Transcript 4 comenius – geothermic energy

Co-finanziato
Dal Programma LLP dell’Unione Europea
Geothermal energy is the energy generated by
geological heat sources, exploiting the natural
heat of the earth.
L’autore è il solo responsabile di questa comunicazione. L’Unione europea declina ogni responsabilità sull’uso che potrà essere fatto
delle informazioni in essa contenute.
Geothermal energy is not widely used
nowadays. In fact this kind of energy accounts
for less than 1% of world total production, yet
there are countries in which this kind of
energy is very important; for example in Iceland
85% of houses are heated thanks to this
source of energy.
The greatest advantages of this kind of
energy is its low cost once the plant has
been built, because it exploits natural
forces. Moreover geothermal plants don’t
damage the environment because they
don’t have waste which is harmful for men,
animals or the earth.
The exploitation of heat is
possible once the underground
heat concentration areas have
been found. These are caused by
volcanic or tectonic
phenomenons and are called
hot springs
Hot springs have been used since the
Paleolithic in China and shortly later by the
Romans as well. Hot springs are used to heat
up houses or greenhouses; low temperature
f luids are used to produce electric energy.
Industries mainly use hydrothermal systems
but there are other kinds of systems, such as
the low enthalpy system which exploits the
heat thanks to a heat pump.
Because of the dangers of active volcanos, the
underground heat is used thanks to the
secondary volcanism phenomenons.These
phenomenons are born as a consequence of the
coming out of magma from the crater.
After the activity of the volcno has stopped,
the magma chamber get cold.
Later it originates phenomenons such as:
Geyser: periodical eruptions of steam charcterized
by a siphon structure, near volcanos.
One of the most famous geysers is Grand Geyser in
Iceland.
Thermal Baths:thermal springs are created
thanks to the degassing of magma in the
magma chamber.
Fumaroles: emanations of gas or heat near
volcanos. They are clefts in the earth in which
gas or steam discharges occur.
Boraciferous soffioni: structures that are born during
the initial phase of the eruption; they allow the
coming out of ga es or steam in the magma chamber.
The boraciferous soffioni in Larderello, Tuscany, are
well-known.
Solfatara: the coming out of steam, carbon
dioxide(CO2), hydrogen, from whose oxidation we
get sulphur. These phenomenons in some cases
originate sulphur exploitable sulphur deposits.
Bradyseism. The lowering or raising of the soil level,
caused by the change in volume of the magma
chamber near the surface, which gets empty and
then fills up
CENTRALE GEOTERMICA
Geothermal plants exploit the heat of the
earth depths because the inner temperature of
our planet grows as we go down towards the
centre of the earth.
This increase in temperature, called
geothermal gradient, is average 3 degrees every
100 metres depth but in some places, where
there are geothermal systems, it is much
higher, up to 250-3050 degrees at about 200040000 mt depth.
Through the clefts in the rock layers, the
heated vapours go up to the surface and are
intercepted and produced by the
geothermal wells.
The steam erogated by the wells is conveyed
into pipes called steam pipes and sent to the
turbine, where enery is transformed into
mechanical rotating energy.
The axis of the turbine is connected to the
engine of the alternator, that, rotating,
trasforms mechanical energy in alternating
electric energy which is transmitted to the
transformer. It raises the value of tension to
132000 volts and conveys it into the
distribution network.
distribuzione.
The steam coming out of the turbine is turned
into a liquid in a condenser while the gases,
contained in the steam, are dispersed in the
atmosphere.
.
A cooling tower allows the cooling of the water
produced by the condensation of the steam
and it also provides the condenser with cold
water.The condensed water out of the plant is
reinjected in the deep rocks from which the
steam has been taken.
When the wells supply a liquid phase with a
temperature below about 180 degrees C, the
heat of the f luid is used to make another low
boiling point liquid (usually isobutane or
isopentane) evaporate. Once it has been
transformed into steam it will be conveyed in
the turbine, starting up the over described
process.
Geothermal sources are
essentially of three types:
-Hydrothermal
-pressurized
-petrolchemical
Hydrothermal systems are
characterized by the presence of
water or steam in underground
pockets heated by rocks at a high
temperature and they are the only
geothermal systems to be exploted by
industry. Hydrothermal systems are
divided into:dominant steam systems
or water dominat sysyems according
to the predominance of one or the
other phase.
Geopressurized systems
are very deep
deposits(between 2500
and 9000mt)at relatively
low temperatures but at a
very high pressure and full
of natural gases.
Petrothermal systems or HDR ( Hard Dry
Rock) are made of blocks of dry rocks at a
small depth, the use of which is not dependent
on the possibility to break the rock and insert
water that may be able to take away the
necessary thermal energy. To this aim large
exchange surfaces are needed, because of the
moderate thermal conductibility of the rock.