Losses effect on solid oxide fuel cell stack performance

Transcription

1 Losses effect on solid oxide fuel cell stack performance H. Mahcene **, H. Ben Moussa *, H. Bouguetaia **, B. Bouchekima **, D. Bechki ** * Département de Mécanique, Faculté des Sciences de l'ingénieur, Université de Batna- ALGRI ** Laboratory of Renewable nergy Ouargla University- ALGRIA B.P N 511 Ouargla- ALGRI Tel//Fax: HSOFC@gmail.com hmohcin@yahoo.fr ABSTRACT This paper presents an investigation of electric performance and maintenance of fuel cells. These cells (SOFC) have many characteristics that make them favorable as energy conversion devices, which rely on transport reactants (oxygen and hydrogen) and produces (water and heat) with the possibility of use in co-generation systems, because of its relatively high efficiency and very low environmental destruction. Thus fuel cell power plants can be configured in the range of electrical output, ranging from watts to megawatts. This investigation does not stop to the construction of the cells, but goes to maintenance requirement, and that by following the potential out put of the cell with the current during the use period. The actual performance is decreased from its equilibrium value because of irreversible losses. Many phenomena contribute to irreversible losses in actual fuel cells. These losses originate primarily from three sources: Activation polarization, Ohmic polarization, Concentration polarization Key-words: Fuel Cells, Losses, SOFC, lectric Performance, Renewable nergy, Polarization.

2 I. INTRODUCTION The fuel cell is an electrochemical device that converts chemical energy of its reactants directly to electricity and heat without combustion. Fuel cells are commonly classified according to the electrolyte used and operating temperature. This investigation addresses the development of the solid oxide fuel cell (SOFC), which is a high- temperature fuel cell employing a solid ion conducting membrane as the electrolyte. The fuel cell is an old innovation, but for along time it was hardly than a curiosity in the field of energy technology. Fundamental technical breakthroughs, however, have been achieved during the past decades, and fuel cells are now rapidly approaching commercialization in several applications. This development is catalyzed by the fact that the global energy use is increasing steadily and environmental problems related to energy production and transportation are growing. Solid oxide fuel cells (SOFCs) hold the promise of playing a major role in future power grids either as gas turbine (GTs) or as stand alone power units because of relatively high efficiency and very low environmental distraction (virtually no acid gas or solid emissions). The efficiency of fuel cells is not limited by the Carnot limitations of heat engines. High efficiency makes fuel cells attractive for a large variety of applications, included road vehicles; decentralized power production, residential energy systems and possibly even smaller applications, compact size and the lack of local emissions also increase the attractiveness of fuel cells. The aim was to create a solid foundation for scientific work on (SOFC) technology. In the simulation part of this work, basic characterization methods were established and basic problems of SOFC design were addressed with emphasis on the control of operating conditions, the separation of over-potentials, and flow field plate design. The work also included an extensive literature study on the subject. SOFCs operate at temperature from 600 C to 1000 C. To avoid thermal shock during heat-up as well as operation, inlet flow(fuel and air) temperatures and speeds must be carefully controlled to maintain the temperature gradient within the SOFC stack. Furthermore, increased fuel flow tends to decrease the fuel utilization but it can cause local fuel depletion and cold spots that exacerbate temperature non uniform II. FUL CLLS SOFCs THORY In fuel cells the reduction of oxidation at the cathode and the oxidation of hydrogen at the anode produces water and heat according at the following reaction: When using hydrogen fuel, the electrochemical reactions of the SOFC: At the anode of SOFC H +O H O +4 e. The reduction of oxygen in cathode is given by: O + 4 e O The overall cell reaction is 4H + O H O + Heat

3 II.1 Fuel Cell Thermodynamics II.1.1 Conversion of Gibbs Free nergy into lectrical nergy lectrochemical energy conversion is the conversion of the free-energy change associated with a chemical reaction directly into electrical energy. For a fuel cell, the maximum work available is related to the free energy of reaction, whereas the enthalpy (heat) of reaction is the pertinent quantity in the case of thermal conversion (such as a heat engine). For the state function ΔG= ΔH TΔS Where ΔG, ΔH, T, ΔS are Gibbs energy free, enthalpy, temperature and entropy respectively. III. LCTROCHMICAL MODL The excel spreadsheet based SOFC stack model, which was mentioned earlier, is included in the aspenbased system model. It predicts stack voltage responses to changes in fuel composition, fuel flow rate, stack temperature and current demand, with response characteristics that are adjustable to changes in stack component materials, and geometric well as to electrode porosity. According to the discretization shown in Fig.1. An electrochemical analysis based on deviation from ideal performance is proposed at each slice. Considering the electrodes to be equilibrium surfaces [r] the Nernst voltage is defined by Nernst = 0 RT p H p T,P + ln nf p 1 O H O Where 0 T,P is the equilibrium potential for a given temperature and pressure, 0 T,P = Δ = Δ G r H r H r Δ = ΔG r nf 0 -T. Δ S r + Δ 0 H r Δ c pr Δ + ( T ). c p r T dt dt Δ c pr = c ph O - H c p - 1 c p H O Such as ph O = p. H O X, p H = p. X H, p O = p. X O So Nernst = 0 RT T,P + ln p nf 1. X X H H O Where p, X are gas pressure and molar fraction respectively. In the models described here, however, we retain Butler-Volmer formalism for the charge transfer steps; this approach provides quantities information about important functional dependencies such as the. X 1 O

4 reaction orders in the exchange current density. Irreversibility, also know polarizations arise that cause voltage losses from the ideal voltage value and have a negative impact on fuel performance. These are three types of losses: - Activation polarization (η Act ) -Ohmic polarization (η ohm ) - Concentration polarization (η Con ) III.1 Activation Polarization (η Act ) The activation polarization of electrolyte-supported SOFC occurs in both electrodes. The activation polarization of the anode is obviously lower than of the cathode due to its lower exchange current density. The activation overvoltage depends principally on current density. This parameter can be considered as the forward and reverse electrode reaction rate at the equilibrium potential. High exchange current density means high electrochemical reaction rate; in this case, good fuel cell performance expected. The value of the exchange current density can be improved by increasing the fuel cell operating temperature or using catalytic materials with lower activation energies. But in low and medium-temperature fuel cells, the activation polarization plays the most important role on total losses causing voltage drop from ideal voltage. Where the first term represents the reversible cell potential at standard temperature and pressure and the second term corrects for changes in gas pressures. Under high activation over-potential, the second term, the Butler-Volmer can be neglected and the formula can be rearranged. RT η act = nαf ln( i ) i0 Where i, i 0, R, F, n, α are the net current density, the exchange current density or the current densities for forward and reverse reaction at equilibrium, the molar gas constant, faraday s constant, the electron number, and the charge transfer coefficient respectively. The charge transfer coefficient α value depends on the reactions involved and on the materials. Its typical anode value is 0.5 and the cathode value often

5 varies between 0.1 and 0.5. Generally, the anode activation over-potential is smaller than that of cathode because it is usually verified that (i 0,C >> i 0,A ). However, it is possible to summarize the anode and cathode activation losses using the following formula: η act = S S ) ( C i A + ln S ( ) + ( ) A SC i SA + SC SA + S 0,A i 0,C C Where i 0,A, i 0,C, S A, S C are the anode and the cathode exchange current density respectively, and the anode and the cathode cell active surfaces respectively. When the activation over-potential is low, it is possible to expand the Butler-Volmer equation as a Taylor series; neglecting the higher terms, the result is η act = RT nαf ln( i i0 ) III. Ohmic Losses(η ohm ) The Ohmic over-potential η ohm, due primarily to resistance of ion transport in the cathode, anode, electrolyte and mostly interconnections. Since these resistances obey Ohm s law, the overall Ohmic overpotential can be written as: i η ohm = σ l e Where i, σ eff eff, l e are nominal current density of the cell, the electric conductivity of the diffusion layer and thickness respectively. 3.3 Concentration Losses (η Conc ) To avoid a heavily reliance on correlation to determine the limiting current density. The concentration loss is evaluated using the Fick law formula. RT 1 i η conc = nf 1 + ln α 1 i il Where i l is the limiting current density (corresponding to a surface concentration value of zero), i represents each in the reactions. Concentration over-potential becomes significant when large amount of current drawn the fuel-cell. The partial pressures of the gases at the reaction sites, which correspond to the volume concentration of the gases, will be less than in the bulk of the gas stream when a large amount of current is drawn. If such partial pressures or concentration of gases are unsustainable, concentration losses will cause excessive voltage losses and the fuel-cell will cease to operate. The global effect is a reduction in hydrogen partial pressure. A similar phenomenon also occurs at air electrode. III.4 lectrical Power and Heat Transfer Losses The cell potential cell is being calculated by subtracting these over-potential from the Nernst potential:

6 cell = 0 T,P - η Act - η Ohm - η Conc The electrical power produced by the fuel-cell is calculated by: W = cell.i VI.4. SOFC MODL VI.1. Governing quations The governing equations for the steady state and one dimension heat and mass transfer are: Continuity: (ρu) = 0 Momentum equations: (ρuu) = p +.(μ u) nergy equation: Concentration equation: (ρuc p T) = p +.(k T) +. q (ρuy i ) = p +.(ρd i,j Y i ) + S m Where ρ, u, p, C p, S m, μ, k, q., D i,j, Y i are density, velocity, pressure, calorific capacity, thermal conductivity, viscosity, heat source or flux and binary diffusion coefficient and mass fraction respectively. VI.. Discretization Method The equations are solved in their differential form. Instead, the finite volume method is applied, which uses the integral form conservation equations as a starting point. The integration of the transport equations results in linearized equations. In order to solve these, the solution domain is subdivided into a finite number of contiguous control volumes (CV s), and the conservation equations are applied to each CV. At the centered of each CV lies a computational node at which the variable values are calculated, Fig.. Interpolation is used to express variable value at the CV surface in terms of the nodal (CV-center) values. The complete set of equations is not solved simultaneously (in other words by a direct method). Quite apart from the excessive computational effort which would entail, this approach ignores the nonlinearity of the underlying differential equations.

7 V. RSULTAS AND DISCUSSIONS Details of different losses as a function of current density in a fuel-cell operated at 103K are shown in Fig.3. The activation overvoltage depends principally on current density at this operating temperature, which is 5300 A/m for anode and 000 A/m for cathode. This parameter can be considered as the forward and reverse electrode reaction rate at the equilibrium potential. Results show that Ohmic and activation losses in the cathode are responsible for the major over- potentials in the fuel-cell The cathode concentration over-potential becomes significant when the fuel-cell operates near to the limiting current density. Compared with the anode, the cathode exhibits higher activation over-potential, which is due to the poor apparent exchange current density at the cathode/electrolyte or anode/electrolyte (LSM-YSZ/YSZ) interface, as will be seen in Fig.3. Since the cathode exchange current density directly affects the electrochemical reaction rate in the cathode leads to high cathode activation losses in the fuel-cell. The effect of temperature of the interconnector, electrolyte, cathode and anode is presented in Fig. 4a-d, respectively. The resistances of this fuel-cell component are determined by the conductivity of the materials used and their respective thickness according to the formulae below. The results show that the resistances of the interconnector, electrolyte, cathode and anode.

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9 σ Fuel utilization 93% Air pressure 7% T lectrical (Ω -1 m -1 ) and thermal λ (Wm -1 K -1 ) conductivities Interconnect lectrolyte Cathode Anode 1100 exp T exp T exp T T exp T T λ Diffusion coefficient D O D O Values (cm s -1 ) T 0.181* T 0.753* Tab.1. The Cell Parameters The performance of fuel-cell as a function of temperature is presented in Fig.5. In the form of fuel-cell voltage versus current density. Results shown that the higher the temperature, despite the lower Nernst potential. As more current is drawn from the fuel-cell, however, a higher cell voltage can be maintained under higher temperature operating at the same current density. There is significant reduction of both Ohmic and activation over-potential at higher temperature operating. IV. CONCLUSIONS The performance model was calculated from semi- empirical correlations based on kinetic data, which are very sensitive to temperature variations. The model allowed determination of temperature, flow and current distributions for different geometries. The model predicted temperature, mass and electrical distribution for the entire stack which consisted of an array of unit cells. The finite-volume method was used for solving the governing equations

10 RFRCNCS [1] H. Mohcene, H. Ben Moussa, H.Bouguettaia, D.Bechki. Power Make Up Conference in Arab Countries 7-8 November, 006, Beirut-Lebanon. [] H.Ben moussa, D. Haddad, K.oulmi, International Chemical ngineering Conference V (JICC05), from the 1th to14th of September (005), Amman (Jordan.) [3] K.Oulmi, H. Ben moussa, D. Haddad, lectrochemical energy conversion and storage (fuel cells), the 56 th International society of electrochemistry, Sept 5 30, (005) Bexco, Busan, Korea. [4] High Temperature Solid Oxide Fuel Cell. Fundamentals, Design and Applications. Sub hash C Singhal Kevin Kendall, LSVIR, 004 [5] Sadik Kakaç, Anchsa Pramuanjaroenkij, Xiang Yang Zhou, Areview of numerical of solid oxide fuel cells, International Journal of Hydrogen nergy, 18, (006), 1-6. [6] Dayadeep S. Monder, K.Nandakumar, Karl T.Chuang, Journal of Power Sources 16 (006) [7] Huayang Zhu, Robert J.Kee, Vinod M. Janardhanan and Al, Journal of lectroche- micalsociety, 15 (1) A47-A440 (005). [8] Pei-Wen Li, Minking K. Chyu, Simulation of the chemical/ electrochemical reactions and heat/mass transfer for a tubular SOFC in a stack, Journal of Power Sources 14 (003) [9] GrayDavid.Govmore, Analyses and Technology Transfer for Fuel Cell Systems., CALIFIRONIA NRGY COMMISSION, June 000. [10] Heuveln, Fredrik Hendrik van Characterisation of Porous Cathodes for Application in Solid Oxide Fuel Cells, 4 January [11] SOFC mathematic model for systems simulations. Part one: from a micro-detailed to macro-blackbox model, International Journal of Hydrogen nergy, 30, (005) [1]. Riensche,. Achenbach, D. Froning, M. R. Haines, W. K. Heidug, A, Clean combined-cycle SOFC power plant-cell modelling and processanalysis Journal of Power Sources, 86 March (000) [1] S.C. Singhal, Advances in solid oxide fuel cell technology, Solid State Ionics 135 (000)