A aquifer: a large permeable body of underground rock capable of yielding quantities of water to springs or wells. Underground aquifers of hot water and steam form geothermal reservoirs.
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B baseload plants: electricity-generating units that are operated to meet the minimum load on the supply 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 vaporised and used to drive a turbine/generator set.
boiling point: temperature at which a single substance, such as water, changes from a liquid to a gas (steam) at a given pressure. Some liquids boil at a lower temperature than water, a principle utilised in binary power plants. Boiling point is also affected by pressure. The greater the pressure, the higher the boiling point. This principle is put to work in geothermal (flash) power plants when geothermal water is brought up wells. Some of the hot water flashes to steam when the pressure is released as it rises to the surface or passes through surface equipment. This phenomenon also occurs naturally, resulting in such features as geysers.
brine: a geothermal solution containing appreciable amounts of sodium chloride or other salts.
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C caldera: a bowl-shaped landform, created either by a huge volcanic explosion (which destroys the top of a volcano) or by the collapse of a volcano’s top.
cap rocks: rocks of low permeability that overlie a geothermal reservoir.
carbon dioxide (CO2): a gas produced by the combustion of fossil fuels and other substances. CO2 also occurs naturally in large amounts in molten magma, which is involved in the explosive eruption of volcanoes. See Greenhouse Effect.
cascading heat: a process that uses a stream of geothermal hot water or steam to perform successive tasks requiring lower and lower temperatures.
chloride spring: geothermal springs can be divided into two types based on the temperature and composition of the water they produce. Most springs produce warm, weakly mineralised water. Chloride springs, produce hot or boiling, heavily mineralised alkaline water that is high in chloride and silica. All geysers and springs that produce sinter terraces are chloride springs, for example those at Orakei Korako and Waiotapu. Chloride springs are vulnerable to damage from the extraction of the geothermal fluid for other uses which divert the chloride water away from the springs.
condensate: liquid water formed by condensation of steam.
condense: to change from a gas to liquid. In conventional condensing geothermal power plants, steam is vented from turbines into a condenser where cooled water is sprayed on the steam to condense it. The condensate can be recycled using a cooling tower to extract more heat. An equivalent system exists for binary power plants, but with the organic liquid being recycled in a closed loop.
condenser: equipment that condenses turbine exhaust steam into condensate.
cooling tower: a structure in which heat is removed from hot condensate through heat exchange with air.
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D direct use: use of geothermal heat without first converting it to electricity, such as for space heating and cooling, food preparation, industrial processes, or bathing.
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: superheated steam without a water phase.
dry-steam reservoir: a geothermal reservoir system in which subsurface pressures are controlled by steam rather than by water.
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E efficiency: the ratio of the useful energy output of a machine or other energy-converting plant to the energy input. Technology with a higher energy efficiency will require less energy to do the same amount of work.
emission: the release or discharge of a substance into the environment; generally refers to the release of gases or particulates into the air.
enhanced geothermal systems: rock fracturing, water injection, and water circulation technologies used to sweep heat from the unproductive areas of existing geothermal fields or new fields lacking sufficient production capacity.
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F fault: a fracture or fracture zone in the Earth’s crust along which slippage of adjacent rocks has occurred.
flash plant: pressure vessels designed to effectively separate flash steam from the liquid phase.
flash steam: steam produced when the pressure on a geothermal liquid is reduced. Also called flashing.
fracture: a crack in the Earth’s crust along which no movement has occurred.
fumarole: a hole or vent from which superheated gas and steam discharges under pressure.
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G geothermal: of or relating to the Earth’s interior heat.
geothermal energy: the Earth’s interior heat made available by extraction of geothermal fluids.
geothermal gradient: the rate of temperature increase in the Earth as a function of depth.
geothermal heat pumps: devices that take advantage of the relatively constant temperature of the Earth’s subsurface, 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.
geothermal power plant: a facility which uses geothermal steam or heat to drive turbine-generators to produce electricity. Three different types make use of the various temperature ranges of geothermal resources: dry steam, flash and binary.
geothermal reservoir: a large volume of underground hot water and steam in porous and fractured hot rock. The hot water in geothermal reservoirs occupies only 2 to 5% of the volume of rock, but if the reservoir is large enough and hot enough, it can be a powerful source of energy. Geothermal reservoirs are sometimes overlain by a layer of impermeable rock. While geothermal reservoirs usually have surface manifestations such as hot springs or fumaroles, some do not.
geyser: a natural hot spring that sends up a fountain of water and steam into the air; some geysers “spout” at regular intervals and some are unpredictable.
global warming/greenhouse effect: the trapping of heat in the atmosphere. Incoming solar radiation goes through the atmosphere to the Earth’s surface, but outgoing radiation (heat) is absorbed by water vapour, carbon dioxide, and ozone in the atmosphere. At certain levels this is beneficial because it keeps the planet warm enough for life as we know it. However, an increase in the normal amount of carbon dioxide and other gases may contribute to a human-caused warming trend that could have serious effects on global climate, the global ecosystem, and food supplies.
greenhouse effect: the presence of trace atmospheric gases make the earth warmer than would direct sunlight alone. These gases (carbon dioxide [CO2], methane [CH4], nitrous oxide [N2O], tropospheric ozone [O3], and water vapour [H2O]) allow visible light and ultraviolet light (shortwave radiation) to pass through the atmosphere and heat the earth’s surface. This heat is re-radiated from the earth in the form of infrared energy (longwave radiation). The greenhouse gases absorb part of that energy before it escapes into space. This process of trapping the longwave radiation is known as the greenhouse effect.
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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.
hot springs: a natural spring that puts out water warmer than body temperature and therefore feels hot; may collect in pools or flow into streams and lakes. A geothermal phenomenon.
hydrothermal: hydro means water and thermal means heat. Literally hydrothermal means hot water. Steam and hot water reservoirs are hydrothermal reservoirs. Hot dry rock resources and magma resources are not considered to be hydrothermal resources.
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I impermeable: does not allow liquids to pass through easily — certain rock types and clay soil are impermeable.
injection: the process of returning spent geothermal fluids to the subsurface; also referred to as reinjection.
injection well: a well through which geothermal water is returned to an underground reservoir after use. Geothermal production and injection wells are constructed of pipes layered inside one another and cemented into the earth and to each other. This protects any shallow drinking water aquifers from mixing with deeper geothermal water.
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K kilowatt (kW): one thousand watts of electricity.
kilowatt hour (kWh): one thousand watthours.
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L lava: molten magma that has reached the Earth’s surface.
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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 crust and above the core of liquid iron and nickel.
megawatt (MW): a unit of power, equal to a thousand kilowatts (kW) or one million watts(W). The watt is a unit of power (energy/time), the rate energy is consumed or converted to electricity. Assessment of the energy in geothermal systems is commonly in terms of equivalent electrical power or MWe, which takes into account the efficiency of conversion.
mineralised fluids: water and steam containing minerals such as silica, lithium and boron. Also called geothermal water or geothermal fluids.
mud pool: thermal surface feature which occurs where there is not enough water to support a geyser or hot spring even though there may be some hot water below. Steam and gas vapours bubble up through mud formed by the interaction of gases with rock.
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O operations and maintenance (O&M) cost: operating expenses are associated with operating a facility (e.g. engineering costs). Maintenance expenses are that portion of expenses consisting of labour, materials, and other direct and indirect expenses incurred for preserving the operating efficiency or physical condition of utility plants that are used for power production, transmission, and distribution of energy.
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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.
petajoule (PJ): a joule is a unit of energy. A petajoule is 1015 joules.
porosity: the ratio of the aggregate volume of pore spaces in rock or soil to its total volume, usually stated as a percentage.
porous: full of small holes (pores); able to be filled (permeated) by water, air, or other materials.
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S salinity: a measure of the quantity or concentration of dissolved salts in water.
separated water: water discharged from a separator.
separator: a pressure vessel used to separate water and steam, normally by centrifugal action.
sinter: a mineral crust or deposit formed from the minerals (mainly silica) in geothermal water, especially from geysers.
steam: the vapour form of water that develops when water boils. Steam is made of very tiny heated water particles (molecules) which are bouncing around and bumping into each other at very high speeds. These heated water molecules are also spreading out and expanding in every direction they can. If we confine or trap water in a container, with a pipe as an opening, and heat the water to steam, it will create great pressure in the container and will rush out the pipe with a great deal of force. This force can be put to work turning a turbine connected to an electricity generator.
subsidence: a sinking of an area of the Earth’s crust due to fluid withdrawal and pressure decline.
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T turbine: a machine for generating rotary mechanical power from the energy of a stream of fluid (such as water, steam, or hot gas). Turbines convert the kinetic energy of fluids to mechanical energy through the principles of impulse and reaction, or a mixture of the two.
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W water phases: the change of water from one state to another. The change from ice to liquid is melting; the reverse process is freezing. The change from liquid to gas is evaporation and the product is water vapour or steam; the change from water vapour to liquid is called condensation. Evaporation and condensation are both important functions in geothermal phenomena and in geothermal technology.
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