Geothermal Glossary

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Air source heat pump
  • A device that extracts heat from air, either to heat a building in winter or cool a building in summer.
  • Natural condition of the environment at any given time.
  • Water-bearing stratum of permeable sand, rock, or gravel.

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.

Cap rock
  • Impermeable rocks such as shale, that overlies the reservoir rock and traps natural gas and crude oil in the reservoir. Also, impermeable rock overlying a geothermal reservoir. Also called sealing rock.
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.

Direct use
  • Use of geothermal heat without first converting it to electricity, such as for space heating and cooling, food preparation or industrial processes.
District heating
  • A type of direct energy use in which a utility system supplies multiple users with hot water or steam from a central plant or well field.
Dry steam
  • Very hot steam that does not occur with liquid.

  • 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.

  • A fracture or fracture zone in the Earth's crust along which displacement of one side relative to the other 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.

  • Of or relating to the Earth's interior heat.
Geothermal energy
  • Energy available in the ground and rocks beneath the Earth’s crust. Geothermal energy resources include hot temperature sources such as hot rock, molten rock and geysers and lower temperature sources in the ground one or two metres below the surface.
Geothermal gradient
  • The rate of temperature increase in the Earth as a function of depth. Temperature increases an average of 25° C for every kilometre of descent (1° Fahrenheit for every 75 feet).
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.
Ground source heat pumps
  • Ground source heat pumps are similar to air source heat pumps except instead of absorbing heat from air, they absorb it from the earth. (Also known as or earth energy systems)

  • Hot dry rock. Subsurface geologic formations of abnormally high heat content that contain little or no water.
Heat exchanger
  • A device such as a coiled copper tube immersed in a tank of water that is used to transfer heat from one fluid to another through a separating wall.
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 heat, hot water and/or steam. Originally used to describe high-temperature water and steam reseources such as geysers and deep hot water reservoirs, the term now includes lower temperature earth energy resources.

Injection (geothermal)
  • The process of returning spent geothermal fluids to the subsurface. Sometimes referred to as reinjection.

  • Molten rock within the Earth, from which igneous rock is formed by cooling.
  • The layer of the Earth lying beneath the Earth's crust and above the Earth's molten outer core of iron and nickel. The mantle can be divided into two parts, an upper solid but ductile upper mantle and a more rigid lower mantle.

  • The capacity of a substance (such as rock) to transmit a fluid, such as crude oil, natural gas, or water. 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 the many rigid plates that make up 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 capacity od a reservoir to store fluids. The ratio of the aggregate volume of pore spaces in rock or soil to its total volume, usually stated as a per cent.

Reservoir (Geothermal)
  • A porous and permeable underground rock formation containing a natural accumulation of high-temperature water that is confined by impermeable rock or water barriers, and is separate from other reservoirs.

  • A measure of the quantity or concentration of dissolved salts in water.
  • Sinking of an area of the Earth's crust due to fluid withdrawal and pressure decline.

  • 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 rotary engine that converts the energy of a moving stream of water, steam or gas into mechanical energy. Turbines are classified as hydraulic (water) turbines, steam turbines or gas turbines. Turbine-power generators produce most of the world's electricity. Windmills that generate electicity are known as wind turbines.

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

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