Hydroelectricity

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hydroelectric power

A hydro station generates power by the controlled release of water from the reservoir of a dammed elipsis

Electricity produced from generators driven by water turbines that convert the energy in falling or fast-flowing water to mechanical energy. Water at a higher elevation flows downward through large pipes or tunnels (penstocks). The falling water rotates turbines, which drive the generators, which convert the turbines' mechanical energy into electricity. The advantages of hydroelectric power over such other sources as fossil fuels and nuclear fission are that it is continually renewable and produces no pollution. Norway, Sweden, Canada, and Switzerland rely heavily on hydroelectricity because they have industrialized areas close to mountainous regions with heavy rainfall. The U.S., Russia, China, India, and Brazil get a much smaller proportion of their electric power from hydroelectric generation. Seealso tidal power.

Learn more about hydroelectric power with a free trial on Britannica.com.

Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

Hydroelectricity

Hydroelectricity is electricity generated by hydropower, i.e., the production of power through use of the gravitational force of falling water. It is the most widely used form of renewable energy. Once a hydroelectric complex is constructed, the project produces no direct waste, and has a considerably different output level of the greenhouse gas carbon dioxide than fossil fuel powered energy plants. Worldwide, hydroelectricity suppled an estimated 715,000 MWe in 2005. This was aproximately 19% of the world's electricity (up from 16% in 2003), and accounted for over 63% of electricity from renewable sources.

Some jurisdictions do not consider large hydro projects to be a sustainable energy source, due to the human, economic and environmental impacts of dam construction and maintenance.

Electricity generation

Most hydroelectric power comes from the potential energy of dammed water driving a water turbine and generator. In this case the energy extracted from the water depends on the volume and on the difference in height between the source and the water's outflow. This height difference is called the head. The amount of potential energy in water is proportional to the head. To obtain very high head, water for a hydraulic turbine may be run through a large pipe called a penstock.

Pumped storage hydroelectricity produces electricity to supply high peak demands by moving water between reservoirs at different elevations. At times of low electrical demand, excess generation capacity is used to pump water into the higher reservoir. When there is higher demand, water is released back into the lower reservoir through a turbine. Pumped storage schemes currently provide the only commercially important means of large-scale grid energy storage and improve the daily load factor of the generation system. Hydroelectric plants with no reservoir capacity are called run-of-the-river plants, since it is not then possible to store water. A tidal power plant makes use of the daily rise and fall of water due to tides; such sources are highly predictable, and if conditions permit construction of reservoirs, can also be dispatchable to generate power during high demand periods.

Less common types of hydro schemes use water's kinetic energy or undammed sources such as undershot waterwheels.

A simple formula for approximating electric power production at a hydroelectric plant is: $P = hrk$ , where $P$ is Power in watts, $h$ is height in meters, $r$ is flow rate in cubic meters per second, and $k$ is a conversion factor of 7500 watts (assuming an efficiency factor of about 76.5 percent and acceleration due to gravity of 9.81 m/s², and fresh water with a density of 1000 kg per cubic metre. Efficiency is often higher with larger modern turbines and may be lower with very old or small installations due to proportionately higher friction losses).

Annual electric energy production depends on the available water supply. In some installations the water flow rate can vary by a factor of 10:1 over the course of a year.

Industrial hydroelectric plants

While many hydroelectric projects supply public electricity networks, some are created to serve specific industrial enterprises. Dedicated hydroelectric projects are often built to provide the substantial amounts of electricity needed for aluminium electrolytic plants, for example. In the Scottish Highlands there are examples at Kinlochleven and Lochaber, constructed during the early years of the 20th century. The Grand Coulee Dam, long the worlds largest, switched to support Alcoa aluminum in Bellingham, Washington for America's World War II airplanes before it was allowed to provide irrigation and power to citizens (in addition to aluminum power) after the war. In Suriname, the Brokopondo Reservoir was constructed to provide electricity for the Alcoa aluminium industry. New Zealand's Manapouri Power Station was constructed to supply electricity to the aluminium smelter at Tiwai Point. As of 2007 the Kárahnjúkar Hydropower Project in Iceland remains controversial.

Small-scale hydro-electric plants

Although large hydroelectric installations generate most of the world's hydroelectricity, some situations require small hydro plants. These are defined as plants producing up to 10 megawatts, or projects up to 30 megawatts in North America. A small hydro plant may be connected to a distribution grid or may provide power only to an isolated community or a single home. Small hydro projects generally do not require the protracted economic, engineering and environmental studies associated with large projects, and often can be completed much more quickly. A small hydro development may be installed along with a project for flood control, irrigation or other purposes, providing extra revenue for project costs. In areas that formerly used waterwheels for milling and other purposes, often the site can be redeveloped for electric power production, possibly eliminating the new environmental impact of any demolition operation. Small hydro can be further divided into mini-hydro, units around 1 MW in size, and micro hydro with units as large as 100 kW down to a couple of kW rating.

Small hydro schemes are particularly popular in China, which has over 50% of world small hydro capacity.

Small hydro units in the range 1 MW to about 30 MW are often available from multiple manufacturers using standardized "water to wire" packages; a single contractor can provide all the major mechanical and electrical equipment (turbine, generator, controls, switchgear), selecting from several standard designs to fit the site conditions. Micro hydro projects use a diverse range of equipment; in the smaller sizes industrial centrifugal pumps can be used as turbines, with comparatively low purchase cost compared to purpose-built turbines.

Advantages

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. Fuel is not required and so it need not be imported. Hydroelectric plants tend to have longer economic lives than fuel-fired generation, with some plants now in service having been built 50 to 100 years ago. Operating labor cost is usually low since 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, farming fish in the reservoirs is common. Multi-use dams installed for irrigation can support the fish farm with a relatively constant water supply. Large hydro dams can control floods, which would otherwise affect people living downstream of the project. When dams create large reservoirs and eliminate rapids, boats may be used to improve transportation.

Disadvantages

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.

A further concern is the impact of major schemes on birds. Since damming and redirecting the waters of the Platte River in Nebraska for agricultural and energy use, many native and migratory birds such as the Piping Plover and Sandhill Crane have become increasingly endangered

Greenhouse gas emissions

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 metre 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.

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.

Discussions to exclude hydropower facilities from obtaining carbon credits under the Clean Development Mechanism are starting to take place, most recently at the UN Climate Change Conference 2007 in Bali, Indonesia.

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.

Comparison with other methods of power generation

Hydroelectricity eliminates the flue gas emissions from fossil fuel combustion, including pollutants such as sulfur dioxide, nitric oxide, carbon monoxide, dust, and mercury in the coal. Hydroelectricity also avoids the hazards of coal mining and the indirect health effects of coal emissions. Compared to nuclear power, hydroelectricity generates no nuclear waste, has none of the dangers associated with uranium mining, nor nuclear leaks. Unlike uranium, hydroelectricity is also a renewable energy source.

Compared to wind farms, hydroelectricity power plants have a more predictable load factor. If the project has a storage reservoir, it can be dispatched to generate power when needed. Hydroelectric plants can be easily regulated to follow variations in power demand.

Unlike fossil-fueled combustion turbines, construction of a hydroelectric plant requires a long lead-time for site studies, hydrological studies, and environmental impact assessment. Hydrological data up to 50 years or more is usually required to determine the best sites and operating regimes for a large hydroelectric plant. Unlike plants operated by fuel, such as fossil or nuclear energy, the number of sites that can be economically developed for hydroelectric production is limited; in many areas the most cost effective sites have already been exploited. New hydro sites tend to be far from population centers and require extensive transmission lines. Hydroelectric generation depends on rainfall in the watershed, and may be significantly reduced in years of low rainfall or snowmelt. Long-term energy yield may be affected by climate change. Utilities that primarily use hydroelectric power may spend additional capital to build extra capacity to ensure sufficient power is available in low water years.

In parts of Canada (the provinces of British Columbia, Manitoba, Ontario, Quebec, Newfoundland and Labrador) hydroelectricity is used so extensively that the word "hydro" is often used to refer to any electricity delivered by a power utility. The government-run power utilities in these provinces are called BC Hydro, Manitoba Hydro, Hydro One (formerly "Ontario Hydro"), Hydro-Québec and Newfoundland and Labrador Hydro respectively. Hydro-Québec is the world's largest hydroelectric generating company, with a total installed capacity (2005) of 31,512 MW.

Countries with the most hydro-electric capacity

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 load factor. The installed capacity is the sum of all generator nameplate power ratings. Sources came from BP Annual Report 2006 List of the largest hydoelectric power stations

Country	Annual Hydroelectric Energy Production(TWh)	Installed Capacity (GW)	Load Factor
(2007)	486.7	145.26	0.37
	350.3	88.974	0.59
	349.9	69.080	0.56
	291.2	79.511	0.42
	157.1	45.000	0.42
	119.8	27.528	0.49
	112.4	33.600	0.43
	95.0	27.229	0.37
	74	-	-
	61.8	-	-
	61.5	25.335	0.25

Old hydro-electric power stations

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.

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.

Major schemes under construction

Name	Maximum Capacity	Country	Construction started	Scheduled completion	Comments
Three Gorges Dam	22,500 MW	China	December 14, 1994	2009	Largest power plant in the world. First power in July 2003, with 12,600 MW installed by October 2007.
Xiluodu Dam	12,600 MW	China	December 26, 2005	2015	Construction once stopped due to lack of environmental impact study.
Xiangjiaba Dam	6,400 MW	China	November 26, 2006	2015
Longtan Dam	6,300 MW	China	July 1, 2001	December 2009
Nuozhadu Dam	5,850 MW	China	2006	2017
Jinping 2 Hydropower Station	4,800 MW	China	January 30, 2007	2014	To build this dam, 23 families and 129 local residents need to be moved. It works with Jinping 1 Hydropower Station as a group.
Laxiwa Dam	4,200 MW	China	April 18, 2006	2010
Xiaowan Dam	4,200 MW	China	January 1, 2002	December 2012
Jinping 1 Hydropower Station	3,600 MW	China	November 11, 2005	2014
Pubugou Dam	3,300 MW	China	March 30, 2004	2010
Goupitan Dam	3,000 MW	China	November 8, 2003	2011
Boguchan Dam	3,000 MW	Russia	1980	2012
Chapetón	3,000 MW	Argentina
Jinanqiao Dam	2,400 MW	China	December 2006	2010
Guandi Dam	2,400 MW	China	Novermber 11 2007	2012
Tocoma (Manuel Piar)	2,160 MW	Venezuela	2004	2014	This new power plant would be the last development in the Low Caroni Basin, bringing the total to six power plants on the same river, including the 10,000MW Guri Dam.
Bureya Dam	2,010 MW	Russia	1978	2009
Ahai Dam	2,000 MW	China	July 27, 2006
Lower Subansiri Dam	2,000 MW	India	2005	2009

Proposed major hydroelectric projects

Name	Maximum Capacity	Country	Construction starts	Scheduled completion	Comments
Red Sea dam	50,000 MW	Middle East	Unknown	Unknown	Still in planning, would be largest dam in the world
Grand Inga	40,000 MW	Democratic Republic of the Congo	2010	Unknown
Baihetan Dam	12,000 MW	China	2009	2015	Still in planning
Wudongde Dam	7,000 MW	China	2009	2015	Still in planning
Maji Dam	4,200 MW	China	2008	2013
Songta Dam	4,200 MW	China	2008	2013
Liangjiaren Dam	4,000 MW	China	2009	2015	Still in planning
Jirau Dam	3,300 MW	Brazil	2007	2012
Pati Dam	3,300 MW	Argentina
Santo Antônio Dam	3,150 MW	Brazil	2007	2012
Guanyinyan Dam	3,000 MW	China	2009	2015	Still in planning
Lianghekou Dam	3,000 MW	China	2009	2015
Lower Churchill	2,800 MW	Canada	2009	2014
Liyuan Dam	2,400 MW	China	2008
Dagangshan Dam	2,300 MW	China	2009	2015
Changheba Dam	2,200 MW	China	2009	2015
Ludila Dam	2,100 MW	China	2009	2015

Cost

United States

In the United States, a study is required before constructing a hydroelectric project. In 2008, a study could cost up to $50,000 for a run of a stream. Both federal and state licenses were required. A license typically cost between $150,000 and $1 million. A project earns money from the sale of energy, the sale of capacity, and the sale of renewable energy credits.