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, 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 four hydroelectricity plants larger than 10 GW: the Three
Gorges Dam and Xiluodu Dam in China, Itaipu Dam
across the Brazil/Paraguay border, and Guri Dam in
Venezuela.
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]
It is also a flexible source of electricity since the amount produced by the
plant can be changed up or 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.
2) than fossil fuel powered energy plants.
History
Hydropower has been used since ancient times to grind flour
and perform other tasks. In the mid-1770s, French engineer Bernard Forest de Bélidor published Architecture
Hydraulique which described vertical- and horizontal-axis hydraulic
machines. By the late 19th century, the electrical generator was developed and could
now be coupled with hydraulics. The growing demand for the Industrial Revolution would drive development
as well. In 1878 the world's first hydroelectric power scheme was developed at Cragside in Northumberland,
England by William George Armstrong. It
was used to power a single arc lamp in his art gallery.The old Schoelkopf Power
Station No. 1 near Niagara Falls in the U.S. side began to produce
electricity in 1881. The first Edison hydroelectric power plant, the Vulcan Street Plant, began operating September
30, 1882, in Appleton, Wisconsin, with an output of about
12.5 kilowatts.[8]
By 1886 there were 45 hydroelectric power plants in the U.S. and Canada. By
1889 there were 200 in the U.S. alone.
At the beginning of the 20th century, many small
hydroelectric power plants were being constructed by commercial companies in
mountains near metropolitan areas. Grenoble,
France held the International
Exhibition of Hydropower and Tourism with over one million visitors. By
1920 as 40% of the power produced in the United States was hydroelectric, the Federal
Power Act was enacted into law. The Act created the Federal Power Commission to regulate
hydroelectric power plants on federal land and water. As the power plants
became larger, their associated dams developed additional purposes to include flood
control, irrigation and navigation.
Federal funding became necessary for large-scale development and federally
owned corporations, such as the Tennessee Valley Authority (1933) and
the Bonneville Power Administration
(1937) were created. Additionally, the Bureau of Reclamation which had began a
series of western U.S. irrigation projects in the early 20th century was now
constructing large hydroelectric projects such as the 1928 Hoover Dam. The U.S. Army Corps of Engineers was also
involved in hydroelectric development, completing the Bonneville
Dam in 1937 and being recognized by the Flood Control Act of 1936 as the premier
federal flood control agency.
Hydroelectric power plants continued to become larger
throughout the 20th century. Hydropower was referred to as white coal
for its power and plenty. 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. 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.
Generating methods
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.
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.
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).
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.
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.
Measurement of the tailrace and forebay rates at the Limestone Generating Station in Manitoba, Canada.
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.
Panoramic view of the Itaipu Dam, with the spillways (closed
at the time of the photo) on the left. In 1994, the American Society of Civil Engineers
elected the Itaipu Dam as one of the seven modern Wonders of the World.
Small
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
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
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). A
Pico-hydro setup is typically 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.
Calculating available power
A simple formula for approximating electric power production
at a hydroelectric plant is: , where
- is Power in watts,
- is the density of water (~1000 kg/m3),
- is height in meters,
- is flow rate in cubic meters per second,
- is acceleration due to gravity of 9.8 m/s2,
- is a coefficient of efficiency ranging from 0 to 1. Efficiency is often higher (that is, closer to 1) with larger and more modern turbines.
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.
Advantages and disadvantages
Advantages
The Ffestiniog Power Station can generate 360 MW of
electricity within 60 seconds of the demand arising.
Flexibility
Hydro is a flexible source of electricity since plants can
be ramped up and down very quickly to adapt to changing energy demands. Hydro
turbines have a start-up time of the order of a few minutes. It takes around 60
to 90 seconds to bring a unit from cold start-up to full load; this is much
shorter than for gas turbines or steam plants. Power generation can also be
decreased quickly when there is a surplus power generation. Hence the limited
capacity of hydropower units is not generally used to produce base power except
for vacating the flood pool or meeting downstream needs. Instead, it serves as
backup for non-hydro generators.
Low power costs
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, and no
imports are needed. The average cost of electricity from a hydro plant larger
than 10 megawatts is 3 to 5 U.S. cents per kilowatt-hour.
Hydroelectric plants have long economic lives, with some
plants still in service after 50–100 years. Operating labor cost is also
usually low, as 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. Additionally, some data shows that in most countries large
hydropower dams will be too costly and take too long to build to deliver a
positive risk adjusted return, unless appropriate risk management measures are
put in place.
Suitability for industrial applications
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.
Reduced CO2 emissions
Since hydroelectric dams do not burn fossil fuels, they do
not directly produce carbon dioxide. 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. One measurement of greenhouse gas related and other externality
comparison between energy sources can be found in the ExternE project by the Paul Scherrer Institut and the University of Stuttgart which was funded by
the European Commission. According to that study,
hydroelectricity produces the least amount of greenhouse
gases and externality of any energy source. Coming in second place
was wind,
third was nuclear energy, and fourth was solar
photovoltaic.
The low greenhouse gas impact of hydroelectricity is found
especially in temperate climates. The above study was for local
energy in Europe;
presumably similar conditions prevail in North America and Northern Asia, which
all see a regular, natural freeze/thaw cycle (with associated seasonal plant
decay and regrowth). Greater greenhouse gas emission impacts are found in the
tropical regions because the reservoirs of power plants in tropical regions
produce a larger amount of methane than those in temperate areas.
Other uses of the reservoir
Reservoirs created by hydroelectric schemes often provide
facilities for water sports, and become tourist attractions
themselves. In some countries, aquaculture
in reservoirs is common. Multi-use dams installed for irrigation
support agriculture
with a relatively constant water supply. Large hydro dams can control floods,
which would otherwise affect people living downstream of the project.
Disadvantages
Ecosystem damage and loss of land
Hydroelectric power stations that use dams would submerge large
areas of land due to the requirement of a reservoir.
Large reservoirs required for the operation of hydroelectric
power stations result in submersion of extensive areas upstream of the dams,
destroying biologically rich and productive lowland and riverine valley
forests, marshland and grasslands. The loss of land is often exacerbated by habitat fragmentation of surrounding areas
caused by the reservoir.
Hydroelectric projects can be disruptive to surrounding
aquatic ecosystems
both upstream and downstream of the plant site. 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.
Siltation and flow shortage
When water flows it has the ability to transport particles
heavier than itself downstream. This has a negative effect on dams and
subsequently their power stations, particularly those on rivers or within
catchment areas with high siltation. Siltation can
fill a reservoir and reduce its capacity to control floods along with causing
additional horizontal pressure on the upstream portion of the dam. Eventually,
some reservoirs can become full of sediment and useless or over-top during a
flood and fail.
Changes in the amount of river flow will correlate with the
amount of energy produced by a dam. Lower river flows will reduce the amount of
live storage in a reservoir therefore reducing the amount of water that can be
used for hydroelectricity. The result of diminished river flow can be power
shortages in areas that depend heavily on hydroelectric power. The risk of flow
shortage may increase as a result of climate
change. One study from the Colorado
River in the United States suggest that modest climate changes, such as an
increase in temperature in 2 degree Celsius resulting in a 10% decline in
precipitation, might reduce river run-off by up to 40%. Brazil in
particular is vulnerable due to its heaving reliance on hydroelectricity, as
increasing temperatures, lower water flow and alterations in the rainfall
regime, could reduce total energy production by 7% annually by the end of the
century.
Methane emissions (from reservoirs)
Lower positive impacts are found in the tropical regions, as
it has been noted that the reservoirs of power plants in tropical regions
produce substantial amounts of methane. This is due to plant material in flooded areas
decaying in an anaerobic environment, and forming methane,
a 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.
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.
Relocation
Another disadvantage of hydroelectric dams is the need to
relocate the people living where the reservoirs are planned. In 2000, the World
Commission on Dams estimated that dams had physically displaced 40-80 million
people worldwide.
Failure risks
Because large conventional dammed-hydro facilities hold back
large volumes of water, a failure due to poor construction, natural disasters
or sabotage can be catastrophic to downriver settlements and infrastructure.
Dam failures have been some of the largest man-made disasters in history.
The Banqiao Dam failure in Southern China directly resulted
in the deaths of 26,000 people, and another 145,000 from epidemics. Millions
were left homeless. Also, the creation of a dam in a geologically inappropriate
location may cause disasters such as 1963 disaster at Vajont Dam
in Italy, where almost 2000 people died.
Smaller dams and micro hydro
facilities create less risk, but can form continuing hazards even after being
decommissioned. For example, the small Kelly
Barnes Dam failed in 1967, causing 39 deaths with the Toccoa Flood, ten
years after its power plant was decommissioned.
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.
Compared to wind farms, hydroelectricity power plants have a more
predictable load factor. If the project has a storage reservoir, it can
generate power when needed. Hydroelectric plants can be easily regulated to
follow variations in power demand.
World hydroelectric capacity
The ranking of hydro-electric capacity is either by actual
annual energy production or by installed capacity power rating. Hydro accounted
for 16 percent of global electricity consumption, and 3,427 terawatt-hours of
electricity production in 2010, which continues the rapid rate of increase
experienced between 2003 and 2009.
Hydropower is produced in 150 countries, with the
Asia-Pacific region generated 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. Brazil, Canada, New Zealand,
Norway, Paraguay, Austria, Switzerland,
and Venezuela
have a majority of the internal electric energy production from hydroelectric
power. Paraguay
produces 100% of its electricity from hydroelectric dams, and exports 90% of
its production to Brazil and to Argentina. Norway produces
98–99% of its electricity from hydroelectric sources.
There are now three hydroelectric plants larger than 10 GW:
the Three Gorges Dam in China, Itaipu Dam
across the Brazil/Paraguay border, and Guri Dam in
Venezuela.
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 capacity factor. The installed capacity is the sum
of all generator nameplate power ratings.
Major projects under construction
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