Hydroelectric dam diagram
The waters of Llyn Stwlan, the upper reservoir of the Ffestiniog Pumped-Storage Scheme in north Wales, can just be glimpsed on the right. The lower power station has four water turbines which can generate 360 megawatts of electricity within 60 seconds of the need arising.
Hydroelectricity, hydroelectric power, is a form of hydropower (the use of energy released by water falling, flowing downhill, moving tidally, or moving in some other way) to produce electricity. Specifically, the mechanical energy of the moving water is converted to electrical energy by a water turbine driving an generator. Most hydroelectric power is currently generated from water flowing downhill, but a few tidal harnesses exist that draw power from the tide. Hydroelectric power is usually generated at dams or other places where water descends from a height, or coasts with a large tidal swing (such as the Bay of Fundy). Hydroelectricity is a renewable energy source, since the water that flows in rivers has come from precipitation such as rain or snow, and tides are driven by the rotation of the earth.
The energy that may be extracted from water depends not only on the volume but on the difference in height between the water crest (or source) and the water outflow. This height difference is called the head. The amount of potential energy in water is directly proportional to the head. For this reason, it is advantageous to build dams as high as possible to produce the maximum electrical energy.
While many hydroelectric schemes supply public electricity networks, some projects were created for private commercial purposes. For example, aluminium processing requires substantial amounts of electricity, and in Britain's Scottish Highlands there are examples at Kinlochleven and Lochaber, designed and constructed during the early years of the 20th century. Similarly, the 'van Blommestein' lake, dam and power station were constructed in Suriname to provide electricity for the Alcoa aluminium industry.
In many parts of Canada (particularly the provinces of Quebec and to some extent Ontario) hydroelectricity is used so extensively that the word "hydro" is used to refer to any electricity delivered by a power utility. The government-run power utilities in these two provinces are called "Hydro Quebec" and "Ontario Hydro" (although Ontario uses nuclear power as well).
Hydroelectric power, using the potential energy of rivers, now supplies 20% of world electricity. Norway produces virtually all of its electricity from hydro, while Iceland produces 83% of its (2004). Apart from a few countries with an abundance of it, hydro capacity is normally applied to peak-load demand, because it can be readily stored during off hours (in fact, pumped-storage hydroelectric reservoirs are sometimes used to store other electricity for use during peak hours). It is not a major option for the future in the developed countries because most major sites in these countries having potential for harnessing gravity in this way are either being exploited already or are unavailable for other reasons such as environmental considerations.
Advantages and disadvantages
The chief advantage of hydro systems is their capacity to handle seasonal (as well as daily) high peak loads. In practice the utilisation of stored water is sometimes complicated by demands for irrigation which may occur out of phase with peak electrical demands. Times of drought can cause severe problems, since water replenishment rates may not keep up with desired usage rates.
Concerns have been raised that large hydroelectric projects might be disruptive to the surrounding ecosystem. For instance, studies have shown that dams along the Atlantic and Pacific coasts of North America have reduced salmon populations by preventing access to spawning grounds upstream.
The creation of large bodies of water involved with damming rivers for Hydroelectric power is also considered an environmental risk. For example, pine needles that fall in the water can turn the water acidic.
Largest hydro-electric power stations
- Itaipu, Brazil/Paraguay, completed 1983, 12,600 MW
- Guri, Venezuela, completed 1986, 10,300 MW
- Grand Coulee, USA, completed 1942, expanded in 1980 to 6,900 MW
- Sayano-Shushenk, Russia, completed 1983, 6,400 MW
- Krasnoyarsk, Russia, completed 1968, 6,000 MW
- Churchill Falls, Canada, completed 1971, 5,428 MW
- La Grande 2, Canada, completed 1979, 5,328 MW
- Bratsk, Russia, completed 1961, 4,500 MW
- Tucurui, Brazil, completed 1984, now 4,245 MW, expanding to 8400 MW
These are installed power figures. If rated by annual power production, the order is different.
Countries with the most hydro-electric capacity
- Canada, 341,312 GWh (66,954 MW installed)
- USA, 319,484 GWh (79,511 MW installed)
- Brazil, 285,603 GWh (57,517 MW installed)
- China, 204,300 GWh (65,000 MW installed)
- Russia, 160,500 GWh (44,000 MW installed)
- Norway, 121,824 GWh (27,528 MW installed)
- Japan, 84,500 GWh (27,229 MW installed)
- India, 82,237 GWh (22,083 MW installed)
- France, 77,500 GWh (25,335 MW installed)
These are 1999 figures and include pumped-storage schemes.