This list contains terms related to geothermal energy and technologies./

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Natural condition of the environment at any given time. /


Water-bearing stratum of permeable sand, rock, or gravel./

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Baseload Plants

Electricity-generating units that are operated to meet the constant or minimum load on the system. The cost of energy from such units is usually the lowest available to the system./

Binary-Cycle Plant

A geothermal electricity generating plant employing a closed-loop heat exchange system in which the heat of the geothermal fluid (the "primary fluid") is transferred to a lower-boiling-point fluid (the "secondary" or "working" fluid), which is thereby vaporized and used to drive a turbine/generator set./


A geothermal solution containing appreciable amounts of sodium chloride or other salts./


British thermal unit. The quantity of heat required to raise the temperature of one pound of water one degree Fahrenheit at standard conditions. (Equal to 252 calories.)/

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Cap Rocks

Rocks of low permeability that overlie a geothermal reservoir./

Cascading Heat

A process that uses a stream of geothermal hot water or steam to perform successive tasks requiring lower and lower temperatures./


Water formed by condensation of steam./


Equipment that condenses turbine exhaust steam into condensate./

Cooling Tower

A structure in which heat is removed from hot condensate./


Earth's outer layer of rock. Also called the lithosphere./

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Direct Use

Use of geothermal heat without first converting it to electricity, such as for space heating and cooling, food preparation, industrial processes, etc./

District Heating

A type of direct use in which a utility system supplies multiple users with hot water or steam from a central plant or well field./


Boring into the Earth to access geothermal resources, usually with oil and gas drilling equipment that has been modified to meet geothermal requirements./

Dry Steam

Very hot steam that doesn't occur with liquid./

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The ratio of the useful energy output of a machine or other energy-converting plant to the energy input./

Enhanced Geothermal Systems

Rock fracturing, water injection, and water circulation technologies to sweep heat from the unproductive areas of existing geothermal fields or new fields lacking sufficient production capacity./

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A fracture or fracture zone in the Earth's crust along which slippage of adjacent Earth material has occurred at some time./

Flash Steam

Steam produced when the pressure on a geothermal liquid is reduced. Also called flashing./


A vent or hole in the Earth's surface, usually in a volcanic region, from which steam, gaseous vapors, or hot gases issue./

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Study of the planet Earth, its composition, structure, natural processes, and history./


Of or relating to the Earth's interior heat./

Geothermal Energy

The Earth's interior heat made available to man by extracting it from hot water or rocks./

Geothermal Gradient

The rate of temperature increase in the Earth as a function of depth. Temperature increases an average of 1° Fahrenheit for every 75 feet in descent./

Geothermal Heat Pumps

Devices that take advantage of the relatively constant temperature of the Earth's interior, using it as a source and sink of heat for both heating and cooling. When cooling, heat is extracted from the space and dissipated into the Earth; when heating, heat is extracted from the Earth and pumped into the space./


A spring that shoots jets of hot water and steam into the air. /

Geysers, The

A large geothermal steam field located north of San Francisco./

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Hot dry rock. Subsurface geologic formations of abnormally high heat content that contain little or no water./

Heat Exchanger

A device for transferring thermal energy from one fluid to another./

Heat Flow

Movement of heat from within the Earth to the surface, where it is dissipated into the atmosphere, surface water, and space by radiation./

Hydrothermal Resource

Underground systems of hot water and/or steam./

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The process of returning spent geothermal fluids to the subsurface. Sometimes referred to as reinjection./

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Known Geothermal Resource Area. A region identified by the U.S. Geological Survey as containing geothermal resources./


1,000 watts—a unit of electric power. Abbreviated kW./


The energy represented by 1 kilowatt of power consumed for a period of 1 hour, equal to 3,413 Btus. Abbreviated kWh./

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The simultaneous demand of all customers required at any specified point in an electric power system./

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Molten rock within the Earth, from which igneous rock is formed by cooling./


The Earth's inner layer of molten rock, lying beneath the Earth's crust and above the Earth's core of liquid iron and nickel./

Multiplier Effect

The multiplier effect is sometimes called the ripple effect because a single expenditure in an economy can have repercussions throughout the entire economy. The multiplier is a measure of how much additional economic activity is generated from an initial expenditure./

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Peaking Plants

Electricity generating plants that are operated to meet the peak or maximum load on the system. The cost of energy from such plants is usually higher than from baseload plants./


The capacity of a substance (such as rock) to transmit a fluid. The degree of permeability depends on the number, size, and shape of the pores and/or fractures in the rock and their interconnections. It is measured by the time it takes a fluid of standard viscosity to move a given distance. The unit of permeability is the Darcy./

Plate Tectonics

A theory of global-scale dynamics involving the movement of many rigid plates of the Earth's crust. Tectonic activity is evident along the margins of the plates where buckling, grinding, faulting, and vulcanism occur as the plates are propelled by the forces of deep-seated mantle convection currents. Geothermal resources are often associated with tectonic activity, since it allows groundwater to come in contact with deep subsurface heat sources./


The ratio of the aggregate volume of pore spaces in rock or soil to its total volume, usually stated as a percent./

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A natural underground container of liquids, such as water or steam (or, in the petroleum context, oil or gas)./

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A measure of the quantity or concentration of dissolved salts in water./


A sinking of an area of the Earth's crust due to fluid withdrawal and pressure decline./

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Total dissolved solids. Used to describe the amount of solid materials in water./

Thermal Gradient

The rate of increase or decrease in the Earth's temperature relative to depth./

Transmission Line

Structures and conductors that carry bulk supplies of electrical energy from power-generating units./


A bladed, rotating engine activated by the reaction or impulse, or both, of a directed current of fluid. In electric power applications, such as geothermal plants, the turbine is attached to and spins a generator to produce electricity./

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A geothermal reservoir system in which subsurface pressures are controlled by vapor rather than by liquid. Sometimes referred to as a dry-steam reservoir./

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Well Logging

Assessing the geologic, engineering, and physical properties and characteristics of geothermal reservoirs with instruments placed in the wellbore./

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