Hydroelectricity is the term referring to
electricity generated by
hydropower; the production of electrical power through the use of the gravitational force of falling or flowing water. It is the most widely used form of
renewable energy, accounting for 16 percent of global electricity generation – 3,427 terawatt-hours of electricity production in 2010,
[1] and is expected to increase about 3.1% each year for the next 25 years.
Hydropower is produced in 150 countries, with the Asia-Pacific region generating 32 percent of global hydropower in 2010. China is the largest hydroelectricity producer, with 721 terawatt-hours of production in 2010, representing around 17 percent of domestic electricity use. There are now three hydroelectricity plants larger than 10 GW: the
Three Gorges Damin China,
Itaipu Dam across the Brazil/Paraguay border, and
Guri Dam in Venezuela.
[1]
The cost of hydroelectricity is relatively low, making it a competitive source of renewable electricity. The average cost of electricity from a hydro plant larger than 10 megawatts is 3 to 5 U.S. cents per kilowatt-hour.
[1] Hydro is also a flexible source of electricity since plants can be ramped up and down very quickly to adapt to changing energy demands. However, damming interrupts the flow of rivers and can harm local ecosystems, and building large dams and reservoirs often involves displacing people and wildlife.
[1] Once a hydroelectric complex is constructed, the project produces no direct waste, and has a considerably lower output level of the
greenhouse gas carbon dioxide (CO
2) than
fossil fuel powered energy plants.
History
Hydroelectric power plants continued to become larger throughout the 20th century. Hydropower was referred to as
white coal for its power and plenty.
[9] Hoover Dam's initial 1,345 MW power plant was the world's largest hydroelectric power plant in 1936; it was eclipsed by the 6809 MW
Grand Coulee Dam in 1942.
[10] The
Itaipu Dam opened in 1984 in South America as the largest, producing 14,000 MW but was surpassed in 2008 by the
Three Gorges Dam in China at 22,500 MW. Hydroelectricity would eventually supply some countries, including
Norway,
Democratic Republic of the Congo,
Paraguay and
Brazil, with over 85% of their electricity. The United States currently has over 2,000 hydroelectric power plants that supply 6.4% of its total electrical production output, which is 49% of its renewable electricity.
[4]
Generating methods
Turbine row at Los Nihuiles Power Station in
Mendoza, Argentina
Cross section of a conventional hydroelectric dam.
Conventional (dams)
Most hydroelectric power comes from the
potential energy of
dammed water driving a
water turbine and
generator. The power 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. A large pipe (the "
penstock") delivers water to the turbine.
[11]
Pumped-storage
This method 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 most commercially important means of large-scale
grid energy storage and improve the daily
capacity factor of the generation system. Pumped storage is not an energy source, and appears as a negative number in listings.
[12]
Run-of-the-river
Run-of-the-river hydroelectric stations are those with small or no reservoir capacity, so that the water coming from upstream must be used for generation at that moment, or must be allowed to bypass the dam. In the United States, run of the river hydropower could potentially provide 60,000 MW (about 13.7% of total use in 2011 if continuously available).
[13]
Tide
A
tidal power plant makes use of the daily rise and fall of ocean 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. Tidal power is viable in a relatively small number of locations around the world. In Great Britain, there are eight sites that could be developed, which have the potential to generate 20% of the electricity used in 2012.
[14]
Underground
An
underground power station makes use of a large natural height difference between two waterways, such as a waterfall or mountain lake. An underground tunnel is constructed to take water from the high reservoir to the generating hall built in an underground cavern near the lowest point of the water tunnel and a horizontal tailrace taking water away to the lower outlet waterway.
Sizes and capacities of hydroelectric facilities
Large facilities
Although no official definition exists for the capacity range of large hydroelectric power stations, facilities from over a few hundred
megawatts to more than
10 GW are generally considered large hydroelectric facilities. Currently, only three facilities over
10 GW(
10,000 MW) are in operation worldwide;
Three Gorges Dam at
22.5 GW,
Itaipu Dam at
14 GW, and
Guri Dam at
10.2 GW. Large-scale hydroelectric power stations are more commonly seen as the largest power producing facilities in the world, with some hydroelectric facilities capable of generating more than double the installed capacities of the current
largest nuclear power stations.
Small
Main article:
Small hydro
Small hydro is the development of
hydroelectric power on a scale serving a small community or industrial plant. The definition of a small hydro project varies but a generating capacity of up to 10
megawatts (MW) is generally accepted as the upper limit of what can be termed small hydro. This may be stretched to 25 MW and 30 MW in
Canada and the
United States. Small-scale hydroelectricity production grew by 28% during 2008 from 2005, raising the total world small-hydro capacity to
85 GW. Over 70% of this was in
China(
65 GW), followed by
Japan (
3.5 GW), the
United States (
3 GW), and
India (
2 GW).
[15]
Small hydro plants may be connected to conventional electrical distribution networks as a source of low-cost renewable energy. Alternatively, small hydro projects may be built in isolated areas that would be uneconomic to serve from a network, or in areas where there is no national electrical distribution network. Since small hydro projects usually have minimal reservoirs and civil construction work, they are seen as having a relatively low environmental impact compared to large hydro. This decreased environmental impact depends strongly on the balance between stream flow and power production.
Micro
Micro hydro is a term used for
hydroelectric power installations that typically produce up to
100 kW of power. These installations can provide power to an isolated home or small community, or are sometimes connected to electric power networks. There are many of these installations around the world, particularly in developing nations as they can provide an economical source of energy without purchase of fuel.
[16] Micro hydro systems complement
photovoltaic solar energy systems because in many areas, water flow, and thus available hydro power, is highest in the winter when solar energy is at a minimum.
Pico
Pico hydro is a term used for
hydroelectric power generation of under
5 kW. It is useful in small, remote communities that require only a small amount of electricity. For example, to power one or two fluorescent light bulbs and a TV or radio for a few homes.
[17] Even smaller turbines of 200-300W may power a single home in a developing country with a drop of only 1 m (3 ft). Pico-hydro setups typically are
run-of-the-river, meaning that dams are not used, but rather pipes divert some of the flow, drop this down a gradient, and through the turbine before returning it to the stream.
