from generators driven by water turbines that convert the potential energy in falling or fast-flowing water to mechanical energy. In the generation of hydroelectric power, water is collected or stored at a higher elevation and led downward through large pipes or tunnels (penstocks) to a lower elevation; the difference in these two elevations is known as the head. In the course of its passage down the steep pipes, the falling water rotates turbines. The turbines in turn drive generators, which convert the turbines' mechanical energy into electricity. Transformers change the alternating current produced by the generators into a very high-voltage current that is suitable for long-distance transmission. The structure that houses the turbines and generators, and into which the pipes or penstocks feed, is called the powerhouse. Hydroelectric power plants are usually emplaced in dams that impound rivers, thereby raising the level of the water behind the dam and creating a relatively high head. The potential power that can be derived from a volume of water is directly proportional to the working head, so that a high-head installation requires a smaller volume of water than a low-head installation to produce an equal amount of power. In some dams, the powerhouse is constructed on one flank of the dam; part of the dam itself is used as a spillway over which excess water is discharged in times of flood. Where the river flows in a narrow, steep gorge, the powerhouse may be in the dam itself. In some areas where electric-power demand varies sharply at different times of the day, pumped-storage hydroelectric stations are used. During off-peak periods, some of the extra power available is used to pump water into a special reservoir. Then, during periods of peak demand when the power required by the system exceeds the base-load value, the water is allowed to flow down again to generate additional electrical energy. Pumped-storage systems are efficient and, in most cases, constitute the most economical way to meet peak loads. In certain coastal areas, such as the Rance River estuary in Brittany, France, hydroelectric power plants have been constructed to take advantage of the rise and fall of tides. When the tides come in, the water is impounded in special reservoirs. The water trapped in these basins is then released and the tidal flow used to power hydraulic turbines, which in turn drive electric generators (see tidal power). Falling water is one of the three principal sources of energy used to generate electric power, the other two being fossil fuels and nuclear fuels. Hydroelectric power has certain advantages over these other sources: it is continually renewable owing to the recurring nature of the hydrologic cycle; and it produces neither thermal nor particulate pollution. Hydroelectric power is of varying importance in different countries. Nations such as Norway, Sweden, Canada, and Switzerland are able to rely heavily on hydroelectricity because they all have mountainous regions that are subject to heavy rainfall and that lie in close proximity to industrialized areas that require large amounts of electricity. These countries have built hydroelectricity-producing dams at many of the favourable sites within their borders. Hydroelectric power is utilized on a relatively large scale by various other countries, including the United States, Russia, China, India, and Brazil, but it contributes a much smaller proportion of these nations' total electric-power production.