Performance assessment of a hybrid fuel cell and micro gas turbine power system

Transcription

1 energyequipsys/vl 1/ 013/ Energy Equipment and Systems Perfrmance assessment f a hybrid fuel cell and micr gas turbine pwer system Ahmad Tahmasebi a Ahmad Sedaghat b* Rasl Kalbasi a Mahdi Mghimi Zand c a Islamic Azad University, Naafabad Branch, Naafabad, I.R. f Iran b Department f Mechanical Engineering, Isfahan University f Technlgy, Isfahan , I.R. f Iran ABSTRACT In this paper, a hybrid slid xide fuel cell (SOFC) and micr gas turbine (MGT) pwer system is parametrically studied t evaluate the effect f different perating cnditins. The SOFC/MGT pwer system includes SOFC reactr, cmbustin chamber, cmpressr and turbine units, and tw heat exchangers. The effects f fuel utilizatin, temperature, and pressure are assessed n perfrmance f the hybrid SOFC/MGT pwer system using energy and exergy analyses. This study reveals that the main exergy lss ccurs in the external refrmer and the maximum achievable utput pwer is abut 7kW fr the hybrid system. Finally, the prmising first law thermal efficiency f up t 83% is achieved when the secnd law efficiency enhances t 65% fr the hybrid system. c Schl f Mechanical Engineering, Cllege f Engineering, University f Tehran, P.O.Bx , Tehran, Iran Article histry: Received 1 May 013 Accepted 15 June 013 Keywrds: Energy & Exergy, Micr Gas Turbine, Slid Oxide Fuel Cell, Thermal Efficiency. 1. Intrductin The demand fr pwer-generatin systems f high efficiency with lw emissin is f increasing imprtance. Recently, the fuel cell (FC) has been regarded as an enabling clean energy technlgy with great ptential wrldwide because it can cnvert the chemical energy f the fuel directly t energy; therefre, they can theretically achieve high electrical efficiency. Fuel cells prduce very lw levels f pllutant emissins such as NOx, SOx, and CO. They are als amenable t high-vlume prductin as standardized pwer mdules. Over the years many types f fuel cells have been develped with their names based n the type f electrlyte utilized. The first type f cells was knwn as phsphric acid fuel cells (PAFC). *Crrespnding authr: Department f Mechanical Engineering, Isfahan University f Technlgy, Isfahan , I.R. f Iran address: sedaghat@cc.iut.ac.ir, (Ahmad Sedaghat) These cells used a phsphric acid electrlyte and platinum electrdes.the prblems encuntered with these cells included leakage f the electrlyte and the high cst f platinum electrdes. The next type f cells was knwn as the mlten carbnate fuel cell (MCFC). These cells use a mlten salt as an electrlyte and perate at 600 C. The type f cell presently under develpment is the prtn exchange membrane fuel cell (PEM). These cells have an acidic electrlyte in a plymer matrix and are presently being sught by the autmbile industry t prduce electric hybrid vehicles. The latest type and prbably the best suited fr central pwer generatin is the slid xide fuel cell (SOFC). These cells use a slid ceramic electrlyte f yttria zircnia which the perating temperature ranges frm abut 600 t 1000 C [1]. An imprtant advantage f a high-temperature fuel cell is the high temperature exhaust gas which can be used as an additinal heat surce fr ther purpses. With cmbining a gas turbine cycle (GT) t a high temperature fuel cell, a hybrid cycle is frmed and the

2 60 Ahmad Sedaghat et al./energyequipsys /Vl 1/ 013 utlet exergy is recvered and the thermal efficiency is enhanced. The theretical feasibilities f such hybrid pwer systems have been investigated by numerus research grups. Several SOFC based pwer generatin systems develped by Siemens- Westinghuse were simulated using the develped SOFC stack mdel []. Vey and Lundberg [3] shwed that an atmspheric pressure SOFC based pwer generatin cycle has an thermal efficiency range f 45%-50% and the SOFC and gas turbine hybrid cycle can prvide up t 70% f electrical efficiencies [3]. Fr small scale electrical pwer generatin with a range f 50kW, a thermal efficiency f 65% can be expected [4]. In general, simplified r empirical, mathematical r validated mdel are used t predict the behavir f hybrid fuel cell and gas turbine. Stephensn and Richey [5] used simplified and empirical relatin based n perfrmance curves frm Siemens-Westinghuse. Lunghi and Umbertini [6] used a mathematical mdel with detailed electrchemical lss descriptin, simplified assumptin f unifrm cell and gas temperature and current density. Sme advanced fuel cell mdels are utilized by Harvey and Richey [7] and Cstamagna and Massard [8]. Sme studies such as Cmpanari [9], Jhanssn [10], Lunghi and Umbertini [6] fcused n relatively large systems allwing fr higher pressure rati and intercled and reheated gas turbine. But smaller systems is studied mderate inlet cnditins by Campanari [11] and Magistri [1]. In this study, a cmbined SOFC and micr gas turbine are cnsidered. This system is designed based n in twn natural gas utilities. With electrchemical and thermdynamic prperties f SOFC and distinguishing different lsses in SOFC, the SOFC current density during the prcess. The generated heat f reheated gas partly dissipated t the which cmbine SOFC with a micrturbine with and cell vltage are calculated. The effects f temperature, pressure, and fractin f reactant gases as mar parameters, cell dimensin and prprtin f generated pwer in each sectin as minr parameters are investigated at different wrking cnditins.. System Cnfiguratins A schematic diagram f the tubular SOFC/MGT hybrid pwer system is given in Fig.1. Althugh slid xide fuel cells can be fed by a number f fuels (methane, natural gas, bigas, syngas, etc.), methane at ambient temperature is cnsidered here as the fuel. The cmpressed fuel (methane) is heated in heat exchanger and then refrmed by using steam and heat frm the fuel cell. In the refrmer, the fllwing reactin is reprted: CH 4 H C 3H & C H C H The refrmer prductins are fed t ande sectin f SOFC. Ambient air is cmpressed by a turbine driven cmpressr and then heated in the heat exchanger by the ht gas stream frm the turbine exhaust. The ht, high-pressure air is then fed t the cathde. Natural gas and air are channeled thrugh the ande and cathde cmpartments, respectively. The electrchemical reactin ccurs and electrical energy is prduced assciated with heat generatin refrm envirnment, partly used t the natural gas and partly used t heat up the feedstck gases. The high-temperature prductins and unused gases, which cmpund f unutilized refrmed natural gas and depleted air, are guided t a cmbustr where residual fuels (hydrgen, methane and carbn mnxide), reacts with the excess air. The cmbustin prductins with high temperature and pressure are Fig.1. Schematic diagram f the tubular SOFC/MGT hybrid pwer system.

3 Ahmad Sedaghat et al./ energyequipsys/ Vl 1/ channeled t turbine. In turbine, expansin ccurred and thermdynamic prperties such as pressure and temperature are reduced. The specificatin f hybrid system cmpnents are given in Table 1. The turbine utlet has high temperature and it can be used fr increasing f cmpressr utlet in heat exchanger (1). The fllwing assumptin has been made in mdeling: (a) all cmpnents are adiabatic, (b) unifrmity f temperature within all cmpnents, (c) feeding cathde stream is cmpsed f O and N, while feeding ande is СH4, CO, CO, H and HO (d) Fuel cell electrchemical reactin is as 1 O e O Cathde H O H O e ande 1 H O H O Overall reactin (e) The electrchemical reactin f CO has nt been taken int accunt (f) Temperature f gases stream at the utlet f the refrmer and SOFC stack are equal t the refrmer and stack temperature, respectively. (g) Gases d nt leak utside frm the system; (h) Chemical reactins prceed t equilibrium states (i) steady-state peratin is achieved; 3. Electrchemical Ptential Fr any pwer prducing device a maximum wrk ptential is defined fr that system. Fr the fuel cell system, the maximum wrk ptential is knwn as the electrchemical ptential. Fr a simple cell perating reversibly in steady state at cnstant pressure and temperature, the first law f thermdynamics is expressed by: where Si and S represent the entering and exiting entrpies, respectively. Fr a reversible system, the entrpy generated is zer and the heat transfer can be expressed by: Q T S S ) (3) ( i Substituting (3) int (1) and realizing that a reversible system prduces the maximum wrk: H H i W max T ( H S S W 0 i i TS ) ( H i max TS W max G (4) Therefre t calculate the cell's maximum ptential vltage, the change in the Gibbs' free energy is necessary t be knwn fr the verall reactin, als this amunt is equal t EF, then W max Wmax EF E (5) F where the number in (5) represents the number f electrns, E is the maximum vltage (reversible vltage) btainable, and F is the Faraday cnstant Overptentials in SOFC In general, fuel cell achieves its maximum reversible vltage in case f n current impsed by the external lad. Due t irreversibility in the cell peratin, the cell vltage, V, is always lwer than reversible vltage E. This difference termed plarizatin and is the sum f three types f lsses including, activatin, hmic and cncentratin. ) H H i Q W 0 (1) Activatin Lsses where Hi and H represent the entering and exiting enthalpies, respectively. Q represents the input heat and W the utput wrk. The secnd law f thermdynamics can be written as Q Si SO Sgen () T Kinetics f electrchemical reactins is measured by the current density. Bth ande and cathde semireactins must be catalyzed, supplying sme additinal energy in rder t speed up the verall electrchemical reactin. This energy, i.e. the activatin energy, is usually measured by the verptential, which is the difference Table 1. Specificatin f the hybrid SOFC/MGT system. Fuel Inlet Temperature (C) Fuel Inlet Pressure (kpa) SOFC Temperature (C) SOFC Pressure (kpa) Air Inlet Temperature(C) 0 Number Of Cells 10 Air Inlet Pressure (kpa) 100 Cell Area (cm ) 130 External Refrmer Efficiency (%) 80 Cmbustr Isentrpic Efficiency (%) 98 Internal Refrmer Efficiency (%) 80 Cmbustr Pressure drp (%) 4 Cmpressr Isentrpic Efficiency (%) 87 Air Stich 6 Turbine Isentrpic Efficiency (%) 89

4 6 Ahmad Sedaghat et al./energyequipsys /Vl 1/ 013 between equilibrium and nn-equilibrium vltage. At equilibrium, the electrnatin current density, i equals the deelectrnatin current density, i r the number f ins ging frm electrde t electrlyte equals the number ging frm electrlyte t electrde. This equality f charge transfer, which represents a quantitative measure f the rate f reactin, is knwn as the equilibrium exchange-current density, i. These exchange-current densities can vary depending n reactin as well as electrde material and temperature. The energy barrier between the electrde and electrlyte results in a lss knwn as activatin lss, significant at lw currents. The activatin vervltage is calculated using the Butler Vlmer equatin [13], nf nf i i exp Vact exp (1 ) Vact RT RT RT 1 i (6) Vact sinh F i, where is the charge exchange cnstant and set t the value f 0.5, i is the exchange current density which are taken frm Cstamagna & Henegger [14]: 0.5 PO E act, i, cathde cathde exp Pref RT PH P E H O act, i, ande ande exp Pref Pref RT Ohmic Lsses chatde ande (7) Ohmic lsses are caused by resistance t the cnductin f ins thrugh the electrlyte and the electrns thrugh the electrdes and current cllectrs and by cntact resistance between the cell cmpnents. The hmic lsses is calculated frm the resistivities f individual layers such as ande, cathde and electrlyte and is given by [15] V hm i R i A exp( B / T) A s, (8) The crrespnding cefficients f A, B, are given in Table [16]. Table. Cefficients used in equatin (8) [16]. Cmpnent δ(cm) B(K) A(cmΩ) Ande Cathde Electrlyte Intercnnectin Mass Cncentratin Lsses One f the mar lsses knwn t exist in any fuel cell, resulting frm mass transfer limitatins, is knwn as cncentratin plarizatin. In the ideal case, a fuel cell wuld btain a vltage drp equal t the difference in ptential between the tw electrdes when n current was flwing. At this pint the cncentratin f fuel r xidant at the respective electrde wuld be the same as that in the free stream. The cncentratin lss is given by RT i Vcnc n(1 ) (9) F il,where i is the current in the fuel cell and il is the limiting current in that cell as determined frm mass transfer relatins [17]. The limiting current density arises frm the fact that the maximum achievable current per unit area is limited by the rate at which the fuel r xidant can reach the reactin sites. Cncentratin verptential becmes significant when large amunts f current are drawn frm the fuel Fig.. Plarizatin curve[13].

5 Ahmad Sedaghat et al./ energyequipsys/ Vl 1/ cell. The partial pressure f the gases at the reactin sites, which crrespnds t the vlume cncentratin f the gases, will be less than that in the bulk f the gas stream when a large amunt f current is drawn. If such partial pressure r cncentratin plarizatin are unsustainable, cncentratin plarizatin will cause excessive vltage lsses and the fuel cell will cease t perate. All the abve mentined ver vltages are evaluated at the utlet SOFC temperature. The verall vltage f the single cell can be calculated as a functin f current density, temperature, pressure, chemical cmpsitin and gemetric/material characteristics by calculating the difference between the reversible ptential and the ver vltages as shwn in Fig., i.e. plarizatin curve. At the high vltages, a slight decrease in vltage des nt result in a substantial increase in current density. This is due t the dminance f activatin plarizatin term at high vltages and lw current densities. At these high vltages, the marity f energy which wuld be utilized t increase current density is nw utilized t vercme the activatin energy barrier. Belw O.9V, the current density increases fairly linearly with a decrease in vltage due t the dminance f hmic lss term. Belw O.5V the linear trend breaks dwn and the increases in current density are much less fr cntinued decreases in perating vltage. This trend is due t the cncentratin plarizatins r mass transfer limitatins increasing dramatically. Therefre the cell vltage can be calculated as fllws: Vcell = E Vhm Vcncetratin Vactivatin (10) 4. System Mdeling 4.1. Cmbustin Chamber Mdel Since the fuel utilizatin factr [13, 18] is usually lwer than 85%, hydrgen mlar flw at SOFC utlet is nt negligible. In the cmbustr, hydrgen and in lwer percentages carbn mnxide and methane are available. The xidant is the depleted xygen f the cathde utlet stream. The air mass flw rate is much higher than the fuel ne in rder t cntrl stack temperature. Cnsequently, xygen mass flwrate is much higher than the required cmbustin stichimetric value. Cmbustin reactins, assumed at chemical equilibrium and driven int cmpletin, are: CH O CO H O H CO 4 1 O 1 O H O CO (11) The cmbustin chamber is adiabatic and utlet temperature is calculated n the basis f a simple energy balance n the cmbustr cntrl vlume. 4.. Heat Exchanger Mdel As mentined, the SOFC temperature perating range is frm abut 600 t 1000 C. Althugh the verall chemical and electrchemical reactins are largely exthermic, air and fuel must be preheated befre entering the stack in rder t avid high temperature gradients. Such prcesses are perfrmed by means f tw tubes in tube cunter flw heat exchangers. The heat exchanger perfrms tw-sided energy and mass flw rate balances. The energy balances fr the ht and cld fluids are expressed by: mcld ( H ut H in ) cld Qleak ) (1) ( m ( H H ) Q ) 0 ht in ut ht The ttal heat transfer between the tube and shell sides (heat exchanger duty) can be defined in terms f the verall heat transfer cefficient, the area available fr heat exchange, and the lg mean temperature difference: Q UAT F (13) LM t These devices are simulated n the basis f the NTU methd [19], implementing temperaturedependent specific heats. lss 4.3. Turbmachineries Mdel The centrifugal cmpressr peratin is used t increase the pressure f an ideal gas stream with relative high capacities and lw cmpressin rati. The turbine peratin is used t decrease the pressure f a high pressure inlet gas stream t prduce an utlet stream with lw pressure and velcity. Fr a centrifugal cmpressr, the isentrpic efficiency is given as the rati f the isentrpic (ideal) pwer required fr cmpressin t the actual pwer required: Efficiency Cmpressr(%) pwer required pwer required isentrpic actual 100% (14) Fr the turbine, the efficiency is given as the rati f the actual pwer prduced in the expansin prcess t the pwer prduced fr an isentrpic expansin: Efficiency turbine pwer prduced actual (%) 100% pwer prduced isentrpic (15) In general, the input r utput wrk fr a mechanically reversible prcess can be determined frm: W V dp (16) With simplificatin, the required wrk fr cmpressr r the prduced wrk by the turbine is calculated by [19]: n P 1 p W F 1( MW ) CF n 1 1 p1 n1 n 1 (17) Fr the centrifugal cmpressr: Pwer Requiredactual = Entalphyutlet Entalphyinlet and fr the turbine, Pwer prducedactual = Entalphyinlet Entalphyutlet. In the case f air cmpressr, the inlet status crrespnds at the

6 64 Ahmad Sedaghat et al./energyequipsys /Vl 1/ 013 envirnmental ne, whereas temperature and pressure f the GT inlet stream can cnsiderably vary accrding t the changes in the design and perating parameters. Air cmpressr and gas turbine are assumed t be cupled n a unique shaft; as a cnsequence, they must have the same rtr speed SOFC Stack Mdel The SOFC stack cnsists f many single cells t increase the vltage utput and hence the electrical pwer fr practical applicatins. In the SOFC stack mdel, several parameters such as gas utilizatin, energy efficiency, thermdynamic prperties (pressure, temperature), electric pwer and interactin between the cells are cnsidered. A cmplete analysis f energy and mass flw rate are perfrmed in HYSYS sftware package, and the simulatin results are derived. The mdel cnsiders mass flw rate variatin due t chemical reactin such as hydrgen xidatin, methane refrming, and water gas shift reactin f carbn mnxide. The hydrgen and xygen is cnsumed and the rest amunt f hydrgen and xygen is btained frm faraday's law. It ffers a cnvenient and time saving means fr chemical prcess studies, including system mdeling, integratin and ptimizatin. It is used in this study as a prcess simulatin tl t investigate ptential SOFC based pwer generatin cycles including SOFC stack and the balance f plants. T facilitate the study, a natural gas feed tabular SOFC stack mdel is develped using existing HYSYS functins and unit peratin mdels with minimum requirements fr linking f a subrutine. This apprach fully utilizes the existing capabilities f this prcess simulatr and prvides a cnvenient way t perfrm detailed prcess study f SOFC based pwer generatin cycles. The SOFC prduced pwer is: P niv (18) cell where n dentes the SOFC cell numbers. Then the net pwer frm the hybrid SOFC/MGT system is determined by P P P P (19) net sfc turbine 5. Mdel Validatin cmpressr In rder t validate the current mdeling, different lsses are calculated in HYSYS and the results are cmpared with Calise and his clleagues [0] in Fig.3 t 6. Fig.3. Activatin lsses cmparisn between present wrk and Calise [0].

7 Ahmad Sedaghat et al./ energyequipsys/ Vl 1/ Fig.4. Ohmic lsses cmparisn between present wrk and Calise [0]. Fig.5. Cncentratin lsses cmparisn between present wrk and Calise [0].

8 66 Ahmad Sedaghat et al./energyequipsys /Vl 1/ 013 Fig.6. Plarizatin curve cmparisn between present wrk and Calise [0]. 6. Exergy Analysis 6.1. Exergy Cncept The secnd law f thermdynamics has prved t be a very pwerful tl in the ptimizatin f cmplex systems. A system can delivers the maximum pssible wrk as it underges a reversible prcess frm the specified initial state t the state f its envirnment, that is, the dead state. This represents the useful wrk ptential f the system at the specified state and is called exergy. Rather, it represents the upper limit n the amunt f device wrk can deliver withut vilating any thermdynamic laws. Frm the first lw f thermdynamic, it is evident that the energy is cnserved. Based n cnservatin f energy principle, energy cannt be created r destryed during a prcess. Unlike the energy, exergy is nt cnserved. Fr an islated system during a prcess always decreases r, in the limiting case f a reversible prcess, remains cnstant. In ther wrds, it never increases and exergy is destryed during an actual prcess. This is knwn as the decrease f exergy principle. Irreversibilities such as frictin, mixing, chemical reactins, and heat transfer thrugh a finite temperature difference, unrestrained expansin, nnquasiequilibrium cmpressin r expansin always generate entrpy, and in general anything that generates entrpy always destrys exergy. The exergy destryed is prprtinal t the generated entrpy and expressed as fllwing: EX T S (0) destryed gen, where EX is exergy, Sgen is entrpy generatin and is cmputed accrding t entrpy balance. S in S S S (1) ut gen Als exergy destryed can be expressed frm exergy balance EX in EX ut EX destryed EX sys () Exergy destryed represents the lst wrk ptential and is als related t the irreversibility [1]. In this study exergy destryed is calculated frm bth entrpy and exergy balance. 6.. Exergy Cmpnents Exergy f a stream f matter in the absence f nuclear, magnetism, electricity and surface tensin effects, is the sum f EX EX EX EX EX (3) ke pe In the present study, the changes in the kinetic and gravitatinal ptential energies are cnsidered t be negligible, and then the thermal and chemical exergy are given by: EX th ( H H ) T ( S S ) EX X ( ) (4) ch 0 00 where X is the mle fractin, is the chemical 0 ptential f the species which is evaluated at T0 and P 0, is the chemical ptential which is 00 calculated in the reference envirnment Exergy Balances In rder t calculate the irreversibility, exergy balance f each cmpnent in Fig.1 shuld be carried ut. Fr heat exchanger and turbmachinery cmpnent, destryed exergy can be btained frm Eq.(5). sys th ch

9 Pwer (W) Ahmad Sedaghat et al./ energyequipsys/ Vl 1/ Fr external refrmer, SOFC and cmbustr, the exergy destryed can be calculated frm entrpy balance. ( S gen) External Re frmer Q / T m 3S3 m 1S1 m S ( E ) T S destryed ExternalRe frmer E destryed gen Q T0 m 3S3 m 1S1 m S T0 m 3S3 m 1S1 m m S m S m S m S Q ( E ) S ( E destryed destryed ) SOFC cmbustr T (5) The destryed exergy is calculated fr each cmpnent and listed in Table 3. The inlet exergy is referred t Methane which is entered at 5 C. Increasing temperature decreases the verptantial lsses and irreversibilities in SOFC; therefre, it imprves the verall efficiency as well as the secnd lw efficiency. 7. Parametric Study fr the Hybrid System In this sectin, the effect f different parameters such as temperature, pressure, cathde and ande mlar flw has been studied The Temperature f SOFC Althugh the pen circuit vltage decreases with increasing temperature accrding t Eq.(4), the perfrmance at perating current densities increases with increasing temperature due t reduced mass transfer plarizatins and hmic lsses. The dependency f SOFC perfrmance n temperature is illustrated in Fig.6. The sharp decrease in cell vltage as a functin f current density at 800 C is a result f the high hmic plarizatin f the slid electrlyte at this temperature. The hmic plarizatin decreases as the perating temperature increases and crrespndingly, the current density at a given cell vltage increases. Increasing the temperature als affects the turbine and its utput pwer will increase. Figure 7a, shws temperature changes n prduced pwer changes f cell. As can be bserved increase in cell utput pwer is higher than the gas turbine. S that the cell shares in prducing ttal pwer in temperatures 800, 900 and 1000 are respectively equal with 50.5%, 59% and 61%. 7.. The Pressure f Cycle With increasing f pressure in cycle, the inlet enthalpy f turbine increased and then pwer prduced is imprved. Figure 7b shws the prduced pwer in SOFC and turbine if pressure is changed. SOFC fuel cell shws an enhanced perfrmance by increasing cell pressure. The effects f pressure f the cell vltage are als shwn in Fig.8 favrably imprving yield f the cell at higher current densities. Table 3. Destryed exergy in each cmpnent. Temperature ( C) Inlet Exergy (kw) Refrmer Cmpressr Turbine Destryed Exergy (kw) SOFC Cmbustr Heat Exchanger 1 Heat Exchanger Outlet Exergy (kw) Secnd Lw Efficiency (%) 800 C 900 C 1000 C SOFC Turbine SOFC/MGT Temperature ( O C) (a)

10 Pwer (W) 68 Ahmad Sedaghat et al./energyequipsys /Vl 1/ SOFC Turbine SOFC/MGT Pressure (kpa) (b) Fig.7. Pwer prduced by SOFC, turbine, and SOFC/MGT at different: (a) temperatures; (b) pressures. Fig.8. Plarizatin curve at different current density The Mass Flw Rate Anther design variable which can impact the verall perfrmance f the fuel cell is the Mass flw rate (the current density f the fuel cell). The purpse f this sectin is t bserve the effects f changing ne r bth f the gaseus flw rates entering the cell. Figure 9, shws the current density effects n the prduced pwer and n the first and secnd law efficiencies The Mass Flw Rate f Fuel Figure 9a, als shws the pwer curves fr different fuel mlar rates, since the fuel mlar rates are dependent n current densities, fr the SOFC, turbine, and the cmbined SOFC/MGT systems. The flw rate f air was varied t prvide six times the stichimetric amunt needed t react all f the fuel. By increasing the fuel mass flw rate, the utput current value f the fuel cell increases and thus

11 Ahmad Sedaghat et al./ energyequipsys/ Vl 1/ activatin, hmic and cncentratin lsses increase and cell vltage and therefre utput pwer f the cell will be reduced. Secnd law efficiency in this case because f the cell entry t the cncentratin drp regin will find a large reductin. By increasing the fuel flw rate and therefre flw rate passing thrugh the turbine, the pwer will increase. Figure 9b shws a drp in bth first and secnd lw efficiency with increasing mass flw rates. The decrease in efficiency can be explained by a review f the ver ptential lsses. It was shwn that increases in flw rate actually shwed a decrease in utput pwer. With increases in flw rate, bth the air and fuel blwers were required t mve mre gas resulting in larger size, higher cst, and a greater amunt f wrk required. This increases in parasitic pwer alng with a decrease in utput pwer results in a lwer verall efficiency. Likewise, the increase in size and cst f all cmpnents f supprt equipment drives the cst f each kilwatt f electrical pwer. What cannt be seen frm this figure is the envirnmental impact. With n gains in pwer fr increases in mass flw rate, the exhaust stream becmes rich in hydrgen, carbn mnxide, and xygen resulting in mre prducts being cmbusted. Since ne f the advantages f a fuel cell is its imprved emissins ver a nrmal cmbustin pwer cycle, higher flw rates are determined frm bth emissins and efficiency standpint. (a) (b) Fig.9. (a) Pwer and (b) efficiency prduced by SOFC/MGT at different current densities.

12 Pwer (kw) 70 Ahmad Sedaghat et al./energyequipsys /Vl 1/ The Mass Flw Rate f Air With increasing air flw due t increasing f partial pressure f cathde, activatin lsses area reduces. By reducing irreversibility f activatin regin, the utput fuel cell vltage and pwer increases. Als with increasing air flw rate, flw rate passing thrugh the turbine increases. By increasing the mass flw rate passing thrugh the turbine, the utput pwer will increase. Figure 10a, shws utput pwer in terms f current density (mass flw rate f the fuel cell). Als first and secnd law efficiencies are shwn in Fig.10a. Due t increased utput pwer, first law efficiency increases and due t cnstant entry exergy and reductin f fuel cell lsses, secnd law efficiency increases Effect f Fuel Utilizatin in Pwer Prduced Using internal refrmer and als high prsity materials in the cell have extremely psitive effect n the cell functin. In Fig.10b the pwer utput in SOFC and Turbine based n fuel utilizatin are cmpared. As it is apparent frm the diagram, increasing the ability t use fuel causes the increase f current density and f curse the utput pwer will increase. And while the increase in fuel cnsumptin by cell, the amunt f utput hydrgen is reduced and therefre the utput f cmbustin area temperature will reduce. By decreasing the entrance temperature f turbine, the utput pwer will decrease. But the Fig.10b reveals that by increasing the cnsumptin f hydrgen in the cell, the ttal prduced pwer in system will nt change. It seems that increasing the fuel cell pwer and decreasing the prduced pwer in the turbine are neutralizing their effects. (a) Fuel Utilizatin SOFC Turbine SOFC/MGT (b) Fig.10. Effect f air and fuel utilizatin n pwer prduced.

13 Ahmad Sedaghat et al./ energyequipsys/ Vl 1/ Maximum Output Pwer f the Cmbined System By drawing efficiency based n the ttal utput pwer f the system, the maximum utput pwer level will be btained. Figure 11 shws these changes. As bserved with increasing the utput pwer, the efficiency is decreased. Maximum utput pwer is estimated t be arund 7 kw. The Maximum pwer is clse t the cncentratin lsses regin. It is dangerus t apprach this regin because upn arrival t this regin the cell cntrl will be lst.here with depicting the Grssman diagram fr the system in design pint (Fig.1) and by cnsidering the input exergy in basis 100, the better illustratin fr exergy analysis is presented. As Fig.1 represents, the secnd law efficiency f system is calculated 65%. The mst exergy lsses are seen in external refrming due t endthermic chemical reactins in this part f system. Als using heat exchangers caused reductin in exergy lsses frm kw t kw. Fig.11. The efficiency versus pwer. Fig.1. Grssman diagram fr the system in design pint.

14 7 Ahmad Sedaghat et al./energyequipsys /Vl 1/ Cnclusin In this paper, a parametric study was cnducted t assess the effects f different parameters n perfrmance f a hybrid SOFC/MGT pwer system. The parameters studied include temperature, pressure, the cathde flw rate, the ande flw rate, and percentage f fuel use. Using energy and exergy analyses, the pwer prductin and the first and secnd law efficiencies f the hybrid SOFC/MGT system were determined. The first and secnd law efficiencies are calculated and the prprtin f the pwer prduced by each cmpnent f the hybrid system was evaluated. It is bserved that, the main exergy lss ccurs in the external refrmer. Further finding is that the maximum achievable utput pwer frm the hybrid SOFC/MGT system is apprximately abut 7kW. Mrever, the thermal efficiency f the hybrid SOFC/MGT system can be increased t 83% while aviding critical cnditins. Detecting the external refrmer as the cmpnent which destrys mre amunt f exergy, the secnd law efficiency is calculated t be 65% fr the hybrid SOFC/MGT system. Nmenclature A heat transfer area (m) A.I Ande Input A.O Ande Output C Cmpressr C.C Cmbustin Chamber C.I Cathde Input C.O Cathde Output Cp Heat specific capacity (kj.kg -1.K -1 ) E Open circuit reversible ptential (V) E.R Enternal Refrmer Eact Activatin energy (kj.kml -1 ) F Faraday cnstant (As ml -1 ) G Gibbs free energy (kj.kml -1 ) H.E (1) Heat Exchanger (1) H.E () Heat Exchanger () i current density (ma.cm - ) i L limiting current density (ma.cm - ) i Exchange current density (ma.cm - ) Μ chemical ptential (kj.kml -1 ) m mass flw rate (kg.s -1 ) P electrical pwer (kw) p pressure (bar) Q Heat (kj) area specific resistance (.cm - ) T temperature (K), Turbine U heat transfer cefficient (kw.m -.K -1 ) U f fuel uilizatin factr V cell ptential (V) activatin verptential (V) V act V cnc V hm cncentratin verptential (V) hmic verptential (V) W Wrk (kj) Xi cmpnent mlar fractin Greek Symbls T LM Mean lgarithmic temperature δ cmpnent thickness (cm) η Efficiency ρ density (kg/m 3 ) Subscripts act Activatin i Inlet Outlet ref reference References [1] Appleby, A. and Fulkes, F., A Fuel Cell Handbk, nd, Keiger Publishing C, Flrida, [] K. Hassmann, SOFC pwer plants, the Siemens Westinghuse apprach, Fuel Cells 1 (001) 1. [3] Vey S.E., Lundberg W. (1999) Slid xide fuel cell pwer system cycles ASME Jurnal, 99- GT-356. [4] S. campanari, fuel lad and part lad perfrmance predictin fr integrated SOFC and micrturbine system, J. Eng. Gas Turbine Pwer 1 (000) [5] Stephensn, D. and Ritchey, I., "parametric Study f Fuel Cell and Gas Turbine Cmbined Cycle Perfrmance", Prceeding f ASME, 97-GT-340, [6] Lunghi, P. and Ubertini, S., "Slid Oxide Fuel Cells and Regenerated Gas Turbines Hybrid Systems; a Feasible Slutin fr Future Ultra High Efficiency Pwer Plants" Prceeding f the Electrchemical Sciety, SOFC-VII, Tsukuba, Japan, 001. [7] Harvey, S.P and Richter, H.J., "Gas Turbine Cycles with Slid Oxide Fuel Cells parts I and II" Jurnal f Energy Resurces Technlgy, Vl. 116, [8] Cstamagna. P., Massad, A. and Bednt, P., "Techn-Ecnmical Analysis f SOFC Reactr- Gas Turbine Cmbined Plants", Prceeding f the furth Eurpean Slid Oxide Fuel Cell Frum, Lucerne, Switzerland, July, 000. [9] Campanari, S. and Macchi E., "thermdynamic Analysis f Advanced Pwer Cycles Based upn Slid Oxide Fuel Cell, Gas Turbine and Rankine Bttming Cycles", ASME Cnference, 98-GT Stckhlm, Sweden, [10] Jhanssn, K., Bafalt, M. and Palssn, J., "slid Oxide Fuel Cells in Future Gas Turbine Cmbined Pwer Plants", nd CIMAC Internatinal Cngress n Cmbustin Engines, 1998, Cpenhagen, Denmark. [11] Campanari, S., "Full lad and part lad perfrmance predictin fr integrated SOFC and micrturbine systems", prceedings frm ASME, 99-GT-065, Indianaplis, USA, 1999.

15 [1] Magistri, L., Massard, A., Rdgers, C. and McDnald, C., "A Hybrid System Based n a persnal Turbine (5kW) and a SOFC Stack: A Flexible and High Efficiency Energy Cncept fr the Distributed Pwer Market", Prceeding f ASME TurbExp 001, 001-GT-009, New Orleans, Luisiana, Usa, June, 001. [13] Larminie, J. and Dicks, A., Fuel Cell Systems Explained, Jhn Wiley, NewYrk, 000. [14] Cstamagna P, Henegger K. Mdelling f slid xide heat exchanger integrated stacks and simulatin at high fuel utilizatin.j Electrchem Sc 1998;145(11). [15] Calise, F., Dentice d Accadiaa, M. Palmba A. and Vanlib, L. Simulatin and exergy analysis f a hybrid Slid Oxide Fuel Cell (SOFC) Gas Turbine system, Jurnal f pwer surces, Vl.158, pp. 5-44, 006. [16] S.H. Chan, H.K. H, Y. Tian. " Multi-level mdeling f SOFC gas turbine hybrid system", Internatinal Jurnal f Hydrgen Energy 8 (003) [17] Chan,S. and Tian, Y. Multi-level mdeling f SOFC gas turbine hybrid system, Internatinal Jurnal f Hydrgen Energy 8, 003. [18] S.C. Singhal, K. Kendall, High temperature Slid Oxide Fuel Cells,Elsevier, 003. [19] Aspen Tech, User Dcumentatin fr Aspen Plus, Versin 10.1, Cambridge, Massachusetts, USA, 000. [0] Calise, F., Dentice d Accadiaa, M. Palmba A. and Vanlib, L. Simulatin and exergy analysis f a hybrid Slid Oxide Fuel Cell (SOFC) Gas Turbine System, Jurnal f pwer surces, Vl.158, pp. 5-44, 006. [1] Wark, K., Advanced thermdynamics fr engineers, McGraw-Hill, New Yrk, Ahmad Sedaghat et al./ energyequipsys/ Vl 1/

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