38 The Open Renewable Energy Journal, 009,, 38-4 Open Acce Development of Solid Oxide Fuel Cell Baed Sytem Stand Alone Mode D. Vera * and F. Jurado * Department of Electrical Engeerg, Univerity of Jaén, 3700 ES Lare (Jaén), Spa Abtract: Several technologie are beg ued ditributed generation application with variable degree of ucce. Among thoe are: wd turbe, mall-cale hydropower plant, bioma, microturbe, photovoltaic array and fuel cell. In a fuel cell, the chemical energy corporated the fuel i converted directly to electricity and heat. Fuel cell technology i one of the mot teretg energy converion ytem offerg nearly zero emiion, flexibility of operation and high converion efficiency. Increag demand for power and the depletion of foil fuel are providg opportunitie for the development of fuel cell a power generatg ytem. In thi paper the modelg of Solid Oxide Fuel Cell (SO) baed generation ytem, fuel proceor (methane reformer) and power conditiong unit (verter) are preented. With acceptable limit, the effect of load variation on output voltage, efficiency and fuel flow demanded are vetigated. SO baed generation ytem ue a tack conitg of 384 fuel cell connected erie, thi provide 0-40kW output power tand-alone mode. The ytem ha been analyzed when a tep load i applied. Matlab-Simulk i ued for the modelg and imulation of the fuel proceor, fuel cell generator and power conditiong unit. eyword: Ditributed generation, fuel proceor, fuel cell, control, power conditiong unit. INTRODUCTION Hydrogen can be produced from the thermochemical gaification of many bioma material, uch a municipal olid wate, agricultural or foret wate or wood chip from hort rotation foretry plantation. The ynthei ga comg from a gaifier will corporate methane and carbon monoxide addition to the hydrogen Eentially, a fuel cell comprie of two porou electrode eparated by an ion-conductg electrolyte. The functiong prciple of a SO i that hydrogen move through the anode and react with oxygen. Throughout thi reaction, water and heat are created, at the ame time freeg electron. The electron go through an external circuit to the cathode, developg electrical power. In the cathode, oxygen move through the electrode and the electron are aborbed. Throughout thi reaction, oxygen ion are contituted, which are carried through the electrolyte via empty pace, endg the electric circuit. The chemical reaction that occur with the SO are a follow: O + e - /O - Cathode reaction () H + O - H O + e - Anode reaction () H + /O H O Overall reaction (3) A the name dicate, the SO poee a ceramic electrolyte, commonly yttria-tabilied zirconia (YSZ), which i a ubtance that conduct oxygen ion at temperature above 800 C, []. Material uually employed for the *Addre correpondence thee author at the Department of Electrical Engeerg, Univerity of Jaén, 3700 ES Lare (Jaén), Spa; E-mail: dvera@ujaen.e; fjurado@ujaen.e anode and the cathode are nickel-ysz-cermet and trontium doped lanthanum manganite, repectively. The terconnect lk the anode of one cell to the cathode of the adjacent. The ubtance can be ceramic or metallic dependg on the workg temperature and fuel cell deign. The cell i electrolyte-upported when workg at about 000 C and electrode-upported when workg at about 800 C. Fuel cell can work on different fuel varyg from hydrogen to gaified olid fuel, uch a bioma. Fuel from renewable orig are of particular importance for the future, but nowaday the mot recognized for fuel cell i natural ga. More formation on fuel cell can be encountered Blomen and Mugerwa [] and Larmie et al. []. The output from the fuel cell i DC power. When the ytem generation provide power to a reidential load, or to the electrical grid, a power conditiong unit i needed [3]. The poitive propertie of fuel cell for high efficiency power generation at any cale and of bioma a a renewable energy ource which i not termittent, location dependent or very unmanageable to tore, dicate that a combed heat and power ytem conitg of a fuel cell tegrated with a wood gaifier may provide a combation for deliverg heat and electricity cleanly and efficiently. SYSTEM GENERATION MODEL SO The electrochemical model of SO decribed thi paper i baed on a model developed by adulle et al. [4]. The aumption for the fuel cell model are a follow: The gae are ideal. The fuel cell i fed with hydrogen and air. 876-387/09 009 Bentham Open
Development of Solid Oxide Fuel Cell Baed Sytem The Open Renewable Energy Journal, 009, Volume 39 The electrode channel are mall enough that the preure drop acro them i negligible. The ratio of preure between the ide and outide of the electrode channel i large enough to aume choked flow. The fuel cell temperature i table. The Nernt equation applie. The loe are a follow: activation, concentration and ohmic. The upper limit theoretic work i the change Gibb free energy of the cell reaction, and the matchg reverible voltage i the electromotive force given by the Nernt equation: 0,5 RT p p V0 = N0E0 + ln H O nf e p HO Due to irreveribility the cell performance, the cell voltage, V, i variably lower than the ideal voltage, V 0. Thi deviation i called polarization and i the ummation of three loe: activation, concentration and ohmic polarization. V = V 0 V loe (5) V loe = V act + V conc + V ohm (6) Activation loe: The general expreion that repreent the activation loe for both electrode come expreed by Butler-Volmer equation, [5]. RT i RT i Vact = enh + enh nf e i0a nf e i0c Concentration loe: There exit expreion propoed by Chan to calculate the concentration loe, both for cathode and anode, [5], but preent paper, there ha been adopted a more implified empirical expreion propoed by Hahn, [6], and that give u a good approximation of thoe loe. V conc = m c e ni (8) Ohmic polarization loe: In general, ug the Ohm law, thee loe can be defed accordg to Eq. (4). V ohm = ri (9) The efficiency of a fuel cell i calculated a the power divided by the LHV of the fuel (methane): = fc m & fuellhv (0) fuel In thi paper, the fuel cell model ue a contant average cell voltage and a contant average fuel utilization to calculate the production of the fuel cell accordg to Faraday law, Eq. (0): ( ) = n y UN FV () e i a (4) (7) A Simulk model [7] wa preented to imulate the fuel cell baed on the analyi performed above (Fig. ). The parameter ued the imulation are ummarized Table. V Fig. (). Simulk model of SO. I Fig. (). Methane reformer. Fuel roceor Fuel proceor i baed a methane reformer with team. The tranfer function of the reformer model can be expreed a Eq. (), [8]. Methane reformed i realized with team (CV=3). q Re f I Re f r U q H opt ia ( / m) ia ( / m) N0 FU U max r Limit U m r T f ideal q H T e Re f q CH 4 CV = q H CH4 + ( + ) + () Reformer output (hydrogen flow) depend of the utilization factor (U) and fuel cell current (I). A I controlled i ued to control the flow rate of methane the reformer, [8]. Fig. () preent the block diagram of methane reformer. k + k q = q q ideal ( ) 3 CH4 H H 3 ower Conditiong Unit q H / r H _ O Activation Loe Concentration Loe I q O kr r r H HO H HO (3) ower Conditiong Unit (CU) conit a DC/DC converter to raie the output voltage to DC bu voltage and V act V conc I Nernt Equation I F Reformer O O V, Q q H
40 The Open Renewable Energy Journal, 009, Volume Vera and Jurado DC/AC verter to convert bu voltage to AC. In thi paper the DC/DC converter model i omitted. The electrical model of verter i baed on a model developed by El-Sharkh et al., [9]. The verter put are fuel cell voltage (V ), modulation dex (m) and phae angle on the AC voltage (). The verter output are AC voltage, active and reactive power. In thi paper, the load termal voltage i 400V AC for reidential and commercial application. V = mv (4) AC AC mvvl = ( ) (5) X FUX ( ) = q H (6) mv N L 0 Load termal voltage i kept contant (400V AC) ug a I controller. I i ued to control modulation dex, [9]. Voltage phae angle () i controlled by the put hydrogen flow ( q H ). Inverter block diagram i howed Fig. (3). Table. Model arameter arameter Value Converion factor, CV 3 Load termal voltage, V L 400V Abolute temperature of fuel cell, T 73 Faraday contant, F 96487000C/kmol Univeral ga contant, R 834J/(kmol ) Standard reverible cell potential, E 0.8V Number of cell tack, N 0 384 No roper model contant, r = 4F 0.995 0-6 kmol/( A) Maximum ue of fuel, U max 0.9 Mimal ue of fuel, U m 0.7 Optimal ue of fuel, U opt 0.8 Valve molar contant for hydrogen, H 8.43 0-4 kmol/( A) I V L Valve molar contant for water, HO.8 0-4 kmol/( A) Valve molar contant for oxygen, O.5 0-3 kmol/( A) q H FUX mv N Fig. (3). DC/AC verter. L k + k = ( ) (7) 3 m VL VAC 0 m V DC-AC Inverter SIMULATIONS AND RESULTS Matlab-Simulk i ued for the modelg and imulation of the fuel proceor, fuel cell generator and power conditiong unit, [0]. Fig. (4) and (5) how the voltage and power output of the model, repectively. ower demand conit a tep load of 0-40kW. The itial tranient are due to tartup of model and the itial condition of the tranfer function beg zero. It can be een that the fuel cell produce approximately a power tep of 0-37kW with a 360-40A current demand at teady tate. The ytem deliver a voltage of 400 V AC. The model ue a tack conitg 384 fuel cell connected erie, [0]. We can notice, Fig. (4) and (5), the voltage and power function don t obta tantaneouly, that i to ay, there exit a delay of approximately 0. Thi i due to time repone of SO ytem. The great ertia of the chemical reaction that take place the fuel cell origate the voltage of exit doe not become table even them 350 approximately (Fig. 4). The Roll-Royce Company [] realize important advance, managg to create prototype baed on fuel cell and batterie ytem to olve thi tranitory problem. V AC AC Q Repone time for hydrogen flow, H 6. Repone time for water flow, HO 78.3 Repone time for oxygen flow, O.9 Effective ohmic reitance, r 0.6 Electric repone time, T e.5 Fuel proceor repone time, T f Number of electron exchanged, n e Rate of exchange for SO, 0,5 Anode current denity, i 0a 6,300 A/m Cathode current denity, i 0c 3,000 A/m Experimental contant, m c 5 0-4 V Experimental contant, n 8 0-4 m /A Reaction total area of fuel cell, A 0,6m Hydrogen-oxygen ratio, r H_O.45 ower factor, F.0 Lower heatg value, LHV CH4 Le reactance, X Reformer time contant,, I ga contant for verter, k, k 3 Methane reference, q ref CH4 800500 kj/kmol 0.05 0 0.0000005 kmol/ I ga contant for reformer, k 0.5 In Fig. (6), we can ee the global efficiency of ytem generation baed on SO. The calculated efficiency correpond with the Eq. (9) (ug LHV of the CH 4 ).
Development of Solid Oxide Fuel Cell Baed Sytem The Open Renewable Energy Journal, 009, Volume 4 plant by mean of other technologie. Beide low CO emiion. Fig. (7, 8 and 9) preent reactive output power, AC voltage phae angle () and fuel flow demanded ( q CH4 ). We can oberve the reactive power (compared with active power) i practically equal to zero a conequence reitive load (unit power factor). Fig. (4). AC voltage output. Fig. (7). Reactive power. Fig. (5). AC power output. Fig. (8). AC voltage phae angle. Fig. (6). Sytem generation efficiency. For a load of 0 kw (rated power), we can oberve the ytem global efficiency i approximately 50 %. Thi efficiency i greater that obtaed one the conventional power CONCLUSIONS In thi paper, SO baed generation ytem ue a tack conitg of 384 fuel cell connected erie, thi provide 0-40kW output power tand-alone mode (reidential load). The ytem ha been analyzed when a tep load i applied. Firt, dynamic model of the fuel cell tack i preented, repreentg properly the low dynamic aociated with the ga flow and the fuel proceor operation. Then, a uitable control architecture i preented for the overall ytem, it objective beg to regulate the put fuel flow order to meet a deirable output power demand, achievg at the ame time a near optimal operation of the fuel cell. Mat-
4 The Open Renewable Energy Journal, 009, Volume Vera and Jurado Fig. (9). Fuel flow demanded. lab-simulk i ued for the modelg and imulation of the fuel proceor, SO generator and DC/AC verter. NOMENCLATURE CV = Converion factor (kmol of H per kmol of CH 4 reformer reaction) E 0 = Standard reverible cell potential (V) F = Faraday contant (C/kmol) i = Stack current denity (A/m ) I = Total tack current (A) i 0a, i 0c = Anode and cathode current denity (A/m ) LHV fuel = Lower heatg value of fuel (kj/kg) m = DC/AC verter modulation dex m c, n = Experimental contant (pecific for SO) &m fuel = Fuel flow (kg/) N 0 = Number of cell tack N a = Reactant flow to the anode (kmol/) n e = Number of electron exchanged the electrochemical reaction AC = AC output active power (W) = Electrical power produced by fuel cell tack (W) p i = artial preure of pecie i r = Effective ohmic reitance of SO tack () R = Univeral ga contant (J/kmol ) = Laplace tranform T = Temperature ok fuel cell tack () U = Average fuel utilization V AC = AC output voltage (V) V 0 = Reverible cell voltage V = Cell voltage (V) V L = Load termal voltage (V) X = Le reactance () y i = Mole fraction of fuel pecie i the anode ga = AC voltage phae angle (rad) G 0 = Change Gibb free energy (V) V act = Activation loe (V) V conc = Concentration loe (V) V ohm = Ohmic polarization loe (V) = Fuel cell efficiency REFERENCES [] Larmie, J.; Dick, A. Fuel Cell Sytem Explaed,.l., John Wiley & Son, 000. [] Blomen, L.; Mugerwa, M. Fuel Cell Sytem..l., lenum re, 993. [3] Jurado, F.; Valverde, M.; Cano, A. Effect of a SO plant on ditribution ytem tability. J. ower Source, 004, 9, 70-79. [4] adullé, J.; Ault, G.W.; McDonald, J.R. An tegrated SO plant dynamic model for power ytem imulation. J. ower Source, 000, 86, 495-500. [5] Chan, S.H.; hor,.a.; Xia, Z.T. A complete polarization model of a olid oxide fuel cell and it enitivity to the change of cell component thickne. J. ower Source, 00, 93, 30-40. [6] Hahn, A. Thei, Univerity of ittburgh, 000. [7] The Math Work. MATLAB and SIMULIN Software. Verion 7.. 005. [8] El-Sharkh M.Y.; Siworahardjo, N.S.; Uzunoglu, M.; Onar, O.; Alam, M.S. Dynamic behavior of EM fuel cell and microturbe power plant. J. ower Source, 007, 64, 35-3. [9] El-Sharkh, M.Y.; Rahman. A.; Alam, M.S.; Sakla, A.A.; Byrne,.C.; Thoma, T. Analyi active and reactive power control of a tand-alone pem fuel cell power plant. IEEE Tran. ower Syt., 004, 9(4), 0-08. [0] Vera, D. Mater Thei, Univerity of Jaén, 007. [] Roll-Royce Company. http://www.roll-royce.com/energy/tech/ fuelcell.jp Received: December 03, 008 Revied: January 5, 009 Accepted: January, 009 Vera and Jurado; Licenee Bentham Open. Thi i an open acce article licened under the term of the Creative Common Attribution Non-Commercial Licene (http://creativecommon.org/licene/by-nc/3.0/) which permit unretricted, non-commercial ue, ditribution and reproduction any medium, provided the work i properly cited.