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($[1, Desisnation:e74 83 Reapprovedee1)

1. Scope1.1 This guidedefinesand examinespower plant perfor-mance ermsand criteria for use n evaluationand compar-isonof geothermalenergyconversionand power generationsystems.The special nature of these geothermal systemsmake performance criteria commonly used to evaluateconventional ossil uel-firedsystems f limited value. Thisguide dentifies the limitations of the lessuseful criteria anddefines an equitable basis for measuring the quality ofdifferingthermal cyclesand plant equipmentfiorgeothermalresources.1.2 This standarddoesnot purport to addressall of thesafetyproblems, tf any, associatedwith its use. It is theresponsibiiityof the user of this standardto establishappro-priate safetyand healthpracticesand determine he applica-bility of regulatory imitations prior to use.2. Significance nd Use

2.1 Thermal efliciencyand heat rate are frequentlyuti-fized to evaluate he thermodynamic quality of fossil fuel-lhed powerplants.zEvaluationof geothermalsystemsusingsimilar definitions of thermal efficiencyand heat rate can bemisleadingand, for mostpurposes,nadequatelyassesseshequality of alternativecycles.A utilization factor, definedasthe ratio of het work output to the ideal work available romthe geofluid,providesa more equitablebasis or evaluationof the thermodynamic xcellence f geothermal ystems.-3. Calculations

3.1 FossilFuel-fired Power Plants-Thermal efficiencyind heat rate are usefuland valid criteria or evaluationand

StandardGuide orSpecifying hermalPerformancef Geothermal owerSystemslThis standard is issued under the fixed designation E 974; the number immediately following the designation indicates the ;'sa1.1original adoption or, in the caseofrevisioq the year oflast revision. A number in parenthesesndicates the year oflast reapproval. Asuperscript epsilon (r) indicates an editorial change since the last revision or reapproval.

INTRODUCTIONThe following sectionsdescribea guide for determining the thermodynamic excellenceofgeothermalpowersystems.This guidemay be r.lsedo establishandcompareperformanceevelsofalternativegeothermalplant designs sing equal or different resource onditionsand s intendedasa means or supplying nformation in support of geothermalplant optimization.It is also he purposeof this guideto promote the common useof pertinent comparisoncriteriafor geothermalpowersystems,and to discourage he use of somecriteria which may range romlessuseful to misleading.

comparisonof fossil fuel-fired power plants. Thermal effi-ciency s the ratio of net work generatedo the heat that istheoretically available from the fuel. Conventional usagewithin the electric generatingndustry defines hermal efE-ciency in fractions)as:n, = (3.6x 106HR)

where:3.6 x 106 joule equivalent J) of I kWh, and IIR = heatrate, the ratio of energysupplied o the net output, J/kWh.3.1.1 For fossil fuel-fired power plants heat rate is ex-presseds: HR = (Mrx FC/W)where:Me : fuel flow rate, kg/h,f'C : fuel higherheatingvalue,J/kg,andW : net output, kW.3.1.2 Thermal efliciencyand heat rate nclude the effec-tivenessof energyconversionassociatedwith the fuel com-bustion, the effect of heat rejected in exhaust gasesandcondensate, nd allowance or equipmentand balanceofplant auxiliary power losses.Thus, thermal e{Iiciency andheat rate provide an equitablebasisfor ranking and com-paring ossil uel-firedplantsofalternativedesign.3.2 GeothermalPowerPlants-Evaluation of geothermalenergyconversionprocessessing a similar defrnition ofthermalefficiencyand heatrate can be misleading nd formost purposesdoes not adequatelyassesshe quality ofalternativecycle configurations.The heat supplied o thegeothermal ycle s in general:

(dimensionless) ( l )

(2)

' t This guide is under the jurisdiction of ASTM Committee E44 on Solar,Geothermal,and Other AlternativeEnergySources nd is the direct responsibilityof SubcommitteeE44.20 on Utilization.Current edition approvedNov. 28, 1983.PublishedMay 1984.2Kestin,A. J., DiPippo, R., I(halifa, H., and Rylev,D. J., "SourceBook on theProduction of Electricity From Geothermal Energy," DoE/RA/28320-2, U.S.DepartmentofEnergy, 1980,pp. 243-257.

Qo(ho h,)(Jlh)and the heat rate becomesexceptions re discussednnextparagraph):

HR = (Q"(h" h,)lw) (J/kwh)

(3 )the(4)

where:Qo = total mass low rate of thegeothermalluid (geofluid)to the cycle,kg/h,563

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2 0op

zRHHr- .f, l 15H4tlr]r

([[llegzam o Fr r24 0 280 320

1 4 0 1 6 00

I I | . . , ; ' . 1 . .' : l ' :i ., .

-/..-t"'-

/-/--- - To - - 2 3 2 o c ( 4 5 0 ' F )- / T i = 1 5 .6 o C ( 6 0 o F )' P c = 1 . 0 k P a ( 3 r n . H s )808 Eng ine Ef f ic idncY

&o : specificenthalpy of the total two-phaseor single-phasewell flow, J/kg, andh, = specificenthalpyofthe fluid rejected rom the cycleand returned o the reservoiror environment,J/kg.3.2.1 The resulting hermal cycleefliciency or a typicalsingle lashsteam ycle, spresentedn Fig. I asa function ofthe temperatureof the rejected luid, f,,. This figure showscontinuously ncreasing hermal efliciencyas hotter fluidsare returned o the reservoir.However, t can be shown hatfor any geotherrnal ycle, there is an optimum value forrejectiontemperature hat will produce maximum poweroutput per unit of mass low from the well. Consequently,the continuously ising efficiencyshownby Fig. I can bemisleading.When optimum rejection temperature s ex-ceeded,he well flow required o produceeachkilowatt ofpower increases xponentiallyand may exceedeconomiclimits.3.2.2 Calculationof the heat rate usingEq 4 is valid forgeothermalbinary cycles Fig. 2a), wherein heat is trans-ferred from the geothermal luid to a secondaryworkingfluid andthe total well flow is returned o the reservoir. hisheat ate equation equiresmodification or cycleswhereinafraction of the geothermalluid is flashednto steam Fig.2band2c)and s rejected scondensate.he modifiedequationbecomes:H R = Qoh,- Qo(M,h,+ M.h")W

rejectediquid fraction,

1 0 0 L 2 0

REJECTION TEMPERATURE, TT "CFlG. 1 ThermalEfficiency or Single-Flash teamCycle

M" : sum of the steam apor ractionsproduced t oneomorepressureevels,andh. = specificenthalpyofrejectedcondensate,/kg.Terms must be added to reflect heat rejected from wellluentsventedasnoncondensibleapors.3.3 Utilization Factor-A more useful basis for coparingthe thermodynamicexcellence f geothermal neconversionprocessess providedby the utilization facThe utilization factor is defined as the ratio of net wooutput to the idealwork available rom thegeofluidbetwits initial state(supply condition) and the sink condit(lowest emperature vailable or heat rejection).Utilizatfactor( U) canbe expresseds:

w 3'6x lo6J rless)u: Q"E,* --kW[- (clmensrotwhere:Ei = ideal specificwork availableo the process ithin tnaturalboundsof the environment, /kg,Oo : well headmass low rate, kg/h, andW = net usefulpower,kW.3.3.1 Utilization factor fulfills the objectiveof evaluathe quality of thermal cyclesand for ranking the powpotential of geothermal esources.t is important that lpower be explicitly defined as net power,as theremaysignifrcantdifferencesn auxiliarypowerrequirements,differingmethodsof providingauxiliarypower, n the cabeingevaluated. his appliesparticularly o auxiliarypowallowances or venting gasesor for transportingworkfluids.

where:M , :

(J/kwh) (5)

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($lb gzcGEOFLUID FRO}.IPRODUCTION WELL

m! t

FlG. 2a Schematic of Geotluid/Organic RankineCycle

STEAT{ TURBINE -GENERATOR

EPARATOR

LIOUID TOINJECTION I{ELL

GEOFLUID FRO}4PRODUCTION WELLFlG. 2b Schematic of Single-FlashSteam Cycle

ORGAI{IC .TURBINEGENERATOR

TO GEOFLUIDINJECTION

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filll egzcH1

COTIDENSERHEATOUT

QrFLUID FROMPRODUCTION LIQUID TOWELL ]NJECTIOI{ WE],L

FlG. 2c Schematicof Dual-FlashSteamCycleT O FT240 280

L20 1 4 0REJECTTON TEMPERATURE,Tr oC

FlG. 3 Utilazation actorFor Single-Flash teamCycle

r|1

Mc

3 4

T o = 2 3 2 " C ( 4 5 0 " F )T , = 1 5 . 6 o c ( 6 0 " F )nJ = r .0 kPa (3 in . Hs )80% E ng i ne Ef f i c i ency

320

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filltegz+3.3.2 Utfl:ranon factor for a typical single flash steamcycle, s presentedas a function of rejectiontemperature nFig.3, the rejectiontemperaturewhich results n the highestutilization is referredto as the optimum temperature.Formultiflash cycles(including flash-binary)optimum valuescan be identified for each flash level. For binary cycles,optimumtemperatures end to vary with the secondaryluid

selected.Optimum rejection temperaturecan alsobe identi-fied for hybrid cyclessuch as those employing total flowmachineswith steambottoming cycles.3.3.3 Examinationof Figs. I and 3 indicateshat thermalefficiencyand heat rate are not comparablewith utilizationfactor.Thermal efliciency, or geothermal ystems,ncreaseswith increasing rejection temperature, whereasutilizationfactorshowsan optimum rejection tmperature or a given: iupply condition. Utilization factor is presentedas a guidefor design evaluationsand the selectionof suitable designconditiqns or equipment specifications.3.4 ldeal SpecirtcWork-The idealspecificwork (d), isthetheoretical maximum work which can be obtained froma systemat an initial stateat pressure P") andenthalpy( ft")andsink conditions P, and 7,). The dealspecificwork canh calculatedusingthe second aw of thermodynamics.

Ei = (ho - h^ ) - f.(S" - S") (7)where:ho and h^ = geothermal luid enthalpiesat the inlet andsink conditions.

The Amedcan Societyfor Testing and Materials akes no position rcspectingthe validity of any patent r,?hts ass erted n connectionwith any item mentionad in this standard. Users of thrs standard arc exprcssty advised that determinationol the validity of any suchpEitentlghts, and the risk of infringement of such rights, are entirely their own responsibility. ifhls stahdard is subiect to revision at any time by the responslb/etec hnical committeeand mustbe rcviewed every five years andit nat revised,either reapqoved or withdrawn. Yourcomments are invitd either for rcvisian of this standardor for addiionat standardsand should be addrcssed to ASTM Headquarters. Your commentawitl receivecarcfut consideation at a meeting of the responsibletechnical committee, which you may aftend. lt you feet that your commentshave not recelved a faii hearingyiu should makeyourviews known to the ASTMCommitteeon Standads, 1916 RaceSt., phitadetphia, A 1910A.

S" and5 = geothermal luid entropiesat inlet and sinkconditions,J/kg.& andTu = sink tenrperalure, (., 3.!.1 The useof thermal properties or pure water in thecalculation= f 4 wifl result in a utilization factor whichreflects he influenceof salinity and other geofluid contami-nants on the relative performance of different sites. The

initial enthalpy, (h), must be explicitly and consistentlydefined for valid comparisons; or instance, the choice ofwell headtemperaturewould not be appropriatefor plantsemploying downhole pumps or downhole heat exchangers.Therefore, he initial temperaturshould be defined for thewell flow entering he downholesystem.3.5 Sink Condition-Unless specialenvironmental condi-tions exist, in deriving the ideal specific work, the sinkcondition of saturatedwater at 15.6'C (60'F) is recom-mended. :3.6 GeofluidRate-:-For comparisonsof alternativecyclesor apparatus for a specific site or resource,where idealspecificwork, (-01),s constant,he coniparison anproceedon the basisof specificpower, WIQ- or its reciprocalwhichis usually termed water-rateor geofluid-rate.Thb geofluid-rate measureshe well flow rate to produi:e a kilowatt ofpowerand is a major index of field divelopmentcosts.Forcomparisonswhere auxiliary lossesare presumedconstant,thegeofluid-ratecalculationis often foreshortenedo a basisof gross o\vergeneration, owever,his shouldbe usedwithcaution.

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