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Acknowledgments
Overtheyearsthefollowingassociationshaveprovideduswithan invaluableforumfortheinvestigationofgeothermaltopics:theStanford GeothermalWorkshop,oneofthegeothermalworld’slongest-running technicalworkshops;theGeothermalResearchCouncil(GRC);the InternationalGeothermalAssociation(IGA);andtheEuropean GeothermalEnergyCouncil(EGEC).Amongtheindividualswewould liketothankareProf.JohnLundoftheOregonGeoHeatCenter,who gaveusunstintingadviceandencouragement,andProf.RolandHorneof StanfordUniversity,whoshowedbyhispersonalexamplethehighlevel ofacademicrigorneededinthisfield.Inthesamelight,specialthanksare duetoProf.BurkhardSanneroftheEuropeanGeothermalEnergy Council.
WewouldalsoliketothankAndrasDianovszkyandMarkZsemkofor recreatingseveralimportantdiagramswhichhadgottenlostinthe shuffle.Lastbutnotleast,ourspecialthankstoDavidFenertyforhis editingandproofreadingsuggestions.
AnikoToth Miskolc,Hungary September,2016
1.1INTRODUCTION
Geothermalenergyisenergycontainedwithinthehightemperature massoftheEarth’scrust,mantle,andcore.SincetheEarth’sinterioris muchhotterthanitssurface,energyflowscontinuouslyfromthedeep, hotinterioruptothesurface.Thisistheso-calledterrestrialheat-flow.The temperatureoftheEarth’scrustincreaseswithdepthinaccordancewith Fourier’slawofheatconduction.Thustheenergycontentofaunitof massalsoincreaseswithdepth.
AlloftheEarth’scrustcontainsgeothermalenergy,butgeothermal energycanonlyberecoveredbymeansofasuitableenergy-bearing medium.Tobepractical,theenergy-bearingmediamustbe:hot enough(high-specificenergycontent),abundantenough,easilyrecoverable,inexpensive,manageable,andsafe.Watersatisfiesthese requirementsperfectly.Thespecificheatofwateris4.187kJ/kg C.Inthe steamphase,latentheatisaddedtoit.Hotwaterandsteamcanbe recoveredeasilythroughdeep,rotary-drilledwells.Throughtheuseofa suitabledesignedheatexchanger,heatcanbeefficientlytransferredfrom
thewaterorsteam.Steamisanespeciallysuitableworkingfluidforenergyconversioncycles.
Nowadays,geothermalenergyproductionismainlyachievedfromhot waterandsteamproductionviadeepboreholes.Anotherrapidlygrowingproductiontechnologyinvolvesexploitingtheenergycontent ofnear-surfaceregionsbyusingshallowboreholeheatexchangersand heatpumps.
Itislikelythatthenaturalheatofvolcanoesandothergeothermal sourceswerealreadybeingusedintheremotePaleolithicera,butconcrete evidenceonlydatesfrom8000to10,000yearsago.Wearethereforeforced touseindirectmethodswhenspeculatingonmankind’searliestrelationshipwithgeothermalphenomenaandproductsoftheEarth’sheat.
Theusesofnaturalhotwaterforbalneologyandtheexploitationof hydrothermalproductsforawiderangeofpracticalapplications increasedremarkablyduringthemillenniumprecedingtheChristianera. ThisuseeventuallyextendedtotheboundariesofancientRome, achievingmaximumuseduringthe3rdcenturyA.D.,theRoman Empire’sapex.AfterRome’sdeclineinthe6thcentury,geothermal exploitationalsodeclinedthroughoutSouthernEurope,aperiodof disusewhichlasteduntilthebeginningofthesecondmillennium.There isevidencethatgeothermalresourceswerestillbeingexploitedinthe centuriesthatfollowed,inChinaandmanyothercountries,butonavery limitedscaleandonlyinrudimentaryforms.
DeepintheRemontalouRivervalley,atthesouthedgeofAuvergnein theCentralFrenchmassif,thetownofChaudes Aigueshasan82 Chot spring,oneofthehottestnaturalthermo-mineralspringsinEurope.The regionhasbeeninhabitedsinceprehistorictimes.Themainspring,called lepar,isoneofabout30gushingspringsconcentratedinasmallarea. Frommid-OctobertotheendofApril,a5-kmnetworkofpipesbringsthe hotwaterfromfiveofthesespringstoheat150homes.Housesbuiltabove thespringsusethehotwaterdirectlybelowforheating,andhavedoneso sincethe14thcentury(Cataldietal.,1999).
GeothermalwaterwasfirstusedforboricacidproductioninLarderello, Italy,in1827.BoricacidproductionwasanItalianmonopolyinEurope, andbecamealarge-scaleindustryinthemiddleofthe19thcentury.
Othercountriesalsobegantodeveloptheirgeothermalresourcesonan industrialscale.V.Zsigmondy,forexample,becamealegendinHungariangeothermalhistoryafterhedrilledEurope’sdeepestwell(971m) inBudapestin1877.Sincethatdate,theresultinggeothermalwaterhas beenusedforbalneologyinthefamousSzechenyiSpa.In1892,thefirst geothermaldistrict-heatingsystembeganoperationsinBoise,Idaho, USA.In1928,Iceland,anotherpioneerintheutilizationofgeothermal energy,alsobeganexploitingitsgeothermalfluids(mainlyhotwaters)for domesticheatingpurposes.
knownastheMohorovicicdiscontinuity,wherethespeedofpropagation ofseismicwavessuddenlyincreasesfrom7km/sto8.1km/s.The Mohorovicicdiscontinuitycanbefoundbeneaththecrustandabovethe mantle.Themantleextendstoadepthof2900km,wherethereitchanges intothemuchdenserliquidcore.Thecoreiscomposedlargelyofmolten iron.Withinthisliquidcoreisasolidifiedironinnercorewitharadiusof about1350km.Onalargescale,thesearethemaincomponentsofthe Earth’sstructure,asshownin Fig.1.2
Fromthegeothermalpointofview,onlythecrustandtheuppermantle areofimportance.Directinformationaboutthemantleisavailablefrom deepboreholesonly.ThethreedeepestareinSakhalin,Qatar,andthe KolaPeninsula.Theyhaveabottomholedepthofabout12km.Allother dataderivefromindirectgravimetric,seismic,dipole-resistivity,and othergeophysicalinformation.
Thecrustisnotahomogeneoussphericalshell.Thecontinentalcrust beneaththecontinentsandtheenclosedseasismainlygranitecomposite, richinsilicawithadensityof2670kg/m3.Theoceaniccrustismainly basaltic.Itispoorinsilicawithadensity2950kg/m3.Thethicknessofthe continentalcrustisvariable.Beneaththehighrangesitcanbe70 75km thick,butbeneaththesinkingsedimentarybasinsitsthicknessisonly 20 25km.
Beneaththecrust,theuppermantleisrigid.Thisistheso-calledlithosphere.Itsthicknessisapproximately80 100km.Underthelithosphere,thepropagationspeedoftheseismicwavesdecreasesina
FIGURE1.2 StructureofEarth’sinterior.
Thedensitydifferencebetweenthemantlematerialandthemelted lithosphereplateissubstantiallygreater(600kg/m3)thanthedensity differencecausedbythethermalexpansionwhichis50kg/m3.Thusthe Archimedeanliftingforcecaninduceamoreintensiveupliftingflowin theregionofthedissolvedlithosphereplate.There-meltedintermediary andacidicmagmaisaccretedfrombelowtothecontinentalcrust.Thusit willberaised.Atthesametime,theextremelystrongconvectiveheatflowpropagatesfurtherinthesolidcrustasaveryintensiveconductive heatflux.Thustheorogenicareasaremoreactivegeothermally,andtheir terrestrialheat-flowissubstantial.Thesolidcrustmayevenbemelted here,andvolcanicareascandevelop.Thisoccurstypicallyalongtheplate boundariesofthePacificcoast.
Thereareotherregionsoutsidetheplateboundarieswherethe terrestrialheat-flowisanomalouslyhigh.SuchregionsaretheCarpathian Basin,theParisBasin,ortheKubanregionatthenorthernsideofthe CaucasusMountains.Thereasonforthehighheat-floworiginatesinthe thinningcontinentalcrustduetothetensionstressesandsubcrustal erosion.ThecrustintheCarpathianBasinmaybeasthinasonly20km. Thiswindowleadsnecessarilytothehigherterrestrialheat-flow.Itis obviousthatthethincrustenablesahigherheatfluxsince:
where d isthethicknessofthecrust,kistheaverageheatconductivity,Tm isthetemperatureatthetopofthemantle,andT0 isthesurfacetemperature.Thegeothermalgradientisalsohigher:
Thegeothermalgradient g intheCarpathianBasinishigherthanthe continentalmeanvaluewhichis0.045 0.065 C/m.Thusrelativelyhot rockmassescanbefoundinrelativelyshallowdepth.Thismeans favorablenaturalconditionstoaccessgeothermalreservoirs,torecover thegeothermalenergy.
1.3GEOTHERMALRESERVOIRS
Thegeothermalfieldisageographicalnotiondesignatinganyregion ontheEarth’ssurfacewheresuchsurfacemanifestationsasgeysers,fumaroles,orboilingmud-pondsindicateanactivegeothermaldomain underground.Thesephenomenaarecharacteristicofactivevolcanicregions.Wheregeothermalfieldsexistbuthavenospectacularsurface manifestations,highterrestrialheat-flowandabove-averagegeothermal 1.WHATISGEOTHERMALENERGY?
withageothermalgradientofatleast40 50 C/km.Itshouldbeobvious thatalongwithsatisfactoryrechargeofthereservoir,asufficientlyporous (20%)andpermeable(500 1000mD)formationisalsonecessary.In Europe,suchconditionsexistmainlyinsunkensedimentarybasinslike theCarpathianorParisbasins.
Inageothermalsystem,temperatureincreaseslinearlywithdepth. Porosity,however,decreasesexponentiallywithdepthassedimentunderneathiscompactedbythelithostaticpressureofsedimentaboveit:
Theexpression Eq.(1.3) approximatesthisdistributionquitewell,in which f0 istheporosityatthesurface,zisthedepth,andAdependson thetypeofthesediment.Thetendencyisshownin Fig.1.5.Theporosity andthetemperaturedistributionsdeterminetheregionwhererecoverablehotwaterreservoirscanbefound.
Thesemedium-temperaturehotwaterfieldsarequitecommonworldwide,andmaybeconfined,artesianaquifersoropen,hydrostaticsystems. Theirtemperatureistypicallylowerthan150 C.Theymaybeworthdirect useasdistrict-heating,agricultural,andindustrialprocessheatsources.
Theconceptualmodelofsuchamedium-temperaturehotwater reservoirisshownin Fig.1.6.Porousandpermeablesedimentarylayers aresettledmainlyhorizontallyontheimpermeablebedrock.Thereare hardlypermeablemainlyclayeylayersbetweenthepermeable
FIGURE1.5 Porosityandtemperaturedistributionalongdepth.
f
(1.3)
FIGURE1.7 Conceptualmodelofaconvectiveheatedreservoir.
gradientisanear-surface(<5km)freshmagmaticintrusion,witha temperatureof650 1000 C.Thisintensiveheatingproducesahigh (>1W/m2)heat-flow.Thereareadditionalnecessaryconditions.The permeabilityshouldbegreaterthan1darcy.Thismaypossibleinafractured carboniferousreservoirorincoarsedeltaicsediments.Asufficientlylarge verticalthicknessoftheaquiferisalsonecessary.Thusconvectiontransfers theheatalongalongerpath,moreefficiently.Thecold-waterrechargetothe deeperformationscanalsoincreasetheheattransferintensity.
Thetemperaturedistributionoftheconductiveheatedreservoirisnot linear.Insidetheaquifer,thetemperaturechangeisverysmall;thetemperaturegradientislargebetweenthetopofthereservoirandthesurface only.Thushightemperaturereservoirfluidcanbefoundatshallow depths,whichcanbeattainedbyshallow,largediameterboreholesmore economically.
Bothconvectiveandconductiveheatedreservoirscontainhotwaterin liquidphase.Thepressureofthewaterincreasesalongthedepthin accordanceofthelawofhydrostatics:
Thegeothermalfluidisobviouslyatitshottestinthedeepestpartofthe reservoir.Asitslowerdensitycausesittoriseup,andasitpassesthrough thefracturesorporechannels,itspressuredecreases.Meanwhile,a fractionofthesteamwillseparateoutofthewaterandflowupward,with theliquidflowingbackdown.Thisbringsthesteamtoalowerpressure region,whereitsexpansionisincreasedandtheprocessisintensified. McNitt(1977) explainedthisphenomenon.Hisbasicideawastocompare theamountofheatneededtoevaporateaunitmassofwaterwiththe amountofheatreleasedasthesteambubblecondenses.
Itisevidentthatthesteamphasebeginsassoonasthereservoir temperatureapproximatesthetemperaturefortheenthalpymaximumon thesaturationcurve.Thepressureshouldchangewithdepthhydrostatically,wherethetemperatureislessthan235 Clevel.Abovethatlevel, temperatureandpressureremainalmostconstantthroughoutthe dry-steamreservoir. Fig.1.11 showspressuredistributionwithincreasing
Entropy, s (kJ/kg K)
FIGURE1.10 Enthalpy entropydiagramofwater.
developtheHDRconcept.InHDRprojects,multi-fractured,interconnectingflowsystemsareproduced.Whereverylow-permeability formationsareenhancedbyhydraulicfracturingtocreateartificial reservoirs,theresultingsystemsareknownasEGS(enhancedgeothermal system).ThemostsuccessfulHDR/EGSprojecttodatewasimplemented inSoultzsousForets,France,wherea1.5MWpowerplantcurrently operates(see Fig.1.12).
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