| 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. |
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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. |
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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.
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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. |
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cap rocks:
rocks of low permeability that overlie a geothermal
reservoir. |
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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. |
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cascading heat:
a process that uses a stream of geothermal
hot water or steam to perform successive tasks
requiring lower and lower temperatures. |
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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. |
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condensate:
liquid water formed by condensation of steam. |
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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. |
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condenser:
equipment that condenses turbine exhaust steam
into condensate. |
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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. |
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drilling:
boring into the Earth to access geothermal
resources, usually with oil and gas drilling
equipment that has been modified to meet geothermal
requirements. |
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dry steam:
superheated steam without a water phase. |
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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. |
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emission:
the release or discharge of a substance into
the environment; generally refers to the release
of gases or particulates into the air. |
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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. |
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flash plant:
pressure vessels designed to effectively separate
flash steam from the liquid phase. |
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flash steam:
steam produced when the pressure on a geothermal
liquid is reduced. Also called flashing. |
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fracture:
a crack in the Earth's crust along which no
movement has occurred. |
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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. |
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geothermal energy:
the Earth's interior heat made available by
extraction of geothermal fluids. |
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geothermal gradient:
the rate of temperature increase in the Earth
as a function of depth. |
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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. |
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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. |
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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. |
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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. |
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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.
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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. |
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heat exchanger:
a device for transferring thermal energy from
one fluid to another. |
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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. |
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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. |
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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. |
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injection:
the process of returning spent geothermal
fluids to the subsurface; also referred to
as reinjection. |
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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. |
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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. |
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mantle:
the Earth's inner layer of molten rock, lying
beneath the crust and above the core of liquid
iron and nickel. |
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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. |
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mineralised
fluids: water and steam containing minerals
such as silica, lithium and boron. Also called
geothermal water or geothermal fluids. |
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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. |
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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. |
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petajoule (PJ):
a joule is a unit of energy. A petajoule is
1015 joules. |
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porosity:
the ratio of the aggregate volume of pore
spaces in rock or soil to its total volume,
usually stated as a percentage. |
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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. |
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separated water:
water discharged from a separator. |
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separator:
a pressure vessel used to separate water and
steam, normally by centrifugal action. |
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sinter:
a mineral crust or deposit formed from the
minerals (mainly silica) in geothermal water,
especially from geysers. |
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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. |
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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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