The U.S. Department of Energy’s hydropower glossary contains definitions for technical terms related to hydropower. Visit Hydropower Basics to learn more about the renewable energy source and Types of Hydropower Plants to view hydropower plant illustrations.

Hydropower Terms

Note: Many of these terms are broadly used across energy sectors, and the definitions listed below refer specifically to their usage within the hydropower industry.

Adjustable-speed technology: In hydropower, refers to machines that have the ability to enable the power consumed (pumps) or generated (turbines) to be varied, thus providing greater flexibility.

Ancillary services: Capacity and energy services (e.g., non-spinning operating reserve, frequency support, voltage support) 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. Also called grid services.

Balancing authority: Entity responsible for integrating resource plans ahead of time, maintaining load-interchange-generation balance within a balancing area, and supporting interconnection frequency in real-time.

Baseload: Minimum energy demand on a given electrical power system over a specific period of time.

Basin-scale: Encompassing the activities that occur within the area of land drained by a river and its tributaries.

Biodiversity: The variety of life in the world or in a particular habitat or ecosystem.

Biogenic: Produced or brought about by living organisms

Biologically-based design: Design of hydropower equipment, such as turbines, that takes into account its direct or indirect biological effects on fish and other aquatic species.

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

Bulk power: Power from generation facilities necessary to maintain reliability of the transmission system.

Bypass reach: The portion of a natural waterway between the intake and the tailrace where any and all flow usually comes from the spillway.

Capacity factor (net): Ratio of a power plant’s actual output over a period of time to its potential output if it were possible for it to continuously operate at full nameplate capacity over the same period of time.

Cavitation: Phenomenon that affects hydropower turbines when vapor bubbles form and implode due to rapid pressure changes, generating shock waves that create cavities on the metal surface.

Civil works: Infrastructure of a hydropower project, such as dams, conduits, powerhouses, tunnels, and penstocks.

Closed-loop pumped storage hydropower: Consists of two reservoirs that are not connected to naturally flowing sources of water.

Condition-based maintenance: A maintenance program that recommends maintenance actions based on information collected from monitoring equipment through its life cycle.

Conduit: A manmade structure for conveying water, such as canals, tunnels, and/or pipelines.

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

Critical infrastructure: Assets that are considered vital to the energy, economy, health, and/or safety security of the United States, such as storage reservoirs for water supply and flood management, dams for power production, and the electrical transmission grid.

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.

Cyber surrogate capabilities: Systems designed to help identify intrusions into the hydropower network by assessing suspicious network traffic or inconsistencies in system signals/operation.

Denitrification: A decrease of dissolved atmospheric nitrogen at a reservoir.

Digitization: The translation of analog systems to digital control systems not only solves traditional challenges for hydropower but also enables access to a new range of opportunities for the industry.

Digitalization: How translated digital control systems (see: digitization) are used to modify business practices and enhance how hydropower plants operate fundamentally.

Digital Transformation: The application of digital capabilities to not only solve traditional challenges for hydropower but also enable access to a new range of opportunities for the industry.

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

Distributed generation: Small, grid-connected energy generation systems located close to the load they serve.

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.

Economic dispatch:  The operation of a generating unit within a power system at a designated output level to meet demand for electricity and generate energy at the lowest possible cost.

Electrical demand: Rate at which electricity is being consumed at a given instant or averaged over a specified period of time.

Electricity generation: The amount of electricity a generator produces during a specific period of time.

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

Energy imbalance services (reserves): A market service provided for the management of unscheduled deviations in individual generator output or load consumption.

Entrainment: The forced passage of fish in water flowing into a turbine or cooling water intake at a power plant.

Environmental flows: Flows required to protect natural, cultural, and recreational resources.

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

Fish passage structure: Structure on or around a dam to facilitate the movement of migrating fish.

Fixed-speed technology: Pump and turbine units that are operated at a constant speed.

Flexibility: The ability of the power system or individual unit to quickly 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.

Flow regime: The magnitude, duration, timing, seasonality, and rate of change of flows in a natural waterway.

Forebay: Impoundment or reservoir immediately above a dam or intake structure at a hydropower plant.

Frequency regulation: Efforts by a balancing authority to maintain scheduled frequency in the grid.

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.

Grid: An electricity transmission and distribution system.

Head loss: Energy lost as water flow, moving from the headwater to the tailwater of a dam, experiences friction due to factors such as the turbines, valves, and turbulence.

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

Hydraulic head: A measure of liquid pressure, expressed in terms of the height of a column of water, which represents the total energy of the water.

Hydroacoustics: Underwater sound; also a technology to monitor fish passage, abundance, and distribution.

Hydrologic cycle: Earth’s natural water cycle includes the processes of evaporation, condensation, precipitation, interception, infiltration, percolation, transpiration, runoff, and storage.

Hydropeaking: The discontinuous releases of water through turbines to meet peak energy demands which causes downstream water flow fluctuations.

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. Also called hydroelectric power.

Impoundment: Body of water created by a structure that obstructs flow, such as a dam.

Independent Power Producer: Any entity that owns or operates an electricity generating facility that is not included in the utility’s rate base.

Independent System Operator: Organization that coordinates, controls, and monitors operation of the electrical power system within a specified geographic region.

Intake: Structure that diverts water from a natural waterway into the turbine.

Interconnection: Major points in the United States electrical grid where large regional grids connect with each other.

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.

Load-following reserves: Additional capacity available to accommodate load variability and uncertainty. 

Marine and hydrokinetic technologies: Devices that capture energy from waves, tides, ocean currents, the natural flow of water in rivers, and marine thermal gradients--these devices are also broadly referred to as marine energy or marine renewable energy technologies. Typically, these technologies do not leverage hydraulic heads as part of their power capture approach.

Modernization: Refers to upgrading or adding new hydropower system capabilities.

Nameplate capacity (installed): The maximum rated output of a generator or other electric power production equipment under specific conditions designated by the manufacturer.

New stream-reach: Denotes waterways that have not been developed for hydropower--also referred to as greenfield sites.

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

Non-spinning operating reserves: Additional capacity that is not connected to the system but can be made available to meet demand within a specified time. Also known as supplemental reserves.

Open-loop pumped storage hydropower:  Consists of two reservoirs that are continuously connected to naturally flowing sources of water.

Peaking: Operating mode in which power is produced only during periods of peak demand.

Peaking power plant: Power plants operated to help balance the fluctuating power requirements of the electricity grid.

Penetration: Fraction of energy produced by select generating sources (such as wind and solar) compared with total generation.

Penstock: A closed conduit or pipe for conducting water from the forebay to turbines in the powerhouse.

Power: The rate of production or consumption of energy; electric power is the rate at which electrical energy is transferred by an electric circuit.

Powerhouse: The structure that houses generators and turbines at a hydropower facility.

Practical resource: Portion of the technical resource that is available when other constraints—including economic, environmental, and regulatory—are factored in.

Pumped storage hydropower (PSH): Type of hydropower project where energy can be stored and generated by moving water between two reservoirs of differing elevations.

Qualified Hydroelectric Facility: A facility owned or solely operated by a non-Federal entity that generates hydroelectric energy for sale and which is added to an existing dam or conduit.

Ramp rate: Rate at which flows from the powerhouse into the tailwater and downstream into the natural waterway are increased or decreased.

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

Reactive supply: Portion of electricity supposed to sustain the electric and magnetic fields of alternating current (AC) equipment, such as transformers.

Regional Transmission Operator: Organizations responsible for moving and monitoring electricity over specific interstate areas. Similar to Independent System Operators, which coordinate, control, and monitor operation of the electrical power system within a specified geographic region.

Regulating reserves: Capacity available for providing fast, real-time balancing services.

Rehabilitation: Process of expanding, upgrading, and improving efficiency of existing hydropower facilities.

Relicensing period: Period during which a hydropower licensee must file a notice of intent to declare whether the licensee intends to seek a new license for its project (at least 5 years before a license expires) and during which the licensee must actually file the application for a new license (at least 2 years before a license expires).

Reregulating reservoir: Reservoir located downstream from a hydropower peaking plant with the capacity to store fluctuating discharges and release them according to environmental flow needs.

Reservoir: Body of water that builds up behind a dam. See also impoundment.

Resource potential: Amount of power that could be generated from a particular resource; see also theoretical, technical, and practical potential.

Rotor: Rotating inner portion of a generator consisting of a series of windings that surround the field poles.

Rough zone: Part of the range between minimum and maximum output that should be avoided due to deteriorating impacts on plant equipment, e.g., due to vibration.

Runoff: Precipitation, snowmelt, glacial melt, or irrigation water that appears in uncontrolled surface streams, rivers, drains, or sewers.

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.

Salmonid: Any of various fishes of the family Salmonidae, which includes the salmon, trout, grayling, and whitefish.

Aerating turbines: Turbines that use low pressures created by flows exiting the turbine to induce additional airflows.

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

Spinning reserves: Additional, rapidly available capacity available in generating units that are operating at less than their capability.

Storage: The storing of water in a reservoir during periods of high inflow that can be used later to generate electricity.

Sustainable hydropower; sustainability: For hydropower, a project or interrelated projects that are sited, designed, constructed, and operated to balance social, environmental, and economic objectives at multiple geographic scales (e.g., national, regional, basin, site) and to internalize all social, environmental, and economic benefits and costs in a manner that provides a long-term net benefit to the public owners of the resource.

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

Tailwater: The water immediately downstream of the powerhouse or dam.

Technical resource: Portion of a theoretical resource that can be captured by using a specific technology.

Theoretical resource: Annual average amount of physical energy that is hypothetically available.

Transformer: A device for changing alternating current (AC) to higher or lower voltages.

Transmission: Conveyance of electrical energy from generation facilities to local distribution systems.

Turbidity: Measure of the relative clarity of a fluid, commonly used as a measure of water quality.

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

Variable renewable generation resource: A renewable energy source that fluctuates due to natural circumstances not controlled by the operator, such as wind and solar.

Watershed: Land that water flows across or under on its way to a stream, river, lake, or ocean.

Weir: A barrier built across a stream or river to alter its flow characteristics.

Wholesale power market: Type of market where any entity that can generate power and connect to the grid can compete to sell their power output; the disposition of such and who is involved varies regionally.

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

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