The upper reservoir and dam of the Ffestiniog Pumped-Storage Scheme in north Wales
.The power station at the lower reservoir has four water turbines which can generate 360 megawatts of electricity within 60 seconds of the need arising. The water of the upper reservoir (Llyn Stylan) can just be glimpsed on the right.
Hydroelectricity, or hydroelectric power, is a form of hydropower, (i.e.,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 a 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 British Columbia, Quebec and Newfoundland and Labrador) 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 provinces are called "BC Hydro," "Hydro-Québec" and "Newfoundland Hydro" respectively.
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 requirements(2004), Austria produces 67 % of all electricity generated in the country from hydro (over 70 % of its requirements). 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-peak hours (in fact, pumped-storage hydroelectric reservoirs are sometimes used to store electricity produced by thermal plants 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 utilization of stored water is sometimes complicated by demand for irrigation which may occur out of phase with peak electricity demand. Times of drought can cause severe problems, since water replenishment rates may not keep up with desired usage rates.
Lakes created by hydroelectric schemes often provide excellent leisure facilities for water sports, and become a tourist attraction in themselves.
Concerns have been raised by environmentalists 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, even though most dams in salmon habitat have fish ladders installed. Salmon smolt are also harmed on their migration to sea when they must pass through turbines. This has led to some areas barging smolt downstream during parts of the year. Also, turbine designs that are easier on aquatic life are an active area of research.
The creation of large bodies of water involved with damming rivers for Hydroelectric power is considered an environmental risk by some. For example, pine needles that fall in the water can turn the water acidic, though this is equally true for natural lakes.
One of the most newly discovered and most troubling environmental consequence of hydroelectric power include the fact that such power plants produce substantial amounts of carbon dioxide, and then serve to convert carbon dioxide already existing in the atmosphere to methane. This is due to plant material in newly flooded and re-flooded areas being innundated with water, decaying in an anaerobic environment, allowing for the formation of methane, a very potent greenhouse gas. The methane is released into the atmosphere once the water is discharged from the dam and turns the turbines. In some cases, more greenhouse gasses are released from a dam for electricity generation than generating the equivalent amount of energy from burning oil .
Another big disadvantage of hydroelectric dams is the need to relocate the people living where the reservoirs are planned. In many cases, no amount of compensation can replace ancestral and cultural attachments to places that have spiritual value to the displaced population.
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
Churchill Falls, Canada, completed 1971, 5,428 MW, expandable to 9,252 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.
New Scientist report on greenhouse gas production by hydroelectric dams.