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US 2012/0117979 A1 May 17,2012 HYBRID CYCLE SOFC -INVERTED GAS TURBINE WITH CO2 SEPARATION FIELD OF INVENTION [0001] Thepresentinventionrelatestopowergeneration andcogenerationofheatandpower. STATE OF THE ART [0002] Tofaceglobalwarmingandgrowingenergyprob lems, concerted efforts are required. The rising demand for electrical power and the necessity to decrease fossil fuel consumptionpushfordevelopmentofnewpowergenerating systems,withhigheref?cienciesandreducedenvironmental impacts. [0003] Anattractivewaytoreachamorerationalenergy conversion of fossil or bio fuels is the decentralized power generationandcogenerationofheatandpower.Amongmajor weaknesses of existing small systems at the building level, consistingmainlyofinternalcombustionorStirlingengines, arelowelectricale?iciency,highmaintenancecosts,together with noise and vibration. Recently introduced mini gas tur bines in the range of 40 to 120 kWel reduce the three latter problems but at an even lower e?iciency. Moreover they are not available in the smaller power range typical of many multi-familyhouses.MoltenCarbonFuelCells(MCFC) and Solid Oxide Fuel Cells (SOFC) are emerging as major can didates to alleviate all the above mentioned drawbacks. [0004] Howeverthefuelcannotbeentirelyconvertedelec trochemicallyinthefuelcellaloneandpartofitiscombusted downstream of the fuel cell with low energy e?iciency. One existing approach suggests to further improve the electrical ef?ciency by combining the fuel cell with a gas turbine in a hybridsystem. [0005] BothMCFCandSOFCaresuitablepartnersforgas turbines, even though SOFC are considered to be the best candidates for these applications because of their higher workingtemperature(around950°C.).The?rstapplications and simulations show a potential of ef?ciency of the order of 60%orhigherwhichwouldmakedecentralizedgenerationas or more ef?cient than large combined cycle power plants. [0006] Inspiteoftheadvantagesofthishybridtechnology, many technical barriers have to be overcome to develop a highly ef?cient system. Fuel cell and gas turbines are rather different power generation systems: the operational mode of a standalone gas turbine is dynamic, whereas the fuel cell is static.Thematchingbetweenthetwosystemsiscrucial.Itis essentialtoguaranteeasafeandstableoperationpointforthe fuel cell, to avoid structural or physical stack damages. Fuel cellpressurizationrepresentsoneofthegreatestchallenges. [0007] ConsideringtheSOFC,twodifferentclassesare [0015] Adifferentwaytofullyintegrateagasturbinewith nowadays available: the tubular and planar geometry. Planar geometry is today the most ef?cient and compact of the two and object ofmuch research, however itisthe more problem aticunderpressurizedconditions. [0008] TheHybridCyclecanbeclassi?edintotwoclasses withrespecttotheintegrationlevelbetweenthegasturbine fullydetailedbyWilson[10],ischaracterizedbytheexpan andthefuelcell[1]. sionintheturbinebeforethecompression.Thistechnologyis [0009] The?rstandsimplestHybridCycleclassconsistsin notinusetodayforpowergenerationpurposesduetothe maintaining the two systems separate and uses a heat exchangerasinterface.An exampleisshowninFIG.1.Inthis case the fuel cell exhaust heats the compressed air in the micro gas turbine recuperator whereas anode and cathode gases are preheated with the gas turbine exhaust gas and heat lower e?iciency obtainable with respect to a traditional gas turbine because ofthe smaller power density. Inoue [11] and Tanaka [12] showed performance evaluations based on simu lationsandexperimentalmeasurementsofagasturbinebased onInvertedBrayton-JouleCycle. releasedfromcombustionofresidualfuelcontainedinthe fuelcellexhaustgas.Thissolutionfacilitatescontrolofthe entiresystem,andcanallowtheSOFC toworkatatmospheric pressure. Inevitably, the drawbacks are high energy losses and therefore a low e?iciency. [0010] Toincreasethesystemef?ciencyitisnecessaryto fullyintegrategasturbinesandfuelcells.Thisisthedirection taken in the second Hybrid Cycle class in which research efforts are more concentrated, and different design solutions havebeendeveloped.Inthemostcommon solution(FIG.2) thecompressorsuppliescompressedairatthefuelcellcath ode. Usually, for planar geometry, the differential pressure between the cathode and anode ishard to maintain; therefore the whole fuel cell is working under the same pressure. For tubular geometry differential pressure between the cathode and anode is easier to maintain. In each case pressurizing the fuelcellspresentsachallenge.Afterpassingthroughthefuel cells,theanodicandcathodic?ows aremixedandthemixture goesintoacombustorintowhichtheunconsumedfuelisfully oxidized. Following this, the hot gases including both the anodic and cathodic ?ows expand in the turbine and are ejectedaswastegases.Usuallyairfactors(ratiobetweenreal and stoechiometric air ?ow) are high because the gas ?ows arealsotheretotaketheextraheatduetoirreversibilityinthe fuel cell, so that the produced steam is rather diluted in the exhaust and therefore dif?cult to condense. [0011] Theworld’s?rstSOFC/GThybridsystem,includ ing a pressurized tubular SOFC module integrated with a microturbine/generator,was deliveredtoSouthernCalifornia Edison for operation at the Irvine’s National Fuel Cell Research Center. This system was the ?rst-ever demonstra tionoftheSOFC/gasturbinehybridconceptandreached53% electrice?iciencyfor220kW [2]. [0012] Otherexperimentalapplicationshavebeendevel oped,butallarerestrictedtothetubularSOFC stacks.Only recently, Lim [3] operates for a few hours a pressurized HybridCycle,accordingwiththeFIG.3,witha25kW micro gasturbineanda5kW classplanarSOFC,designedtowork underatmosphericconditions. [0013] Manystudieshaveassessedthefeasibilityandoper ating condition of a variety of integrated high ef?ciency design alternatives. Palsonn [4] showed the possibility to reach, also with a low pressure ratio (typical of small to medium size dynamic turbomachinery), more than 65% of e?iciency. Massardo [5] analyzed pressurized and atmo spheric systems with ef?ciency up to 75%. [0014] OtherapproachesarerelatedtotheHybridSystem design with a thermo-economic analysis. Autissier [6] dem onstrated the possibility to reach 70% ef?ciency for 6700 $/kWwitha50kW pressurizedsystem. fuel cells is proposed by Tsujikawa [7, 8, 9]. He proposed a Hybrid Cycle with the fuel cell operating at atmospheric pressure and the gas turbine driven in an Inverted Brayton JouleCycle(FIG.3). [0016] TheInvertedBrayton-JouleCycle,whichhasbeenPDF Image | HYBRID CYCLES OF GAS TURBINE WITH CO2 SEPARATION
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