HYDROPOWER

Hydroelectricity is electricity produced by hydropower. It is a renewable source of energy, produces no waste, and does not produce carbon dioxide (CO2) which contributes to greenhouse gases. Hydroelectricity now supplies about 715,000 MWe or 19% of world electricity (16% in 2003), accounting for over 63% of the total electricity from renewables in 2005.

Although large hydroelectric installations generate most of the world's hydroelectricity, small hydro schemes are particularly popular in China, which has over 50% of world small hydro capacity.

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.

The free renewable source of energy provided by falling water that drives the turbines. Hydropower is the most important of the regenerable energy sources because of its highest efficiency at the energy conversion. There are two types of hydroelectric power plants: a) run-of-river power plants for the use of affluent water; b) storage power plants (power stations with reservoir) where the influx can be regulated with the help of a reservoir. Mostly greater differences in altitudes are being used, like mountain creeks. Power stations with reservoirs are generally marked by barrages with earth fill dam or concrete dams. Though hydropower generally can be called environmentally acceptable, there exist also some problems: a) change of groundwater level and fill up of the river bed with rubble. b) Risk of dam breaks. c) Great demand for land space for the reservoir. d) Diminution, but partly also increase of value of recreation areas. As the hydropowers of the world are limited, the world energy demand however is rising, finally the share of hydropower will decrease. (Source: PORT / PHC / PZ) 

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 barrages and reservoirs, can also be dispatchable to generate power during high demand periods.

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.

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.

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. These emissions represent carbon already in the biosphere, not fossil deposits that had been sequestered from the carbon cycle.

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-burning. 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.

The Three Gorges Dam project in Hubei, China, is the world's largest hydroelectric generating system. 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 17,215 MW. The whole project is planned to be completed in 2011. The total generating capacity will be 25,615 MW by then.

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, 12,500 MW, 12,600 MW, and 6,000 MW respectively. The total generating capacity of those four dams is 38,500 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.

The James Bay Project in Quebec, Canada, is the world's second largest hydroelectric generating system. The eight generating stations of the complex have a total generating capacity of 16,021 MW. The Robert Bourassa station alone has a capacity of 5,616 MW. A ninth station (Eastmain-1) is currently under construction and will add 480 MW to the total. Construction on an additional project on the Rupert River was started on 11 January 2007. It will add two stations with a combined capacity of 888 MW.