Frequently Asked Questions


The International Energy's Renewable Energy Essentials: Hydropower (IEA, 2010) which is available for free download (pdf, 1.5MB), is a very useful resource.

The purpose of the following links is to help people searching for information in various areas related to hydroelectricity. The answers are provided for informational purposes only and the mention of organizations and/or links to other web sites is not an endorsement of any organization or web site content.

Generating hydroelectricity starts with the annual hydrologic, or water cycle, providing seasonal rain and runoff from snow pack.

The runoff from rain and snow collects in lakes, streams and rivers and flows to dams downstream.

The water funnels through a dam, into a powerhouse and turns a large wheel called a turbine.

The turbine turns a shaft that rotates a series of magnets past copper coils in a generator to create electricity.

The water then returns to the river.

From the powerhouse, transmission lines carry electricity to communities.   

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The mechanical power of falling water is an age old tool. It was used by the Greeks to turn water wheels for grinding wheat into flour, more than 2,000 years ago . The availability of cheap slave and animal labour, however, restricted its widespread application until about the 12th century. During the Middle Ages, large wooden waterwheels were developed with a maximum power output of about 50 hp. Modern large-scale water-power owes its development to the British civil engineer John Smeaton, who first built large waterwheels out of cast iron.

Water-power played an important part in the Industrial Revolution. It gave impetus to the growth of the textile, leather, and machine-shop industries in the early 19th century. Although the steam engine had already been developed, coal was scarce and wood unsatisfactory as a fuel. Water-power helped to develop early industrial cities in Europe and the United States until the opening of the canals provided cheap coal by the middle of the 19th century.

Dams and canals were necessary for the installation of successive waterwheels when the drop was greater than 5 m (16 ft). Large storage-dam construction, however, was not feasible, and low water flows during summer and autumn, coupled with icing during the winter, led to the replacement of nearly all waterwheels by steam when coal became readily available.

The earliest hydroelectric plant was constructed in 1880 in Cragside, Northumberland, England. The rebirth of water-power came with the development of the electric generator, further improvement of the hydraulic turbine, and the growing demand for electricity by the turn of the 20th century. By 1920 hydroelectric plants already accounted for 40 per cent of the electric power produced in the United States.

The basic principle of operation of most major installations has remained sthe same since then. Plants depend on a large water-storage reservoir upstream of a dam where water flow can be controlled and a nearly constant water level can be assured. Water flows through conduits, called penstocks, Okinawa Seawater Pumped Storage Power Plantwhich are controlled by valves or turbine gates to adjust the flow rate in line with the power demand. The water then enters the turbines and leaves them through the so-called tailrace. The power generators are mounted directly above the turbines on vertical shafts. The design of turbines depends on the available head of water, with so-called Francis turbines used for high heads and propeller-turbines used for low heads.

In contrast to storage-type plants, which depend on the impounding of large amounts of water, a few examples exist where both the water drop and the steady flow rate are high enough to permit so-called run-of-the-river installations; one such is the joint US-Canadian Niagara Falls power project.

In the 1700's, Americans recognized the advantages of mechanical hydropower and used it extensively for milling and pumping. By the early 1900's, hydroelectric power accounted for more than 40 percent of the United States' supply of electricity. In the 1940's hydropower provided about 75 percent of all the electricity consumed in the West and Pacific Northwest, and about one third of the total United States' electrical energy. With the increase in development of other forms of electric power generation, hydropower's percentage has slowly declined and today provides about one tenth of the United States' electricity.

The early hydroelectric plants were direct current stations built to power arc and incandescent lighting during the period from about 1880 to 1895. The years 1895 through 1915 saw rapid changes occur in hydroelectric design and a wide variety of plant styles built. Hydroelectric plant design became fairly well standardized after World War I with most development in the 1920's and 1930's being related to thermal plants and transmission and distribution.

The following lists some important events in hydroelectric history:

1826 French engineer, Benoit Fourneyron, developed a high efficiency (80%) outward flow water turbine in which water was directed tangentially through the turbine runner causing it to spin. Another French engineer, Jean V. Poncelet, designed an inward-flow turbine in 1826 that used the same principles. It was not built until 1838 when S. B. Howd obtained a U.S. patent for a similar design.1848 James B. Francis improved on these designs to create a turbine with 90% efficiency.

1870 the world's earliest hydroelectric project at Cragside, Rothbury, England supplied electric light.1880 the first industrial use of hydropower to generate electricity occurred in Grand Rapids Michigan when 16 brush-arc lamps were powered using a water turbine at the Wolverine Chair Factory in Grand Rapids, Michigan1881 in Niagara Falls, New York a brush dynamo was connected to a turbine in Quigley's flour mill to light city street lamps.1882 in Appleton, Wisconsin the first hydroelectric station to use the Edison system was the Vulcan Street Plant.

1887 the San Bernadino, California, High Grove Station was the first hydroelectric plant in the West of the U.S.1889 at Oregon City, Oregon, the Willamette Falls station was the first AC hydroelectric plant. It transmitted single phase power 13 miles to Portland at 4,000 volts, stepped down to 50 volts for distribution.

1891 at Frankfort on Main, Germany, the first three phase hydroelectric system was used for a 175 km, 25,000 volt demonstration line from plant at Lauffen.

1895 the first publicly-owned hydro-electric plant in the Southern Hemisphere was completed at Duck Reach, Tasmania and supplied power to the city of Launceston for street lighting.

1898 Decew Falls 1, St. Catherines, Ontario, Canada was completed. Owned by Ontario Power Generation, four units are still operational. On 25 August 1898 this station transmitted power at 22,500 Volts, 66 2/3 Hz, two-phase, a distance of 56 km to Hamilton, Ontario. Using the higher voltage permitted efficient transmission over that distance. (Recognised as an IEEE Milestone in Electrical Engineering & Computing by the IEEE Executive Committee in 2002)

1901 at Trenton Falls, New York, saw the first installation of high head reaction turbines designed and built in the U. S.1905 at Sault Ste. Marie, Michigan, the first low head plant with direct connected vertical shaft turbines and generators was built

1906 at Ilchester, Maryland, a fully submerged hydroelectric plant was built inside Ambursen Dam.1911 R. D. Johnson invented the differential surge tank and Johnson hydrostatic penstock valve.

1912 at Holtwood, Pennsylvania, there was the first commercial installation of a Kingsbury vertical thrust bearing in hydroelectric plant.

1914 S.J. Zowski developed the high specific speed reaction (Francis) turbine runner for low head applications.

1916, there was the first commercial installation of fixed blade propeller turbine designed by Forrest Nagler.

1917 the hydracone draft tube was patented by W. M. White.

1919 Viktor Kaplan demonstrated an adjustable blade propeller turbine runner at Podebrady, Czechoslovakia.

1922 was the first time a hydroelectric plant was built specifically for peaking power.

1929 the Rocky River Plant at New Milford, Connecticut, was the first major pumped storage hydroelectric plant.

For more information : A recommended resource detailing the history of hydroelectricity is the two volume set of Hydroelectric Development in the United States 1880-1940, prepared for the Task Force on Cultural Resource Management, Edison Electric Institute, Duncan Hay, New York State Museum, 1991. This book details American hydroelectric developm ent from the first use of hydroelectric power around 1880 up to 1940 by which time there were over 1500 hydroelectric facilities on line producing about one third of the United States' electrical energy. This compares to today when less than one tenth of the energy is hydroelectric.

History of Hydro and Power Plants in the Niagara Region 

The Snowy Mountains scheme, New South Wales, Australia, is one of the world's most complex integrated water and hydroelectric power schemes, which took 25 years to build. Information on the development is available on the Snowy Hydro website.

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If a magnet is inserted into a coil of conducting wire an instantaneous current occurs in the wire which will produce a voltage which can be observed with a voltmeter; when the magnet is removed from the coil another instantaneous but opposing voltage can be observed.

This effect, whereby the relative motion of a magnet and an electric coil produce a current, is known as electromagnetic induction and was simultaneously discovered in 1831 by Michael Faraday (1791-1867, England) and Joseph Henry (1799-1878, America). Faraday developed the first dynamo (generator) in which the continuous rotation of a conducting copper plate between the poles of a magnet produced a continuous current.

Primitive electric generators were in use for various experimental purposes very soon after Faraday's 1831 discovery, but it was about 50 years before commercial generators came into use after Werner Siemens (1816-1892, Germany) perfected a generator in which part of the generator's working current is used to power the field windings, eliminating both the need for permanent magnets and one of the basic limits to generating electric power. In a generator, mechanical energy is converted into electrical energy via a magnetic field.

In a hydroelectric power plant the motion of water is used to move big fan like blades in a turbine to then turn a shaft connected to a generator. The generator has a powerful electromagnet (a rotor) which is turned inside a "'coil" of copper bars (a stator). This produces "electromotive force," or the process of exciting electrons to jump from atom to atom. When electrons flow along a wire or other conductor, jumping from atom to atom, they create an electric current, or a flow of electricity.

Generators cannot store the energy they create. Once the mechanical energy from the flow of water is converted into electricity it must be used immediately. Therefore, electric demand must be well anticipated to avoid wasting the resources used to make electricity.

More general information and diagrams can be found at the Foundation for Water and Energy Education (FWEE), the Canadian Hydropower Association or the U.S. DOE , Department of Energy Efficiency and Renewable Energy.

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This varies according to the size of the generators.

For example, the Bureau of Reclamation's generators in the United States range in size from 350 kilowatts (kW) to 805,000 kW.

The maximum power that could be generated per day from each of those units therefore ranges from 8,400 kilowatt-hours (kWh) (350 kW times 24 hours) to 19,320,000 kWh per day (805,000 kW times 24 hours).

The average size of these generators is about 76,000 kW which would result in an average maximum production of about 1,824,000 kWh per day (76,000 kW times 24 hours).

Since the average household in the United States uses about 1,000 kWh of electricity per month or 33.3 kWh per day, this average unit could supply around 50,000 houses. However hydroelectric units are usually not base loaded (operated at full load or at a given load level continuously) as they are subject to water limitations at time and are also much more useful to the system as load followers. This is because they can respond quickly to changing load needs and are therefore able to follow the ups and downs of the system throughout the day.

The actual production varies each year from about 25 % to 45% of the maximum rated output of the units due to water availability (drought or flood years, etc.) and system requirements.

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The actual cost of producing power will vary from power plant to power plant with one of the main variables being the size of the plant.

For example, since it could take as many people to operate and maintain a small one unit generator as it would to operate and maintain two larger generators, the cost of operation and maintenance per kilowatt produced would be higher for the smaller plant.

In general, the larger the hydroelectric plant, the cheaper the cost per kilowatt to produce the electricity.

When compared to other means of producing electricity, hydroelectric production costs run about one third those of either fossil-fueled (coal or oil) or nuclear power plants, and is less than one fourth the cost of gas turbine electricity production.

The main contributing factor for the difference in this cost of production is the fuel costs for the other means of producing electricity.

The original plant cost for a hydroelectric plant is somewhat cheaper than either fossil fuel or nuclear plants.

Gas turbine plants are the cheapest to build but the most expensive to operate.

The International Energy Agency  regularly reports on electricity cost production. Projected Costs of Generating Electricity - 2015 (IEA, 2015) is available from the IEA Bookshop. Download the summary report. 

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Hydroelectricity enjoys several advantages over most other sources of electrical power. These include a high level of reliability, proven technology, high efficiency, very low operating and maintenance costs, and the ability to easily adjust to load changes.

Because many hydropower plants are located in conjunction with reservoirs, hydropower projects often provide water, flood control, and recreation benefits. In addition, hydropower does not produce waste products that contribute to air quality problems, acid rain, and greenhouse gases. It is a renewable resource that reduces the use of other fuels (oil, gas, and coal).

Disadvantages of hydroelectricity include high initial costs of facilities; dependence on precipitation (no control over amount of water available); changes in stream regimens (can affect fish, plants, and wildlife by changing stream levels, flow patterns, and temperature); inundation of land and wildlife habitat (creation of reservoir); and displacement of people living in the reservoir area.

The environmental impacts of hydroelectric projects are discussed in great detail in the publicationHydropower and the Environment (IEA Hydropower IA, 2000) 533kb.


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In some situations, increased rating and efficiency can be obtained by runner replacement. For pre-1960 turbines, it is frequently possible to obtain output increases as high as 30 percent, and efficiency increases of more than 5 percent, by replacing existing runners with runners of improved design.

The former requirement that generators deliver rated output with no more than a 60C temperature rise, and the conservative safety factors provided by early generator manufacturers, result in the possibility of substantial increases in machine capacity by installing windings using modern insulation technology which can provide increased electrical capacity with the same physical size as earlier manufactured windings.

Therefore, it is often possible to increase the capacity of older units by installing new stator windings and improved runners, and by upgrading various auxiliary equipment. Apart from technical limitations, the economic value of capacity is most important in justifying a refurbishment.

The practical limit of uprating is reached when the cost of replacing equipment to obtain additional capacity equals the economic worth of that added capacity.

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The IHA and the IEA estimate the world’s total technical feasible hydro potential at 14,000 TWh/year, of which about 8000 TWh/year is currently considered economically feasible for development.

At present about 808 GW in operation or under construction. Most of the potential for development is in Africa, Asia and Latin America with Asia having the greatest economically feasible potential at 3600 TWh/year.

South America has an economically feasible potential of 1600 TWh/year and Africa's potential is 1000 TWh/year. In the United States the Department of Energy has identified 5,677 sites with undeveloped capacity of about 30,000 MW.

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Small-scale hydropower is usually 10 MW or less in size.

One of the common definitions for micro hydropower is a rated capacity of 300 kW or less. The 300 kW limit is because this is about the maximum size for most stand alone hydro systems not connected to the grid, and suitable for "run-of-the-river" installations.

Small-scale hydro is normally run of the river design and is one of the most environmentally benign energy conversion options available, because it does not attempt to interfere significantly with river flows.

The break point between small-scale and large-scale hydro differs from country to country ranging between 10 and 50 megawatts (MW).

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It is almost impossible to provide an accurate answer to this question since much of the micro hydro and small-scale hydro is not connected to a grid or major electrical system, and is therefore never measured. The definition of small-scale hydro power also varies from country to country.

However, it is estimated that about 5 percent of the total hydro potential is in the small hydro range which would yield a result of 100 billion to 150 billion kilowatt hours per year.

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A good starting point for this information can be found on the following sites:

  • RETScreen Clean Energy Management Software is a system for evaluating the feasibility, energy production, life-cycle costs and greenhouse gas emissions (GHG) reduction for central-grid, isolated-grid and off-grid small hydro projects, ranging in size from multi-turbine small and mini hydro installations, to single-turbine micro hydro systems. It is also a comprehensive repository of technical information and source material on clean energy. It is available as a free download from Natural Roesources Canada, with information about the system in 36 languages. 

  • The U.S. Department of Energy Hydropower Program site, and the site offer news, resources and downloads in relation to micro hydro development.
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The basic elements required for a potential hydropower development are streamflow and an available drop, or "head", through which the streamflow can be used to convert the potential hydraulic energy into electrical energy. The power generated is represented by the equation:

P = eHQg


P = Electric Power Output in kilowatts (kW)

e = Efficiency range 0.75 to 0.88 (75% to 88%)

H = Head, in metres (m)

Q = Design flow, in cubic metres/sec (m3/s)

g = acceleration of gravity, normally 9.81 m/s/s

For small-scale hydroelectric applications, if an Efficiency value of 81% is assumed, the following equation can be used:

P (kW) = 7.95 x H (m) x Q (m3/s)

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