6.4 geothermal energy - treccani geothermal energy is the heat contained within the earth that gives...

6.4.1 Introduction

Geothermal energy is the heat contained within theEarth that gives rise to numerous geologicalphenomena. The expression geothermal energy,however, is used nowadays to indicate the fraction ofthe Earth’s thermal energy that can, or could, beextracted and exploited by man. Geothermal resourcesrepresent an important form of renewable andsustainable energy that is currently adopted in manyparts of the world (Dickson and Fanelli, 2003).

Volcanoes, geysers, thermal springs, fumaroles andother such surface phenomena will certainly have ledour ancestors to suspect that parts of the Earth’sinterior were hot. However, it was only between theSixteenth and Seventeenth century, when the firstmines were excavated to a few hundred metres depth,that temperature was discovered to increase withdepth.

The first temperature measurement using athermometer was probably that of M. De Gensanne ina mine near Belfort, France, in 1740 (de Buffon,1778). From 1870 onwards, the Earth’s thermal regimewas studied with modern scientific methods, but itwasn’t until the Twentieth century, and the discoveryof the role of radiogenic heat, that such phenomena asthe Earth’s heat balance and its thermal history werefully understood. All modern thermal models of theEarth must include the heat continuously generated bythe decay of the long-life radioactive isotopes ofuranium (238U, 235U), thorium (232Th) and potassium(40K) present in the Earth’s interior (Lubimova, 1969).Other sources of heat, whose contribution is, however,less easy to define, include the Earth’s inherited(thermal) heat, gravitational energy and the kineticenergy of the tides. Realistic thermal models were notavailable until the 1980s, when it was demonstratedthat there is a lack of equilibrium between the heat

produced by the decay of radioactive isotopes presentin the Earth’s interior and the heat dispersed from theEarth’s surface into the atmosphere; in other words, itbecame clear that our planet is slowly cooling down.

In the heat balance developed by Frank D. Staceyand David E. Loper, the total heat dissipated from theEarth’s surface was evaluated at 42�1012 W(conduction, convection and radiation); the heat flowfrom the mantle alone, which represents 82% of theEarth’s total volume (Fig. 1), was estimated at10.3�1012 W (Stacey and Loper, 1988). More recentcalculations, using far more data, give a surface heatflow value that is 6% higher than that reported byStacey and Loper. Cooling of the Earth’s mantle isconsequently taking place at a slightly faster pace thanestimated by these authors, but our planet is, in anycase, cooling very slowly. The temperature of themantle, approximately 4,000°C at its base, hasdecreased by 300-350°C at the most over three billionyears. It has been estimated that the total thermalenergy contained within the Earth, assuming anaverage surface temperature of 15°C, is in the order of12.6�1024 MJ, and that the thermal energy contained inthe crust is in the order of 5.4�1021 MJ (Armstead,1983). The Earth’s thermal energy is thereforeimmense, but only a part of it can be exploited byman. So far, its utilization has been limited to areas inwhich the geological conditions allow a vector (water in the liquid or vapour phase) to transport thethermal energy from deep hot zones to, or near to, the surface, creating what are commonly known asgeothermal resources.

Brief geothermal historyThe first and simplest utilization of the Earth’s

heat, for cooking food, dates back to pre-historictimes. Neolithic man used the naturally warm watersto cure diseases, and in his rites and ceremonies. There

595VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY

6.4

Geothermal energy

is archeological evidence that the Etruscans madewidespread use of these waters for bathing, and theRomans continued and elaborated on this practice,utilizing the water in thermal spas and heating theirhomes from the First century bc until the fall of theEmpire. There is evidence that geothermal resourcescontinued to be utilized in the centuries that followed,in many countries all over the world, including China,but on a very limited scale and in the mostrudimentary of forms. It was not until the Nineteenthcentury that geothermal energy was exploited on atruly industrial scale (Ciardi and Cataldi, 2005).

In the early 1800s, in a village in Tuscany, centralItaly, that was later to become known as Larderello, asmall chemical industry was set up to extract boricacid from the hot waters gushing naturally from the ground or extracted in shallow wells (Nasini, 1930).The boric acid was obtained by evaporating the hot boron-enriched water in metal boilers, using woodfrom the nearby forests as fuel. In 1827, FrancescoLarderel, manager of the business since 1818, inventeda system for exploiting the heat content of the boricfluids in the extraction process, instead of burningtimber from the rapidly depleting forests.

It was during this same period that the mechanicalenergy of the natural steam was first used to lift water,in simple gas-lift systems, and to drive the pumps andwinches used in drilling operations or in the boric acid

industry. The chemical industry at Larderello held themonopoly in boric acid production in Europe between1850 and 1875. Low-pressure geothermal steam wasfirst used at Larderello to heat homes, businesses andgreenhouses in the 1910-40 period, and thisapplication has gradually expanded since then.Exploitation of geothermal energy was, however, notconfined to Larderello, or Italy: in 1892, the firstdistrict-heating system was put into operation inBoise, Idaho (USA); in 1928, Iceland began exploitingits geothermal waters in space heating, and haspioneered a variety of utilizations since then.

The first successful attempt at generatingelectricity from geothermal resources took place atLarderello on July 4th, 1904, when Piero Ginori Conti,who had taken over the boric acid industry from theLarderel family, lit the first light bulb, using an enginedriven by geothermal steam and connected to adynamo. This experiment is an important milestone inthe history of geothermal energy, marking thebeginning of a form of utilization that was eventuallyto be adopted in many other countries.

The production of electric energy from geothermalresources at Larderello was not just a technicalbreakthrough, but was also to prove a commercialsuccess. By 1916, the geothermoelectric capacityinstalled in the area had reached 12,000 kWe; in 1942,before the devastation of the Second World War, this

596 ENCYCLOPAEDIA OF HYDROCARBONS

POWER GENERATION FROM RENEWABLE RESOURCES

asth

enos

pher

e

6,370 km2,900 km

inner core

outer core

crust

mantle

lith

osph

ere

man

tle

crus

t

Fig. 1. The Earth’s crust,mantle and core. Upper right, a detail of the crust and upper partof the mantle.

figure had risen to 127,650 kWe. The Italian examplewas swiftly followed by other countries. In 1919, thefirst geothermal well was drilled in Japan, followed bythe United States in 1921. In 1958, the firstgeothermal power plant began operating in NewZealand; in 1959, in Mexico; and in 1960, in theUnited States; many other countries were to begingenerating electricity from this source in the years tofollow.

Current use of geothermal energyAfter the Second World War, many countries were

attracted to geothermal energy, considering iteconomically competitive with other forms of energy.There are currently 24 nations utilizing geothermalenergy in the generation of electricity (geothermalpower). These are listed in Table 1, which also reportsthe geothermoelectric capacity installed in the world in2005 (8,934 MWe) and, for comparison, in 1995(6,833 MWe). In 1995, the geothermoelectric capacityinstalled in the developing countries represented 38%of the total worldwide, but by 2005 it had increased toabout 48%. Geothermal energy can play a significantrole in the national energy balance in these countries:in 2002, the electricity produced from geothermalresources represented 27% of the total electricitygenerated in El Salvador, 22% in the Philippines, 15%in Costa Rica and 11% in Kenya.

Nowadays, more than 70 countries exploit theirgeothermal resources for non-electric uses (the directuse of geothermal heat). In 2005, the total installedcapacity worldwide in this type of plant amounted to28,269 MWt while the energy consumed was273,372 TJ/yr. The most widespread non-electric usesin the world are: heat pumps, 32% of the total; bathing(including balneotherapy and heating ofswimming-pools), 30%; space heating, 20% (77% ofwhich is for district-heating); greenhouse and soilheating, 7.5%; industrial heat processes, 4%;aquaculture, 4%; and a variety of other small-scaleapplications, such as crop-drying, refrigeration,pavement and road de-icing, about 2.5% (Lund et al., 2005).

6.4.2 Nature of geothermalresources

The Earth as a thermal engineThe geothermal gradient expresses the increase in

temperature with depth. Down to the depths accessiblewith modern drilling techniques, the averagegeothermal gradient is 2.5-3°C/100 m. Consequently,if the temperature in the first few metres below theground surface is 15°C (which roughly corresponds to

the average annual temperature of the outside air), wecan expect a temperature of 65-75°C at a depth of2,000 m, 90-105°C at 3,000 m and so forth for severalthousand metres. There are, however, vast regions inwhich the geothermal gradient is nowhere near theaverage value. In large geologically youngsedimentary basins, the geothermal gradient may belower than 1°C/100 m, whereas in recently upliftedareas, it might well be higher than average. In certaingeothermal areas, the gradient could be ten timeshigher than the norm. The difference in temperature


GEOTHERMAL ENERGY

Table 1. Installed geothermal generating capacitiesworldwide (MWe) in 1995 (Huttrer, 2001)

and in 2005 (Bertani, 2005, revised)

1995 2005

Australia 0.17 0.17

Austria – 1.25

China 28.78 28

Costa Rica 55 162.5

El Salvador 105 151

Ethiopia – 7.3

Philippines 1,227 1,931

France (Guadeloupe) 4.2 15

Germany – 0.23

Japan 413.705 535

Guatemala – 33

Indonesia 309.75 797

Iceland 50 202

Italy 631.7 790.5

Kenya 45 129

Mexico 753 953

Nicaragua 70 77.5

New Zealand 286 435

Papua - New Guinea – 6

Portugal (Azores) 5 16

Russia 11 79

Thailand 0.3 0.3

Turkey 20.4 20.4

USA 2,816.7 2,564

Total 6,832.705 8,934.15

between the deep hotter zones and shallow colderzones creates a flow of heat from the former to thelatter. The mean terrestrial heat flow is 65 mWm�2 incontinental areas and 101 mWm-2 in oceanic areas,with an overall weighted mean of 87 mWm�2 (Pollacket al., 1993). These values are based on 24,774measurements taken in 20,201 sites, covering about62% of the Earth’s surface. Heat flow estimates for theareas not covered by these measurements were basedon studies of the distribution of the geological units.

The temperature increase with depth, volcanoes,geysers, fumaroles, and hot springs are all tangible andvisible manifestations of the Earth’s internal heat; thisheat is responsible for other less perceptiblephenomena that are, nevertheless, of such enormitythat the Earth has been compared to an immensethermal engine. The phenomena are referred tocollectively as plate tectonics, a theory thatrevolutionized our geological knowledge of the Earth,and in which geothermal resources also play a role.

The Earth’s structure consists of the crust, whosethickness varies from about 20-65 km in continentalareas to 5-6 km in oceanic zones; the mantle,approximately 2,900 km thick; and the core, whichhas a radius of approximately 3,470 km (see againFig. 1). The physical and chemical properties of thecrust, mantle and core are variable. The outermostshell of the Earth, called the lithosphere, comprisesthe crust and the uppermost part of the mantle; with athickness ranging from less than 80 km in oceanicareas to over 200 km in continental zones, it acts as arigid body. Beneath the lithosphere is theasthenosphere, which is made up of the upper layer ofthe mantle, and is less rigid, or more plastic. In otherwords, on a geological scale in which time ismeasured in millions of years, the asthenospherebehaves like an extremely viscous fluid.

The differences in temperature between the variousparts of the asthenosphere create movements that formconvective cells. The heat produced by the decay ofradioactive isotopes and the heat rising from greatdepths sustain the extremely slow movement of thesecells. Enormous volumes of material, hotter and lessdense than the overlying material, rise from greatdepth towards the surface, while the material closer tothe surface, being cooler and denser, tends to sinkdownwards, where it is heated up and starts to riseonce more.

In the zones where it is thinner, and particularly inoceanic areas, the lithosphere is thrust upwards andfractured by the partially molten material rising fromthe asthenosphere through the ascending parts of theconvective cells. It is this mechanism that created (andstill creates) the spreading ridges that extend for over60,000 km below the oceans, emerging in some areas(Iceland), and at times even creeping betweencontinents, as in the Red Sea. A relatively smallfraction of molten material emerges from the crest ofthese ridges and solidifies to form new oceanic crust.Most of the material ascending from theasthenosphere, however, divides into two branches andflows in opposite directions back under thelithosphere. The continuous generation of new crustand the pull exerted by the two branches in oppositedirections causes the ocean floor on either side of theridges to drift apart. The ocean floor (oceaniclithosphere) would consequently tend to expand wereit not for the compensatory and equivalent reduction(or absorption) of lithosphere in other parts of theEarth. This phenomenon takes place in so-calledsubduction zones, such as along the margins of thePacific Ocean, where the lithosphere plunges beneaththe adjacent lithosphere and descends to very hot,deep zones of the mantle. During its descent (Press



oceanic plate

oceanic crustcomposite volcano

asthenosphere

asthenosphere

trenchisland arc

trench

mid-oceanridge

subductionzone rift

valley

continental plate

continental crust

lithosphere

Fig. 2. Schematic cross-section which shows the dynamics of plate tectonics.

and Siever, 1997), part of the lithospheric materialmelts and rises to the surface through fractures in thelithosphere (Fig. 2), forming volcanoes on thecontinental margins (as in the Andes) or on island arcs(as in Japan and the Aleutine Islands).

Spreading ridges, transform faults and subductionzones form the margins of six large and several othersmaller lithospheric plates. The plate marginscorrespond to weak, densely fractured zones and arecharacterized by an intense seismicity, a large numberof volcanoes and, due to the ascent of hot moltenmaterial towards the surface, a high terrestrial heatflow. There is consequently a close relationshipbetween plate tectonics and the distribution ofgeothermal resources (Sommaruga and Zan, 1995),especially high-temperature resources, which aregenerally situated along these plate margins (Figs. 3and 4).

Geothermal systemsA geothermal system can be schematically defined

as “convecting water in the upper crust of the Earth,which, in a confined space, transfers heat from a heatsource to a heat sink, usually the free surface wherethe heat is absorbed (dispersed or used)” (Hochstein,1990). A geothermal system is made up of threeelements (Fig. 5): a heat source, a reservoir and a fluid,

which is the carrier that transports the heat. The heatsource can be either a very high-temperature(�600°C) magmatic intrusion that has risen torelatively shallow depths (5-10 km), or, as in certainlow-temperature systems, the Earth’s normal heat. Thereservoir is a volume of hot permeable rocks that yieldtheir heat to the circulating fluids; it is generallyoverlain by a cover of impermeable rocks andconnected to surficial recharge areas, through whichmeteoric waters can totally or partly replace the fluidslost through natural manifestations, such as springs orfumaroles or extracted in wells. In most cases, thegeothermal fluid is meteoric water in the liquid orvapour phase, depending on its temperature andpressure. This fluid often contains chemicals and gasessuch as CO2, H2S, etc.

The mechanism underlying geothermal systems islargely governed by fluid convection laws. Fig. 6describes this mechanism for anintermediate-temperature hydrothermal system.Convection occurs because of the heating andconsequent thermal expansion of fluids in a gravityfield; heat, which is supplied at the base of thecirculation system, is the energy that drives thesystem. Heated fluid of lower density tends to rise andto be replaced by colder, denser fluid coming from themargins of the system. Convection, by its nature, tends


GEOTHERMAL ENERGY

1 2 3 4

PacificPlate

PacificPlateAfrican

PlatePhilippine

Plate

North American Plate Eurasian Plate

IndianPlate

Antarctic Plate Antarctic Plate

Australian Plate

NazcaPlate

South AmericanPlate

CocosPlate

GuadeloupeGuatemala

Mexico

ThailandNicaraguaCosta Rica

El SalvadorEthiopia

AustriaItaly

TurkeyAzores

Iceland

Germany

Russia

ChinaJapan

Philippines

Indonesia

Australia

NewZealand

PapuaNew Guinea

Kenya

USA

Fig. 3. Tectonic plates, spreading ridges, subduction zones, crustal fractures and countries that produce electric energy of geothermal origin. 1, spreading ridges crossed by transversal fractures (transformed faults); 2, subduction zones, where the lithosphere plunges into the asthenosphere; 3, rift valleys; 4, large crustal fractures.



Geysers

Casa Diablo

Coso

Salton Sea

East Mesa

Roosevelt

Beowawe

SteamboatDixie Valley

StillwaterSodaLake

Brady

Heber

Cerro Prieto

Las Tres Virgenes

Los HumerosM E X I C O

CENTRAL AMERICA

JAPAN

EASTERN AFRICA

PHILIPPINESINDONESIA

NEW ZEALAND

USA

ICELAND

SOUTHERN EUROPE

U S A

GUATEMALA

NICARAGUA

COSTA RICA

E T H I O P I AP H I L I P P I N E S

I C E L A N D

I TA LY

AUSTRIA

T U R K E Y

N E WZ E A L A N D

J A PA N

I N D O N E S I A

K E N YA

EL SALVADOR

Los Azufres

Zunil Amatitlan

AhuachapànBerlin

San JacintoMomotombo

Miravalles

Aluto-Langano

Olkaria

Lahendong

Mt. Apo

Tongonan

Palinpinon

Sibayak

DiengKamojang

Ngawha

Kawerau

OhaakiRotokawa

Mori

Sumikawa Matsukawa

KakkondaUenotai

Onikobe

Yanaizu

TakigamiOtake

HatchobaruOgiri

MokaiWairakei

Nesjavellir

Krafla

Svartsengi

SalakW. Windu

Darajat

TiwiMak-BanBac-Man

Monte Amiata

Kizildere

Travale

Larderello

Altheim

Fig. 4. Main geothermal areas showing the geothermal fields that produce electricity.


GEOTHERMAL ENERGY

geothermalwell

impermeable caprock(thermal conduction)

impermeable rock(thermal conduction)

flow of heat(conduction)

magmaticintrusion

hot springor steam

vent

coldmeteoricwaters

recharge area

hotfluids

reservoir(thermal

convection)

Fig. 5. Schematic diagram of a geothermal system.

dept

h (t

hous

ands

of

ft)

dept

h (k

m)

20

1

2

3

4

5

6

7

15

10

0

5

temperature (°F)

temperature (°C)

10°C at surface

boiling begins

hot w

ater

low

den

sity

high

den

sity

cold

wat

er

rocks of lowpermeability

rockpermeable

rock

heatheat

magma

crystalline

0

0 600200 400

boiling beginsA

AE

ED

D

B

B

C

CF

F

G

G

curve 1

curve 2

400 800 1,200

hot springor geyser0

Fig. 6. Model of a geothermal system. Curve 1 is the boiling-point curve of water; curve 2 shows the fluid temperature profile along a typical circulation path from recharge at point A to discharge at point E (White, 1973).

to increase temperatures in the upper part of a system,as temperatures in the lower part decrease (White,1973).

Of all the elements of a geothermal system, theheat source is the only one that must be natural; theothers can be artificial. For example, the fluidsdischarged from a geothermal power plant afterextraction of their thermal energy can be injectedback into the reservoir through wells drilledspecifically for this purpose. In this way, the naturalmeteoric recharge of the reservoir can be boosted byartificial recharge. Reinjection of the spent fluidsfrom power plants has also been adopted for someyears now as a means of reducing the environmentalimpact of the geothermal plants. Artificial rechargecan also be used to replenish and sustain old orexhausted geothermal fields, as in the case of TheGeysers geothermal field in California. In this field,one of the largest in the world, production started todecrease rapidly at the end of the 1980s because ofover-exploitation and consequent fluid depletion inthe reservoir. In order to overcome this problem,about 60 km of pipeline were laid in order totransport 820 l/s of water to The Geysers, torecharge the reservoir. This project has led to thereactivation of a number of power plants that hadbeen abandoned because of a lack of fluids.

In the Hot Dry Rock Project (HDR), launched atLos Alamos (USA) in the early 1970s, both the fluidand the reservoir are artificial. High-pressure wateris pumped through a specially drilled well into adeep body of compact hot rock, causing its hydraulicfracturing. The water permeates these artificialfractures, extracting heat from the surrounding rock,which acts as a natural reservoir. The reservoir isthen crossed by a second well, which extracts theheated water. The system therefore consists of aborehole, which is used for hydraulic fracturing, anartificial reservoir, into which this borehole injectscold water and, finally, a borehole that is used toextract the hot water (Fig. 7). The entire system,complete with surface utilization plant, forms aclosed loop, avoiding all contact between the fluidand outside environment (Proceedings […], 1987).The Los Alamos HDR project, which wasabandoned after a few years of experiments, was theforerunner for other similar projects. These projectsgained new impetus after the discovery that deeprocks have a certain innate fracturation, and thatmethods and technologies should be tailored to localgeological conditions. HDR projects have beendeveloped (with alternating success depending onthe funds available) in Australia, France, Germany,Japan and the United Kingdom. A few are on thebrink of finally becoming operative.

Classification of geothermal resourcesThe most common criterion for classifying

geothermal resources is that based on the enthalpy ofthe fluids that transport heat from depth to the surface.Enthalpy, which increases as the temperature rises, isused to express the heat content (thermal energy) ofthe fluids and gives an approximate idea of their value.The resources are classified as low, medium or highenthalpy (temperature) resources according to criteriathat are based on the energy content of the fluids andtheir potential forms of utilization. Low enthalpyresources are therefore fluids with a temperaturebelow 90°C, which is the bottom limit for generatingelectricity in binary-cycle plants; medium enthalpyresources have temperatures in the 90-150°C range,and high enthalpy resources have temperatures above150°C, which is the bottom limit for generatingelectricity in conventional power plants.

Geothermal systems are also frequently dividedinto water-dominated and vapour-dominated (or drysteam) systems (White, 1973). In water-dominatedsystems, liquid water is the continuous,



Fig. 7. Schematic diagram of an artificial geothermal system (Hot Dry Rock Project).

pressure-controlling fluid phase; some vapour may bepresent, generally as discrete bubbles. Thewater-dominated systems, whose temperatures rangebetween �125 and �225°C, are the most widespreadin the world. Depending on temperature and pressureconditions, they can produce hot water, water andsteam mixtures, wet steam and, in some cases, drysteam. In vapour-dominated systems, liquid water andvapour normally co-exist in the reservoir, with vapouras the continuous pressure-controlling phase. Theseare high-temperature systems and normally theyproduce dry or superheated steam. Geothermalsystems of this type are quite rare; the best known areLarderello in Italy and The Geysers in California.

Geothermal systems can also be classified as eitherdynamic or static, depending on the state ofequilibrium of the reservoir (Nicholson, 1993), whichis based on reservoir fluid circulation and themechanism of heat transfer. In dynamic systems, watercontinuously recharges the reservoir, is heated andthen discharged from the reservoir to the surface orinto underground permeable rock formations. The heatis acquired by the system through conduction and fluidcirculation; the dynamic systems comprisehigh-temperature (T�150°C) and medium-to-lowtemperature (T�150°C) systems. In static systems,there is very little recharge of the reservoir or none atall, and heat transfer is by conduction only; thiscategory includes the low-temperature andgeopressurized systems. Geopressurized systems aretypically found in large sedimentary basins (such asthe Gulf of Mexico) at depths of 3-7 km.Geopressurized reservoirs are made up of permeablesedimentary rocks, hosted within low-conductivityimpermeable layers. They contain pressurized hotwater (which was trapped when the sediments weredeposited), at values close to lithostatic pressure, andgreatly exceeding hydrostatic pressure. These systemscan also contain significant quantities of methane.Geopressurized systems could produce thermal andhydraulic energy (pressurized hot water) and methanegas, but they have not as yet been exploited on anindustrial scale.

Renewability and sustainabilityGeothermal energy is generally defined as being

renewable and sustainable. The term renewable refersto a property of the energy source, whereas the termsustainable describes how the resource is used.

The most critical factor for the classification ofgeothermal energy as a renewable energy is the rate ofenergy recharge. When exploiting a naturalgeothermal system, energy recharge occurs by theadvection of hot fluids into the system at the same (orcomparable) time as these fluids are extracted from the

system. Thus, geothermal energy can be classified as arenewable energy resource. In the case of hot dryrocks and certain hot aquifers in sedimentary basins(geopressurized systems), energy recharge occurs byheat conduction only; because this process is so slow,hot dry rocks and some sedimentary basins shouldactually be considered as limited energy resources(Stefansson, 2000).

The sustainable utilization of a resource dependson its initial quantity, how fast it regenerates and howfast it is consumed. Consumption can obviously besustained for as long as we want it to be, provided theresource is being regenerated at the same or faster ratethan that at which it is being exploited. The termsustainable development is used by the WorldCommission on Environment and Development todescribe development that “meets the needs of thepresent generation without compromising the needs offuture generations”. In this context, sustainabledevelopment does not mean that all energy resourcesshould be used in a totally sustainable way, but merelythat when a given resource has reached depletion,there must be another resource available that is capableof meeting the demands of future generations. Aspecific geothermal field does not necessarily have tobe exploited in a sustainable way. Projects for thesustainability of geothermal energy should be directedat achieving and maintaining a certain production levelon a national or regional scale, both in electricitygeneration and the direct use sector, for a given periodof, say, 300 years, by putting new geothermal systemsinto production as soon as others are exhausted(Wright, 1998).

6.4.3 Geothermal exploration

The main objectives of geothermal exploration are: a) to identify areas with geothermal resources,determine the type and extension of the resource, andthe location of possible productive zones; b) define theheat content of the fluids present in the reservoir; c) identify any characteristics of the resource thatmight cause damage to the environment; d ) define anyparameters that could create problems duringexploitation. Numerous methods and technologies areavailable for achieving these objectives, many ofwhich are in common use and thoroughly tried andtested over years.

Exploration methodsGeological and hydrogeological studies are the

starting-point of every exploration programme. Theirbasic function is to define the location and extensionof the areas that should be investigated in greater


GEOTHERMAL ENERGY

detail, and to recommend the most appropriateexploration methods. Geological and hydrogeologicalstudies have a very important role in all subsequentphases of geothermal research, up to the siting of theexploration and production wells. They also provideinformation, and data of fundamental importance forinterpreting the data obtained with other explorationmethods, and for constructing a realistic model of thegeothermal system and evaluating the resourcepotential.

Geochemical prospecting (including isotopegeochemistry) can help us to determine whether ageothermal system is water- or vapour-dominated, topredict the minimum temperature in the reservoir, todefine the chemical characteristics of the deep fluidand to identify the origin of the recharge water. It canalso provide useful indications on potential problemsin the reinjection phase and during utilization, such ascorrosion and scaling in pipelines and/or plants,environmental impact, and how to avoid or reducethese problems. Geochemical prospecting includessampling and chemical and/or isotopic analysis of thegeothermal fluid, and of any geothermalmanifestations (thermal springs, fumaroles, etc.) in thestudy area (Gandino et al., 1985a; Krauskopf and Bird,1995).

The target of geophysical surveys is to indirectlyprovide information from the surface, or from depthintervals close to the surface, that can be used todefine the physical parameters of the geologicalformations at depth. These physical parameterscomprise temperature (thermal survey), electricalconductivity (electrical and electromagnetic methods),propagation velocity of elastic waves (seismic survey),density (gravity survey) and magnetic susceptibility(magnetic survey). The seismic, gravity and magneticsurveys, which are traditionally used in oil research,can provide a great deal of information on the shape,dimensions, depth and other important characteristicsof deep geological structures that might act as ageothermal reservoir, but they give few indications asto whether these structures actually contain fluids,which is the objective of geothermal research.Information of this type can be obtained fromelectrical and magnetotelluric surveys, which are themost widely used in geothermal prospecting. Thermalmethods (direct temperature measurements,determination of the geothermal gradient andterrestrial heat flow) can provide a fairly reliabletemperature value for the upper part of the geothermalreservoir (Gandino et al., 1985b; Zan et al., 1990;Parasnis, 1997).

Drilling of exploratory wells is the final phase ofevery exploration programme and is the only means ofdefining the characteristics of a geothermal reservoir

and evaluating its potential. The function of the dataprovided by the exploratory wells is to verify thehypotheses and models elaborated from the results ofsurface exploration. They should also confirm whetherthe reservoir is productive and contains fluids inadequate quantities with suitable characteristics for theutilization envisaged for the resource.

6.4.4 Utilization of geothermal resources

Electricity generation is the most important form ofutilization of high-temperature (�150°C) geothermalresources. The medium-to-low temperature resources(�150°C), which can be used to produce electricenergy in binary-cycle plants, are also suited to a widevariety of other applications, from space heating torefrigeration, soil heating and crop drying, fishfarming and industrial heat processes (Fig. 8).

Electricity generationElectricity generation takes place in conventional

or binary-cycle plants, depending on thecharacteristics of the geothermal resources.

Conventional plants require fluids with atemperature of at least 150°C and are either of theback-pressure (discharging directly into theatmosphere) or condensing type.

The back-pressure units are simpler and cheaper.The steam, which either comes directly from drysteam wells, or from the separator, after separation ofthe liquid component when it comes from wet wells,passes through the turbine and is then discharged intothe atmosphere (Fig. 9). With this type of plant, steamconsumption is 15-25 kilograms per kilowatt-hourproduced; at the same turbine inlet pressure, steamconsumption is approximately double that of acondensing unit. The back-pressure units are, however,useful as pilot plants, as stand-by units hooked onto anisolated well with a low flow-rate, or used to generateelectricity from test wells during field development.They are also used when the steam has a highnon-condensable gas content (�15% by weight).Back-pressure units can be constructed and installedvery rapidly, and put into operation within 13-14months of their order date. This type of machine isgenerally available in small sizes (2.5-5 MWe).

The condensing units (Fig. 10), having moreauxiliary equipment (condensers, compressors, andcooling towers), are more complex than theback-pressure units, and can take twice as long toconstruct and install because of their size. The specificsteam consumption of the condensing units, however,is approximately half that of the back-pressure units



(6-10 kilograms of steam per kilowatt-hour produced).Condensing plants of 55-60 MWe capacity are themost widely used, but 110 MWe units are also beingconstructed and installed.

With the progress made in binary-cycle technologyover the last few decades it is now possible to generateelectricity from medium-to-low temperature fluids andfrom the hot waste fluids discharged from theseparators in water-dominated geothermal fields.Binary plants use a secondary working fluid, generallyan organic fluid such as n-pentane, which has a lowboiling point and high vapour pressure at lowtemperatures compared to steam. The secondary fluidoperates in a conventional Rankine cycle: thegeothermal fluid circulates in the heat exchanger,yielding its heat to the secondary fluid; the heatedsecondary fluid then vaporizes and the vapour


GEOTHERMAL ENERGY

evaporation of highly concentrated solutionsrefrigeration by ammonia absorptiondigestion in paper pulp, kraft

heavy water via H2S processdrying of diatomaceous earth

satu

rate

d st

eam

wat

er

drying fish mealdrying of timber

aluminia via Bayer’s process

drying farm products at high ratescanning of food

evaporation in sugar refiningextraction of salts by evaporation and crystallization

fresh water by distillationmost multiple effect evaporations, concentration of saline solution

drying and curing of light aggregate cement slabs

drying of organic materials, seaweeds, grass, vegetables, etc.washing and drying of wool

drying of stock fishintense de-icing operations

space heatinggreenhouses by space heating

refrigeration (lower temperature limit)

animal husbandrygreenhouses by combined space and hotbed heating

mushroom growingbalneological baths

soil warming

hatching of fish, fish farming

swimming pools, biodegradation, fermentationswarm water for year-round mining in cold climatesde-icing

180

°C

170

160

150

140

130

120

110

100

90

80

70

60

50

40

20

30

Fig. 8. Lindal diagramshowing the possiblenon-electrical uses of geothermal fluids at different temperatures(Lindal, 1973).

steamwater

reinjectionwell

productionwell

steam

separator

turbo-alternator

atmosphericexhaust

water

Fig. 9. Diagram of a back-pressure plant for electricity generation. The flow of the geothermal fluid is indicated in red.

produced drives a normal axial flow turbine connectedto a generator. The vapour is then cooled and returnsto the liquid state, and the cycle begins again (Fig. 11).By selecting the most appropriate secondary fluid,binary plants can be designed to utilize geothermalfluids in the 90-170°C temperature range. The upperlimit depends on the thermal stability of the organicworking fluid, and the lower limit ontechnical-economical factors: below this temperature,in fact, the heat exchangers would have to be of suchhuge dimensions as to make the project uneconomical.Binary plants operate in closed loops, where neitherthe working fluids nor the geothermal fluids come intocontact with the outside environment. Binary plantsare normally constructed in small modular unitsranging from a few hundred kilowatts to a fewmegawatts; these small units can be linked up to formpower plants of a few tens of megawatts. Their costdepends on a number of factors, but particularly on thetemperature of the geothermal fluid, which dictates thesize of the turbine, heat exchangers and cooling

system. The total size of the plant has little influenceon specific cost as a series of standard modular unitscan be joined together to obtain larger capacities asrequired (ORMAT, 1989; DiPippo, 2004).

In the Kalina cycle, a new binary system developedin the 1990s, a mixture of water and ammonia is usedas working fluid. The latter is expanded through thehigh-pressure turbine in superheated conditions andthen re-heated before entering the low-pressureturbine. After the second expansion, the saturatedvapour moves through a recuperative boiler beforebeing condensed in a water-cooled condenser. TheKalina cycle seems to be more efficient than theorganic fluid binary plants but it is also of morecomplex design and operation (DiPippo, 2004).

A conventional plant and a binary plant can belinked in a combined-cycle power plant in order toachieve maximum combined efficiency. In this kind ofsystem, the geothermal fluid from a water-dominatedreservoir is piped to a conventional single-flash powerplant where the steam that feeds the turbine



turbo-alternator cooling tower

steamwater

coolingwater pump

reinjectionwell

reinjectionwell

productionwell

condenser

steam

separator

water

Fig. 10. Diagram of a condensation plant for the generation of electricity. The geothermal fluid circuit is indicated in red;the cooling circuit in blue.

turbo-alternator

cooling tower

coolingwater pump

feedpump

heatexchanger

reinjectionwell

productionwell

condenser

Fig. 11. Diagram of a binarycycle plant for electricitygeneration. The flow of thegeothermal fluid isindicated in red; thesecondary (working) fluid,in green; and the coolingcircuit, in blue.

(conventional plant) is separated from the warm waterthat is circulated in the heat exchanger (binary cycleplant) before being injected back into the reservoir. Inhybrid power plants, the medium-to-low temperaturegeothermal fluid (�150°C) is circulated through aheat exchanger to pre-heat another fluid (generallywater) that is then vaporized by the heat supplied byfossil fuels or biomass.

Direct heat useDirect use of geothermal fluids is the oldest, most

versatile and most common form of utilization ofgeothermal resources. Balneology, space and districtheating, agricultural applications, aquaculture andindustrial processes are the best-known, but heatpumps are the most widespread. There are many otherapplications run on a much smaller scale, some ofwhich are unusual, such as the de-icing of roads,textile processing, etc.

Space and district heating have been widelydeveloped in Iceland; by 2004, a total of 1,350 MWt

was installed for these heating systems, but this typeof application is also extremely common in otherEuropean countries, as well as the United States,China, Japan, etc. The heating elements are thetraditional radiators, radiating panels, etc. The warmgeothermal fluids are used directly in these systemsunless they contain potentially corrosive or scalingsubstances, in which case they are passed through aheat exchanger to yield their heat to a secondary fluid.Geothermal district heating systems require hugecapital investment. The main costs are the initialinvestment in production and injection wells, auxiliaryplants, the distribution network and peaking stations.Compared with conventional systems, however, theoperating costs are significantly lower and consist ofpumping power, system maintenance, control andmanagement. A crucial factor in estimating the initialcost of a geothermal district heating system is thermalload density, i.e. the heat demand divided by theground area of the district. A high heat densitydetermines the economic feasibility of a districtheating project, as the distribution network isexpensive. In regions with a favourable climate, someeconomic benefit can be achieved by combiningheating and cooling. The load factor of a combinedheating/cooling system will be higher than that of aheating system only, and the unit energy price willthus decrease (Gudmundsson, 1988).

Space cooling is a feasible option where absorptionmachines can be adapted to geothermal use. Thetechnology of these machines is tried and tested, andthey are readily available on the market. Theabsorption cycle is a process that utilizes heat insteadof electricity as energy source. The refrigeration effect

is achieved utilizing two fluids: a refrigerant, whichcirculates, evaporates (absorbing heat) and condenses(emitting heat), and a secondary fluid or absorbent.For applications above 0°C (primarily inair-conditioning of buildings and in industrialprocesses), the cycle uses lithium bromide as theabsorbent and water as the refrigerant. For applicationsbelow 0°C, an ammonia/water cycle is adopted, withammonia as the refrigerant and water as the absorbent.Geothermal fluids provide the thermal energy neededto drive the machines, although the efficiency of themachines decreases when fluid temperatures are lowerthan 105°C.

Geothermal space conditioning (heating andcooling) has expanded greatly since the 1980s,following the introduction and increasingly wider useof heat pumps. The various heat pump systems permitus to economically extract and utilize the thermalenergy of low-temperature bodies such as ground andshallow aquifers, ponds, etc. (Sanner et al., 2003; Fig. 12). Ground-coupled and ground-water heat pumpsystems, exploiting temperatures in the 5-30°C range,are currently installed in thirty-three countries and, in 2005, accounted for a total installed capacity of 15,384 MWt.

The agricultural applications of geothermal fluidsconsist essentially of control of plant growth


GEOTHERMAL ENERGY

hot watertank

borehole heatexchanger

low-temperatureunderfloor heating

heat pump

Fig. 12. Example of a house heated by a typical ground-coupled heat pump system (Sanner et al., 2003).

temperature through open-field irrigation orgreenhouse heating. The geothermal water can be usedin open-field agriculture for irrigation, provided itdoes not contain chemical elements harmful to theplants; it can also be circulated in buried pipes to heatthe soil, thus limiting the damage to crops duringsevere winters, extending crop seasons, increasingplant growth and production, and sterilizing the soil(Barbier and Fanelli, 1977).

The most common application of geothermalenergy in agriculture, is, however, greenhouse heating,which is in widespread use in many countries. Thecultivation of vegetables and flowers out-of-season orin an unnatural climate can now draw on a widelyexperimented technology. Numerous solutions areavailable for achieving optimum growth conditions,based on the optimum growth temperature of eachplant, the quantity of light, the CO2 concentration inthe greenhouse, the humidity of the soil and air, aswell as air movement. The temperature in thegreenhouse is maintained at an optimum level bycontrolling the heat loss to the outside and by heatingthe inside ambient. Heating is achieved by forced-aircirculation using heat exchangers or by circulatingwarm water in tubes, or by a variety of combinationsof the same. Using geothermal energy in greenhouseheating can significantly reduce the operating costs,which, in some cases, can account for as much as 35%of the final cost of the product (vegetables, flowers,indoor plants, and seedlings).

As with vegetables, controlled temperatureenvironments will also improve the quality andquantity of farm-reared animals and aquatic species

(Fig. 13). In many cases, geothermal water can be usedprofitably in a combination of animal husbandry andgreenhouse heating. As the energy required to heat abreeding installation is about 50% that required to heata greenhouse of the same surface area, it iseconomically sensible to adopt a cascade system; inthis type of system, the geothermal fluid is circulatedfirst in the greenhouse, where it yields part of its heat,before being channelled through the nearby breedinginstallation, where it cedes the rest of its heat content.Breeding in a controlled temperature ambientimproves animal health, and the hot fluids can also beused to clean, sanitize and dry the animal shelters andwaste products.

Aquaculture, i.e. the controlled breeding of aquaticforms of life, has developed on a worldwide scale,following the increased demand for fish produce.Controlled-temperature breeding for aquatic species ismuch more important than for land species (see againFig. 13): by artificially maintaining an optimumtemperature, it is possible to breed more exoticspecies, improve production and, in some cases,double the reproductive cycle. The most commonspecies farmed in these environments are eel, bass,carp, mullet, cat fish, salmon, sturgeon, tilapia,lobster, shrimp, crab, mussels and oysters. Aquaculturealso includes alligator and crocodile farming, as atourist attraction and for their skins. An alligatorreared at a constant temperature of around 30°Creaches a length of about 2 m in three years, comparedto 1.2 m under natural conditions. The temperaturesrequired for aquatic species generally range from 20 to30°C. The size of their installations will depend on the



0

20

40

60

80

100

0 10 20 30 40

perc

enta

ge o

f gr

owth

(wei

ght g

ain

or p

rodu

ctio

n)at

opt

imum

tem

pera

ture

temperature (°C)

catfish (weight gain)

broilers (weight gain)

swine (weight gain)

shrimp (growth)

cow milk(production)

laying hens (production)Fig. 13. Effect of temperature on the growth and productive life of animals (Beall and Samuels, 1971).

temperature of the geothermal fluids, the temperaturerequired in the fish ponds, and the heat losses from thelatter.

The entire range of geothermal fluids, from steamto warm water, can be used in industrial processes,whose forms of utilization comprise process heating,evaporation, drying, distillation, sterilization, washing,de-icing and salt extraction (see again Fig. 8).

Another widespread direct application ofgeothermal energy is balneology, where the warmwaters are used in thermal spas and swimming pools.

6.4.5 Environmental impact

The degree to which geothermal exploitation affectsthe environment depends on the type of utilizationinvolved (Brown and Webster-Brown, 2003). Theeffects of the direct heat uses (non-electric uses) aregenerally modest (Table 2). Electricity generation inbinary-cycle plants will have a similar low-key impact.The impact on the environment is, however, potentiallygreater in the case of conventional power plants,especially with regard to air quality, but it cannevertheless be kept within tolerable limits.

The first perceptible effect on the environmenttakes place during drilling of the exploration andproduction wells, which may modify the local surfacemorphology and disturb local vegetation and wildlife;

leakage of fluids (liquid or gas) could temporarilypollute surface waters and the atmosphere. Afterdrilling, the installation of distribution pipelines andthe construction of utilization plants may also affectanimal and plant life, as well as the landscape.Discharges into the atmosphere from the power plantswill primarily contain carbon dioxide (CO2), hydrogensulphide (H2S), ammonia (NH3), methane (CH4) anddissolved chemical substances whose concentrationsincrease with temperature. Wastewater emissions areanother potential source of pollution. This wastewatermay contain dissolved chemical substances such assodium chloride (NaCl), boron (B), arsenic (Ar) andmercury (Hg), and may become polluting if dispersed.Therefore, they must be either treated or injected backinto the reservoir (or both). Some geothermal fluidscontain no harmful substances, such as those used inIceland for space heating, but this is an exception tothe general rule. Moreover, the water discharged fromgeothermal plants is generally at a higher temperaturethan the surrounding environment and could thuscause thermal pollution.

Air pollution could become a problem whengenerating electricity in conventional plants, becauseof the gases (H2S, CO2, etc.) that are sometimespresent in geothermal fluids, although there are anumber of systems available for reducing gasemissions. The amount of carbon dioxide dischargedby geothermal plants is, however, lower than theemissions from fossil-fuelled plants: 13-380 grams forevery kilowatt-hour of electric energy produced ingeothermal power plants, compared to the 1,042g/kWh from coal-fired plants, 906 g/kWh fromoil-fired plants and 453 g/kWh from natural-gas plants(Fridleifsson, 2001).

Extraction of large quantities of fluid from thegeothermal reservoir may give rise to subsidencephenomena, i.e. a gradual sinking of the land surface.This slow process can be prevented or reduced byinjecting the waste fluids from the power plants backinto the geothermal reservoir.

References

Armstead H.C.H. (1983) Geothermal energy. Its past, presentand future contributions to the energy needs of man, London,Spon.

Barbier E., Fanelli M. (1977) Non-electrical uses ofgeothermal energy, «Progress in Energy and CombustionSciences», 3, 73-103.

Beall S.E., Samuels G. (1971) The use of warm water forheating and cooling plant and animal enclosures, OakRidge National Laboratory, ORNL-TM-3381.

Bertani R. (2005) World geothermal power generation in theperiod 2001-2005, «Geothermics», 34, 651-690.


GEOTHERMAL ENERGY

Table 2. Potential environmental impactof geothermal direct-use projects(Lunis and Breckenridge, 1991).L�low; M�moderate; H�high

Impact Probability Severity

Air quality pollution L M

Surface water pollution M M

Underground water pollution L M

Land subsidence L L-M

High noise levels H L-M

Well blow-outs L L-M

Conflict with cultural and archeological features

L-M M-H

Socio-economic problems L L

Chemical or thermalpollution

L M-H

Solid waste disposal M M-H

Brown K., Webster-Brown J. (2003) Environmental impactand mitigation, in: Dickson M.H., Fanelli M. (edited by)Geothermal energy. Utilization and technology, Paris,UNESCO, 155-173.

Buffon, Leclerc G.L. de (1778) Histoire naturelle, genéraleet particulière, Paris, Imprimerie Royale, 1749-1789;Supplément.

Ciardi M., Cataldi R. (a cura di) (2005) Il calore della Terra,Pisa, ETS.

Dickson M.H., Fanelli M. (edited by) (2003) Geothermalenergy. Utilization and technology, Paris, UNESCO.

DiPippo R. (2004) Second law assessment of binary plantsgenerating power from low-temperature geothermal fluids,«Geothermics», 33, 565-586.

Fridleifsson I.B. (2001) Geothermal energy for the benefitof the people, «Renewable & Sustainable Energy Review»,5, 299-312.

Gandino A. et al. (1985a) Preliminary evaluation of SoufrièreGeothermal Field, St. Lucia (Lesser Antilles), «Geothermics»,14, 577-590.

Gandino A. et al. (1985b) Preliminary model of the RibeiraGrande Geothermal Field (Azores Islands), «Geothermics»,14, 91-105.

Gudmundsson J.S. (1988) The elements of direct uses,«Geothermics», 17, 119-136.

Hochstein M.P. (1990) Classification and assessment ofgeothermal resources, in: Dickson M.H., Fanelli M. (editors)Small geothermal resources. A guide to development andutilization, New York, UNITAR, 31-57.

Huttrer G.W. (2001) The status of world geothermal powergeneration, 1995-2000, «Geothermics», 30, 1-27.

Krauskopf K.B., Bird D.K. (1995) Introduction togeochemistry, New York, McGraw-Hill.

Lindal B. (1973) Industrial and other applications ofgeothermal energy, in: Armstead H.C.H. (edited by)Geothermal energy. Review of research and development,Paris, UNESCO, 135-148.

Lubimova E.A. (1969) Thermal history of the Earth, in: TheEarth’s crust and upper mantle. Structure, dynamicprocesses and their relation to deep-seated geologicalphenomena, Washington (D.C.), American GeophysicalUnion, 63-77.

Lund J.W. et al. (2005) Direct application of geothermalenergy: 2005 worldwide review, «Geothermics», 34, 691-727.

Lunis B., Breckenridge R. (1991) Environmentalconsiderations, in: Lienau P.J., Lunis B.C. (edited by)Geothermal direct use, engineering and design guidebook,Klamath Falls (OR), Geo-Heat Center, 437-445.

Nasini R. (1930) I soffioni e i lagoni della Toscana e l’industriaboracifera, Roma, Tipografia Editrice Italia.

Nicholson K. (1993) Geothermal fluids. Chemistry andexploration techniques, Berlin, Springer.

ORMAT (1989) Production of electrical energy from lowenthalpy geothermal resources by binary power plants,Roma, UNITAR/UNDP Centre on Small Energy Resources.

Parasnis D.S. (1997) Principles of applied geophysics, London,Chapman & Hall.

Pollack H.N. et al. (1993) Heat flow from the earth’s interior.Analysis of the global data set, «Reviews of Geophysics»,31, 267-280.

Press F., Siever R. (1997) Understanding Earth, New York,W.H. Freeman.

Proceedings of the first EEC/US workshop on geothermal hot-dry rock technology (1987), Brussels (Belgium), 28-30May 1986.

Sanner B. et al. (2003) Current status of ground source heatpumps and underground thermal energy storage in Europe,«Geothermics», 32, 579-588.

Sommaruga C., Zan L. (1995) Geothermal resources inrelation to plate tectonics. World exploration anddevelopment, San Donato Milanese, Aquater.

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Stefansson V. (2000) The renewability of geothermal energy,in: Proceedings of the World geothermal congress 2000,Kyushu (Japan), 28 May-10 June, CD-ROM.

White D.E. (1973) Characteristics of geothermal resources,in: Kruger P., Otte C. (edited by) Geothermal energy.Resources, production, stimulation, Palo Alto (CA), StanfordUniversity Press, 69-94.

Wright P.M. (1998) The sustainability of production fromgeothermal resources, «Geo-Heat Center QuarterlyBulletin», 19, 9-12.

Zan L. et al. (1990) Geothermal exploration in the Republicof Djibouti: thermal and geological data of the Hanle’andAsal areas, «Geothermics», 19, 561-582.

Mario FanelliPiero Manetti

Mary Hana DicksonConsiglio Nazionale delle RicercheIstituto di Geoscienze e Georisorse

Pisa, Italy

Leonardo ZanSnamprogetti

Fano, Pesaro e Urbino, Italy



6.4 geothermal energy - treccani geothermal energy is the heat contained within the earth that gives...

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