Nomenclature: number of electrons e -1. electron charge F constant number, (columbs/moles of e -1 ) atomic number g

Transcription

1 Quanttatve Analyss f Irreversbltes Causes Vltage Drp n Fuel cell (Smulatn) Hssen Ghadaman*, Dr. Yadlah Sabh** Department f Energy Engneerng, Scence and Research Branch Azad Unversty, Islamc Republc f IRAN Sharf Energy Research Insttute(SERI) Abstract Pwer level f a fuel cell depends n ts peratng cndtn, whch s prduct f vltage and current-densty.the hghest level f vltage s dentfed as reversble pen crcut vltage (ROCV), whch represents an deal theretcal case. [3] Cmpared t that s deal peratng vltage whch s usually characterzed as pen crcut vltage (OCV).[3] An evaluatn f devatn f peratng vltage level frm deal peratnal case may prvde nfrmatn n the extent f mprvng effcency and energy effcency f a fuel cell. Therefre, quantfcatn f peratn devatn frm OCV s the man pnt that s dscussed n the present paper. The analyss prcedure f vltage drp s based n step-by-step revew f vltage drps ver actvatn, nternal currents (fuel-crss-ver), hmc & mass- transprt r cncentratn lsses. Accumulated ttal vltage drps wuld be estmated as a sum f afrementned lsses. The accumulated vltage drps wll then be reduced frm OCV t btan the peratng vltage level. The abve numercal analyss has been appled t dentfy the extents f vltage drp. The pssble reducng varables n vltage drps revewed & cncluded that the actvatn lss has cnsderable mpact n ttal vltage drps and t explans the mst part f ttal lsses. It s als fund that the fllwng crrespndence parameters cause decrease n vltage drps: 1. Temperature ncreasng. Pressure ncreasng 3. Hydrgen r xygen cncentratn ncreasng 4. Electrde effectve surface ncreasng 5. Electrde & electrlyte, cnductvty mdfcatn. 6. Electrlyte thckness decreasng t lmted (lwest pssble). 7. Cnnectn mdfcatn. eywrds: fuel cell; vltage drp Nmenclature: N number f electrns e -1 electrn charge F cnstant number, (clumbs/mles f e -1 ) Z atmc number g f mlar Gbbs energy ROCV reversble pen crcut vltage OCV pen crcut vltage T temperature P pressure R gas cnstant, (kj/mle) charge transfer ceffcent v vltage, vlt current densty, ma/cm r specfc surface resstance, k. cm Bc cathde B Ba ande B P1 pressure n pnt n.1 P pressure n pnt n. IL maxmum current densty, ma/cm n partal t ttal pressure rat, Pn/P * Crrespndng authr. Tel.: , Fax: E-mal address: hghadaman@yah.cm ** Prfessr assstant at Sharf unversty f technlgy and Head f Sharf energy Insttute (SERI)

2 1. Intrductn Fuel cells are nwadays technlgy f energy systems. Vltage drps are explaned wth dfferent phrases. Electr chemsts explan vltage drp by ver vltage r verptentals, ths explanatn s nt a useful wrd, because t appears an ncreasng meanng n readers mnd. [5] Sme f engneers call t plarzatn. As plarzatn ften s used fr electrcty whch s useless accumulated, t desn t have equal defntn fr vltage drps. [5] Therm-dynamc scentsts, has an pnn t explan ths vltage drps by rreversbltes, we beleve ths phrase s s general, and t desn t have enugh accuracy n explanatn. Mst f electrcal engneers use vltagedrps n ths case, ths phrase s s expermental and n scence frm t use, but t s s meanngful and very gd adapted wth current-vltage graph. [5] Sme ther scentsts use lsses, t s t cmmn t use, and nether has enugh exactness appntng t the subject. The best explanatn phrase s rreversbltes cause vltage drps, whch s a cmbnatn f revewed phrases n wrds and meanngs. [3] In fact all mentned wrds & phrases have the same meanng and cause reductn n effcency & utput pwer f fuel cell.. Reversble pen crcut vltage (deal theretcal) When we assume reversble prcess, t means that, Gbbs free energy s cnverted t electrcty drectly. In ths cndtn we have ROCV n vltage. [1] In hydrgen fuel cell, base n ne mlecules f generated water r used hydrgen, tw electrns wuld be generated, and meantme ne mle f hydrgen usng, N electrns passng n external wre. [] Prcedure charge = -Z.N.e = -Z.F = -*96485 electrcal wrk = charge * vltage = -.F.E reversble system : electrcal wrk = Gbbs free energy -Z.F.E = g E = ROCV = - Cnclude calculatn T = 100 C P=1atm g f = -5. kj/mle f H Z = F = clumbs ROCV = 1.57 vlts f g f / Z.F 3. Open crcut vltage (deal peratnal) The deal peratn f a fuel cell s defned by Nerst-equatn, whch s hghly depended n electr- chemcal reactns f xygen & dfferent fuels. Nerst equatn explans bascs f peratn n fuel cell as an electr-chemcal prcess vew. [1] In fact t descrbes the relatn f dfferental partal pressure and dfferental ptental. In ther wrds, f we remve the ptental drps f zer current level f ROCV, we reach t pen crcut vltage. Prcedure j J + k m M actvty = a = P/P g f = E = E + g f a H = PH/P j aj a a j k aj a m am - R.T.Ln * Ln a O = PO/P E = E + aho= PHO/P a ROCV=OCV +.Ln ROCV=OCV + v ( 0) Cnclude calculatn Daltn Rule P.V = n.r.t P = PA + PB a H = 1 a O = 1/3 a HO = 1/3 OCV = vlts k m M * Ln j aj a a a a 1/ H O HO k m M 4. Actvatn lsses Electrde surface vltage ncreasng, relevant t current densty lgarthm equatn whch s ftted n the belw graph s called Tafel equatn. Alpha s charge transfer ceffcent, whch s used fr changng the electrchemcal reactns, rate. It s ften bunded between ( ). [3] Als reactn are balanced n zer current densty cndtn, t means that the number f electrns whch mve ff the electrlyte are equal t the cmng back electrns. Ths knd f explaned balancng current s called exchanged current densty (). Whenever we have ncreasng n, we have mre actve electrdes n use. [5] :

3 It shuld be ntced, n lw &mddle temperature fuel cells ths type f lsses (actvatn) s mre cnsderable. In addtn t, actvatn lsses n cathdes are mre than andes, s shuld be cncerned mre. v A. Ln( / ) A Z.. F Cnclude calculatn R = kj/kmle k T = k F = A = 0.03 v 0.03* Ln( / 0.07) (b) Actvatn lsses decreasng methds (b-1) fuel cell temperature ncreasng: It s smply fnd ut frm cncluded graph, ncreasng n temperature cause ncreasng n exchange current densty and results cnsderable drps n actvatn lsses. (b-) mre advanced catalyst usng: Dfferent materal tested n catalyst explans nckel & platnum are better than znc & ther sub-famlar materal. [5] (b-3)ncreasng rughness n electrde surface: Ths rder cause ncreasng n effectve surface rat rare t nmnal surface f electrdes. Actually the man effect s ncreasng n exchange current densty. (b-4) ncreasng n cncentratn f reactants: If we use xygen nstead f ar, the catalyst spaces are ccuped wth reactants mre effectvely. Ths subject has anther duble effect n ROCV ncreasng. [] (b-5) pressure ncreasng: Ths rder has dual effect, frstly ncreasng n catalyst spaces ccupancy, secndly ROCV ncreasng. [5] 5. Fuel crss-ver r nternal currents In fuel cell peratn cndtn, sme f hydrgen (fuel) separated frm ande & dffused, then t crss-ver catalyst t cathde & reacted wth xygen drectly, ths reactn prduce water. [6] It shuld be ntced, n utput current exsts n ths case, and ths type f lsses fuel s called fuel crss ver r nternal currents. In energy lsses evaluatn ths lsses are nt cnsderable, but the effect f that n OCV reductn s mprtant. S, by smlar effects, n equatns and quantfcatn nternal current packed wth actvatn lsses as belw: v A. Ln ( n) / A Z.. F gas usage = n F Cnclude calculatn R = kj/kmle k T = k F = A = n 3( ma / cm ) v 0.03* Ln ( 3) / 0.07 (b) Fuel crss-ver lsses decreasng methd It shuld be suppse the methds n actvatn lsses decreasng are as the same as methds f fuel crss-ver lsses decreasng. But the mst effectve rders n ths case s, ncreasng n reactant cncentratns & pressure ncreasng. [6] 6. Ohmc lsses The smple lsses n mdelng & understandng are Ohmc lsses, whch cnsst f electrcal resstance f electrdes &n transprtatn resstance n electrlyte. [3] It has been cnclude that, the resstance f electrlyte s mre cnsderable. t shuld be

4 cnsder, Ohmc lsses appears mre n hgh temperature fuel cells. v. r Cnclude calculatn r = 30*10-6 v k. cm 6 30*10 * Cncept P v B. Ln( ) P 1 B = Bc + Ba B P = P1( 1- /l) Cnclude calculatn Ba = 0.16 vlts Bc = 0.08 vlts B = 0.04 vlts L = 800 ma v 0.04* Ln 1 ( /800) (b) Ohmc lsses decreasng methds (b-1) usng the electrdes wth hghest pssble electr-cnductvty. (b-) gd desgn fr plates & cell cnnectns. [5] (b-3) electrlyte thckness reducng up t pssble lmtatn: Ths case s rarely dffcult, because electrlyte s a supprt fr electrdes, s t shuld have enugh mechancal stablty. In addtn t, the electrlyte needs t be thck enugh fr ns crculatn pssblty. On the ther hand, the thckness s serusly related t shrt-cut prhbtn. [5] 7. Mass transprt r cncentratn lsses If we use ar n fuel cell, n the mddle f peratn, the xygen cncentratn reduced sde t electrdes. [3] Reductn rate f cncentratn depends n three tems: [6] Requested current quantty f fuel cell Physcally characters f ar crculatn besdes the cathde The speed f xygen substatn n cathde If we nject hydrgen t ande, after fuel cnsumptn n a perd f tme, the hydrgen pressure wll be reduced, als reductn f pressure s depends n smlar tem. It s t mprtant that, ntrgen exstence and xygen cnsumptn make ths lss mre & mre. Fnally, mass transprt r cncentratn lsses are cnsderable f system cnsst fuel refrmng r ar feedng nstead f xygen. (b) Mass transprt lsses decreasng methds (b-1) xygen feedng n cathde nstead f ar (b-) makng an ntellgent system n hydrgen njectn t ande, whch wrks n hydrgen pressure reductn rder n fuel cell. (b-3) usng mre pure hydrgen, nstead f hydrcarbn fuels wth refrmng system. 8. Ttal vltage lsses If we sum up all knd f mentned lsses (actvatn, Ohmc, nternal currents & mass transprt), then less that frm ROCV level, the vltage frmula wll cnclude. Ths vltage frmula wth zer level current densty cndtn wll make pen crcut vltage, thus; v vactvatn vfuelcrssver v hmc v cncentratn v OCV = ROCV - ( ) V = ROCV (+ n).r A.Ln( n - B.Ln (1 ) L n )

5 Cnclude calculatn v v actvatn fuelcrssver hmc 6 30*10 * * Ln 0.07 vcncentratn 0.04* Ln(1 ) * Ln(1 ) V * Ln( ) 30*10 electr-cnductvty (7) Gd desgn fr plates and cell cnnectns r n a bref phrase Cnnectn mdfcatn. 10. References [1].Calculatn sheets f Vrgna tech. Unversty, PEM text fles, Vrgna tech. unversty reprts, 00. [].1 kw PEM stack desgn, Dr.azemn, frst Iranan fuel cell cnference n Sharf unversty, 001. [3].Fuel cell system explaned, J.Larmn, A.Dcks, Jhn Wlly & sns, pblsher, Nv.000, ISBN. [4].Heat &mass transfer effects n PEM fuel cell, N.Vanderberg, J.Huff, ls-alams labratry, 001. [5].Fuel cell Handbk, fve edtn, Parsn Ins., Natnal Energy Technlgy Labratry, Oct.000. [6].One dmensnal transent mdel f heat, mass & charge transfer n PEM, Bardn M.Eaten M.Sc. thess, Vrgna tech. unversty webste, 001. In abve graph the zne n.1 s the dfference f ROCV and OCV levels. Ths zne n lw temp fuel cells s cnsderable and n hgh temperature type s neglgble. The zne n. ncludes knetc lsses whch explan actvatn and nternal current drps. Ths zne n lw temperature fuel cells has bgger slpe n cmparsn wth hgh temperature type. In zne n.3 whch explans n transprt lsses, and btan Ohmc electrlyte lsses. In lw temperature fuel cells current vltage drp rat s lnear and has slw slp, but n hgh temperature type slp s bgger and nn-lnear. On the end, n hgh temperature fuel cells, separatn f znes n.&3 s nt pssble. It was because f depend effects f actvatn, Ohmc and nternal current drps. In zne n.4 whch explans mass transprt lsses s hghly effectve n bth types f fuel cells. 9. Cnclusn Base n quantfcatn f effects n vltage drps n a prttype PEM fuel cell t s als be fund that the fllwng crrespndence parameters cause decrease n vltage drps: (1) Temperature ncreasng () Pressure ncreasng (3) Hydrgen r xygen cncentratn ncreasng (4) Electrde effectve surface ncreasng (5) Electrlyte thckness reducng up t pssble lmtatn. (6) Usng the electrdes wth hghest pssble