Modern Applied Siene August, 2009 Using Mximum Power Cpbility of Fuel Cell in Diret Methnol Fuel Cell / Bttery Hybrid Power System Mehdi Drghi (Corresponding uthor) Islmi Azd University - Jouybr Brnh Jouybr, Irn Tel: 98-911-128-7365 E-mil: Drghi.mehdi@gmil.om Mohmmd Reznezhd Fulty of Eletril Engineering, Bbol University of tehnology Shriti St. Bbol, Irn Tel: 98-911-314-2090 E-mil: mreznejd@stu.nit..ir Abstrt Considering limited bility of Diret Methnol Fuel ell (DMFC) systems to produe power nd importne of optimum methnol onsumption, it is neessry to provide onditions, in whih, the mximum power of DMFC from present fuel flow n be utilized. But, in different onditions, mximum power point (MPP) of DMFC is unique point. So, operting point should be determined bsed on DMFC onditions, nd lod requirements re not tken into ount. This mens tht, output voltge will not be fixed if lod resistne vries. In this pper, new ontrol sheme for DMFC/Bttery hybrid power system is proposed nd nlyzed. In the proposed system, MPP trking ontroller serhes for MPP of DMFC in ny operting onditions. Moreover, bttery hrge ontroller is utilized to keep the bttery voltge in its permissible limits. So, in the proposed sheme, MPP opertion of DMFC nd output voltge regultion n be hieved onurrently. Keywords: Diret Methnol Fuel ell, Bttery, Mximum Power Point Trking 1. Introdution DMFC is one of the newest eletri power soures, whih beome inresingly more ttrtive subjet for mny reserhers beuse: - A solid polymer eletrolyte is used in these kinds of fuel ells (FCs), - Their opertionl temperture is reltively low, nd - Their fuel is low ost nd hs high energy density (Jeong et l, 2008). A lrge number of internl prmeters n impt on produed voltge of DMFC (Jeong et l, 2008; Guo et l, 2004; Sundmher et l, 2001), but in ny ondition, there is just one unique point on V-I urve whih represents MPP. In this point, DMFC n produe its mximum power. Beuse of the limited bility of DMFC systems to produe power from vilble fuel flow, it is neessry to fore the system to operte in ondition whih mthes to DMFC s MPP. This n void exessive fuel onsumption nd low effiieny opertion. A Mximum power point trking (MPPT) ontroller tres the MPP of DMFC using MPPT lgorithm. The ontroller genertes instrutions for DC-DC onverter nd ertin mount of urrent whih orresponds to MPP, is extrted from DMFC. There re severl methods to serh extremum vlue of funtion (Zhi-dn et l, 2008; Wsynzuk, 1983; Hussein etl, 1995; Noguhi et l, 2002; Enslin et l.1997). Among them Perturbtion nd Observe (P&O) (Zhi-dn et l, 2008; Wsynzuk, 1983) is the most ommonly used method beuse of its simple lgorithm. In this pper, DMFC MPPT, its problems nd defiienies, nd proposed solution whih is hybrid Diret Methnol 45
Vol. 3, No. 8 Modern Applied Siene Fuel ell/bttery system is presented nd nlyzed. In setion II, DMFC power system is reviewed. In setion III, the MPPT strtegy is nlyzed. In next setion, setion IV, the proposed hybrid system nd its ontrol sheme is introdued. The results nd onlusion re presented in setions V nd VI respetively. 2. DMFC power system Suh s other kinds of DMFC, there re three different regions in V-I urve of DMFC. At low urrent densities, the voltge is dominted by the eletrohemil kinetis of the nodi methnol oxidtion. As urrent density inreses, the voltge dereses further due to limittions of methnol mss trnsport to the node tlyst lyer nd the eletri resistne of the membrne. In next region, the system pprohes limiting urrent density ilim t whih the voltge dereses rpidly. The level of the limiting urrent density minly depends on the mss trnsport resistne of methnol in the node diffusion lyer. Methnol mss trnsport is bsed on diffusive nd onvetive trnsport mehnisms. In ddition, depending on the operting onditions, it is influened by the relese of CO2 bubbles from the node surfe (Sundmher; 2001). A multi-step retion mehnism is used to desribe retions inside DMFC (Sundmher; 2001), whih n be summrized s: Pt / Ru + Anode : CH3OH() l + H2O() l CO2 ( g) + 6H + 6e 3 + Pt Cthode : O2 ( g) + 6H + 6e 3H 2O( l) 2 3 Overll: CH3OH () l + O2 ( g) CO2 ( g) + 2H2O( l) 2 STD Output voltge of DMFC is funtion of open iruit voltge, U, node overpotentil, η ell, thode overpotentil, η nd membrne ohmi overpotentil, η ohmi. STD Vell U ell 46 = η η η (1) + ohmi Using hrge blne t node nd thode sides following expressions n be hieved. dη dt dη dt 1 = C ( i ell 6Fr = 1 ( i 6 ( M ell F r + n MeOH )) C ) M where, F is Frdy's onstnt, C nd C re double lyer pities of node nd thode respetively nd n MeOH is mss flux density of methnol in membrne. Rte of methnol oxidtion retion t node tlyst lyer, r, nd oxygen redution t thode tlyst lyer, r, n be expressed s: r k x ( α ) α F CL 1 F exp η xco exp η (4) 2 RT RT CL = MeOH ( α ) 1.5 PO α F 1 F 2 exp η exp η STD (5) P RT RT = k r where, k nd k re rte onstnt of nodi nd thodi retions respetively, methnol nd rbon dioxide, α nd α re hrge trnsfer oeffiients, pressures, R is universl gs onstnt, nd T is temperture. CL MeOH (2) (3) x nd x CL 2 re mole frtion of CO P O 2 nd P STD re oxygen nd stndrd Extended studies bout bovementioned reltions nd expressions n be found in (Jeong, 2008; Guo,2004; Sundmher et l, 2001). The output voltge of the fuel ell stk n be lulted by the following expression. Vstk = NVell (6) where N is the number of single ells in stk. The output power of the stk is: Pstk = Vstk I (7) Choosing prmeter vlues from (1), (2) nd (3) the DMFC stk output voltge nd power urves re presented in Figure1.
Modern Applied Siene August, 2009 3. Mximum power point trking strtegy There re vrious methods to find extremum vlue of funtion (whih hve mostly been developed for photovolti rrys), suh s Perturbtion nd Observe (P&O) (Zhi-dn et l, 2008; Wsynzuk, 1983 ), Inrementl Condutne (Hussein etl, 1995), short-iruit urrent method (Noguhi et l, 2002), nd the open-iruit voltge method (Enslin et l.1997 Enslin et l.1997). Considering its simple lgorithm, P&O is the most ommonly used method. In P&O, s n be understood from its nme, perturbtion is pplied to system nd its effet is observed. For DMFC system, n inrement (either positive or negtive) in DMFC urrent, ΔI, is onsidered s perturbtion nd output power vritions, ΔP, will be observed. If output power is inresed by positive onstnt inrement in DMFC urrent, further positive inrement is needed; otherwise perturbtion should be negtive. In similr wy, if output power is deresed by positive perturbtion, further positive inrement is needed; otherwise diretion of perturbtion vrition should be hnged. This lgorithm n be summrized s flowhrt shown in figure 2. For fst trking of mximum power point, the perturbtion rnge should be lrge. This leds to more flututions in output power. If we wnt to redue these flututions, the perturbtion rnge should be smll, but trking time will be long. So, s summry, the quik trking time is in onflit with the suppression of flututions in output power. Mny pprohes hve been proposed to improve the P&O lgorithm, suh s using vrible step size of perturbtion insted of onstnt vlue s following (Huynh et l, 1996): I ΔP k k +1 = I k + M (8) ΔI k where M is oeffiient tht modifies step size, nd I k, ΔI nd ΔP re the urrent vlue, devition of urrent nd devition of power t k th smpling period respetively. It should be noted tht, in spite of pproprite behvior of P&O under stti onditions, it will not funtion well if it enounters dynmi behvior of system (Zhi-dn et l, 2008). To obvite this drwbk, ontinuous hnges in DMFC opertion should be voided. 4. Hybrid system nd its ontrol sheme A DC-DC power onverter between DMFC nd lod n hnge lod equivlent resistne seen by DMFC. At MPP, urrent gin of the onverter, G, hs vlue whih uses the lod equivlent resistne to be equl to the internl resistne of the DMFC. The reltionship between lod equivlent resistne R eq nd lod resistne R Lod is: R eq = G 2 R Lod (9) A MPPT ontroller uses DMFC voltge, urrent nd subsequently its power, to find MPP nd then genertes ontrol instrutions for the power onverter. The onverter fores the fuel ell to work t urrent whih is defined by MPPT ontroller nd orresponds to MPP of DMFC. In this point, the lod equivlent resistne is equl to the internl resistne of the DMFC nd mximum power n be delivered to lod, onsidering mximum power trnsfer priniple. Figure 3 shows generi shemti digrm of suh system. As n exmple of MPPT for DMFC system, Figure 4 shows plot of the DMFC stk power in stndrd onditions. As n be seen, the MPP is found by MPPT ontroller nd DMFC is fored to sty t this operting point. In order to operte in MPP, DMFC output urrent should be onstnt; so, lod n just reeive fixed urrent nd power. Consequently, hnges in lod resistne will result in vritions in output voltge. Figure 5 shows the opertion of system under vrible lod resistne ondition. To overome the desribed defiienies, DMFC/Bttery hybrid system, whih represented in Figure 6, is proposed. In this system, bttery is pled in prllel with the power onverter, nd is responsible to keep the output voltge onstnt. In order to keep the bttery voltge in predetermined limit, bttery hrge ontroller is employed. The ontroller Input is the bttery voltge, nd its output determines the DMFC input fuel flow. This ontroller works in the wy tht, for exmple, if we wnt to keep output voltge in 100±5% of nominl voltge, when bttery voltge inreses to its upper limit, i.e. 100+5%, fuel flow will derese to its minimum vlue, i.e. 0%. On the ontrry, when bttery voltge rises to its lower limit, i.e. 100-5%, bttery hrge ontroller dereses the fuel flow up to its mximum, i.e. 100%. So, with respet to lod requirements, bttery voltge nd subsequently, DMFC input fuel flow, hnge in the wy tht, lod demnd is stisfied nd output bus voltge is kept onstnt. MPPT ontroller tres the MPP of DMFC, in ny fuel flow onditions, nd sends the required instrutions to the DC-DC onverter, so tht MPP opertion of DMFC n be relized. In order to void rpid nd sudden hnges of fuel flow, whih n hve serious nd hrmful dmges for DMFC, low pss filter should be utilized in bttery hrge ontroller output. The filter only psses slow nd soft hnges of fuel flow to the DMFC. To hve n pproprite understnding for designing of this filter, mximum llowed rte of vritions of fuel flow, whih n be tolerted by DMFC, should be tken into ount. Then, the filter n be tuned to 47
Vol. 3, No. 8 Modern Applied Siene relize the desired settings. Figure 7 shows the response of typil low pss filter with dmping ftor of 1, for different ut-off frequenies, when step hnge is pplied s its input. Choosing suh dmping ftor will void ny osilltions. 5. Simultion nd results Prmeters nd reltionships presented in setion II re used to model DMFC stk. A 220V led-id bttery is used in this study. These kinds of btteries re employed in mny eletril systems to store or deliver energy. There hve been mny proposls for led-id bttery models. A omprehensive eletril iruit model for led-id btteries ws proposed in (Slmeh et l, 1992). This model n be simplified to the iruit shown in figure 8 (Wng nd Nehrir, 2007). In this iruit model, Cb is bttery pitne, Rs is self-dishrge resistne, Ri is internl resistne, Co is overvoltge pitne, nd Ro is overvoltge resistne. The bttery is onsidered fully hrged before the pplition of lod, nd permissible rnge for voltge is onsidered 100±5% of nominl voltge. For power eletronis intermedites, Buk-Boost DC-DC onverters re employed. Stte-spe verged model n be used to nlyze the onverters performne (Rshid, 2001). These models n represent the verge behvior of onverters in stedy stte s well s in trnsients. In order to nlyze the performne of the proposed onfigurtion, the system is tested in two stges. First, MPPT ontroller is tested seprtely, nd results re shown. In the next stge, the fuel flow whih determined by bttery hrge ontroller is pplied to DMFC, nd effets of lod resistne hnges on system performne re nlyzed. To investigte performne of the MPPT ontroller, hypothetil step hnges re pplied to input fuel flow of DMFC. Considering suh hnges, DMFC output power vs. its urrent is depited in figure 9.. Figure 9.b shows the output power of DMFC whih is extrted by DC-DC onverter bsed on MPPT ontroller instrutions. As n be seen, MPP of DMFC for different vlues of input fuel flow is tred ontinuously nd opertion of DMFC beomes fixed in this point. Now, by dding the bttery hrge ontroller to system, the effets of lod resistne hnges on system performne will be nlyzed. In this ondition, the ontroller hnges the input fuel flow of DMFC ontinuously. The filter ut-off frequeny is set to 1 Hz nd the dmping ftor is 1. Step hnges of lod resistne re shown in figure 10.. The lulted fuel flow nd DMFC output power re presented in figure 10.b nd figure 10. respetively. Figure 10.d represents the voltge of bttery terminls. As n be seen in bove figures, in spite of the ft tht hnges of lod resistne is in wide rnge, it hs no effet on the MPPT ontroller performne, nd MPP is tred ontinuously. Moreover, bttery voltge is regulted in the determined limits, stisftorily. 6. Conlusion Considering the importne of high effiieny opertion nd optimum fuel onsumption in DMFC power systems, it is neessry to provide onditions in whih DMFC n operte in its MPP, but s desribed in this pper, there re some problems. In order to overome these defiienies, new onfigurtion for Fuel ell / Bttery hybrid power system is proposed nd nlyzed. This system is ble to utilize DMFC mximum power nd regulte output voltge simultneously, using two sets of ontroller. One is DC-DC onverter in ddition to MPPT ontroller, whih uses P&O lgorithm to tre MPP. The next is bttery hrge ontroller whih regultes bttery voltge by hnging the input fuel flow of DMFC. So, both MPP opertion of DMFC nd output voltge regultion n be relized. The proposed system is simulted nd results demonstrte pproprite performne of the system nd ontrollers. Referenes Enslin, J., Wolf, M.S., Snymn, D.B. nd Swiegers, W., (1997). Integrted Photovolti Mximum Power Point Trking Converter. IEEE Trns. on Industril Eletronis, vol. 44, pp. 769-773. Guo, H., M, C., (2004). 2D nlytil model of diret methnol fuel ell. Eletrohemistry Communitions 6, pp. 306 312. Hussein, K., Mut, I. Hoshino, T. nd Oskd, M. (1995). Mximum Photovolti Power Trking: An Algorithm for Rpidly Chnging Atmospheri Conditions. IEE Pro.-Genertion, Trnsmission, Distribution, Vol. 142, No.1, pp. 59-64. Huynh, P., Cho, B.H., (1996). Design nd Anlysis of Miroproessor-Controlled Pek-Power-Trking System. IEEE Trns. Aerosp. Eletron. Syst. 32 (1), pp. 182 190. Jeong, I., Kim, J., Pk, S., Nm, S., Moon, I., (2008). Optimum operting strtegies for liquid-fed diret methnol fuel ells. Journl of Power Soures 185, pp. 828 837. M. H. Rshid, (2001). Power Eletronis Hndbook, New York: Ademi. 48
Modern Applied Siene August, 2009 Noguhi, T., Togshi, S., Nkmoto, R., (2002). Short-Current Pulse-Bsed Mximum-Power-Point Trking Method for Multiple Photovolti nd Converter Module System. IEEE Trns. on Industril Eletronis, vol. 49, pp. 217-223. Slmeh, Z. M., Cs, M. A., nd Lynh, W. A., (1992). A mthemtil model for led-id btteries. IEEE Trns. Energy Convers., vol. 7, no. 1, pp. 93 98. Sundmher, K., Shultz, T., Zhou, S., Sott, K., Ginkel, M., Gilles, E.D. (2001). Dynmis of the diret methnol fuel ell (DMFC): experiments nd model-bsed nlysis. Chemil Engineering Siene 56, pp. 333-341. Wng, C., Nehrir, H., (2007). Lod Trnsient Mitigtion for Stnd-Alone Fuel Cell Power Genertion Systems. IEEE Trns. Energy Conversion, 22(4), pp. 864-872. Wsynzuk, O., (1983). Dynmi Behvior of Clss of Photovolti Power Systems. IEEE Trns. Apprtus nd Systems, Vol. PAS-102, No. 9, pp. 3031-3037. Zhi-dn, Z., Hi-bo, H., Xin-jin, H., Gung-yi, C., Yun, R., (2008). Adptive mximum power point trking ontrol of fuel ell power plnts. J. Power Soures 176, pp. 259 269. Figure 1. DMFC stk output voltge nd power v.s. its urrent urves Figure 2. Flowhrt of P&O lgorithm 49
Vol. 3, No. 8 Modern Applied Siene Figure 3. Shemti digrm of MPPT fot DMFC system Figure 4. MPPT for DMFC system in stndrd onditions 50
Modern Applied Siene August, 2009 Figure 5. Output voltge vritions when lod resistne vries Figure 6. Complete proposed DMFC/Bttery hybrid system 51
Vol. 3, No. 8 Modern Applied Siene Figure 7. Response of typil low pss filter to step input for different vlues of ut-off frequeny Figure 8. Simplified model for led-id btteries 52
Modern Applied Siene August, 2009 () (b) Figure 9. system performne when step hnges re pplied to input fuel flow. ) DMFC output power vs. its urrent, b) output power of DMFC 53
Vol. 3, No. 8 Modern Applied Siene () (b) 54
Modern Applied Siene August, 2009 () (d) Figure 10. System performne under vrible lod resistne ondition, ) Step hnges pplied to lod resistne, b) Vritions of DMFC input fuel flow, ) FC output power, d) Bttery voltge 55