Hydropower Technologies
How hydropower Works

Hydropower is using water to power machinery or make electricity. Water constantly moves through a vast global cycle, evaporating from lakes and oceans, forming clouds, precipitating as rain or snow, then flowing back down to the ocean. The energy of this water cycle, which is driven by the sun, can be tapped to produce electricity or for mechanical tasks like grinding grain. Hydropower uses a fuel—water—that is not reduced or used up in the process. Because the water cycle is an endless, constantly recharging system, hydropower is considered a renewable energy.

The Water (Hydrologic) Cycle

When flowing water is captured and turned into electricity, it is called hydroelectric power or hydropower. There are several types of hydroelectric facilities; they are all powered by the kinetic energy of flowing water as it moves downstream. Turbines and generators convert the energy into electricity, which is then fed into the electrical grid to be used in homes, businesses, and by industry.

Types of Hydropower

There are three types of hydropower facilities: impoundment, diversion, and pumped storage. Some hydropower plants use dams and some do not. The images below show both types of hydropower plants.


The most common type of hydroelectric power plant is an impoundment facility. An impoundment facility, typically a large hydropower system, uses a dam to store river water in a reservoir. Water released from the reservoir flows through a turbine, spinning it, which in turn activates a generator to produce electricity. The water may be released either to meet changing electricity needs or to maintain a constant reservoir level.


A diversion, sometimes called run-of-river, facility channels a portion of a river through a canal or penstock. It may not require the use of a dam.

Pumped Storage

When the demand for electricity is low, a pumped storage facility stores energy by pumping water from a lower reservoir to an upper reservoir. During periods of high electrical demand, the water is released back to the lower reservoir to generate electricity.

Sizes of Hydropower Plants

Facilities range in size from large power plants that supply many consumers with electricity to small and micro plants that individuals operate for their own energy needs or to sell power to utilities.

Large Hydropower

Although definitions vary, we define large hydropower as facilities that have a capacity of more than 50 MW.

Medium Hydropower

Although definitions vary, we define medium hydropower as facilities that have a capacity of 10 MW to 50 MW.

Small Hydropower

Although definitions vary, we define small hydropower as facilities that have a capacity of below 10 MW.

Advantages and Disadvantages of Hydropower

Hydropower offers advantages over other energy sources but faces unique environmental challenges.


Hydropower relies on the water cycle, which is driven by the sun, thus it's a renewable power source. Hydropower is a fueled by water, so it's a clean fuel source. Hydropower doesn't pollute the air like power plants that burn fossil fuels, such as coal or natural gas.
Hydropower is generally available as needed; engineers can control the flow of water through the turbines to produce electricity on demand.
Hydropower plants provide benefits in addition to clean electricity. Impoundment hydropower creates reservoirs that offer a variety of recreational opportunities, notably fishing, swimming, and boating. Most hydropower installations are required to provide some public access to the reservoir to allow the public to take advantage of these opportunities. Other benefits may include water supply and flood control.


Fish populations can be impacted if fish cannot migrate upstream past impoundment dams to spawning grounds or if they cannot migrate downstream to the ocean. Upstream fish passage can be aided using fish ladders or elevators, or by trapping and hauling the fish upstream by truck. Downstream fish passage is aided by diverting fish from turbine intakes using screens or racks or even underwater lights and sounds, and by maintaining a minimum spill flow past the turbine.

Hydropower can impact water quality and flow. Hydropower plants can cause low dissolved oxygen levels in the water, a problem that is harmful to riparian (riverbank) habitats and is addressed using various aeration techniques, which oxygenate the water. Maintaining minimum flows of water downstream of a hydropower installation is also critical for the survival of riparian habitats.

New hydropower facilities impact the local environment and may compete with other uses for the land. Those alternative uses may be more highly valued than electricity generation. Humans, flora, and fauna may lose their natural habitat. Local cultures and historical sites may be impinged upon. Some older hydropower facilities may have historic value, so renovations of these facilities must also be sensitive to such preservation concerns and to impacts on plant and animal life.

Glossary of Hydropower Terms

The glossary of terms defines the components that make up hydro turbines and hydropower plants.

Alternating current (AC)

Electric current that reverses direction many times per second.

Direct current (DC)

Electric current which flows in one direction.

Forbay, Trashrock, Penstock ,

Forbay is the upstream pool before the intake. Trashrock is the net screen to prevent large objects flowing into the turbine. Penstock is a steel closed conduit or pipe for conducting water to the powerhouse.

Head H

Vertical change in elevation, expressed in meters, between the upstream water level and the tailrace water level. Rated head is Hr in meters.

Flow Q

Volume of water, expressed as cubic meters per second, passing a point in a given amount of time. Rated flow is Qr in m3/s.

Efficiency η
A percentage obtained by dividing the actual power or energy by the theoretical power or energy. It represents how well the hydropower plant converts the energy of the water into electrical energy.

Output of Hydro Turbine Nt
in kW

Nt=9.81ηHr Q (KW)

Spiral case

A spiral-shaped steel intake guiding the flow into the wicket gates located just prior to the turbine.

Wicket gates

Adjustable elements that control the flow of water to the turbine passage.


The rotating part of the turbine that converts the energy of falling water into mechanical energy.

Cavitation and Suction head

Noise or vibration causing damage to the turbine blades as a results of bubbles that form in the water as it goes through the turbine which causes a loss in capacity, head loss, efficiency loss, and the cavity or bubble collapses when they pass into higher regions of pressure.
The suction head Hs is the distance from the install level of the center line of turbine runner to the minimum tailrace level.

Draft tube

A water conduit, which can be straight or curved depending upon the turbine installation, that maintains a column of water from the turbine outlet and the downstream water level.


The channel that carries water away from the powerhouse.


The downstream water of the powerhouse.


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