Although no official
definition exist for the capacity range of large hydroelectric power stations,
facilities from over a few hundred megawatts to more than 10 GW is 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.
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. The Grand
Coulee Dam switched to support Alcoa aluminium in Bellingham, Washington,
United States for American World War II airplanes before it was allowed to
provide irrigation and power to citizens (in addition to aluminium 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.
The construction of these large hydroelectric facilities and
the changes it makes to the environment, are often too at very large scales,
creating just as much damage to the environment as at helps it by being a
renewable resource. Many specialized organizations, such as the International
Hydropower Association, look into these matters on a global scale.
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).
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 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. 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 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. 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 being exhausted back to the stream.