SUPERCRITICAL RANKINE CYCLE
Transcript of SUPERCRITICAL RANKINE CYCLE
Supercritical rankine cycle A synopsis of the cycle, it’s background, potential applications and engineering challenges.
Shane Hough04/07/09(ME-517)
Supercritical rankine cycle A synopsis of the cycle, it’s background, potential applications and engineering challenges.
Shane Hough04/07/09(ME-517)
Abstract
TheRankinecyclehasbeenusingwatertogenerateusefulworksincethemid1800’s.Withtheadventofmodernsuper alloys,theRankinesteamcyclehasprogressedintothesupercriticalregionofthecoolantandisgeneratingthermalefficienciesintothemid40%range.Asof2001,therewere360supercriticalfossilfuelplantsoperatingacrosstheglobe[10].Theseplantsareoperatingattemperaturesaround1000oFandpressuresaround3500psia.TwoofthecurrentengineeringchallengesaretoadaptthesupercriticalRankinecycletocoolantsotherthanwater,andtodesignnuclearpowerplantscapableofoperatingatsupercriticaltemperaturesandpressures.Thereismuchworktobedoneintheareaofmaterialdevelopmentbeforenuclearplantswillbecapableofreliableoperationatthetemperaturesandpressuresrequiredtoachievethermalefficienciesover35%.
TableofContents
Introduction:TheSimpleRankineCycle 1
SimpleCycle 1 Figure1.SimpleRankine/SteamCycleDiagram. 1 Figure2.SimpleRankineCycleT-SDiagram. 1 Superheat 1 Figure3.SimpleRankineCyclewithSuperheatT-SDiagram. 1 Efficiency 2 MeasuredParameters 2 Figure4.EffectofTurbineOutletBackpressureforaConstantInputThrottle
Temperature. 2 Figure5.EffectofTurbineInletTemperatureforaConstantOutletBackpressure 3
SupercriticalRankineCycle 3
EfficiEncy� 3� � figurE�6.���SupErcritical�rankinE�cyclE.� 3� DEfinition� 3� MatErial�concErnS 4
NuclearLightWaterPowerApplication 4
SuperCriticalWater-cooledReactor(SCWR) 4 Differencebetweenconventionalnuclearplantdesigns 4 Table1:SCWRPressureVesselSpecifications 4 Materialstresstesting 4 Figure7.SCWRpowerconversioncycleschematic 5 Figure8.SCWR,PWR,&BWRcomparison 5 Figure9.StressCorrosionCracking(SCC)photos 6
NaturalGasProductionwithaSupercriticalGeothermalPowerApplicationBy-product 7
NaturalGasBrine 7 PropaneCycle 7 Design 7 Efficiency 7 Viability 7 Figure10.SupercriticalPropaneCycleSchematic 8
SupercriticalCO2PowerApplication 8
CO2criticalvalues 8
SCWRcomparison 8 Smallercomponents 8 Operationalparameters 8 Materialconcerns 8 Figure11.SupercriticalCO
2RecompressionCycle 9
Table2:CalculatedSystemParameterscorrespondingtoFigure11 9 Figure12.SizeComparisonbetweenTurbinesofVariousCycles/Coolants 9
Conclusion 10
Efficiencyimprovements 10 Materialimprovements 10 Reliability(60yrs) 10 PlantSize 10 Large(1600MW
e) 10
Mid(20MWe) 10
Small(400kWe) 10
EconomicConsideration 10
References 11
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Introduction:TheSimpleRankineCycle
TheRankine cycle is synonymouswith the steamcycle and is the oldest functional heat cycle utilized byman.AbasicdiagramofthecycleisillustratedinFigure1.Figure 2 represents the Temperature - Entropy (T-S)diagramforthesimplestversionoftheRankinecyclewhichcorrespondstosystemswithworkingpressuresupto400psia[1].ThecycleasitisdepictedinFigure2israrelyusedtoday. More commonly, the simple Rankine cycle ismodified such that the steam is superheated prior toenteringtheturbine.Thisisindicatedbyline3-4intheT-SdiagramshowninFigure3.Whenproperlydesigned,thismodificationincreasesW(theworkcapacityofthesystem)withoutsignificantincreasetoQr(theunavailableheatofthe system). Typically, point 4 represents temperaturesup to approximately 900 oF (755 K). Theoretically, it should be possible to superheat the steam to thesametemperatureastheheatsource;whichinaboiler,isontheorderof3500oF(2200K).However,thetemperaturethatcorrespondstopoint4isphysicallylimitedtoadifferentialtemperatureofabout1000oF(811K),themetallurgicallimit[2].Inotherwords,solongastheheattransferacrossthematerialsintheboilerandtheturbinesmaintainsatemperaturedifferentiallessthantheoperationallimitsofthematerial,itshouldnotexperiencecatastrophicfailure.Additionally,superheatingallowsthesteamtostillremainapproximately90%(orgreater)dry as it exhausts from the turbine. This featureof the superheated cycle simplifies theturbinedesignandextendsturbinelifeduetoreducedwearfromwaterimpingementontheblades.
Heat Sink
Heat Source
Qa
Qr
Engine(Turbine)
Superheater
Pump
Boiler
Condenser
1
5 6
1
2
2
343
45
Figure 1. Simple Rankine/Steam Cycle Black numbers correspond to Figure 2; Grey numbers correspond to Figure 3.
T
0 S
1
2 3
45
W = Q - Q
Qr
a r
Figure 2. T-S Diagram, Simple Rankine Cycle.
T
0 S
1
6
2 3
4
5
W = Q - Q
Qr
a r
Figure 3. T-S Diagram, Simple Rankine Cycle with Superheat.
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Theefficiency(η)ofthiscyclecanberepresentedbytheratioofworkcapacitydividedbytheheatputintothesystem(Qa):
η =WQa
ηsimple =h3 − h1 − h4 + h5
h3 − h1
ηsuperheat =h4 − h1 − h5 + h6
h4 − h1
ηsupercritical =h2 − h1 − h3 + h4
h2 − h1
Equation1.
Intermsofenthalpy(h),theefficiencyfortheRankinecyclewithandwithoutsuperheatingcanbedefinedbythefollowingequationsrespectively:
η =WQa
ηsimple =h3 − h1 − h4 + h5
h3 − h1
ηsuperheat =h4 − h1 − h5 + h6
h4 − h1
ηsupercritical =h2 − h1 − h3 + h4
h2 − h1
Equation2.
η =WQa
ηsimple =h3 − h1 − h4 + h5
h3 − h1
ηsuperheat =h4 − h1 − h5 + h6
h4 − h1
ηsupercritical =h2 − h1 − h3 + h4
h2 − h1
Equation3.
Typicalefficienciesforthesecyclesrangefromaround20%-35%. Therearetwomeasurablephysicalandcovariantparametersofthesystemwhichcorrelatetosubstantialaffectsonthethermalefficiencyofthecycle.Thefirstparameteristhesystembackpressure.Foragiveninlet(throttle)temperatureandpressureattheturbine,alowerbackpressureequatestoahighersystemefficiency.Ineffect,anincreaseinthebackpressurereducesthecapacityofthesteamtoexpandthroughtheturbine.TheeffectofvariationsinsystembackpressurecanbeseeninFigure4.Thesecondparameteristhethrottletemperature.Whileholdingboththesystembackpressureandthethrottlepressureconstant,anincreaseinthethrottletemperaturewillincreasethethermalefficiencyofthecycle.ThiseffectisshowninFigure5.Anincreaseinthethrottletemperaturesimplyincreasesthepotentialoftheexpandingsteamtotransferenergytotheturbine.
50
45
40
35
30
30
25
2520
20151051014.7312.289.827.374.912.460.490
Throttle Temperature = 810.93 K (1000 F)
Ran
kine
Ste
am C
ycle
Ther
mal
Eff
icie
ncy
, Pe
rcen
t
Thro
ttle
Pre
ssure
, psi
a
Engine Back Pressure, In. Hg abs (psia)
50003200
1500
2500
1000
500
200
10,000
o
Figure 4. Effect of Turbine Outlet Backpressure for a Constant Input Throttle Temperature [1].
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SupercriticalRankineCycle
TheRankinecyclecanbegreatlyimprovedbyoperatinginthesupercriticalregionofthecoolant.MostmodernfossilfuelplantsemploythesupercriticalRankineSteamCyclewhichpushesthethermalefficiencyoftheplant(seeequation4)intothelowtomid40%range.
η =WQa
ηsimple =h3 − h1 − h4 + h5
h3 − h1
ηsuperheat =h4 − h1 − h5 + h6
h4 − h1
ηsupercritical =h2 − h1 − h3 + h4
h2 − h1 Equation4.
For water, this cycle corresponds to pressures above 3,206.2 psia and temperatures above 705.4 oF(646.3 K) [1]. The T-S diagram for a supercritical cycle canbe seen inFigure6. With theuseof reheat and regenerationtechniques,point3 inFigure6,whichcorresponds to theT-Svaporstateofthecoolantafterithasexpandedthroughaturbine,canbepushedtotherightsuchthatthecoolantremainsinthegasphase†.Thissimplifiesthesystembyeliminatingtheneedforsteamseparators,dryers,andturbinesspeciallydesignedforlowqualitysteam.†Traditionally, a Rankine cycle operates across a phase change and a Brayton cycle operates
entirely in the gas phase. However, as improvements in material make higher temperatures
and pressures available, the difference between a supercritical Rankine cycle and a Brayton
cycle blur. For the purpose of this paper, a supercritical Rankine cycle is one which has
evolved from a simple Rankine cycle or one for which it is desirable to run along the
gas/vapor boundary (dimer and trimer formation only) to maximize heat extraction from
the coolant.
T
0 S
1
4
2
3
Qr
W = Q - Qa r
Figure 6. Supercritical Rankine Cycle.
Ran
kine
Ste
am C
ycle
Ther
mal
Effic
iency
, Pe
rcen
t
Thro
ttle
Pre
ssure
, psi
a
Back pressure = 0.5 In. Hg abs (0.0246 psia)
14.7
50
100
200
500
100015002500
3206.2
500010,000
Dry
Satu
rate
dV
apor
atTh
ro
ttle
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52
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46
44
42
40
38
36
34
32
30
Throttle Temperature, K ( F)o
400 600 800 1000 1200 1400 1600422 533 644 755 866 977 1088
Figure 5. Effect of Turbine Inlet Temperature for a Constant Outlet Backpressure [1].
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Theprimaryconcernwiththiscycle,atleastforwater,isthemateriallimitsoftheprimaryandsupportequipment.Thematerialsinaboilercanbeexposedtotemperaturesabovetheirlimit,withinreason,solongastherateofheattransfertothecoolantissufficientto“cool”thematerialbelowitsgivenlimit.Thesameholdstruefortheturbinematerials.Withtheadventofmodernmaterials,i.e.superalloysandceramics,notonlyarethephysicallimitsofthematerialsbeingpushedtoextremes,butthesystemsarefunctioningmuchcloser to their limits. Thecurrent superalloysandcoatingsareallowing turbine inlet temperaturesofupto1290oF(973K)[3]andfourthgenerationsuperalloyswithRutheniummono-crystalstructurespromiseturbineinlettemperaturesupto2010oF(1370K)[4]inthefuture.
NuclearLightWaterPowerApplication
TheGenerationIVSuperCriticalWater-cooledReactor(SCWR)isa1600MWenuclearpowerplantdesignbasedonthe supercriticalRankinecycle. Thesystem layout is identical to the supercritical systemcurrentlyusedinfossilfuelsplantsexcepttheboilerisreplacedwithanuclearreactor.ThesystemdiagramcanbeseeninFigure7,page5.ConventionalnuclearplantsoperateonthesimpleRankinecyclewithsuperheatingand exhibit thermal efficiencies on the order of 33%. The SCWR is capable of a thermal efficiency of44.8%[5].Oneoftheattractiveaspectsofasupercriticalcycleforanuclearapplicationisthatiteliminatestheneedforpressurizers,primarytosecondaryheatexchangers,steamdryers,andsteamgenerators.Whenoneconsidersthattheprimarycostofelectricityforanuclearplantistheconstructioncost,eliminationofsupportequipmentiscrucial,especiallyifitoffsetsthecostofheavierpressurevesselsandsupportpiping. The primary differences between the SCWR and conventional Boiling Water Reactors (BWR) &PressurizedWater Reactors (PWR) is the reactor outlet temperature is 930 oF (770 K) instead of 570 oF(570 K) and the SCWR operates at approximately 3625 psia [5] as compared to 915 psia and 2265 psiarespectively[6].ThisdifferenceisgraphicallyrepresentedinFigure8,page5. Inorder tohandle thehigherpressuresand temperatures, the reactor vessel must be designedto handle the extreme temperatures and pressures. Thespecifications required to meet this demand are listed inTable1.Moreimportantthanthepressurevesselhowever,arethefuelassembliesandcontrolrodcomponents.Withareactorinlettemperatureof535oF(550K),thereactorcomponents are exposed to a temperature differential of395oF(220K).Unlikethetubesinaboiler,asignificantportion of the fuel assembly structure is not directlytransferring heat to the fluid. This places much greaterstress on those components and consequently, Phillip E.MacDonaldperformedafirstorderstress testonmanyofthecommonmaterialsused for theseassemblies. Of theaustenetic(304L&316L)andNickle(Alloy600,625,690,
Parameter Value
Material SA-533orSA-508Grade3,Class1
DesignPressure
27.5MPa(3990psig)110%ofnominalrating
OperatingTemperature
535oF(553K)
InsideShellDiameter
5.322m(209.5in)
ShellThickness
0.457m(18in)
HeadThickness
0.305m(12in)
VesselWeight
1.7millionlbs
Table 1: SCWR Pressure Vessel Specifications [5]
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Reactor
Figure 7. Schematic of the SCWR power conversion cycle (HPT = high pressure turbine, LPT = low pressure turbine, FWH = feedwater heater) [5].
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2175
3625
T (°F)
P (psig)
536 545
SCWR
BWR
1015
CriticalPoint
3206
L
SV
608 705 932
PWR
Figure 8. SCWR, PWR, & BWR operating conditions on the phase diagram of water [7].
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(a)
(b)
(c)
Figure 9. Cross sections of the 304L stainless steel SCC sample tested in deaerated supercritical water at (a) 750 °F, (b) 930 °F, and (c) 1020 °F
[5].
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&718)basedalloyswhichweretestedinthepressure/temperatureenvironmentpresentinanSCWR,allweresusceptibletoStressCorrosionCracking(SCC),thoughAlloy316LandAlloy690weresufficientlyresistanttowarrantfurtherstudy[5].ExamplesofSCCin304Lstainlesssteelcanbeseenonpage6inFigure9.Oftheferritic-martensitic(HT-9,T-91,&HCM12A)alloystested,manyexhibitedresistancetoSCCbuthadoxidationratesoneorderofmagnitudemorethantheausteneticalloys[5].Thismakestheferritic-martensiticmaterialsillsuitedforthinassemblycomponents.TheseissuesprovideasubstantialengineeringobstacletobeovercomebeforetheSCWRcanbeconsideredaviablenuclearpowerplantdesign.Additionally,noneofthematerialtestingtodatehasbeenperformedineitherahighneutronorhighgammafluxtocharacterizeactualmaterialperformanceinanoperatingreactorvessel.
NaturalGasProductionwithaSupercriticalGeothermalPowerApplicationBy-product
Inmanycases,thesourceforNaturalGasproductionisageothermallyheatedbrine.Inthesecases,thebrinetemperaturesrangefrom240oF(390K)upto360oF(455K)dependinglinearlyuponwelldepth[8].Inthistemperaturerange,thebrinepassesthroughaheatexchangertofeedasupercriticalRankinecycleforPropane,whichhasacriticaltemperatureof206oF(370K)andacriticalpressureof616psia.Theoretically,the brine may be able to run the cycle directly but there are too many contaminants and compositionalvariationsforthistobefeasible.Ifapowercyclelikethiswereemployed,thesitesproducingNaturalGascouldpotentiallygeneratepowerforGriduseor,ataminimum,generatethemajorityoftheplant’selectricalrequirements.IntheUnitedStates,thereareseveralareasalongtheTexasandtheLouisianaGulfCoastwherethistypeofpowercycleisfeasible[8]. Thebenefittothiscycle isthat it isextremelysimpleintermsofsystemcomponents. Thesystemrequires only a single phase heat exchanger, a turbine, an air cooled condenser, and a pump. Figure 10,page 8, depicts a basic system diagram. The nominal operating pressure of this system is approximately1000psia,which suggests thatallof the supportpipingandequipment is commerciallyavailable. Givena 15 Million BTU/hr brine source, this system could generate approximately 400 kW net power with athermalefficiencyof9%[8]. Additionally,thesystemcanbebuilttobeselfregulatingbyusingthepowergridasadynamicbrakefortheturbine-generatorset[8].Ineffect,thisactsasaspeedcontrolfortheturbineduringslightvariationsinsystemdemandundernormaloperatingconditions. Additionalcontrolscanbeimplementedtoautomatethesystembasedonbrinetemperatureandflowrates,allofwhichminimizetheneedtohaveafull-timeoperator,thusreducingoperationalcosts. Despite the downside of a poor thermal efficiency of only 9%, this system is still a viable sourceof energy. Currently,productionplants aredumping theavailableheat to theatmosphere. Thequestionbecomesoneofeconomics;atacostofapproximately$2,131/KW[8](1982dollars≈$5,488todayadjustedfor inflation),will a9%returnpay itselfbackover the lifeof thewell. At today’spriceswithanaverageelectricitycostof$0.11perkWhr,itwouldtakeapproximately6yearstorecoverthecostofthepowerplant.Sinceapowerplantofthissizecanbebuiltonamobileplatform,evenifanindividualwelldoesnotlast6years,thepowerplantcanbemovedtoanewlocationandreused.Basedontheplatformbeingreusable,thisisapotentiallyviablepowerplantdesigndespitethelowthermalefficiencyof9%.
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SupercriticalCO2PowerApplication
ThesupercriticalCO2cycleisonewhichisarguablyaBraytoncycleratherthanaRankinecycle.CO2hasacriticalpressureofapproximately1057psiaandacriticaltemperatureofapproximately88oF(305K).Becausethecriticaltemperatureisachievableduringnormaloperatingconditions,itisfeasibletorunalongthegas/vaporboundarybetween the turbineoutlet and the compressor inlet; thus allowing the system tomaximizetheworkthatcanbeextractedfromthecoolant.Forthisreason,thesupercriticalCO2cyclewillbeconsideredinthispaper.AbasicsystemdiagramisshowninFigure11,page9,andcorrespondingsystemtemperaturesandpressuresintable2,page9. OneoftheGenerationIVnuclearplantdesignsisasupercriticalCO2cycle.ThisdesignoperatesatsimilartemperatureandpressureastheSCWR.Theoutlettemperatureisapproximately1020oF(820K)andtheexitpressureis2900psia[9].Thethermalefficiencyofthiscycleisabout45%butduetotheenhancedheattransferpropertiesofCO2,thecomponentscanbemadesmallerandtherebyreducecomponentcostsbyupto18%comparedtoconventionalBWRandPWRplants[9].Alsoofinterest,thedensityofCO2increasesand compressibilitydecreases substantially as it approaches the critical point. Thispropertyboth reducestheworkrequiredtobeperformedbythecompressorandreducesthefootprintofthephysicalcomponent,thusimprovingefficiencyandcomponentcost.AnexampleoftheconceptualsizedifferencecanbeseeninFigure12,page9. Additionally,thissystemhasreactorinlettemperaturesofapproximately745oF(670K),whichleadstotemperaturedifferentialsofabout275oF(410K)[9].WhiletheconditionsinthesupercriticalCO2cyclearenotasextremeasthoseintheSCWR,theyaresufficienttowarrantextensiveSCCtests,especiallywhereCO2ispotentiallymorecorrosivethanwateratthenominaloperatingtemperatures.
Brine
Turbine
Pump Accumulator
Air CooledCondenser
Primary HeatExchanger
Figure 10. Supercritical Propane Cycle Fed by Geothermally Heated Natural Gas Brine.
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Steam turbine: 55 stages / 250 MWMitsubishi Heavy Industries Ltd, Japan (with casing)
Helium turbine: 17 stages / 333 MW (167 MWe)X.L.Yan, L.M. Lidsky (MIT) (without casing)
Supercritical CO2 turbine: 4 stages / 450 MW (300 MWe)(without casing)
5 m
Compressors are of comparable size
Figure 12. Size Comparison between Turbines of Various Cycles/Coolants [9].
LOWTEMPERATURERECUPERATOR
FLOWMERGE
TURBINE
HIGHTEMPERATURERECUPERATOR
SPLIT
MAIN
PRECOOLER
FLOW
COMPRESSOR
REACTOR2 4
6
7
8 6
8
RECOMPRESSING
188
COMPRESSOR
3
35
3
Figure 11. Supercritical CO2 Recompression Cycle [9].
Point Pressure(psia)
TemperatureoF
1 1101 89.62 2864 142.03 2862 316.44 2858 745.85 2839 1022.06 1131 824.57 1119 335.08 1103 157.3
Table 2: Calculated System Parameters corresponding to Figure 11 [9].
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Conclusion
The supercritical Rankine cycle, in general, offers an additional 30% relative improvement in thethermalefficiencyascomparedtothesamesystemoperatinginthesubcriticalregion. Thecyclehasbeensuccessfullyutilizedinfossilfuelplantsbutthecurrentavailablematerialsprohibitreliableapplicationofthesupercriticalcycletonuclearapplications.Thereismuchworktobedoneinordertoadvancematerialstothepointwheretheywillbeabletoreliablywithstandthestressesofasupercriticalenvironmentinsideanuclearreactorforadesignedlifespanofapproximately60years. Whilemanyofthematerialadvancesareduetotheadventofspecializedcoatings,itisreasonabletosuspectthatnewadvancescouldalsobemadebyimprovementsintheisotopicqualityofthebasemetals.Ithasbeenknownfordecadesthat isotopicallypurematerialshaveconsiderablybetterthermalconductivity.Improvementsinisotopicpuritycanaffectheattransfercharacteristicsbyuptoafactorofthree[11],possiblymore. It should be noted however, the cost of obtaining sufficient quantities of such materials may beprohibitiveandthebenefitgainedistemperaturedependant. OneotherconsiderationforasupercriticalRankinecycleisthesizeoftheplant.Forlargeplantsontheorderofa1600MWethethermalefficienciesareinthe40%region.Bythetimethepowerplantsarereducedtothe20MWerange,thermalefficienciesaredownintothelowtomid20%region[7].Asindicatedbythegeothermalpropaneapplication,whenaplantisinthe400kWeregion,thethermalefficiencyisloweryet.Forthepropanecycle,itwas9%;byextrapolation,asupercriticalsteamcyclewouldbe≈19%efficientwere it applicable. The issue for smaller systems is the cost effectiveness. A supercritical cycle is simplerandreducestheamountofequipmentrequiredtooperatethecyclebutbecauseitoperatesatmuchhigherpressuresandtemperaturesthecostoftheequipmentwhichisrequired,goesupconsiderably.Smallsystemsmayhaveimprovedthermalefficiencybutmaynotbethemostcosteffectivesolutions;forthisreasontheyshouldbeconsideredskepticallybeforebeingimplemented.Justbecauseasystemcanbemademoreefficientdoesnotmeanitisthebestallocationofmoney,ormaterialresources.
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References
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