Hydroelectricity -Glossary of Terms

Access roads

Gravel or dirt roads (rarely paved) that provide overland access to transmission line and pipeline rights-of-way (ROWs) and facilities for construction, inspection, maintenance, and decommissioning. Access roads have an average distance of 5 miles or less, have a nominal width of 15 feet, and exist within the center of a nominal 25-foot-wide ROW.

Alternating current (AC)

Electric current that reverses direction many times per second.

Ampere

A measure of electric current (similar to describing water volume in cubic feet per second).

Ancillary services

Operations provided by hydroelectric plants that ensure stable electricity delivery and optimize transmission system efficiency.

Axial flow turbine

A turbine that typically has two or three blades mounted on a horizontal shaft to form a rotor; the kinetic motion of the water current creates lift on the blades causing the rotor to turn driving a mechanical generator. These turbines must be oriented in the direction of flow. There are shrouded and open rotor models.

Ballast water

Water carried in special (ballast) tanks of ships and used to provide stability needed when carrying less than a full load of cargo and to keep the ship at the proper depth in the water. When the ship is loaded with cargo, the ballast water is released to surrounding waters; when the ship is empty, it takes on more water to keep it upright.

Bubble curtain

A device used during pile-driving without cofferdams that will surround the large-diameter piles and generate bubbles to attenuate peak underwater sound pressure levels, which may adversely affect fish and marine mammals.

Cavitation

Noise or vibration causing damage to the turbine blades as a result of bubbles that form in the water as it goes through the turbine. Cavitation causes capacity loss, head loss, efficiency loss.

Conductor

Material, such as wire or cable, used to carry electricity. This term can also be used in reference to a pipeline that carries water.

Consumption

End use of energy and energy sources, such as electricity, and is typically measured in kilowatt-hours.

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Current

The flow of electrons in an electrical conductor – measured in amperes.

Dam

A barrier constructed to store or divert water for different purposes, including electricity production. Typically made of earth, rock, or concrete.

Direct current (DC)

Electric current that flows in one direction.

Distribution

A process of moving power at lower voltages from substations to customers.

Draft tube

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

Efficiency

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

Electric energy

Power delivered over a period of time; commonly measured in kilowatt-hours (kWh) or megawatt-hours (MWh).

Electric power

Rate of electric energy delivery; also a measure of a power plant’s generating capacity or installed capacity; the basic measures are the kilowatt (kW) and megawatt (MW).

Environmental assessments

Planning and decision-making tool used by industry and regulators to identify the environmental impacts and costs of proposed energy projects, and potential solutions.

Head pond

A pond that is used to store water for a hydroelectric project.

Hydraulic fracturing (Hydrofracturing)

Fracturing of rock at depth with fluid pressure. Hydraulic fracturing at depth may be accomplished by pumping water into a well at very high pressures. Under natural conditions, vapor pressure may rise high enough to cause fracturing in a process known as hydrothermal brecciation. See also Fracturing.

hydropower

Water moves constantly through a vast global cycle, evaporating from lakes and oceans, forming clouds, precipitating as rain or snow, then flowing back down to the ocean.

hydropower facility, impoundment

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 to meet changing electricity needs or to maintain a constant reservoir level.

hydropower facility, diversion

A diversion, sometimes called a “run-of-river” facility, channels a portion of a river through a canal or penstock.

hydropower facility, pumped storage

During periods of high electrical demand, the water is released back to the lower reservoir to generate electricity.

Fish ladder

A series of pools arranged like steps that allow fish to pass upstream over a dam.

Flow

Volume of water, expressed as cubic feet (or cubic meters) per second.

Flow management

Management of hydroelectricity operations to control downstream water flows and their effects.

Generating capacity

A power plant’s ability to produce a specific amount of electricity at a specific moment in time; measured in kilowatts or megawatts – also known as “installed capacity” or “nameplate capacity”.

Generation

The process of converting different forms of energy — thermal, mechanical, chemical, or nuclear, into electricity.

Gigawatt (GW)

A measure of electric power; the equivalent of 1,000 megawatts or 1 million kilowatts.

Gigawatt-hours (GWh)

A measure of electric energy; the equivalent of 1,000 megawatt-hours or 1 million kilowatt-hours.

Grid

A regional or nation-wide network of high-voltage transmission lines.

Head

The vertical change in elevation expressed in either feet or meters, between the reservoir (“head water”) level and the downstream river (“tailwater”) level.

Headwater

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

Hydropower (“Hydro”)

The process of generating electricity by capturing the potential energy of falling water through the use of a water wheel (turbine) to mechanically spin rotating magnets which create electrical current that can be distributed to users by transmission lines.

     Large Hydropower: Although definitions vary, the U.S. Department of Energy (DOE) defines large hydropower as facilities that have a capacity of more than 30 megawatts.

     Small Hydropower: The DOE defines small hydropower as facilities that have a capacity of 100 kilowatts to 30 megawatts.

     Micro Hydropower: A micro hydropower plant has a capacity of up to 100 kilowatts. A small or micro-hydroelectric power system can produce enough electricity for a home, farm, ranch, or village.

Hydrologic Cycle

The natural process by which water evaporates, primarily from the oceans as energy from the sun and wind is absorbed. Water vapor in the form of clouds is transported by winds to higher atmospheric levels and over land where water condenses and falls as precipitation (rain & snow). The cycle continues when water then runs off the land into rivers, and back to the oceans where the cycle repeats.

Hydrologic conduits

Channels for carrying water or other fluids, such as pipes and aqueducts

Installed capacity

The amount of power that can be generated at a given moment by a power plant. Usually measured in kilowatts (kW) or megawatts (MW) – NOTE: actual generation is measured in kilowatt-hours or megawatt-hours.

hydropower facility, diversion

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

hydropower facility, 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.

Intake

The entrance to a turbine unit at a hydroelectric dam.

Kilowatt

A measure of electric power; the equivalent of 1000 watts.

Kilowatt-hour (kWh)

A measure of electric energy; the equivalent of 1,000 watt-hours (e.g. if you burn ten 100-watt light bulbs for one hour, they will use one kilowatt-hour of electricity) – NOTE: residential customers are usually charged for electricity based on a rate of cents per kilowatt-hour.

Load

The total amount of electricity on a specific power system required to meet customer demand at any moment.

     Base Load Plant – Plant operates at maximum output at all times to provide maximum energy into the grid used to meet some or all of a given region’s continuous energy demand.

     Load Following Plant (“Intermediate Peaking”) – A plant that operates at variable power output setting to meeting the changing energy demand of customers, usually slow moving changes such as from night periods to day periods.

     Dispatchable Power (“Fast Peaking”) – Dispatchable generation refers to sources of electricity that can be dispatched at the request of power grid operators; that is, generating plants that can be quickly turned on or off, or can adjust their power output on demand.

Megawatt (MW)

A measure of bulk power; the equivalent of 1,000 kilowatts or 1 million watts; the unit is generally used to describe the output of a generator.

Megawatt-hour (MWh)

A measure of electric energy; the equivalent of 1,000 kilowatt-hours or 1 million watthours – NOTE: Megawatt-hours are determined by a hydropower plant’s capacity and how long the plant is running (e.g. a 1,000-megawatt power plant running at full power for one hour produces 1,000 megawatt-hours (MWhs) of electricity; and if that plant runs all day, it produces 24,000 MWhs).

Penstock

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

Power

In electricity transmission, current delivered at a given voltage is measured in watts or kilowatts.

Powerhouse

The physical structure of an electric generating facility.

Runner

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

Sediment

Materials that sink to the bottom of a body of water, or materials that are deposited by wind, water, or glaciers.

Scroll case

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

Spill

The release of water from a dam or hydropower project without passing it through the powerhouse – NOTE: Typically a situation to be avoided as water “spilled” is lost powergeneration revenue.

Spillway

The structure or portion of a larger structure that is used to release excess water over or around a dam.

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Stratification

A variation in temperature and quality of deep water as surficial water is warmed by the sun fostering biologic growth while colder, oxygen-depleted water sinks to lower levels.

Substation

An electrical facility where the voltage of incoming and outgoing circuits is changed and controlled.

Supersaturation

Change in water quality that occurs when turbulent water passing over a spillway absorbs and entrains free air. This process increases nitrogen levels in the water – NOTE: fish exposed to supersaturated water can be injured or killed when the nitrogen gas produces bubbles in their bloodstream. This effect is called “gas bubble disease” and is similar to “the bends” that can occur in human divers.

Sustainable

Ecosystem condition in which biodiversity, renewability and resource productivity are maintained over time.

Sustainable development

Development that meets the needs of the present without compromising the ability of future generations to meet their own needs (as defined by United Nations World Commission on Environment and Development).

Tailrace

The downstream channel that carries water away from a dam or powerhouse.

Tailwater

The water level downstream of the powerhouse or dam.

Terawatt

A measure of electric power, the equivalent of 1,000 gigawatts or 1 billion kilowatts; the unit is generally used to describe generating capacity at national or international levels.

Terawatt-hour

A measure of electric energy; the equivalent of 1,000 gigawatt-hour or 1 billion kilowatthours. Transformer An electromagnetic device for changing alternating current (AC) electricity to higher or lower voltages.

Transmission

The process of moving electric power at high voltages from the generation facility to local communities. Turbidity Muddy or unclear water quality caused by suspended sediments.

Turbidity

Muddy or unclear water quality caused by suspended sediments.

Turbine

A rotary engine that converts the energy of a moving stream of water, steam or gas into mechanical energy.

     TURBINE TERMS:

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     Generator:  An arrangement of magnets spinning inside a coil of wire to produce electricity.

      Rotor: The moving part of an electric generator – NOTE: the rotor’s outer surface is covered with electromagnets and as the rotor turns inside the stator, the electrons in the copper windings “vibrate” such that their movement generates an electric current.

     Stator: The stator is the stationary part of an electric generator – NOTE: the stator is omprised of a series of vertically oriented copper coils nestled in the slots of an iron core so that as the rotor spins its magnetic field induces a current in the stator’s windings thereby generating electricity.

Wicket gates : Adjustable elements that control the flow of water from the scroll case into the turbine passage. Voltage A measure of the electric pressure that pushes electric current through a circuit (just as pressure causes water to flow in a pipe); measured in volts or kilovolts. Watershed Area draining into a stream or river. Watt A measure of electric power; standard light bulbs are rated at 25, 40, 60 or 100 watts.

Voltage

A measure of the electric pressure that pushes electric current through a circuit (just as pressure causes water to flow in a pipe); measured in volts or kilovolts.

Watershed

Area draining into a stream or river. Watt A measure of electric power; standard light bulbs are rated at 25, 40, 60 or 100 watts.