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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.

Balancing authority: Responsible entity that integrates resource plans ahead of time, maintains load-interchange-generation balance within a balancing area, and supports interconnection frequency in real time.

Black start: A process of restoring a power station to operation without relying on the external electric power transmission network.

Capacity factor (net): The ratio of the net electricity generated, for the time considered, to the energy that could have been generated at continuous full-power operation during the same period.

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.

Closed-Loop Pumped Storage Hydropower: Projects typically consisting of two reservoirs that are not connected to naturally-flowing sources of water.

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

Curtailment: Reduction of output (ramp down or shut down) that is a generation unit's response to a grid operator's request, or to market signals.

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

Dispatch: The operation of a generating unit within a power system at a designated output level to meet demand for electricity.

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.

Energy arbitrage: Purchasing (storing) energy when electricity prices are low, and selling (discharging) energy when electricity prices are high.

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

Flexibility: The ability of the power system to respond to variations in supply and/or demand.

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

Frequency response: Generation ability to increase and decrease output to maintain system frequency

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.

Load: The amount of electrical power delivered or required at any specific point or points on a system.

Load following, Load shifting: Ability of a hydropower plant to adjust its power output as electricity demand changes throughout the day.

Low head: Head of 66 feet or less.

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

Modular: Standardized structures designed so their capacity and function can be scaled by deployment of multiple components that integrate easily.

Nameplate capacity (installed): The maximum rated output of a generator, prime mover, or other electric power production equipment under specific conditions designated by the manufacturer. Installed nameplate capacity is commonly expressed in megawatts (MW) and is usually indicated on a nameplate physically attached to the generator.

New stream-reach: Denotes waterways that are previously undeveloped with hydropower.

Non-powered dams: Dams that do not have any electricity generation equipment installed.

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.

Ramping capability: Ability of a power station to change its output over time.

Reliable (power generation): Probability that a generating unit will perform when used under stated conditions.

Reservoir: See impoundment.

Round trip efficiency: Energy storage typically consumes electricity and saves it in some manner, then hands it back to the grid. Round Trip Efficiency is the ratio of energy stored (per unit of energy) to energy discharged from the storage system (per unit of energy), expressed as a percentage. The higher the round trip efficiency, the less energy we lose due to storage, the more efficient the system as whole. A round trip efficiency of 80% in energy storage systems is preferred when possible. Put simply, this process shows how a storage system loses energy and why the process is a net consumer of energy. These losses are outweighed by the grid services and other benefits such as energy arbitrage – buying and storing electricity when prices are low and producing and selling the same when prices are high.”

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

Run-of-river: Type of hydropower project in which limited storage capacity is available and water is released at roughly the same rate as the natural flow of the river.

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