Hydroelectricity or hydroelectric power is electricity generated from the captured energy of moving water, be it from the sea, or from freshwater sources. It is an established and continuous form of renewable energy. It uses the natural water cycle, and gravity, to generate electricity.

Determining the powerpotential of a site[edit | edit source]

Determining hydropower potential[edit | edit source]

The following mathematical formula allow to estimate the hydropower potential:

Knowing that:

  • P expresses the electrical power in kilowatt-hour of the current generator;
  • Q expresses the waterflow in the penstock in m³/sec;
  • H expresses the difference in water level between the entrance to the penstock and the leakage channel of the turbine, expressed in meters;
  • R is the overall efficiency of the facility, including the loss of load in the penstock, the turbine efficiency and alternator efficiency. A total efficiency of 60% is acceptable for the estimated hydroelectric potential.

As an example, with Q being 0.1 m³/sec and H being 10m we get a potential of 6kW.

The efficiency of the turbine is a characteristic of the machine given by the manufacturer. This efficiency depends on the waterflow. Indeed, if the inflow is, for example, only half of the expected speed (or nominal waterflow), it is easy to understand that the piece of the waterflow that will run between the rotating parts and the fixed parts without reacting on the blades of the machine does little or nothing. So the effective waterflow drops sharply and leads to an efficiency drop.

The potential of a site can be improved if the hydroelectric equipment is not a hydroelectric facility along the water stream but equipped with an accumulation pond. In these circumstances it is possible to, for a few hours of the day, provide sufficient water flow for periods of greater consumption of electricity. When the population of the village sleeps, they generally consume much less electricity. This decrease in electricity requirements accompanies, through the speed regulator discussed above, a decrease in water consumption. The saved amount can be used at times of greater need. This accumulation pond should be considered in the context of a daily regulation. If deals around overcoming the insufficiencies of low water flows, the accumulation intake reaches a size that is outside the context of small power plants.

Overview of formula's[edit | edit source]

The amount of energy released by lowering an object of mass by a height in a gravitational field is[1]:

where is the acceleration due to gravity.

Converting these units, a common field equation to measure the maximum power available in a moving body of water is:


  • Pmax=Maximum Power Available (kW)
  • Qmax=Volumetric Flow Rate (Volume/time)
  • Hmax=Head (Vertical drop in ft)
  • emax=Efficiency of the turbine (use a value of 1 for max power available)
  • K=Unit conversion factor (see table below for some common values)
For Q measured in K is equal to
ft3/min 708 (ft4)/(min*kW)
ft3/sec (CFS) 11.8 (ft4)/(sec*kW)
l/sec 334(l*ft)/(sec*kW)
gal/min (GPM) 5302 (gal*ft)/(min*kW)

To find the actual power you will get from that moving body of water, calculate Pnet with the following changes made.


  • Pnet=The net power extracted from the river, not including loss in delivery from power station to load (kW)
  • Qnet=Volumetric Flow Rate (Volume/time) - Only take a portion of the max flow (%take). For delicate streams this may be a small percentage of the total flow.
    • Qnet=Qmax*%take
  • Hnet=Head (Vertical drop in ft) - This is the actual head that you have available due to losses from friction. Calculate friction loss using tables based on the materials you use for diversion (e.g. PVC).
    • Determine equivalent length of pipe by adding actual length of pipe and equivalent lengths of fittings based on tables using pipe size.
    • Find Frictional Pressure Loss Ratio (FPL) coefficient in ftloss/ftpipe based upon flow rate and pipe size
    • calculate Hloss=equivalent length of pipe * FPL


  • enet=Efficiency of the turbine - Always between 0 and 1, usually between.5 and.9 depending on the turbine type and flow rate. A value of 0.78 is a good guess for modern turbines in average conditions.
  • K=Unit conversion factor (see table above for some common values)

Note that these equations are static in time. You must run these equations for with a resolution high enough to cover periods of variation (e.g. monthly river data).

Parts of a hydroelectric plant[edit | edit source]

An installation, no matter how small, always includes the following elements (following the flow of water):

  1. a main water intake which ensures that as long as there is water in the river, it is directed towards the power plant. This intake is fitted with a valve that can cut the water supply in case of failure of the turbine or in case of repairs of the infrastructure.
  2. an intake channel more or less long will lead water from the main water intake to the head intake or startup room. This channel can also create a greater drop height when choosing a suitable route.
  3. the intake of the startup room ensures filling the pipeline linking the intake to the turbine. The correct creation of the startup room keeps the penstock submerged. The intake can be used as an emergency release to allow the evacuation of excess water by reducing the water needs of the turbine.
  4. the penstock (pipe between the intake of startup room and turbine) creates the water column which allows the startup of the turbine. This pipeline usually of steel for conventional plants, but can be carried out in polyethylene for pico-power. Its diameter is calculated to avoid loss of charges in the flow of water. A large pipe is expensive in purchase and placement but reduces the loss. When the height of the water column is low, there is no penstock but a concrete construction in which water flows to support the water column.
  5. the turbine room of the facility, is a kind of wheel that is rotated by the flow of water flowing through it. The type of wheel is to be chosen depending on the water flow, but also depending on the water pressure. Some of these turbines resemble the old bucketwheels which are particularly suitable for plants that need to operate with significant water flow variations. The turbines may have a vertical or horizontal axis. There are two types of turbines, an impulse turbine and a reaction turbine. An impulse turbine uses the flow of water to move a runner which then discharges pressure, the steam then hits the buckets on the runner and water flows out of the bottom of the turbine. This type of turbine is best used in high head and low flow situations.At these sites either a pelton and a cross-flow turbine should be used. A reaction turbine uses pressure and moving water to generate electricity, the runner is placed in the water and flows through all of the blades to generate electricity. This system is best used at a low head and high flow site. At these sites either a propellor, Francis and kinetic turbines should be used.
  6. The speed regulator controls the speed of the turbine. In times of normal operation, speed is the balance between the hydraulic load that turns the turbine and the resistive load of the electricity production. In case of a ruptured electrical circuit, the resistive electrical is canceled and the speed of the turbine will accelerate because the hydraulic load remains unchanged. Too much speed can be fatal to the equipment and the presence of the regulator corrects the beginning of the acceleration also called the runaway.
  7. The electricity generator, is another essential part as it will convert the mechanical energy of the turbine into electrical energy. This conversion into electricity facilitates the provision on places that can be removed from the turbine. Today, the generator is an alternator that provides an alternating current, a type of electricity that is more widespread than direct current. To provide an alternating current conforming to the needs of network users, the alternator must turn in a speed range set by the manufacturer. Between the generator and the turbine, which rotates much more slowly, a gearbox is installed to make the operation of the two machines compatible. The generator is set synchronous or asynchronous, as determined or set on the frequency of the cycles of the current on the network, being 50 or 60 cycles per second depending on the frequency of the grid. If the plant is the only main power source of the network, install a synchronous alternator.
  8. Electrical cabinet for control and distribution. For the safety of the facilities and also the people, one should distribute the current on the network via a switchboard where the devices (fuses, disconnectors) control the distribution. At the other end of the network similar equipment should be installed to control the distribution with the client.
  9. The electrical distribution network includes wiring that allows the transport of power to the users. This network is either low voltage, i.e. operating voltage of client devices, or high voltage. This is necessary when the transport distance is over 500m. Indeed, to carry 20 kW at a voltage of 220V, one requires a 90 amp electrical cable of at least 10mm2 section. If the voltage is 5000V, a section of 1mm2 is sufficient. The cost of this cable will probably be cheaper than the other one.[verification needed]
  10. The leakage channel is the route of escape of the water that has gone through the turbine. This channel is connected to the stream that resupplies the water volume that is diverted to the water intake. It is important that this channel is not flooded in case the river floods. Indeed, the rise of water level can cause flooding of the engine room (generator and turbine) with risk of damage.

Other equipment that can be used is a sand trap on the feeder canal. The grains of sand that accompany the water in the turbine have an abrasive effect that reduces the life of the machine. The biggest materials (stones, bricks) have an even more destructive effect and gratings should be installed at the entrance to the intake line but also at the penstock to prevent such incidents.

Assessment of electricity needs[edit | edit source]

It is difficult at first to estimate the effective electricity output since it depends on the project which is associated with the construction of the hydroelectric power plant. It is known that the provision of electricity promotes its use, without any regulation system of the consumption (limiting the amount available to the consumer via electrical switchboard and billing of the consumption). The lighting of offices is an important element for the development of the electricity grid, as well as the sponsoring of communication facilities such as the telephone network, operation of television channels and computers. The electrical energy is also useful to activate the cooling chain (storage of medicines, vaccines...). Foremost, we must exclude, in the context of a micro-powerplant, the use of electricity as a means of heating because consumers will become prohibitive for the small network. To give a general idea however, an installed capacity of 2kW per family is a correct value to identify the power needs in a village community.[verification needed]

It is also possible to set, among the regular customers of a small network, priority clients such as those that manage a cooling chain for vaccines and drugs, or a surgical hospital. These customers will be supplied as long as possible. These provisions should be exposed to all users as a way to maintain cohesion and avoid conflicts. This regulating system allows to supply electricity to more customers but for a more limited period.

Types of hydroelectric power plants[edit | edit source]

A large scale hydroelectric power plant forces water, generally held in a dam, through a hydraulic turbine connected to a generator. After the water exits the turbine it returns to the stream or river below the dam.

Because of the dependence on gravity, hydroelectric power is generally more feasible in mountainous regions than in flat country.

There are three different types of hydroelectric power plants, impoundment, diversion and pumped storage. An impoundment system is the most common type and uses the whole width of the river to generate power. A diversion also known as "run-of-river" hydro power plant diverts a portion of the water to generate power but does not require a dam. Part of the water is diverted through an intake before the water falls or drops significantly either by a waterfall or a loss in head. That water then runs through the system and through the turbine and then leaves through the outtake. A pumped storage system is compared to a battery because it stores the energy by pumping the water uphill to a reservoir that has less head. When there is less demand for energy this storage facility pumps the water from a lower reservoir to the upper reservoir. When there is high demand for energy the water is then pumped back down from the high reservoir to the lower reservoir to compensate for the energy demand.

Hydroelectricity has relatively low operations and maintenance costs, especially compared to other forms of renewable energy.

Sizes of hydroelectric plants[edit | edit source]

Today, to transform hydropower to electrical power, it is possible to construct very large but also small-sized plants. This article lists the parameters to determine small hydroelectric facilities. They have a place in the process of the development of small communities and in the process of sustainable development because of the economy in terms of CO²-emissions. See http://www.kuleuven.ac.be-ei-public-publications-EIWP900-09_fr.pdf

The technical literature considers small power plants, those plants that generate a power output less than 2000kW or 2 MW. The small power plants are divided into mini power (maximum power output 500kW), micro-power (maximum power output of 100 kW) and pico-power (0.2 kw to 5kW). These powers outputs are to be considered as orders of magnitude because, depending on the country or region, different figures have been cited. The pico-plants have been installed mostly in Asia, Vietnam (120,000) and the Philippines.

Country Micro (kW) Mini (kW) Small (MW) Source
United States <100 100-1000 1-30 Dragu, 2002
China <500 0,5-25 Dragu, 2002
Italy <3 European Commission, 2000
Portugal,Spain, Ireland, Greece, Belgium <10 European Commission, 2000
France 5-5000 <8 European Commission, 2000
India <100 101-1000 1-1,5 Dragu,2002
ESHA-European Small Hydropower Association <100 101-500 0,5-10 ESHA,1998

Advantages and disadvantages[edit | edit source]


  • hydroelectricity is a long-established technology, and the process itself is very reliable.
  • hydroelectricity has very little impact on air pollution and climate change, as no fuel is burned.
  • hydropower is highly dependent upon precipitation (rain and snow); droughts can effect generating capacity.


  • vegetation in the flood zone when the dam is built will decay in the lake formed by the dam, releasing methane. The net result is still believed to be a very small amount of greenhouse gases for the electricity produced, but it will vary from dam to dam.
  • (when using a hydroelectric dam): the ecological effects of a dam on the downstream side of the structure can be severe, as the water restriction often dramatically changes the normal flow of the river or stream. The range of anadromous fish (such as salmon) upriver is understandably hampered by the concrete and turbines of dams, beyond which is a habitat that has been deprived of a main predator. The buildup of sediment and silt behind the dam can substantially alter the geology of the downstream reaches. The construction of a dam is hence extremely controversial and guaranteed to face strong opposition from environmental groups and owners of affected farms and other property. In practice, only an authoritarian government is likely to choose to build a new dam. To avoid some of these issues, a "run of the river" design is preferable.
  • the ability to build new hydroelectric capacity is extremely limited, especially in developed countries, because most suitable sites are already being used.

In developing countries, the potential for small hydropower installations has never carefully been measured. Past surveys of hydropower potential have focused on possible sites for large dams, as small hydroelectric units were considered un-economical or ill-suited to the goal of providing large blocks of electric power for cities, industrial estates, or aluminum production. With the rapidly increasing costs of energy, however, the economics are now much more favorable for small hydroelectric units, which are also well-suited to the needs of small rural communities, and do not bring the degree of environmental disruption associated with large reservoirs. Many small units do not require reservoirs at all, and use small diversion canals instead. With climate change becoming more of a threat the necessity for increasing renewable energy usage around the world is very important. Bringing electricity to rural communities is also becoming more prudent so through installing micro-hydro plants this could become possible(depending on the access to a river or stream).

The success of waterpower installations can be greatly affected by forest conservation practices in the watershed above. Rapid deforestation brings high rates of soil erosion and subsequent rapid silt filling of reservoirs behind dams. At the same time, greater rain runoff causes increasingly violent floods that threaten hydropower installations. During the months following the floods, low water flows are likely to reduce generating capacity. A program to protect the watershed and the construction of a diversion canal may be necessary to prevent damage to a small waterpower installation.

Economics[edit | edit source]

  1. A small hydroelectric powerplant is very expensive to install, around 1500 € to 3000 € per installed kW; operating costs are however quite low. The material is generally robust and lasts a lifetime if regular maintenance is exerciced for several decades. If we sign up for the perspective of a sustainable development, we should consider that in terms of the economic life-expectancy of the facility, the replacement of the facility will be incorporated by the users of the network agreeing to pay for the supplied power. The billing of the consumption allows to build up the needed capital for the maintenance and future replacement; it avoids abusive consumptions. About the pricing, it must be designed in function of the objectives of the powerplant project. About the pricing, we suggest progressive pricing (the more one consumes, the more the price per kW is increased, obviously within certain limits) which reduces the risk of waste, and sectorial pricing (collective or community consumptions are are charged at more favorable rates). See http://www.ciele.org/filieres/hydraulique.htm
  2. Operating costs are low if the facility is regularly maintained: the damaged parts are replaced quickly (stock of spare parts), abnormal wear is investigated to identify the causes. Losses on the network are eliminated without delay.
  3. Who says maintenance, says manager and personnel trained for the task at hand.
  4. Control of consumption and illegal connections are punished. A too great consumption (overloading the network) will lead to a voltage drop that can cause damage to connected equipment, especially engines. The client is asked to sign a service contract which stipulates his rights and obligations.
  5. Public information campaign on things to avoid and prevention: The area of the water intake in the river is to be avoided by the other users of the river (risk of getting caught up in the waterflow and risk of drowning); intake channel may be used for other purposes (laundry, irrigation), the manager informs the public to incidents such as blocked grates, overflow, etc... The practice of bathing is regulated or prohibited in the absence of a basin with restraints to prevent the risk of drowning or sudden spills of large volumes of water. Information on the dangers of electrocution will be presented at both the facility and at the home.
  6. In the spirit of sustainable development, the facility must take into account other water uses, both existing or planned, at the time of the drafting of the powerplant project. The sharing of the water must be done fairly; ie on the basis of criteria that all stakeholders comply to.
  7. Estimates of budget, deadline for implementation: From an average unit cost of 2000 €/kW installed, it is easy to estimate the investment budget for 10, 100 or 1000kW, as being € 20,000 to € 2,000,000. On the other hand, pico-powerplants of 300W are sold in Vietnam for U.S.D. $20 and are very popular. It is likely that the complexity of a facility of more than 100kW extends the delay of realisation because of forgoing feasibility studies, which probably increases the quality and reliability of the project. Compact installation kits are sold in commerce, requiring only a minimum of know-how for the installation. This equipment series offers the advantage of being available quickly and to be well calibrated for operating conditions. Unless you have control of all parameters (location, project funding, expertise for the construction of infrastructure), an installation project requires several years between the first sketches on paper and the effective operation of the distribution network.
  8. Small powerplants and local development: Many places on earth are not yet equipped with electric network due to lack of interest (economic or political) to install utilities. The low population density is one of the main reasons for this lack. A micro-powerplant offers a solution for decentralised development where the local undertaker or local authorities can act autonomously. A well designed and well maintained installation offers a very low return price, within the reach of small and medium enterprises. The social impact of these small facilities is considerable in the fight against poverty. See http://www.tve.org/ho/doc.cfm?aid=1636&lang=English

The Chinese authorities have helped a great amount of areas removed or isolated from major electricity distribution networks in acquiring small powerplants. Similar projects have been developed, especially in Kenya.

These measures allow the local population to develop activities that meet their expectations and to avoid, for example, migration to the already congested suburbs of major cities. These populations can emerge from the isolation, communicate with the rest of the world and participate in the evolution.

Use of hydroelectricity[edit | edit source]

From backyard, single resident systems made from recycled car parts, to grand scale projects such as those on the Colorado river in the western United States, people are generating energy of water moving down a gradient. The size and therefor, application, of a hydroelectric system can vary widely.

Use in P.R. of China[edit | edit source]

The use of water power in the People's Republic of China has reflected the same pattern, with first water wheels and then turbines being built in great numbers as power demands increased along with technical production capabilities. By 1976 an estimated 60,000 small hydroelectric turbines were in operation in South China alone, contributing a major share of the electricity used by rural communes for lighting, small industrial production, and water pumping.

With the rising cost of energy in the United States today, small hydroelectric units are returning in large numbers. Generating stations along New England rivers are being rehabilitated and put back into operation. The number of companies making small waterpower units has jumped. The U.S. Department of Energy has estimated that 50,000 existing agricultural, recreational, and municipal water supply reservoirs could be economically equipped with hydroelectric generating facilities.

Use of hydroelectric plants depending on size[edit | edit source]

Size Power Output Typical Use
Large >10MW Usually part of a large grid.
Small 1MW-10MW Usually grid intertied. up to 50MW in Canada
Mini 100kW-1,000kW (1MW) Community and industry. Often grid intertied.
Micro 1kW-100kW Small low energy consuming community, small industry, rural high energy consuming household. Usually off-grid.
Pico <1kW Radio tower, low energy consuming household, charging station. Almost always off grid.

Please note that these are averages, many different communities classify hydropower somewhat differently.

References[edit | edit source]

  1. Some high flow, low head situations can use hydropower systems such as a water wheel to convert just kinetic energy of the flowing water with very little change in the potential energy. In those cases, where is the velocity of the water.

See also[edit | edit source]

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Authors KVDP, Chris Watkins, Katelyn Peakes
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Aliases Hydro Energy
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Created December 6, 2007 by Susan Brown
Modified May 26, 2024 by Kathy Nativi
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