geothermal energy utilization in low enthalpy sedimentary environments

Geothermal Energy Utilization in Low-Enthalpy Sedimentary Environments

Scientific Technical Report STR11/06

Ben Norden (ed.)

www.gfz-potsdam.deISSN 1610-0956

Imprint

Telegrafenberg D-14473 Potsdam

e-mail: [email protected]: http://www.gfz-potsdam.de

Printed in Potsdam, GermanyApril 2008

ISSN 1610-0956

This text is available in electronic form: http://www.gfz-potsdam.de/bib/zbstr.htm

B.

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Geothermal Energy Utilization in Low-Enthalpy Sedimentary Environments

Scientific Technical Report STR11/06

zur Erlangung des akademischen Grades

Ben Norden (ed.)

Impressum

Telegrafenberg

D-14473 PotsdamGedruckt in Potsdam

Mrz 2011ISSN 1610-0956

Die vorliegende Arbeitin der Schriftenreihe

Scientific Technical Report (STR) des GFZist in elektronischer Form erhltlich unterwww.gfz-potsdam.de - Neuestes - Neue

Publikationen des GFZ

GeothermalEnergyUtilizationinLowEnthalpySedimentaryEnvironments

EditedbyB. Norden

The authors are thankful for fruitful discussions and valuable remarks ofthe coworkers within the research project The geothermal energypotential inDenmark,especially forcommentsgivenbyAndersMathiesen(GEUS), Lars Henrik (GEUS), Troels Laier (GEUS), Niels Balling (AU), and JesperMagentgaard (DONG).TheDanish Council for Strategic Research isthanked forfinancingthisproject.

Scientific Technical Report STR 11/06 DOI: 10.2312/GFZ.b103-11066

Deutsches GeoForschungsZentrum GFZ

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Content

ListofFigures..................................................................................................................vi

ListofTables..................................................................................................................viii

BenNorden

Introduction.........................................................................................................................1

Whatisgeothermalenergyandwhatisthegeothermalpotential?..........................1

Howaregeothermalresourcesclassified?..................................................................1

Howcanthisenergyreservebeassessed?..................................................................1

StephanieFrick,StefanKranz,SimonaRegenspurg,andBenNorden

A.GeothermalApplicationsandPlantTechnologies..........................................................5

A.1Utilizationofgeothermalreservoirs.........................................................................7

A.1.1Heatsource.........................................................................................................7

A.1.2Thermalenergystorage......................................................................................7

A.2Energysupplyoptions...............................................................................................9

A.2.1Heatsupply.........................................................................................................9

A.2.2Chillsupply........................................................................................................11

A.2.3Powersupply....................................................................................................13

A.2.4Combinedenergysupply..................................................................................16

A.2.5Energynetworks...............................................................................................18

A.2.6Hybridsystems.................................................................................................19

A.3Plantdesign.............................................................................................................20

A.3.1GeneralAspects................................................................................................20

A.3.2Geothermalplantefficiency.............................................................................22

A.3.3Geochemicalaspects........................................................................................23

Fluidcompositionandclassification.................................................................23

Effectofthefluidpropertiesonplantoperation..............................................24

A.3.4Thermalwaterpumps......................................................................................26

A.3.5Heatexchangerssuppliedwiththermalwater................................................28



iv

BenNordenandAndreaFrster

B.ExplorationofGeothermalReservoirs..........................................................................31

B.1TheExplorationConcept.........................................................................................31

B.1.1BoreholeData...................................................................................................31

B1.2SurfaceGeophysicalSurveys............................................................................33

Electricalmethods..................................................................................................33

Seismicmethods....................................................................................................34

Summary................................................................................................................35

BenNordenandIngaMoeck

C.Drilling........................................................................................................................38

C.1.1Drillingrelatedequipmentandtechnologies..................................................38

DirectionalDrilling.............................................................................................41

Coring................................................................................................................42

Logging..............................................................................................................42

Wellcompletion................................................................................................43

C.1.2Geologicaltechnicaldrillingrisks....................................................................44

C.1.3Wellplanning...................................................................................................45

HorizontalWellsandMultiwellsettings..........................................................46

BenNorden,IngaMoeck,M.GuidoBlcher,andGnterZimmermann

D.ManagementofGeothermalSystems..........................................................................47

D.1Reservoiranalysis....................................................................................................47

D.1.1GeologicalModels(staticmodelling)...............................................................48

D.1.2Welltestingandreservoirevaluation..............................................................49

Skinfactor..........................................................................................................52

D.1.3Simulations(dynamicmodelling).....................................................................55

D.2Operationstooptimizethegeothermalsystem.....................................................59

D.2.1Enhanceproductivity........................................................................................59

Chemicaltreatments.........................................................................................59

Thermaltreatments..........................................................................................63



vHydraulicfracturing...........................................................................................63

Treatmentmonitoringandtreatmentevaluation............................................65

BenNordenandStephanieFrick

D.3Regulations,economics,riskassessment,andinsurancesofgeothermalprojects66

D.3.1Economicalaspectsofgeothermalprojects....................................................67

D.3.2Riskassessment,decisionmaking,andinsurances..........................................68

BenNordenandErnstHuenges

E.Outlook..........................................................................................................................71

BenNorden

F.GeothermalSitesandApplications...............................................................................72

F.1HeatPumps..............................................................................................................72

F.2AquiferThermalEnergyStorage:Reichstag(Berlin,Germany)..............................74

F.3HeatandPowerSupply............................................................................................75

F.3.1OregonInstituteofTechnology(USA)..............................................................76

F.3.2GeothermalPlantsinDenmark.........................................................................76

F.3.3HeatandPowerGeneration:GeothermalPlantNeustadtGlewe,Germany..78

F.3.4HeatandPowerGeneration:Unterhaching,Bavaria,Germany......................79

F.3.5GeothermalLaboratoryGroSchnebeck.......................................................80

F.4Greenhouseheating................................................................................................81

F.4.1DrentheStudy,Netherlands.............................................................................83

F.5OtherUses...............................................................................................................83

G.References....................................................................................................................86

H.Index..............................................................................................................................99



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ListofFiguresFigure1:Energysupplyoptionswithdifferentplanttechnologiesfromheatsourceswithtemperaturesbetween20and200CderivedfromLindal(1973)....................................5Figure2: Principle for using a geothermal reservoir as heat source showing a welldoublet,thegeothermalfluidloopandaheatexchangeronthesurface.........................7Figure3:SchemeofanATESdoubletandnotation...........................................................8Figure4: Scheme of awell doubletwith backup/peakload system feeding a districtheatinggrid.......................................................................................................................10Figure5:Annualheat loaddurationcurvesandcorrespondinggeothermalplantdesign...........................................................................................................................................11Figure6:Principleofanabsorptionrefrigerationcycle....................................................12Figure7: Temperatureentropydiagram (Ts diagram) of an example working fluidshowinggeneralnotations(left)andacorrespondingRankinecyclewithsaturatedvapor(right).................................................................................................................................13Figure8: SchematicsetupofRankinecycleusinggeothermalheatasheatsource....14Figure9: Temperatureentropy diagram showing the saturation vapour curves ofdifferentworkingfluids.....................................................................................................15Figure10: Ts diagram showing the correlation of evaporation temperature andgeothermaloutlettemperature(left)anddevelopmentofpoweroutputasafunctionoftheevaporationtemperature(right)................................................................................16Figure11: Schematic setup of a plantwith conventional cogeneration of power andheat....................................................................................................................................17Figure12:Schematicsetupofageothermalplantwithpowerandheatsupplyinserialconnection.........................................................................................................................18Figure13: TemperatureEntropy Diagrams of a potential hybrid power plant. Therelative input of geothermal energy increases from the concept at the top to theconceptatthebottom......................................................................................................20Figure14: Depth distribution of main elements of sedimentary basin fluids plottedversusdepth. They represent 78 to 98wt.% of total fluid salt content (Kharaka andHanor,2004)(left).ModifiedfigureafterRegenspurgetal.,2009.Depthdistributionofdissolvedsolids(logTDS);datafromsedimentarybasins(blackdiamonds;Frsteretal.,2006;Hanoretal.,2004;Gieseetal.,2002;Wolfgrammetal.;2007)andcrystallinerockformations(whitesquares;Frapeetal.,2004)(right)......................................................24Figure15:Schemeofaproductionwellshowingdownholepump,staticfluid levelanddynamicfluidlevel(left)andthepressurecurveduringfluidproduction(right)............27Figure16: Qualitative development of fluid production effort as a function ofgeothermalfluidflowfordifferentreservoirproductivities............................................28Figure17: Illustration of a shellandtube heat exchanger (left) and a plate heatexchanger(right)...............................................................................................................30Figure18:Rotarydrilling.a)RotaryTable,b)TopDrive(PhotobyB.Norden)...............39



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Figure19:BlowoutpreventerunitofadrillriginKetzin(PhotobyB.Norden)...............40Figure20:Drillbits(PDCbit,rollerbit,anddifferentcorebitunits(PhotosbyB.Norden)...........................................................................................................................................41Figure21:Reservoirmanagementdiagram(Ungemachetal.,2005)..............................48Figure22:Geologicalmodelsonbasin,reservoir,andboreholescale(GFZPotsdam)....49Figure23:InterpretationofawelltestdatasetwiththeTheismethod.(a)semilogplotof drawdown and derivative; (b) superposition of the data with the model afterautomaticfitting(Renard,2005).......................................................................................51Figure 24: Sketch of the damaged zone and the pressure profile (nearwellbore skineffect),afterAhmed&McKinney(2005)..........................................................................53Figure25:Improvedandreducednearwellborepermeability(positiveandnegativeskineffects).Darcyslawequationwiththeskinterms(afterAhmed&McKinney,2005)53Figure26:Semilogplotofdrawdowndata(afterAhmed&McKinney,2005)................54Figure27:Exampleofreservoirsimulationmodelsatporescaleandatreservoirscale,linkedby(static)lithofaciesmodelandthegeomodel(afterRingroseetal.,2008).....55Figure 28: Cumulated discounted cash flow and net present value (after Frick et al.,2010)..................................................................................................................................69Figure 29: Geothermal heat pump installation systems (Reference:www.architecture2030.org,19052010).........................................................................73Figure30:Schemesofsummer (top)andwinteroperation (bottom)of thedeepATESsystemoftheReichstag,Berlin.........................................................................................75Figure31:GeothermalplantinThisted(Mahler&Magtengaard,2010).........................77Figure32:Amagergeothermalplant,Copenhagen(Mahler&Magtengaard,2010)......77Figure33:CombinedheatandpowersupplyinNeustadtGlewe(in:Lund,2005)..........79Figure34:SchemeoftheUnterhachinggeothermalplant..............................................80Figure35:Exampleofagreenhouseheatingsystem(modifiedfromRafferty,1998).....82Figure36:Sketchofagreenhouseusing solarheatandunderground thermal storage(Kleinwchteretal.,2006)................................................................................................83



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ListofTablesTable1:Geophysicalexplorationtechniques(modifiedafterTaylor,2007)...................37Table2:Drillingrisksrelatedtogeology(Sperberetal.,2010)........................................46Table3:Typesofwelltestsandkindof informationwhichcouldbeobtained (Renard,2005;Ahmed&McKinney,2005).....................................................................................52Table4:ConceptualmodelsforfracturedrockafterKolditz(1997)................................56Table5:Reservoirparameters(afterVBI,2010)...............................................................57Table6:Stepsinsandstoneacidizing(Portieretal.,2007)..............................................61Table 7: Acid guidelines for the chemical treatment of sandstones according to thecompositionoftheformation(Croweetal.,1992)..........................................................62



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1BenNorden

Introduction

Thegeothermalenergymarketisastronglygrowingsectorworldwide.Asstatedinareviewonthedirectgeothermalenergyuse(Lund&Freeston,2010),the installedthermalpowerfor direct utilization of geothermal energy is estimated at the end of 2009 to amount50,583MW,almosta79% increasedoverdataof2005,growingwithacompoundrateof12.3%annually.Thisgrowth isstrongly linked to thegrowingawarenessandpopularityofgeothermal (groundsource) heat pumps, which could utilize groundwater or groundcoupled temperatures anywhere in the world. The increase in the field of geothermalelectricity generation from 2005 to 2010 is in the range of 20 %, showing an installedcapacityof10,715MWin2010worldwide(Bertani,2010).

Whatisgeothermalenergyandwhatisthegeothermalpotential?Geothermalenergybecomesobviouswhenvolcanoesareactiveorhot springsandotherthermalphenomenaarepresentatthesurface.ThesephenomenaarenormallyrelatedtogeologicallyactivepartsoftheEarth,likecontinental(plate)margins.Theyarelikeawindowtowardsdeeperpartsoftheearth,showingusthattheinterioroftheEarthispartlyhot.Insedimentary basins, the temperature ismore or less constant near the surface reflectingmoreorlesstheannualmeannearsurfacetemperatures.Belowthatzone,thetemperaturemay be variable because of geological structure and different thermal conductivity ofgeological formations or because of groundwater circulation masking the temperaturedistributioncausedbyheatdiffusionfromtheEarthinterior.Ingeneral,thetemperatureinthe Earths crust increaseswith depth.Down to depths of about 10,000m, the averagegeothermalgradientisabout23C/100m(Press&Siever,1984);resultingintemperaturesaround100Cat3kmdepth.

Howaregeothermalresourcesclassified?Geothermal resources are classified based on their reservoir temperatures alone (e.g.Muffler&Cataldi,1978;Hochstein,1990;Benderitter&Cormy,1990;Haeneletal.,1988)orwith reference to their specific exergy index to reflect their ability to do thermodynamicwork(Lee,2001).Inthisreport,theclassicapproachrelatedtothereservoirtemperatureisconsidered. According to Haenel et al. (1988), a lowenthalpy resource corresponds toreservoir temperature of less than 150 C. Highenthalpy resources are present if thetemperatureexceeds150C(seealsoChandrasekharam&Bundschuh,2008).

Howcanthisenergyreservebeassessed?The term geothermal energy is understood not only as the heat containedwithin theEarth.GeothermalenergyismoreoftenusedtoindicatethatpartoftheEarthsheatthatcan, or could, be recovered and exploited byman (Dickson& Fanelli, 2004). In general,geothermalenergy isextracted fromgeothermalreservoirs.AccordingtotheEncyclopedia



2

ofPhysicalScienceandTechnology(Meyers,1992),geothermalreservoirsaredefinedasageometrically definable volume of permeable rock which contains a proven reserve ofthermalenergy,suchaswaterorsteamthatcanbeextractedinapractical,economicway.Inotherwords,ageothermalreservoircouldbeconsideredageothermalsystemconsistingofthreemainelements:aheatsource,areservoir,andafluid.Thefluid isthecarrierthattransferstheheat (Dickson&Fanelli,2004).Geothermalenergy,madeassessablebyman,can be used for different purposes like district or local heating, chill provision, or powergeneration (section A. Geothermal Applications and Plant Technologies, p. 5). Themainadvantage of using geothermal energy for direct use in the low to intermediatetemperature range is that these resources aremore widespread and exist at economicdrillingdepths (Lund, 2007). In addition,partsof a geothermal system couldbeused forexampleasanundergroundthermalenergystoragesystem(Andersson,2007).

The general strategy of geothermal resource development (section B. Exploration ofGeothermal Reservoirs, p. 31) startswith the development of a conceptualmodel of thepotentialgeothermalresource.Atthisearlystage,alsosecureleasesandpermitsplayarole.Thereviewofexistingdata,especiallyongeology(structuralgeology,stressfield,hydraulictransportproperties,(thermal)rockproperties,petrographyandmineralogy),temperature,heat flow,andgeochemistry form thebase to identify thepotentialgeothermal reservoirandtoestimatethesizeandtheheatcontentofit.Thesestepsarepartofthegeothermalexploration(sectionB.1TheExplorationConcept,p.31)resultinginmappingandintegratedmodelling based on the available data. Depending on the exploration targets, differentgeophysical or geochemical exploration surveys may become relevant. Those newexploration data should be integrated in geological site models, on which the wellpositioningandthewellpathplanningforgeothermalwells,theriskassessmentconcerningtheboreholestability,andthesimulationofthereservoirbehaviour (duringproduction) isbased.Thedrillingofnewwells,providing further information to the localconditionsandthustothemodels,shouldbeperformedaccordingtotherequirementsofgeothermalwellsandthesitespecificconditions (sectionC.Drilling,p.38).The laststeps intheassessmentanddevelopmentofageothermalresourcerequiretheinstallationandmanagementofthewhole geothermal system including the geothermal plant technology (section D.ManagementofGeothermalSystems,p.47).Modelsandcomputer simulations topredictthe reservoir behaviour are important tools for these objectives andmay be used for asustainable, reliable, and efficient energy provision, for the planning of further reservoirtreatments,andfortheconsiderationofenvironmentalaspects.

Theevaluationofthefinancialaspectsforthedevelopmentofageothermalsystemdependsstronglyonapropersitecharacterization,which inturnformsthebasisfortheoverallsitedevelopment. If the intended extraction of the thermal energy of the reservoir could beutilizedeconomicallydependsnotonlyonthepropertiesofthereservoir;other importantfactors are the legal and political framework (the outer conditions), the drilling andexploitationtechniqueofthereservoir,thereservoirmanagement,asmartcombinationofavailable(surface)technologiesandthelifetimeoftheinstallations.Inthisreport,wefocus



3onthetechnologicalaspectsofgeothermalenergyutilization.Nevertheless,wecannotomittheouterconditionsastheyplayanimportantroleinthedecisionprocessoftheplanninganderectionofgeothermalfacilities.ThesetopicsareaddressedinsectionD.3Regulations,economics,riskassessment,andinsurancesofgeothermalprojects,p.66.

Finally, several tasks related to the utilization of geothermal energy in lowenthalpysedimentary settings can be identified (section D. Outlook, p. 71). For example, theexplorationtechniquesoftheoilandgas industryare lesseffective ingeo(hydro)thermalenvironments. In contrast to thehydrocarbon industry,one successful geothermalwell isoftennotenoughtoensureeconomicprovisionofgeothermalenergy.Therefore,thecostsofthereservoirexploitation (especiallywhendeeperwellshavetobedrilled)needstobereduced.Inaddition,several(economical)uncertaintiesdoinfluencethegeothermalprojectdecisionsveryindividually:thepriceofenergy(electricityorheat)thatcanbesoldmayvaryduring the time of project; the available transmission for the energy producedmay beunanswered; and the years to bring the resource into production even in the best ofcircumstancesmaybehardlyaddressed(Taylor,2007).Theplanttechnologiesmayfurtherdevelop and thus may give power generation under lowenthalpy conditions morerelevance.Someexamplesofdifferentgeothermalapplicationsinlowenthalpysedimentaryenvironmentsandtheirhistoricaldevelopmentaregiven inthesectionF.GeothermalSitesandApplications(p.72).

Thepresentworktriestocondensethestateoftheartfortheexplorationandexploitationof geothermal energy. Although a lot of experiences from different sitesworldwide areintegrated inthisstudy,somesectionsdoreflecttheGermanexperience.Forexample,theGermanRenewableEnergySourcesAct(EEG,seepage78)enablesalsoaspecificpromotionandgrowthofthegeothermalsectorinGermanybyguaranteeingspecialfeedintariffsfora20yearsperiod.Duringthe lastyears,severalcountriesworldwidehave introducedsimilarrenewable energy promotion policies (someof them restricted to certain energy sourcesonly or requiring a certain renewable energy quote), supporting investigations also ingeothermal applications. Thus, the experiences in Germany may be of some value.Dependent on the politicaleconomical framework, the geothermal exploration andexploitation of deeper reservoirs represents an excellent option to extend geothermalapplicationsalsoinareaswhereshallowgeothermalreservoirsmaynotbeabletomatchthelocalenergydemand.Therefore,alsochallengesrelatedtotheexplorationandexploitationofdeeplowpermeabilityreservoirsareincludedinthisstudy.



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StephanieFrick,StefanKranz,SimonaRegenspurg,andBenNorden

A.GeothermalApplicationsandPlantTechnologies

Inthisstudy,geothermalapplicationsrefertoplantsinwhichaheatcarrierispumpedfromthe geothermal reservoir to the surface.Plant setups, inwhich theheat carrierhasonlyindirectcontacttothereservoir,suchasboreholeheatexchangers,willnotbeaddressedindetail.

Heatthatisextractedfromgeothermalreservoirscanbeusedtosupply:

heatand/or chilland/or power

5dependingon the temperatureof theheatcarrierand the implementedplant technology(Figure1).Forthenonelectricaluseofgeothermalenergy,themainapplicationsareinthefieldofgeothermalheatpumps,balneology,spaceheating,greenhouseheating, industrialprocessheating,andaquaculture(Lund&Freeston,2010).

Figure1: Energy supply options with different plant technologies from heat sources with temperaturesbetween20and200CderivedfromLindal(1973)



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Geothermalreservoirscanbeusedas

heatsourceor thermalenergystorage.

Iftheextractedheatisgeothermalheatthatisnaturallyavailableinthereservoir,then,thegeothermal reservoir is used as heat source. The extracted heat, however, can also besurplusheatthatcanbestoredintheunderground.Inthiscase,thegeothermalreservoirisusedasthermalenergystorage.

Geothermalreservoirscanbeutilizedby:

asingleplantonthesurface(e.g.geothermalbinarypowerplants), a plant in a compound of different energy suppliers (e.g. geothermal heat plants

supplyingheatingsystemstogetherwithsolarandfossilsupplytechnologies),or

ahybridplantthatintegratesotherenergysourcesintheplantconcept(e.g.binarypower plants that are fed with geothermal heat and heat from a biomasscombustion).

These different utilization and supply options result in a variety of different potentialgeothermalplantconcepts.Forthedevelopmentofgeothermalprojects,thecharacteristicsofboth,thereservoiraswellastheparticularsupplyoptions,mustbeknown.Thenecessaryplant technology for geothermal applications is generally available on themarket. Sincemany components arepredominantlyused inother fieldsof application, their adaptationand thorough selection according to the specific preconditions of a project site are veryimportant.Cascadingandcombinedusesdoenhancethefeasibilityofageothermalproject(Gudmundsson, 1988; Lund et al., 2005). For example, lowtomoderate temperaturegeothermal resources can be used in combined heat andpower plants (CHP),where hotwaterswith temperatures as low as100C are first run through abinary (OrganicRankinCycle) power plant and then cascaded for space, swimming pool, greenhouse andaquaculturepondheating,beforebeinginjectedbackintotheaquifer(Lundetal.,2005).

Thefollowingsectionswillgiveageneraloverviewonthesedifferentutilizationandsupplyoptions of geothermal reservoirs and the most important aspects of geothermal plantdesign.



A.1Utilizationofgeothermalreservoirs

A.1.1HeatsourceWhen using geothermal reservoirs as heat source, the heat carrier is pumped from thereservoir through theproductionwell to the surface.Theheat isextracted from the fluidwithaheatexchangerandthecooledfluid isreinjected intothereservoir inthe injectionwell (Figure2). The focus of this study is on typical geothermal applications that useformation water as heat carrier. In research, however, also other plant concepts arediscussedandmightberealisedata fewsites inthe future.Such futurepossibleconceptsrefertogeothermalplantsusingotherheatcarriersthanformationwatersuchasCO2(c.f.Pruess & Spycher, 2007). Kolditz et al. (2010) calculated the relative efficiency of heatextraction (CO2 vs. H2O) depending on fluid pressure and temperature. For reservoirconditionsfoundunderatypicalgeothermalgradientof30Ckm1theresultsshowthatata depth of 800 m to approximately 4 km, CO2 will bemore efficient than H2O. Thesecalculationsdo,however,notconsidertheefficiencyofthewholegeothermalprocesschain.

7The extracted heat can be used to supply heat and/or chill and/or power. DifferentexamplesforplantsusingthereservoirasheatsourcearegiveninsectionA.2Energysupplyoptions,p.9.

Figure2:Principleforusingageothermalreservoirasheatsourceshowingawelldoublet,thegeothermalfluidloopandaheatexchangeronthesurface

A.1.2ThermalenergystorageInAquiferThermalEnergy Storage (ATES) systems,waterbearing sandstones areused asstoragemediumforthermalenergyresultingfromcombinedheatandpowerplants(CHP),



solar energy or industrialwaste heat. The groundwater serves likewise as heat transfermediumandstoragemedium.CommonATESsystemsconsistoftwowellgroupsoratleasttwowells(welldoublet),coldwellsandwarmwells(Figure3).This indicationreferstothetemperaturelevelintheaquiferused.Duetothelargestoragecapacityofwaterbearingsandstones, ATES systems are used to store thermal energy seasonally, mainly for theprovisionofroomheatandcoldinbuildings.WhenchargingtheATESsystemwithheat,thegroundwater is produced from the cold well, heated at the surface by means of heatexchangersandinjectedintotheaquiferviathewarmwell.Duringdischargingtheoperationis reversedand the stillheated groundwater isproduced from thewarmwell, cooledoffwhilepassingthroughheatexchangersandinjectedagainintotheaquiferviathecoldwell.

8

Figure3:SchemeofanATESdoubletandnotation

ManyoftheATESsystemsthataredesignedforheatingpurposesarelinkedtoheatpumps.Butalsodirectuseof the stored thermalenergy ispossible if theextractedgroundwaterprovidessufficientlyhightemperatures.

The storage through borehole heat exchangers (BTES, Borehole Thermal Energy Storage)systemconsistsofanumberofcloselyspacedboreholes,50200mdeep.Theseareservingasheatexchangerstotheunderground.Thestoringprocessismainlyconductive.

Underground Thermal Energy Storage (UTES) systems are typically applied for combinedheating and cooling, normally supportedwith heat pumps for a better usage of the lowtemperatureheatfromthestorage(Andersson,2007).Ingeneral,UTESsystemsofferalargeprimaryenergysavingpotentialforprovidingheatandcold.Inordertotaptheirfullenergy



9savingpotential, anefficient integrationofUTES in energyprovision systems isessential.More information on ATES is provided by Paksoy (2008), Dincer & Rosen (2002) andSchaetzleetal.(1980).

A.2EnergysupplyoptionsGeothermalenergycanbeused to supplydifferent formsofenergy. Inorder to comparedifferentsupplyoptionsforonesite,itneedstobeconsideredthatnotonlytheconversionefficiency of a particular supply option is important, but also the amount of geothermalenergy that is actually being used and the auxiliary power demand. Comparing differentgeothermalsites, itmustbeconsidered thatqualityof theheatsourcevaries.Thequalitytherebydecreaseswithdecreasingheatsourcetemperature.Sincepowerforexamplehasahigherqualitythanheat,itcanonlybegenerated(consideringtheambientconditions)fromheatsourceswithhighenoughtemperatures.Otherissuesrefertogeochemicalaspectsandthehydraulicproductivityofageothermal reservoir. InFricketal. (2010)amoredetaileddiscussionontheevaluationandcomparisonofgeothermalplantsispresented.

A.2.1HeatsupplyGeothermalreservoirscanbeexploited inordertodirectlysupplyheat,suchasfordistrictheatinggridsorprocessheatapplications.Ifthetemperatureofthefluidfromthereservoiris not sufficient to meet the required supply temperatures, the temperature can beincreasedbyheatpumpsorconventionalboilers,whichhowever,needextraenergy.

The (direct) supply of heat from a geothermal reservoir is for example realised bytransferringthegeothermalheattoawateroperatedheatingnetworkoraheatcarrierusedin process heat applications. In contrast to electrical power grids, which are widelydeveloped networks inmany countries due to the good physical transport properties ofelectricity, heating networks refer to local supply structures. Typical district heatingnetworksarefedbyasmallnumberofheatingstations,whichare locatedpreferablyclosetotheheatcustomersinordertoavoidlargeenergylosseswhentransportingtheheat.Thedesign of geothermal plants for direct heat use does therefore inmost cases not onlydepend on the sitespecific reservoir conditions but also on the structure of the heatcustomers within a heating network. The characteristics of heat consumption and theirinfluenceonplantdesignareoutlinedinthefollowing.

AschematicsetupofageothermalheatplantfeedingadistrictheatingsystemisshowninFigure4.Atsomesitesheatplantsmustalsoforeseeabackupsystem(forexampleincasethegeothermalplantistheonlyheatproviderofthedistrictheatingsystem)orapeakloadsystem(incasethegeothermalheatplantisdesignedtocoverthebaseloaddemand).Theheatdemandofaheatingnetworkisdefinedbyitssupplyandreturntemperatureandthedemandedheatcapacitywhich is thesumof thesimultaneousdemandsof theconnectedheatcustomers.Thetemperaturelevelofasingleheatcustomerdependsontheuseoftheheat,suchasforspaceheating,warmwaterproductionoralsoindustrial,e.g.Lindal(1973).



Figure4:Schemeofawelldoubletwithbackup/peakloadsystemfeedingadistrictheatinggrid

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Existing geothermal heat plants typically feed district heating networkswhich aremainlyusedforspaceheating.Spaceheatingrepresentsabout40%oftheworldwideheatdemand(OECD/IEA, 2008). The supply of district heat is characterised by supply temperaturesbetween 50 and 90C and return temperatures between 30 and 70C, sometimes evenlower. Incontrasttoanannuallyconstantheatdemand formany industrialprocesses,thedemand for space heating is characterised by a variable heat demand during the year.Regarding theheatdemandcharacteristics, itmustbeconsidered thatageothermalheatsource is not optimally utilized because with a variable heat demand the continuouslyavailableheatsourcecanonlypartlybeusedovertheyear. Ifthereturntemperatureofadistrictheatingsystemisabovethetemperature,downtowhichthegeothermalheatcanbeused, theheat source isalsonotutilized to its fullextent.Basedon theseconsiderations,differentdesignaspectsofgeothermalplantsarediscussedinthefollowing.

Referringtoagivenheatdemand,geothermalplantdesign,includingboreholeandreservoirstimulationconceptanddimensioningoftheheatexchangerandothersurfaceinstallations,aimstofullycoverthisheatdemandbygeothermalenergy.Inthecasethatthegivenheatdemandvariesduring theyear, theenergeticpotentialof thegeothermalreservoir isonlyused toa smallpart.As shown inFigure5 (left), thegeothermal full loadhoursof suchaplantare low.Anotherpossibility istodesignthegeothermaldrivenpartoftheheatplantfor a part of the heat supply which can, for example, be the base load heat demand(Figure5,right).Forthehoursoftheyear,wherethisbaseloaddemandisexceeded,apeakloadsystemsuchasanotherrenewableheattechnologyoraconventionalboilerisinstalled.Thisway, thegeothermal full loadhourscanbe increasedand theuseof thegeothermalresourcecansignificantlybeimproved.



Figure5:Annualheatloaddurationcurvesandcorrespondinggeothermalplantdesign

11

Another approach to geothermal plant design is to adapt the heat demand side to thecontinuous characteristicsof thegeothermal resource. In the summermonths,where thedemand for space heat is lower, the geothermal heat can, for example, be used for thesupplyofchill.Thiscanbedonebydirectlysupplyingchill(whichrequiresacoolinggrid)orconnectingconsumerswhichusetheheatfordecentralisedchillprovision.Presently,amorecommonway is touse the geothermalheat,availableoff theheatingperiod, toproducepower.

Besides the designof theheat consumer structure in termsofamore continuousheatdemand throughout the year, also the cooling of the geothermal fluid is an importantaspect.Bycascadingheatconsumerswithdifferenttemperaturelevels,theutilizationofthegeothermalresourcecanbeoptimized.

Moreexamples for theheatsupply fromgeothermalreservoirsaregivene.g. inGeoHeatCenterOregon (2005)1 or can be found on theGeothermal Conference PapersDatabasefromtheInternationalGeothermalAssociation(IGA)2.

A.2.2ChillsupplyBesides theprovisionofheat forspaceheatingor industrialprocesses,geothermalenergycanalsobeused for theprovisionof low temperatureheatneeded for refrigeration.TheheatdrivenabsorptionrefrigerationcycleappliedinARSs(absorptionrefrigerationsystems)is themostapplicable for thispurpose. Ingeneral,alsovapourcompressionrefrigeration,which is themostcommonlyusedrefrigerationprinciple,oradsorptionrefrigerationexist.TheprincipleschemeofanARSisshowninFigure6.AtypicalARSusesamixtureconsistingof a refrigerant and an absorbent as working fluid and consists of an evaporator, a

1http://geoheat.oit.edu/pdf/tp115.pdf2http://geothermal.stanford.edu/standard/



condenser, a generator, an absorber, a solution heat exchanger, a solution pump andthrottlingvalves.

Asmostrefrigerationcycles,absorptionrefrigerationisbasedonevaporatingtherefrigerantat low temperature andpressure, compressing the vapour, and condensing it at ahighertemperatureandpressure level.Duringevaporation,therefrigerantabsorbstheheatfromtheheatsourcethathastobechilled.Duringcondensationthisheatistransferredtoaheatsinkatahighertemperaturelevel,suchasawaterorairdrivenrecoolingsystem.

Evaporator, condenser and throttling valve are similar to the ones used in vapourcompressionrefrigeration systems.Due to theuseofamixture insteadofapure fluidasworking fluid,ARSs contain a thermal instead ofmechanical compressor. Such a thermalcompressor is comprisedof threemainelements:absorber, solutionpumpandgenerator(Figure6).Inordertoobtainahigherefficiencyasolutionheatexchangerisusedforenergyrecuperation.

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Figure6:Principleofanabsorptionrefrigerationcycle

InthefollowingparagraphtheprincipleofcycleoperationisexplainedusingtheexampleofanammoniawaterASR.Theammonia (refrigerant)vapourexits theevaporator (Figure6,step 1) and is absorbed by the water (absorbent) inside the absorber at low pressure(approx.2.5bar)andrelativelylowtemperature.TheresultingheatofabsorptionQ hastoberemovedfromthecycle.Theliquidsolution(2)containsarelativelyhighconcentrationofwater. The solutionpasses the solutionpump (23)which raises thepressureup to thepressure level (approx.10bars)of thegeneratorand the condenser. Inside thegenerator(34) the ammonia is driven off thewater bymeans of heat inputQ , such as from ageothermalfluid.Afterwardstheammoniavapour (5)entersthecondenserandthewater(8),which contains a low concentration of ammonia, passes the solution heat exchanger(89),expandstothe lowpressure level insidethethrottlingvalve(910)andentersthe



absorber.Thesolutionheatexchangerreducesboththeheat,neededinthegeneratorandtheonethathastoberemovedfromthecycle.Insidethecondensertheammoniaiscooleduntilitcondenses.TheheatofcondensationQ hastoberemovedaswell.Afterleavingthecondenser (6) the liquidammonia isexpandedby the throttle to the lowerpressure level(67)oftheevaporation. Intheevaporatortheammoniaevaporatescompletelyata lowtemperature and pressure level. The heat needed for evaporation corresponds to therefrigerationcapacityoftheARS.Afterevaporationtheammoniavapour(1)isredirectedtotheabsorber.

More informationongeothermaldrivenARSscanbefound inLund(1998).Sincenetworkssupplyingchillonly scarcelyexistat themoment,geothermaldrivenARSshaveonlybeeninstalledinsmallscaleapplications.

A.2.3PowersupplyThe principle of converting heat into power bymeans of an evaporableworking fluid isreferred to as Rankine cycles. Themost commonworking fluid iswater, but also otherworking fluids exist.Due to the variety of evaporable fluids, Rankine cycles are versatileapplicable, also for low temperature heat sources.Most other types of thermodynamiccyclesusedforlowtemperatureheatareessentiallymodificationsofit(e.g.OrganicRankineCycleorKalina;Kalina,1983).Thebasic formof theRankinecycle is shown inFigure7. Itconsistsofthefollowingchangesofstate:

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- pressureincreaseoftheliquidworkingfluidwithafeedpump(12),- heatingofthepressurizedliquiduptotheevaporationtemperature(23),- evaporationoftheliquidatconstanttemperatureandpressure(34),- expansionofthevapor inanexpansionmachine(45)whichproducesmechanical

work,and- condensingtheexpandedvaporatconstantpressureandtemperature(51).

-

Figure7:Temperatureentropydiagram(Tsdiagram)ofanexampleworkingfluidshowinggeneralnotations(left)andacorrespondingRankinecyclewithsaturatedvapor(right)



Fortheconversionofheatfromgeothermalreservoirsintopower,theheatistransferredtotheworkingfluidcontainedinasocalledbinarypowerunit.Asshownintheschematicsetup of a geothermal driven binary cycle in Figure8, this working fluid is preheated andevaporated. The generated vapour is expanded and the released enthalpy is used by anexpansionmachine. Inmostapplications turbinesareusedasexpansionmachinesdue totheirwideapplicationrangeandwidespreadavailability.Forsmallpowerratingalsoscreworpistonexpanders are available.Theexpansionmachine is connected to a generator inordertoconvertthemechanicalworkintoelectricity.Theexhaustvapourfromtheturbineiscondensed.The cooledworking fluid ispressurised in the feedpumpand recycled to thepreheater.Thepower that isproduced in abinary conversion cycledependson theheatsourceand,onthebinaryside,oncomponentspecificparameters(suchasefficiencyoftheturbine,generatorandfeedpump,temperaturedifferences inheatexchangers)andbinarycycledesign.

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Figure8:SchematicsetupofRankinecycleusinggeothermalheatasheatsource

Existing binary conversion cycles generally use differentworking fluids and cycle layouts(such as cycles with superheating, supercritical cycles, cycles with internal heatrecuperation,cycleswithdifferentevaporationstagesandworkingfluidmixtures).

A general characterisation of differentworking fluids can be expressed according to thesaturationvapourcurveinthetemperatureentropydiagram(Figure9).Awetfluidshowsanegative slopeof the saturation vapour curve.Whenexpanding saturated vapour inanexpansionmachinethestateobtainedisalwayslocatedinthetwophaseregion.Waterandammonia, forexample,are typical wet fluids.A dry fluid, suchas isobutane, shows apositiveslopeofthesaturatedvapourcurve.Afterexpandingsaturatedvapour fromadryfluid, its state is always located in the superheated region. If the saturated vapour curvedoesnotshowanyslope,the fluid iscalledisentropic.Awiderangeofpossibleworking



fluidsareinvestigatedinSalehetal.(2007)withregardtotheachievablecycleefficiencyandconsideringaminimumandmaximumcycletemperatureof100Cand300C,respectively.Salehetal.therebyconsidersubcriticalaswellassupercriticalcycledesigns.

Referringtogeothermalresources,theOrganicRankineCycle(ORC),whichusesanorganicworkingfluid,isthemostcommontype.Atthemomentmorethan150binaryunitswithanaverage capacity between 1 and 3MW are installedworldwide (DiPippo, 2008). AnotheroftendiscussedbutsofarrarelyinstalledtypeistheKalinacycle.OneKalinaplanthasbeeninstalledinHsavik,Iceland(MaackandValdimarsson,2002).AsecondplantisoperatedinUnterhaching,Germany (Knapek and Kittl, 2007). The Kalina cycle is characterised by anammoniawatermixture and an absorption and desorptionprocess within the cycle.However,alsoRankineCycleswithinorganicworkingfluids,suchaspureammonia,orothermixtures are possible. A detailed overview of existing geothermal binary power plants isgiveninDiPippo(2008).

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Figure9:Temperatureentropydiagramshowingthesaturationvapourcurvesofdifferentworkingfluids

Geothermaldrivenbinarycyclesaredesignedaccording to the temperature leveland thecharacteristicsthatareencounteredataspecificsite.Sitespecificcycledesignhastoaimata high utilization of the geothermal heatwith an efficient and reliable power conversiontechnology.Comparedtoconventionalpowerplants,somegeothermalparticularitieshavetobeconsidered.

Thehighestefficiency, forexample,doesnot result in thehighestpoweroutput, such asshown in Figure10. The cycle efficiency increaseswith higher evaporation temperatures.Theevaporationtemperature,however,alsoinfluencesthecoolingofthegeothermalfluid.



The utilization of the geothermal fluid thereby decreases with higher evaporationtemperatures.Forthemaximumpoweroutput,theoptimumevaporationtemperaturehastobefound.

16 Figure10: Ts diagram showing the correlation of evaporation temperature and geothermal outlettemperature(left)anddevelopmentofpoweroutputasafunctionoftheevaporationtemperature(right)

Attentionmustalsobegiventothedesignoftherecoolingsystemofabinarycyclebecausepowerconversionfromgeothermalreservoirshastodealwithlargeamountsofwasteheatandengineeringofsuchcoolingsystemsmustconsidersitespecificpreconditions.

Binarypowerunitsusingheat sourceswith temperaturesup to approximately200C arediscussedinmoredetail,e.g.inSaadatetal.(2010),Fricketal.(2010),Khler(2005),Salehetal.(2007)andDiPippo(2008).

A.2.4CombinedenergysupplyBesidestheprovisionofheat,chillandpower fromseparateplants,geothermalreservoirsalsooffer thepossibility foracombinedenergy supply.This ispossible, forexample,withcogenerationorcombinedheatandpowersupply(CHP)which isknownfromconventionalpower plant design. In this setup the waste heat from the conversion process, whichotherwisewould be discharged to the environment, is used for heat supply (Figure11).Cogeneration is thereforecharacterisedbyasimultaneoussupplyofpowerandheat fromthe sameunitand leads toa significantlyhigherutilizationof theheat sourcedriving theconversionprocess.



Figure11:Schematicsetupofaplantwithconventionalcogenerationofpowerandheat

Theheat contained in theexhaust vapour at theoutletof the turbine from conventionalpower plants can for example be used to supply district or process heat with supplytemperaturesabove100Cor toconvert thisheatandprovidechill. Ingeothermalpowerplants, in contrast, the temperature of the exhaust vapour is normally lower than inconventional power plants. The temperature correlates with the geothermal fluidtemperatures. For geothermal fluid temperatures of up to 170C, the exhaust vapourtemperature typically lies below 60C; for higher fluid temperatures of up to 200C, anexhaustvapour temperatureofabout80Ccanbeexpected (Khler,2005). It isgenerallypossibletoraisetheexhaustvapourtemperaturethoughthismeasurehaslimitedinfluenceandleadstoaconsiderableefficiencydecreaseintheconversionprocess.

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Cogeneration in geothermal plants is therefore dependent on a suitable heat consumerstructure characterised by low supply temperatures. Regarding the relatively largewasteheat amounts of geothermal power plants, cogeneration is, however, also interestingbecausethetechnicaluseofthewasteheatcanreducethedemandforrecooling.

Following thecombinedenergysupplyapproach forgeothermalapplications,moreor lessindependentplantpartsareofteninstalled,whichareconnectedinparallelorserial.

A typical geothermal serial connection is represented by a unit providing power and adownstreamunitprovidingheat(Figure12).Ifabinaryunitoptimisedforpoweroutputhasa geothermal outlet temperature of 70C, the heat unit can, for example, feed a lowtemperatureheatinggridwithasupply temperatureof60Candareturn temperatureof40C.Iftheheatunithastoprovide70Cinstead,thepowerconversioncycleneedstobeadapted inorder tocool thegeothermal fluidonly toabout75or80C,which leads toalowerpoweroutput.Assumingatypicalheatdemandwhichvariesthroughouttheyear,thisreductioninthepoweroutputisonlyrelevantfortimesofheatdemand.



Figure12:Schematicsetupofageothermalplantwithpowerandheatsupplyinserialconnection

Basedontheseconsiderations,aserialconnection isespeciallyadvantageouswhenthe inandoutlettemperaturesoftheconnectedunitscloselymatchtheoptimumtemperaturesoftheirsingleapplication.

18 Inaparallelconnectionofdifferentenergy supplyunits thegeothermalmass flow is splitbetween these units so that the inlet temperatures of all units are the same. Themaindesign criterion is the splitup of the geothermalmass flow. Each energy supply unit isoptimised forthegeothermal fluidtemperatureandacertainmass flow. Incomparisontounitsconnected inseries,unitsconnected inparallelcanbedesignedmore independently.Parallelconnectionscanalsobedesignedeasilyforavaryingenergyprovisionmix.Incaseofavariablemassflow,thepartloadbehaviourofaunitneedstobeconsidered.

Parallelconnectionsareadvantageouswhen thenecessary inlet temperaturesofdifferentenergy supply units are at the same temperature level and especially when thesetemperatures are close to the geothermal fluid temperature. A geothermal plant withparallelsupplyofpowerandheat,forexample,usesallofthegeothermalmassflowforheatprovision in timesof largeheatdemand. In timesof lowerheatdemand (but stillwith asupply temperature close to the geothermal fluid temperature), only a part of thegeothermalmassflowisusedforheatsupplysothattheremainingmassflowcanbeusedinabinarypowerunit.

Geothermal plants can also combine cogeneration, serial and parallel connection in oneplant concept, so that a variety of plant setups are possible depending on the demandcharacteristicsataspecificsite.AnoverviewofexistinggeothermalplantsprovidingpowerandheatisgivenbyLund&Chiasson(2007).

A.2.5EnergynetworksEnergy networks are a compound of different, separately running energy suppliers. Thenetworkisdesignedandoperatedaccordingtotheenergydemandbutalsoaccordingtothecharacteristics of each energy supplier and existing synergies. Energy networks offer thepossibilitytocombinebaseloadandpeakloadsystems,tointegratecomponentsinordertouncoupleenergydemandand supply, suchas storage systems,and tomakeuseofwasteenergystreams.



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Due to thevarietyofsetupsofgeothermalplants, theyalsoofferavarietyofways tobeintegrated in energy networks. Geothermal plants can especially serve as base loadcapacities for heat, chill or power supply. ATES can especially serve to decouple heatdemandandsupplyinanetwork.

The energy supply system of the German Parliament Buildings in Berlin, Germany, forexample, comprises several components that allow an energy efficient and environmentfriendlyenergysupply.Apart fromenergyconversioncomponents (suchascombinedheatand power units, refrigerationmachines), there are two seasonalATES systems involved.OneATESsystemisusedasheatstorageandislocatedatadepthofapp.300m.ThesecondATESservesascoldstorageandusesanaquiferatadepthof40to70m.Theheatstorageischargedwithsurplusheatfromabiofueldrivencogenerationplant(CHP)insummertime.Inwintertime,theATES isdischargedtosupplythebuildingswith lowtemperatureheat.Thetemperaturesare45Cand30Cforflowandreturnrespectively.Thecoldstorageiscooleddown by means of dry cooling in case of the ambient temperatures being low. Insummertime,thecoldgroundwater isusedtocoolthebuildings(Kabus&Seibt,2000;seealsop.74).

A.2.6HybridsystemsHybrid systems refer to technologies that use different energy carrierswithin the sameplant. Insuchplants, the internalprocessesneed tobeoptimized inorder togetahigherenergyoutputcomparedtotheseparateuseoftheenergycarriers.

Regardinggeothermalenergy,hybridpowerplantsaremostlydiscussedatthemoment inwhich the geothermal heat is combined with a heat stream of higher temperature(Figure13). Inthiscase,theprocesstemperaturesoftheconversionprocessandthereforetheconversionefficiencycanbeincreasedcomparedtopowerplantonlyfedbygeothermalenergy. The higher temperature heat can be any heat such as waste heat from otherprocesses (e.g.biogasmotors)or thecombustionofbiomassorbiogas.Dependingon thesiteandtheavailableenergycarriers,geothermalenergycanbeusedtocomeupforpartialortotalpreheatingoftheworkingfluidoralsofullevaporation(Figure13).

One hybrid power plant is presently under construction inNeuried,Germany (Kreuter&Schrage,2010).Astolfietal.(2010)haverecentlydiscussedasolargeothermalhybridpowerplant.



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Figure13: TemperatureEntropy Diagrams of a potential hybrid power plant. The relative input ofgeothermalenergyincreasesfromtheconceptatthetoptotheconceptatthebottom.

A.3Plantdesign

A.3.1GeneralAspectsGeothermalplantscanbasicallybedivided intodifferenttechnicalsubsystemssuchasthegeothermalreservoir,thefluid loopconnectingtheundergroundwiththesurface,andtheenergy supplyunit.The challenge indesigninggeothermalplants is tomeet thedifferentdesign characteristics of these subsystems and manage their efficient and reliableinteraction. Boundary conditions and design criteria of geothermal and their subsystems



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therebyvaryfromsitetosite.Basedonthisvarietyinplantdesign,thedesignoptimizationof a geothermal plant at a specific site requires the comparison of different technicalsolutions.Inthiscontextsimulationtoolsareanimportantinstrument.

For planning a geothermal plant, the characteristics of the reservoirmust be known orevaluated.Besides thereservoir temperature,the temperatureof theheatcarrierson thesurface, and the volume flow rate that can be pumped to the surface, also the auxiliarypower demand for operating the geothermal reservoir and technical restrictions areimportant. Technical restrictionsmay need to be considered such as allowable operationtemperatures and pressures based on geochemical considerations or the design of deepwells (diameters, inclination, etc.), causing also constraints for the installation andconfigurationofdownholepumps.

Thegeothermalfluidloopcarriesthegeothermalenergyandisthereforeaverydecisivepartof geothermal plants. It needs to integrate different functions such as the production ofgeothermal fluid from the reservoir, its transport onto the surface, the processing, andfinally its reinjection into the reservoir.Regarding thedesignof thegeothermal fluid loopandthecomponentscontained,suchasproductionpump,pipelines,heatexchangers,filtersandinjectionpump,thefollowingaspectsneedtobeconsidered.

Propertiesofgeothermalfluidsdifferdependingonthesitespecificreservoircharacteristicsand the change of temperature and pressure during energetic utilization. Knowing thegeochemicalcompositionofthefluidandassessingitsalterationinthegeothermalfluidlooporinthereservoirduringreinjectionmustbeanintegralpartofgeothermalplantdesign.

Theoperationalreliabilityisstronglyaffectedbyinteractionofthegeothermalfluidwiththeplant materials. Especially corrosion and scaling are technical risks which need to beconsidered for optimizing design and successful operation of the geothermal fluid loop.Adequate measures include selection of durable materials and monitoring of physicochemicalconditionsofthefluid.

Thereliabilityofthegeothermalfluid loop isbasedonthechoiceofsuitablematerialsandcomponent layoutsaccording to thecharacteristicsof thegeothermal fluidand local fluidpropertieswhichareexpectedduringoperation. Improperdesigncan leadtodeteriorationofgeothermalplantperformanceorevenplantfailureifcomponentsofthegeothermalfluidloop have to be replaced. For the operation of a geothermal plant, corrosion of plantmaterials and cloggingof thepipes (scaling) are the two riskswhich affect stronglyplantreliability.

Aspectsofoverallplantdesignforgeothermalreservoirsarediscussedinmoredetailine.g.Kranz&Bartels(2010);Frick,Kaltschmitt,&Schrder(2010);Frick,Kranz,&Saadat(2010);Saadatetal.(2010)andHuenges(2010).



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A.3.2GeothermalplantefficiencyTheefficiencyofaprocessisdefinedastheratiooftheeffectiveorusefuloutputtothetotalinputinasystemandistypicallyusedtoevaluatetheperformanceofasystem.Forenergysupplyingplantsefficiencyreferstotheratiooftotalenergy inputtousefulenergyoutput.The calculation of the energetic efficiency for evaluation and comparison purposes,however,mustbebasedon a reasonabledefinitionof the systemboundaries for energyinput and output in order to consider the (different) particularities of the compared andevaluatedsystems.

Inmanyplantstheenergy input isrelatedtoadefined inputoffuel(e.g.fossilfiredpowerplants,gas firedboilers)whichhastobedeliveredtotheplantsite.Geothermalplants, incontrast,useanenergysourcethat isdirectlyavailableatthesitesothattheefficiencyofgeothermal plants should also consider the ratio of used energy to available energy.Furthermore,thequalityofthegeothermalenergy inputvariesfromsitetositedependingon the reservoir temperature and geochemical composition. This needs to be taken intoaccount, for example,when comparing the efficiencies of geothermal plants at differentsites.

AccordingtoKagel(2008)andSaadatetal.(2010),theefficiencyofageothermalplantcouldbecalculatedusingdifferentapproachesdependingontheevaluationpurpose:

1.) First lawefficiency():ratioofusefulenergy(e.g.heatornetpower)totheenergyinput (First Law of Thermodynamics, Law of Conservation of Energy). In case ofrefrigerationcyclesandheatpumps,thisefficiency isalsoreferredascoefficientofperformance(COP).Thefirstlawefficiencyisthemostcommonwaytoevaluatetheperformanceofaplantbutdoesnotdistinguishbetweenthequalityofenergythatisreceivedordeliveredbyaplant.Comparing theperformanceof twopowerplantswith different heat source temperatures, for example, based on the first lawefficiencyisnotsufficient.Theplantsmighthaveidenticalfirstlawefficiency.Butthepower plant using theheat sourcewith the lower temperature uses the availableexergymoreefficiently.Thefirstlawefficiencyis,however,veryusefulforcomparingdifferentplantconceptsoranalyzingtheoverallplantperformanceataspecificsite.

2.) Second lawefficiency: ratioof first lawefficiencyof the realplant to the first lawefficiency of a thermodynamically ideal cycle operating under the samethermodynamic conditions. This measure is based on the Second Law ofThermodynamics, which does not only consider the conservation of energy in asystem but also the irreversibilities of thermodynamic processes due to theproduction of entropy and the destruction of exergy related therewith.Using thisefficiency forevaluationpurposes,theworkoutput iscomparedtothepotentialofthe input to do work. Regarding power plants, the second law efficiency hencecomparesthefirstlawefficiencyoftherealplantswiththeCarnotefficiencyforthesameupperandlowerprocesstemperatures.



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3.) Resourceutilization:ratioof thegeothermalheatusedbyaplant to themaximumusableheatavailablefromthegeothermalsource.Themaximumusablegeothermalheatcanbedeterminedbytheambienttemperatureaslowertemperaturelimitoraminimumallowabletemperature.Comparingtwogeothermalplants,theplantwiththehigherutilizationratioisusingthegeothermalresourcetoalargerextentandreinjectslessheatunusedintothereservoir.

4.) EfficiencybasedonPowerandFluidmeasurements:here,theamountofnetpowerproduced per unit flow of geothermal fluid is considered. For given resourceconditions,thehigherthisvaluethemoreefficientaplantis.Suchmeasurementsareclassified as Specific Power Output (SPO),measured inwatt hours per pound orkilogramofgeothermal fluid,orSpecificGeofluidConsumption (SGC),which is theinverseoftheSPO.DividingtheSPObytheavailableenergy isonewaytomeasuresecondlawefficiency.

While efficiency is important, it is only one characteristic among many that must beconsideredwhen choosing themost appropriate energy option for a particular location(Kagel,2008).

A.3.3GeochemicalaspectsThegeothermal fluid isdefinedasaheatedmultiphasesubstanceconsistingofmainlygasand liquid,which flows within pores of a geological formation (= bedrock) of the deepgeothermalreservoir.Asaconsequenceof temperatureandpressurechangeduringupliftand processing, aswell as by reinjection of the chilled fluid into the reservoir, variouschemicalandphysicalprocesses (e.g.degassing,mineralsolutionandprecipitation)canbeexpectedtooccurwithinthefluidandby interactionwiththesurroundingmaterials(rock,tubing,casing).Thesereactionscandamagetheplantduetocorrosionandfoulingoftubesor clogging of the reservoir pores during reinjection. Thus, for proper design of thegeothermalfluidloop,thefluidneedstobecharacterizedintermsofitscompositionanditseffectstoalterboth,theusedmaterialsandthereservoir.

Anintroductiontofluidtypesandcompositionsaswellasthemaincompoundsofthefluidsandpotentialinteractionswiththeplantmaterialswillbegiveninthefollowing.

FluidcompositionandclassificationThe geological source of the fluid bedrock as well as interactions of the fluid with thesurrounding rocks duringmigration determines the chemical composition of geothermalfluids.Roughly,fluidscanbedividedaccordingtotheirgeologicalsource:sedimentarybasinfluids and crystalline rock fluids (Drever et al., 2004). The fluids are usually classifiedaccordingtotheirmajorchemicalcompounds(i)oraccordingtotheirsalinity(ii).



(i)Dependingontheirdominantions,fluidsaregroupedindifferenttypesrepresentingthemajorcomponentsofthesystem.Insedimentarybasinfluids,themostfrequentlyoccurringcompounds are sodium (Na+), calcium (Ca2+) and chloride (Cl) (Figure14, left) aswell asoccasionallytheorganicanionacetate(CH3COO

).

(ii)Due to thestrongvariation insalinity from fewmilligrams toseveralhundredg/l totaldissolved solids (TDS), fluids are classified in freshwater ( 100 g/l; Davis, 1964). In general, salinityincreaseswithincreasingdepth(Figure14).Mostgeothermalfluidsarebrinesalthoughlesssaltywatersare lesscorrosiveandthuseasiertohandleandwouldconsequentlybemorefavorableingeothermalsystems.Inlowenthalpygeothermalsystems,lowersalinitieshaveonlybeenreportedfromfew,evaporatefreeregions.

Extensiveoverviewsabout fluid formationandcompositionconsideringa largenumberofsamplesfromallovertheworldaregivenbyHanor(1994)andKharakaandHanor(2004)forsedimentarybasinfluidsandbyFrapeetal.(2004)fordeepfluidsfromcrystallinerocks.24

Figure14: Depth distribution of main elements of sedimentary basin fluids plotted versus depth. Theyrepresent78to98wt.%oftotal fluidsaltcontent (KharakaandHanor,2004) (left).Modified figureafterRegenspurgetal.,2009.Depthdistributionofdissolvedsolids(logTDS);datafromsedimentarybasins(blackdiamonds;Frsteretal.,2006;Hanoretal.,2004;Gieseetal.,2002;Wolfgrammetal.;2007)andcrystallinerockformations(whitesquares;Frapeetal.,2004)(right)

EffectofthefluidpropertiesonplantoperationGeothermal fluids represent a mixture of many inorganic ions, dissolved silica, organicmaterials and dissolved gases. Some of these compounds are highly dominant in the



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solution, but of less importance for operating the geothermal plant. Other are highlyreactiveandeventraceamountscanhaveeffectsontheproductionprocess.

Variationof fluidconditions,e.g. inducedbypressure (p)and temperature (T)changecanprovokeachangeinfluidcompositionduetodegassingorprecipitationofmineralsbecausemineralsolubilityandformationdependsstronglyonpTconditions.TheseprocesseswouldfurtheraffectthepHvalueor induce interactionofthefluidwiththeplantmaterials.Thusknowledgeofthefluidcompositionandpropertiescan indicatenotonlythereservoirtypeandtheoriginofthewaterbutalsothepotentialriskforplantcorrosionandformationofscaling(=mineralprecipitation).

Anoverviewofthemostrelevantpropertiesofageothermal fluid isgiven inSaadatetal.(2010).Themostrelevantpropertiesare:

Temperature, pressure: Both, pressure and in particular temperature changes affectstronglythesolubilityofmineralsandthusneedtobeconsideredinordertoassessscalingofmineralsandfoulingofcasing.

Physicochemicalconditions (pHandredoxvalue):AtgivenpTconditionsofasolution,theparameters pHvalue and redox potential determine the dominant species in which acompound(e.g.ametal)occurs insolution.Thisknowledge is importanttoestimate ifthiscompound of certain concentration would remain in solution or precipitate. Silicaprecipitation isespeciallyundesired inplants,because itcanhardlyberemovedandsolelytheadditionofHF,whichisofhighhandlingrisk,candissolvetheseprecipitates.

Cations and anions of importance:Na+ and Ca2+ are the highest concentrated cations ingeothermal fluids which are mainly enriched due to halite (NaCl) and calcite (CaCO3)dissolutionormineraltransformation.Othermetals,suchasiron(Fe),lead(Pb),manganese(Mn), magnesium (Mg), barium (Ba), are often oversaturated and thus play also animportant role in terms ofmineral precipitation (scaling). The anion showing by far thehighest concentration in geothermal fluids is Cl. Its enrichment is a consequence of itsfrequencyinsaltsandmineralsaswellasofitsrelativelylowsolubility.Sinceitisknownforits corrosive properties if oxygen is available in the system, it needs special attention ingeothermalplants.

Dissolvedgasses:Fluidscontainvariousamountsofdissolvedgasses(0.051litergasperliterfluid) consistingmainlyof amixtureofnitrogen (N2) and carbondioxide (CO2) aswell asoccasionallyhydrogensulphide(H2S),methane(CH4)andtracesofhelium(He).ThesolubilityofbothN2andCO2 increaseuponcoolingof thewater.Agasphasemay form ifpressuredrops below bubble point. Of special interest for plant design is the content of noncondensablegases(NCG)suchasCO2andN2,whicharenoteasilycondensedbyfluidcoolingandthusmaynotbesimplyinjectedbackintothereservoir.Moreover,NCGsalsovaryovertime at a given field, which can cause problems with the fluid production equipment.Dependingonthelocalflowconditions,theNCGwouldthenaccumulateintheinstallations



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ofthegeothermalfluid loop(suchasheatexchangers, injectionwell).Toavoidscalingandcorrosion induced by reaction of gas with plant material, a removal system (vacuumequipment)would be necessary. Of special concern is the gas hydrogen sulphide (H2S),because sulphide can react withmetals such as Pb, Cu or Fe to form various sulphidemineralswhichwouldclogthepipes,whereasthefreeprotons(H+) inducetheircorrosion.Carbondioxideandmethanearegreenhousegasesandmeasureshavetobetakentoavoidtheirescapeintotheatmosphereiftheyarepresentinthegeothermalsystematall.

Theamountandcompositionofgassescanvarydramaticallyfromreservoirtoreservoirandfromwell towellwithin the same reservoir.Consequently, it is verydifficult to establishaccuratedesignbasesforthegasfractioningeothermalplants.

A.3.4ThermalwaterpumpsPumping and injecting the geothermal fluid isoften to a considerable amount related toauxiliarypower.Thepowerneededdependsmainlyonthereservoircharacteristicsandthedesignofthethermalwaterloop.Toenergeticallyoptimizetheplantoperation,theauxiliarypowerinputmustbeameliorated/minimized.

Undertypicalgeologicalconditions,thepressureingeothermalreservoirswillnotovercomethehydrostaticpressureof thewaterheadwhichbuildsup in thewell. In this case, fluidproduction technology to lift thegeothermal fluid from the reservoir isneeded.Designinggeothermalplants,thereliabilityofthefluidproductiontechnologyandtheauxiliarypowertorun theequipment isacrucialaspect.More informationondownholepumpsaregivene.g. in Economides andUngemach (1987), Culver and Rafferty (1998), Aksoy (2007), andLegarth (2003). Flashman and Lekic (1999) give a brief overview of other technical fluidproductionoptions.

Theuseofdownholepumps installedbelow the fluid level in theproductionwell ismostcommon for geothermal plants. Downhole pumps are distinguished depending on theirmode of operation in a) lineshaft pumps or b) electricalsubmersible pumps. Lineshaftpumpsaredrivenbyashaftandanelectricalmotor locatedaboveground.Thislimitstheirapplicationtoverticalwellsandinstallationdepthsofupto600m(Aksoy,2007).Incontrast,electricalsubmersiblepumpsaredrivenbyanelectricalmotorlocatedintheproductionwellandhaveawiderrangeofapplication.

Although the physical life of geothermal downhole pumps is typically limited to severalyears, its lifetimecouldbeaffecteddependingon thesiteand theoperatingconditions intheproductionwell.Ithastobeconsideredthatthereplacementofadownholepumpcantakeup a significant amountof time. Such as for thedesignofother components in thegeothermal fluid loop, aspects of corrosion, scaling, and thermal expansion must beconsideredfordesignandoperationofdownholepumps.Afurtherlimitingconditionforthephysical life of downhole pumps are high temperatures of the geothermal fluid. This isespeciallyaproblemforelectricalsubmersiblepumps.Sincethecoolingofthemotormust



be realised by the streaming hot geothermal fluid, overheating of themotor is an issue.Other aspects, which can limit the physical life of downhole and especially electricalsubmersiblepumps, is theaggressivenessof theproduced fluid,which canbeaproblem,e.g.forseals.Unintentionaldegassingofthegeothermalfluidinthepumpduetopotentiallocal pressure drops results inmaterial stresses andmay lead tomaterial failure.Otherproductiontechnologiesdoexistbutpresentlyhavelessrelevanceforgeothermalsystems.Gas liftbasedon inertgases,forexample, isnotconsideredasolutionforcontinuousfluidproductionduetohighcosts.Theturbopumptechnology,whichconsistsofaturbinedrivensubmersible pump supplied by highpressured water from the surface, has only beeninstalledatonesitesofarduetoitslowefficiency(BoissavyandDubief,1995).

Dependingonthereservoirpressure,theproductionwell isfilledwithgeothermalfluiduptoacertain level,which iscalled the static fluid leveland isusuallymeasured indistancefrom the surface. Ifgeothermal fluid isproduced from the reservoir, the fluid level in theproductionwell lowers.The fluid levelduringproductionmainlydependsontheproducedflowrateandthereservoircharacteristicsandiscalleddynamicfluidlevel.Comparedtothestatic fluid level, thedynamic fluid level is therefore lower anddecreaseswith increasingflowrate.Accordingly,theinstallationdepthofthedownholepumpmustbeadaptedtothedesignflowrateandthefluid leveldrawdownrelatedtherewith.Furthermore, ithastobeconsideredthatdownholepumpsneedaminimumintakepressure(approx.10to20bar)sothatthepumpmustbe installedatasufficientdepthreferringtothe lowestdynamicfluidlevel which occurs during operation in order to ensure this intake pressure with theoverlyingwaterhead(Figure15).Accordingtothestateoftheart,withelectricsubmersiblepumpsinstallationdepthsofupto3,000mcanberealised(Legarth,2003).

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Figure15:Schemeofaproductionwellshowingdownholepump,static fluid levelanddynamic fluid level(left)andthepressurecurveduringfluidproduction(right)



Theefforttoproducethegeothermalfluidfromthereservoirisderivedfromtheflowrate,the pressure increase applied by the downhole pump, and the efficiency of the pump.Downholepumpstypicallyhaveanefficiencyofapproximately50to75%dependingonthetemperature conditions and the contents of dissolved gases in the produced fluid. Thepressure increaseof thedownholepumpmustbe regulatedaccording to thepressure toovercome thedifference inheightbetween thedynamic fluid level in theproductionwelland the surface and thenecessarywellheadpressure.The latter is set inorder to avoiddegassingandtoensuretheneededinjectionwellheadpressure(incaseswherethereisnoinjection pump). The hydraulic conditions in the production well and the pipes of thegeothermalfluidloopandthereforepressurelossesduetofrictionmustalsobeconsidered.

Designingthefluidproductionalsoneedstotaketechnicalrestrictionsintoaccount,suchasamaximum installationdepth.Fordownholepumps,themaximumproduciblefluidflow isapprox.600m3/handthemaximummotorcapacityisabout1,200kW(Legarth,2003).

From an energetic viewpoint, another very important aspect for the design of fluidproductionistheincreaseoftheproductioneffortwithincreasingflowrate.Thepumpingofahigherflowrateresults ina largerdrawdownofthefluid levelanda largerdifference inheightwhichthedownholepumphastocompensate.Theproductioneffortthereforeshowsaquadraticdependenceontheflowrate(Figure16).Theinfluenceofanincreasingflowrateonthefluidproductioneffortistherebylargerforlowerreservoirproductivities.

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Figure16:Qualitative development of fluid production effort as a function of geothermal fluid flow fordifferentreservoirproductivities

Atgeothermalsites,wherethepressureatthebottomofthe injectionwellresulting fromthewaterheadintheinjectionwellandthewellheadpressure,isnotsufficienttoovercomethereservoirpressure,an injectionpumpmustbe installedonthesurface.This isthecasewhentheinjectivityofareservoirislowerthanitsproductivity.

A.3.5HeatexchangerssuppliedwiththermalwaterHeatexchangersused ingeothermalplantsareessentialcomponents forproducingpoweraswell as providing heat and/or chill.Heat exchangers should enable the optimum heat



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transfer from one fluid to another fluid for any given application. Depending on thegeothermal application, various types of heat exchangers are required. Because of thedifferent boundary conditions in geothermal applications no offtheshelf heat exchangerwillmeetallrequirementsandalmosteveryheatexchangerhastobedesignedseparately.

Theneededheat transfer rateofaheatexchanger,which is theheat transferredperunittime,hastobeconsideredinthepreliminaryplanningofthegeothermalplant.Concerningabinarypowerplant,thenecessaryheattransferrateofaspecificheatexchangerisdefinedby thebinarycycledesigncalculation.Theheat transfer rate isdeterminedby theoverallheattransfercoefficientandthedrivingtemperaturedifferencebetweenthefluidstreams,allowingalsotocalculatetheneededheattransfersurfacearea.

Thematerialsused intheconstructionofaheatexchangercanbean important issue.Thechemicalpropertiesofthegeothermalfluidaswellasthefluidonthesecondarysideoftheheat exchanger (such as the working fluid in a binary cycle) require corrosion resistantmaterialssuchasstainlesssteelandtitanium,orcoatings.

Due to the different requirements of heat transfer applications many types of heatexchangers are available and therefore a wide range of design approaches have beendeveloped.Ageneraldistinctionismadebetweentheflowarrangementsofthefluidswithintheheatexchanger. Inparallelflow,bothfluidsentertheheatexchangeratthesameendand flow in the same direction. In counter flow, both fluids enter the heat exchanger atoppositeendsand flow inoppositedirections.The third flowarrangement iscalledcrossflow.Here, the fluidsmoveperpendicular toeachother through theheatexchanger.Thedifferentflowarrangementsoftheheatexchangerresult indifferenttemperatureprofiles,whichneedtobeconsidered.Inmostcases,acombinationofthementionedarrangementsisused.

Focusing on constructionrelated classifications, shellandtube heat exchangers and plateheat exchangers are commonly used in geothermal applications (Figure17). Plate heatexchangersare characterizedbyahigh surfacetovolume ratiowhich leads toa compactconstructionform.Onceinstalled,theycaneasilybeexpandedwhenmoreheatcapacityorheatexchangesurface isneeded.Plateheatexchangersarecommonlyusedaspreheaters(e.g.RaffertyandCulver,1998).Theiruseasevaporators isalsopossibleas it isthecase inchilling technologies. However, the design of plate heat exchangers is based on moredetailedcalculationscomparedtothedesignofshellandtubeheatexchangers(e.g.ZhuandZhang, 2004). In comparison, shellandtube heat exchangers have a larger surfacetovolume ratio and larger realizable temperature gradients. Furthermore, fouling can be alarger issue than for plate type heat exchangers (e.g. Chandrasekharam and Bundschuh,2002).



Figure17:Illustrationofashellandtubeheatexchanger(left)andaplateheatexchanger(right)

Heat exchangers are designed for special applications and specific operational modesdepending on inlet and outlet temperatures and mass flow rates. When operating ageothermalplant,particularlyabinarypowerplant,differentoperationalmodesoccurduetochangingparametersandboundaryconditions.Consideringacombinedheatandpowerprovision,thetemperaturesandtheheatcapacityofthedistrictheatingsystemwillaffecttheoperatingconditions.Forthatreason,theheatexchangerdesignhastobefocusedonfull loadaswellaspart loadconditions.Thepart loadbehaviourbecomesacrucial factorwhenoperationoffthedesignpointispredominant.

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Heatexchangerdesignmustconsiderfouling,i.e.precipitationofmineralssuchassilicaanddepositswhichaccumulateontheheattransfersurface.Thisisaveryimportantissueduetothe changes of the chemical equilibrium of geothermal fluids which results from thetransport and cooling of the fluid and the pressure and temperature changes relatedtherewith.The layerofdepositsrepresentsanadditionalheattransferresistancebetweenthehotandthecoldfluid.Therefore,theknowledgeofthermophysicalfluidpropertiessuchasspecificheatcapacity,viscosityanddensityisimportant.Withregardtogeothermalfluidsthemanifoldchemistrymayhinderthecalculationofpropertieswiththerequiredaccuracy.Methodsforcalculatingdensity,heatconductivityandviscositydependingontemperature,pressureandsalinityaregivene.g. inMcDermottetal.(2006).Methodsforcalculatingthespecificheatcapacityaregivene.g.inThomsen(2005)andMillero(2009).Incasesufficientmethodsforcalculatingthesepropertiesarenotavailable,thedataneedtobemeasured.

An additional issue is theneed for low temperaturedifferencesbetween the geothermalfluidandthefluidonthesecondaryside,particularlytheworkingfluidinbinarycycles.Thesmalltemperaturedifferenceisimportantfordecreasingtheexergydestructionduringheattransfer. It must be considered, however, that low temperature differences lead to anincreasingheattransfersurface.



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BenNordenandAndreaFrster

B.ExplorationofGeothermalReservoirs

B.1TheExplorationConceptIncontrasttoclassicalgeothermalexploration,wheretheexistenceofgeothermalreservoirsat depth often is indicated by surface manifestations (e.g. hot springs), the respectivereservoirs in sedimentary basins are hidden. However, a basic comprehension of thegeologicalandgeothermalconditionsofasedimentarybasin isthebasis intheapplicationfor licences of target areas to be further explored and finally used tomine geothermalenergy.

Todelineategeologicalformationspresumablyhostinggeothermalreservoirsanexplorationconcept needs to be developed. This concept is heavily dependent on the state ofexplorationthatthebasinhasundergonepriortogeothermalexploration.Manybasinsareexplored forhydrocarbons so thata comprehensionof themajor featuresof thebasin isalready available. If those legacy data are for a geothermal assessment, they should bescreened forquality.However,thehydrocarbon industryoftenholdssubsurface (analogueor digital) data and reports (with orwithout raw data) that are often not accessible togeothermal projects or only at an oilbased price. If those data sourceswould bemadeavailableforthegeothermalcommunity(asitisthecaseforexampleintheNetherlandsandintheUSA),fasterandbetterquantificationsofgeothermalresourcesandreservoirscouldbemade.Unfortunately, case studieson the successor failureof geothermalexplorationusinglegacydataarenotpublishedmakingitimpossibletoevaluateindetailtheusefulnessoftheirimplementationintotargetedgeothermalexploration.

Theexploration concept tobedevelopedwill slightlydifferaccording to the two typesofgeothermal resources: (1) hydrothermal resources/reservoirs and (2) petrothermalresources,which bothmay occur in sedimentary basins, but forwhich slightly differentapproachesapply.While the fluidsof the type(1) reservoirsoften canbepumped to thesurface without stimulating the reservoir, the type(2) reservoirs show lowporosity/permeability and lowornonatural fluid flow so that theyneed tobe artificiallystimulatedbyfracoperations,requiringalsotheuseofanadditionalartificialheattransfermediumtoproducetheheatfromthereservoir(HotDryRocksystem).

B.1.1BoreholeDataIngeneral,ifnobackgroundgeologicaldataofasedimentarybasinareavailable,explorationstarts as a green field approach. Drilling of some exploration wells in conjunction withgeophysical surveys conducted at the surface (see B 1.2 Surface Geophysical Surveys)provide the first basic insight, preferentially on the structure and lithology of the basin.Althoughthedrillingofseveralkmdeepexplorationwellsisexpensive,theyprovidetheonly



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sources on which the reservoir lithology and related reservoir performance can besufficientlyinvestigated.

Firstattentioningeothermalexplorationneedstobedrawntothetemperatureatdepthasthis parameter decides the type of geothermal use and the cost of drilling. If boreholetemperature data (logs and/or BHTs, e.g. Frster, 1999) are available only from shallowdepth(wheretemperaturesofhydrothermalfluidsaretoolowforageothermaluse)orarefrom locationsoutsidethetargetarea,temperatureprofilesneedtobeextrapolated fromonearea toanotheror calculated forgreaterdepthusingdataon surfaceheat flow (e.g.Nordenetal.,2008,Blackwelletal.,2006).Thesurfaceheatflowthereforeisanimportantparameteronwhichthegeothermalpotentialofanareacanbequantified.Todetermineareliable heat flow value in a heat conduction/diffusion domain a highresolutiontemperaturelogaswellasvaluesofthermalconductivityoftheloggedsectionarenecessary(e.g.FuchsandFrster,2010).

Boreholesalsoprovidethefirstinformationonthestructuringofthesedimentarysequenceand, if abundant, also on the areal extent of geological formations being perspective asgeothermalreservoirs(e.g.Feldrappeetal.,2008).Thisinformation,usuallyintegratedwithseismicdata, canbe shownbymaps, cross sectionsor3Dmodels.Examplesonhow thisinformation isprovidedaregiven inHurterandHaenel (2002) in theAtlasofGeothermalResourcesinEurope.

Borehole analyses (cores, logs, tests) add important information on the hydraulicparametersofperspectivereservoirs,whicharetheeffectiveporosityandpermeability(e.g.Wolfgramm et al., 2008). Again, this information is responsible in decisionmakingwithregard to the typeof the geothermaluse (e.g.hydrothermal versusEGS). In sedimentarybasinsofhighporosity/permeability, thegeneralhydrogeologyshouldbe investigatedandthemajorfluidflowdirectionsandvelocitiesdelineated.Highfluidvelocitiescanessentiallyoverprint theconductive temperature field resulting in temperatureanomaliesother thanthose caused by the geological structure and different thermal conductivities of thegeologicalformations.

Mineralogicalandgeochemicalanalysesof the reservoir rockaswellasananalysisof thechemistryof reservoir fluids shouldbeavailable (e.g.Wolfgrammetal.,2008) inorder toassess the fluidrock interactions thatmayoccur if thephysicochemicalconditionsof thereservoirwillchangeovertime.

Data on the regional and local stress/strain field are necessary if the reservoir/formationneeds tobeartificiallystimulatedby thegenerationof fractures (e.g.Moecketal.,2009).The sameapplies to themechanicalproperties,onwhich thedesignof fracoperations isbased(e.g.Zimmermannetal.,2010).

Thermal conductivity and thermal resistivity (e.g. Norden and Frster, 2006; Fuchs andFrster,2010;Hartmannetal.,2007)aswellas hydraulicrockproperties from laboratory



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measurementsofcoresandfromtheanalysisofwell logsplayanimportantrole inmodelsthat address the lifeti

geothermal energy utilization in low enthalpy sedimentary environments

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