The Three Gorges Dam project in Hubei, China, is the world’s largest hydroelectric power plant complex as regard generating capacity. It includes 2 generating stations. They are the Three Gorges Dam (22,500 MW when completed) and Gezhouba Dam (3,115MW). The total generating capacity of this complex is currently at 21,515 MW. The whole project is planned to be completed in 2011. The total generating capacity will be 25,615 MW by then. In 2008, this complex generated 97.9 TWh of electricity.

The Itaipu power plant in Brazil-Paraguay border,is currenly the largest single hydroelectric power plant in the world as regards the generation of energy. With 20 generator units and 14,000 MW of installed power, In 2008, the Itaipu power plant reached a new historic record for electricity production by generating 94.7 TWh (94,684,781 MWh).In generation Itaipu capacity will remain the most important, because the hydrological regime of the Paraná River has increased flow of water that the Yangtze River .

The Jinsha River (the upper stream of Yangtze River) complex is the largest hydroelectric generating system currently under construction. It has 3 phases. Phase one includes 4 dams on the downstream of Jinsha River. They are Wudongde Dam, Baihetan Dam, Xiluodu Dam, and Xiangjiaba Dam, with generating capacity of 7,400 MW, 14,000 MW, 12,600 MW, and 6,000 MW respectively. The total generating capacity of those four dams is 40,000 MW. Construction of Xiluodu Dam started on December 26 2005. Construction of Xiangjiaba Dam started on November 26 2006. Phase one is planned to complete in 2015. Phase two includes 8 dams on the middle stream of Jinsha River. The total generating capacity is 21,150 MW. Phase three includes 8 dams on the upper stream of Jinsha River. The total generating capacity is 8,980 MW. The total capacity of the complex is 68,630 MW.

Disadvantages

Recreational users must exercise extreme care when near hydroelectric dams, power plant intakes and spillways.

Environmental damage

Hydroelectric projects can be disruptive to surrounding aquatic ecosystems both upstream and downstream of the plant site. 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 spawn are also harmed on their migration to sea when they must pass through turbines. This has led to some areas transporting smolt downstream by barge during parts of the year. In some cases dams have been demolished (for example the Marmot Dam demolished in 2007) because of impact on fish. Turbine and power-plant designs that are easier on aquatic life are an active area of research. Mitigation measures such as fish ladders may be required at new projects or as a condition of re-licensing of existing projects.

Generation of hydroelectric power changes the downstream river environment. Water exiting a turbine usually contains very little suspended sediment, which can lead to scouring of river beds and loss of riverbanks. Since turbine gates are often opened intermittently, rapid or even daily fluctuations in river flow are observed. For example, in the Grand Canyon, the daily cyclic flow variation caused by Glen Canyon Dam was found to be contributing to erosion of sand bars. Dissolved oxygen content of the water may change from pre-construction conditions. Depending on the location, water exiting from turbines is typically much warmer than the pre-dam water, which can change aquatic faunal populations, including endangered species, and prevent natural freezing processes from occurring. Some hydroelectric projects also use canals to divert a river at a shallower gradient to increase the head of the scheme. In some cases, the entire river may be diverted leaving a dry riverbed. Examples include the Tekapo and Pukaki Rivers.

Greenhouse gas emissions

Bonnington hydroelectric power station, River Clyde, Scotland.

The reservoirs of power plants in tropical regions may produce substantial amounts of methane and carbon dioxide. This is due to plant material in flooded areas decaying in an anaerobic environment, and forming methane, a very potent greenhouse gas. According to the World Commission on Dams report, where the reservoir is large compared to the generating capacity (less than 100 watts per square meter of surface area) and no clearing of the forests in the area was undertaken prior to impoundment of the reservoir, greenhouse gas emissions from the reservoir may be higher than those of a conventional oil-fired thermal generation plant. Although these emissions represent carbon already in the biosphere, not fossil deposits that had been sequestered from the carbon cycle, there is a greater amount of methane due to anaerobic decay, causing greater damage than would otherwise have occurred had the forest decayed naturally.

The pipes supplying water from the River Clyde to Bonnington hydroelectric power station, Scotland.

In boreal reservoirs of Canada and Northern Europe, however, greenhouse gas emissions are typically only 2% to 8% of any kind of conventional fossil-fuel thermal generation. A new class of underwater logging operation that targets drowned forests can mitigate the effect of forest decay.

In 2007, International Rivers accused hydropower firms for cheating with fake carbon credits under the Clean Development Mechanism (CDM), for hydropower projects already finished or under construction at the moment they applied to join the CDM. These carbon credits – of hydropower projects under the CDM in developing countries – can be sold to companies and governments in rich countries, in order to comply with the Kyoto protocol.

Population relocation

Another disadvantage of hydroelectric dams is the need to relocate the people living where the reservoirs are planned. In February 2008, it was estimated that 40-80 million people worldwide had been physically displaced as a direct result of dam construction. In many cases, no amount of compensation can replace ancestral and cultural attachments to places that have spiritual value to the displaced population. Additionally, historically and culturally important sites can be flooded and lost. Such problems have arisen at the Three Gorges Dam project in China, the Clyde Dam in New Zealand and the Ilısu Dam in Southeastern Turkey.

Dam failures

Failures of large dams, while rare, are potentially serious — the Banqiao Dam failure in Southern China resulted in the deaths of 171,000 people and left millions homeless. Dams may be subject to enemy bombardment during wartime, sabotage and terrorism. Smaller dams and micro hydro facilities are less vulnerable to these threats. The creation of a dam in a geologically inappropriate location may cause disasters like the one of the Vajont Dam in Italy, where almost 2000 people died, in 1963.

Affected by flow shortage

Changes in the amount of river flow will correlate with the amount of energy produced by a dam. Because of global warming, the volume of glaciers has decreased, such as the North Cascades glaciers, which have lost a third of their volume since 1950, resulting in stream flows that have decreased by as much as 34%. The result of diminished river flow can be power shortages in areas that depend heavily on hydroelectric power.

Northern hemisphere

Appleton, Wisconsin, USA completed 1882, A waterwheel on the Fox river supplied the first commercial hydroelectric power for lighting to two paper mills and a house, two years after Thomas Edison demonstrated incandescent lighting to the public. Within a matter of weeks of this installation, a power plant was also put into commercial service at Minneapolis.
Niagara Falls, New York. For many years the largest hydroelectric power station in the world. Operation began locally in 1895 and power was transmitted to Buffalo, New York, in 1896.
Decew Falls 1, St. Catharines, Ontario, Canada completed 25 August 1898. Owned by Ontario Power Generation. Four units are still operational. Recognized as an IEEE Milestone in Electrical Engineering & Computing by the IEEE Executive Committee in 2002.
Claverack Creek, in Stottville, New York, believed to be the oldest hydro power site in the United States. The turbine, a Morgan Smith, was constructed in 1869 and installed 2 years later. It is one of the earliest water wheel installations in the United States to generate electricity. It is owned today by Edison Hydro.[citation needed]
The oldest continuously-operated commercial hydroelectric plant in the United States is built on the Hudson River at Mechanicville, New York. The seven 750 kW units at this station initially supplied power at a frequency of 38 Hz, but later were increased in speed to 40 Hz. It went into commercial service July 22,1898. It is now being restored to its original condition and remains in commercial operation.
The oldest continuously-operated hydroelectric generator in Canada is located in St. Stephen, New Brunswick, Canada. Part of the construction of the Milltown Cotton Mill, this rope-driven generator originally powered the electric lights for the mill when it opened in 1882, and in 1888 started providing power to homes in the town. NB Power now owns and operates this as part of the Milltown Dam hydroelectric station.

Southern hemisphere

A small hydroelectric station, generating 650 kW, opened at Waratah, Tasmania, in 1885
Duck Reach, Launceston, Tasmania. Completed 1895. The first publicly owned hydro-electric plant in the Southern Hemisphere. Supplied power to the city of Launceston for street lighting.
Chivilingo was the first hydroelectric plant in Chile and the second in South America. With first power produced in 1897, it has two Pelton wheel turbines each turning a 215 kW generator. It was installed to provide power to mines and the city of Lota, Chile.
The Snowy Mountains Scheme has turbines all along the tunnel so it, in some perspectives it is also another hydroelectric station. However it only operates during peak hours of the day and mostly during the evening and early night. It supplies electricity to all over the state of NSW.

The ranking of hydro-electric capacity is either by actual annual energy production or by installed capacity power rating. A hydro-electric plant rarely operates at its full power rating over a full year; the ratio between annual average power and installed capacity rating is the capacity factor. The installed capacity is the sum of all generator nameplate power ratings. Sources came from BP Statistical Review – Full Report 2007 List of the largest hydoelectric power stations. Norway produces 98-99% of its electricity from hydroelectric.

Brazil, Canada, Norway and Venezuela are the only countries in the world where the majority of their internal electric energy production is from hydroelectric power.
China –          563.3TWh    17.18%
Brazil –          371.5TWh    85.56%
Canada –       368.2TWh    61.12%
USA –             250.8TWh      5.74%
Russia –         179.0TWh    17.64%
Norway –       135.3TWh    98.25%
India –            122.TWh    15.80%
Venezuela –   83.9TWh    67.17%
Japan –           83.6TWh    7.21%
Sweden –        66.2TWh    44.34%

Paraguay –     64.0TWh
France –         63.6TWh    11.23%

Advantages

The upper reservoir and dam of the Ffestiniog pumped storage scheme. 360 megawatts of electricity can be generated within 60 seconds of the need arising.

Economics

The major advantage of hydroelectricity is elimination of the cost of fuel. The cost of operating a hydroelectric plant is nearly immune to increases in the cost of fossil fuels such as oil, natural gas or coal, and no imports are needed.

Hydroelectric plants also tend to have longer economic lives than fuel-fired generation, with some plants now in service which were built 50 to 100 years ago. Operating labor cost is also usually low, as plants are automated and have few personnel on site during normal operation.

Where a dam serves multiple purposes, a hydroelectric plant may be added with relatively low construction cost, providing a useful revenue stream to offset the costs of dam operation. It has been calculated that the sale of electricity from the Three Gorges Dam will cover the construction costs after 5 to 8 years of full generation.

Greenhouse gas emissions

Since hydroelectric dams do not burn fossil fuels, they do not directly produce carbon dioxide (a greenhouse gas). While some carbon dioxide is produced during manufacture and construction of the project, this is a tiny fraction of the operating emissions of equivalent fossil-fuel electricity generation.

Related activities

Reservoirs created by hydroelectric schemes often provide facilities for water sports, and become tourist attractions in themselves. In some countries, aquaculture in reservoirs is common. Multi-use dams installed for irrigation support agriculture with a relatively constant water supply. Large hydro dams can control floods, which would otherwise affect people living downstream of the project.