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The word geothermal comes from the Greek words
geo (earth) and therme (heat), and means the heat
of the earth.
Geothermal energy is ultimately derived from
the heat contained in the core of the earth and
from radioactive decay within its mantle. At high
temperatures and pressures within the mantle,
melting of mantle rock forms magma which rises
towards the surface carrying the heat from below.
 In
some regions where the earth's crust is thin or
fractured, or where magma bodies are close to
the surface, there are high temperature gradients.
Deep faults, rock fractures and pores allow groundwater
to percolate towards the heat source and become
heated to high temperatures. Some of this hot
geothermal water travels back to the surface through
buoyancy effects to appear as hot springs, mud
pools, geysers, or fumaroles. If the ascending
hot water meets an extensively fractured or permeable
rock zone, the heated water will fill pores and
fractures and form a geothermal reservoir. These
reservoirs are much hotter than surface hot springs,
reaching temperatures of more than 350°C,
and are potentially an accessible source of energy.
These high temperatures can be achieved in liquid-dominated
reservoirs because increasing hydrostatic pressure
with depth allows elevated temperatures without
boiling. Many undisturbed geothermal reservoirs
in New Zealand have temperature and pressure profiles
such that the fluid is close to boiling point
to depths of more than 1 km.
Geothermal areas are commonly close to the edges
of continental plates, and New Zealand's location
on an active plate boundary (between the Indo-Australian
and Pacific Plates) has resulted in the development
of numerous geothermal systems and a world-class
geothermal energy resource.
The characteristics of geothermal systems vary
widely, but three components are essential:
- a subsurface heat source that may be igneous
magma bodies or heat stored in other rocks
- fluid to transport the heat
- faults, fractures or permeability within sub-surface
rocks that allow the heated fluid to flow from
the heat source to the surface or near-surface.
Geothermal resources can be classified into three
categories:
- High temperature, usually magmatic-related
resources. These have temperatures of 200- 350°C
at economically-drillable depth. They are of
limited occurrence, and form individual convective
geothermal systems of up to 50 sq km in area.
Technologies may be developed in the future
to exploit even deeper, hotter resources.
- Moderate to low temperature resources, of
non-magmatic origin, usually associated with
deep faults. Maximum temperatures at drillable
depth do not exceed 140°C, and are often
less. These are more widespread than the high
temperature resources, but the individual systems
are no larger. The distinction between these
and the first kind is not entirely clear-cut,
as cooled outflows from hotter resources can
also fall into the same temperature range.
- Very low temperature resources, which are
widespread but close to ambient temperature.
Links
IEA
Geothermal Energy
Geothermal
Education Office
International
Geothermal Association
Geothermal
Resource Information Clearinghouse
US
Department of Energy - Geothermal
Geothermal
Resources Council
Renewable
Energy Policy Project
EECA
fact sheet
Generalised Geothermal System
of the Taupo Volcanic Zone
(After Henley & Others,
1986)
See Other Geothermal Energy
Content
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