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.
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.
A simple formula for approximating electric power production at a hydroelectric plant is: , where is Power in watts, is height in meters, is flow rate in cubic meters per second, and 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.
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.
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.
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
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.
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.
|Country|| Annual Hydroelectric|
|Installed Capacity (GW)||Load Factor|
|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|
|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|
|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|