Geothermal energy is the energy generated by geological heat sources,

exploiting the natural heat of the earth.

Co-finanziatoDal Programma LLP dell’Unione Europea

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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 fluids 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 2000-40000 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 fluid 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.