Resources:
Glossary

A

 

AMBIENT

Natural condition of the environment at any given time.

AQUIFER

Permeable sand, rock, or gravel containing or conducting water, especially usable water.

 

B

 

BASELOAD PLANTS

Power plants generating the minimum level of demand on an electrical supply system over 24 hours. Base load power sources are those plants which can generate dependable power to consistently meet demand. They are the foundation of a sound electrical 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.

 

BRINE

A geothermal water containing appreciable amounts of dissolved solids.

 

BTU

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.)

 

C

 

CAP ROCKS

Rocks of low permeability that overlie a geothermal reservoir, sealing in the hot water or steam.

 

CASCADING HEAT

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

 

CONDENSATE

Water formed by condensation of steam.

 

CONDENSER

Equipment that condenses turbine exhaust steam into condensate or another vapor to the liquid form in a binary power plant.

 

COOLING TOWER

A cooling tower is a heat rejection device which rejects waste heat to the atmosphere through the cooling of a water stream to a lower temperature.

 

CRUST 

Earth’s outer layer of rock. Also called the lithosphere.

D

 

DIRECT USE

Use of geothermal heat without first converting it to electricity, such as for space heating and cooling, food preparation, greenhouse heating, 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. Geothermal district heating supplies this heat from hot water or steam wells.

 

DRILLING

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.

 

E

 

EFFICIENCY

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

 

ENHANCED GEOTHERMAL SYSTEMS

An Enhanced Geothermal System (EGS) is a man-made reservoir, created where there is hot rock but insufficient or little natural permeability or fluid saturation. In an EGS, fluid is injected into the subsurface under carefully controlled conditions, which cause pre-existing fractures to re-open, creating permeability.

 

F

 

FAULT

A crack in the earth’s crust resulting from the displacement of one side with respect to the other

 

FLASH STEAM

Steam produced when the pressure on a geothermal liquid is reduced. Also called flashing. This can happen in the well, in the geothermal reservoir or in a pressure separation vessel on the surface.

 

FUMAROLE

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

 

G

 

GEOLOGY

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

 

GEOTHERMAL

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 that temperature increases with respect to depth in the Earth’s interior. Away from tectonic plate boundaries, it is about 25 °C per km of depth (1 °F per 70 feet of depth) near the surface in most of the world.

 

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.

 

GEYSER

A natural hot spring in which water intermittently boils, sending a tall column of water and steam into the air.

 

GEYSERS, THE

A large geothermal steam field located north of San Francisco.

 

H

 

HDR

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

Naturally circulating underground systems of hot water and/or steam.

 

I

 

INJECTION

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

 

K

 

KGRA

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

 

KILOWATT

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

 

KILOWATT-HOUR

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

 

L

 

LCOE

Levelized Cost Of Electricity. This is the cost of all aspects of developing and operating the power generation project. It is calculated by accounting for all of a system’s expected lifetime costs (including construction, financing, fuel, maintenance, taxes, insurance and incentives), which are then divided by the system’s lifetime expected power output (kWh). All cost and benefit estimates are adjusted for inflation and discounted to account for the time-value of money.

 

LOAD

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

 

M

 

MAGMA

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

 

MANTLE

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.

 

P

 

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.

 

PERMEABILITY

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.

 

POROSITY

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

 

R

 

RESERVOIR

A subterranean accumulation of fluid such as hot water or steam, or hydrocarbons held in porous or fractured and permeable rock

 

S

 

SALINITY

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

 

SUBSIDENCE

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

 

T

 

TDS

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

 

TURBINE

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.

 

V

 

VAPOR-DOMINATED

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

 

W

 

WELL LOGGING

Assessing the geologic, engineering, and physical properties and characteristics of geothermal reservoirs.