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Glossary of Hydropower Terms

The glossary of terms defines the components that make up hydro turbines and hydropower plants. Visit Types of Hydropower Plants to view hydropower plant illustrations.

Alternating current (AC): Electric current that reverses direction many times per second.

Ancillary services: Capacity and energy services provided by power plants that are able to respond on short notice, such as hydropower plants, and are used to ensure stable electricity delivery and optimized grid reliability.

Cavitation: The phase changes that occur from pressure changes in a fluid that forms bubbles, resulting in noise or vibration in the water column. The Implosion of these bubbles against a solid surface, such as a hydraulic turbine, may cause erosion, and lead to reductions in capacity and efficiency pressure.

Control gate: A barrier that regulates water released from a reservoir to the power generation unit.

Direct current (DC): Electric current which flows in one direction.

Diversion: A facility that channels a portion of a river through a canal or penstock.

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

Efficiency: 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 potential energy of water into electrical energy.

Fish ladder: A transport structure for safe upstream fish passage around hydropower projects.

Flow: Volume of water, expressed as cubic feet or cubic meters per second, passing a point in a given amount of time.

Generator: Device that converts the rotational energy from a turbine to electrical energy.

Head: Vertical change in elevation, expressed in feet or meters, between the head (reservoir) water level and the tailwater (downstream) level.

Headwater: The water level above the powerhouse or at the upstream face of a dam.

Hydropower: The harnessing of flowing water—using a dam or other type of diversion structure—to create energy that can be captured via a turbine to generate electricity.

Impoundment: A body of water formed by damming a river or stream, commonly known as a reservoir.

Low head: Head of 66 feet or less.

Micro hydro: Hydropower projects that generate up to 100 kilowatts.

Penstock: A closed conduit or pipe for conducting water to the powerhouse.

Power house: The structure that houses generators and turbines

Pumped storage: A type of hydropower that works like a battery, pumping water from a lower reservoir to an upper reservoir for storage and later generation.

Reservoir: See impoundment.

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

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

Small hydro: Hydropower projects that generate 10 MW or less of power.

Spillway: A structure used to provide the release of flows from a dam into a downstream area.

Tailrace: The channel that carries water away from a dam.

Tailwater: The water downstream of the powerhouse or dam.

Transformer: Device that takes power from the generator and converts it to higher-voltage current.

Turbine: A machine that produces continuous power in which a wheel or rotor revolves by a fast-moving flow of water.

Ultra low head: Head of 10 feet or less.

Wicket gates: Adjustable elements that control the flow of water to the turbine.

Learn more about hydropower technology