195 b.m. sudaroli

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  • 1. Heat and Mass Transfer Characteristics of Direct Methanol Fuel Cell: Experiments and Model By B. Mullai Sudaroli and Prof. Ajit Kumar KolarDepartment of Mechanical Engineering Indian Institute of Technology Madras 4th International Conference on Advances in Energy Research (ICAER 2013) Indian Institute of Technology Bombay, Mumbai, India.

2. Introduction Direct Methanol Fuel Cell(DMFC) Fuel cell is an electrochemical device which converts chemical energy of fuel and oxidant in to electrical energy. Fuel is methanol. Conventional Energy Thermal EnergyChemical Energy2ICAER 2013Mechanical EnergyDMFCElectrical Energy 3. Advantages and Applications Advantages Electrical Efficiency 40-60% No harmful emissions No moving parts-no noise and less maintenance Low temperature Operates as long as fuel is supplied Direct use of fuel- Reformer is not required High energy density-10 times higher than hydrogen Safe, easy to handle and transport Applications Portable applications (mW-W) Mobile phones, Laptops Sensors (W) Distributed power generation(kW) 3ICAER 2013 4. Working Principle Anode reaction CH3OH+H2O Cathode reaction 3/2O2+6H+ +6eOverall reaction CH3OH+3/2O26H+ +6e- + CO23H2O2H2O+CO2The crossover reaction at cathode 2CH3OH+3O2 4H2O+2CO2 Methanol and water crossover Methanol and water crosses through the membrane and affects ORR due to oxygen deficiency at the cathode catalyst layer, hence there is loss in potential and fuel utilization. 4ICAER 2013 5. Objective and Scope Objective Heat and mass transfer characteristics of DMFC and its effect on cell performance. Scope To develop a full cell model for anode side of DMFC and to predict methanol and temperature distribution. To study the methanol and water transfer process through the membrane with varying methanol concentration and cell current density. The effect of double channel serpentine flow field on cell performance 5ICAER 2013 6. Experimental ProgrammeExperimental Setup6ICAER 2013Double channel flow field plate 7. Geometric parameters Active area of the cell (mm)50X50Diffusion layer thickness(mm)0.14Catalyst layer thickness(mm)0.03Membrane thickness(mm)0.18Chanel width, depth and rib width (mm)1Operating conditions Methanol flow rate (ml/min) Air flow rate (ml/min)600Cell temperature (C)60Methanol concentration 7140.25,0.5,1MICAER 2013 8. Mathematical Model Governing equationsSmeoh Mass conservation equation ( u ) mMmeoh ja 6F.Mh 2 o Sh 2 o jc 6F.Mco 2 Sco 2 jc 6FMco 2 j m MmeohSmeoh Mh 2 oSh 2 o 6Fm Mo 2 So 2 Mh 2 oSh 2oMo 2 SO 2 jc 4FMomentum conservation equation uu P .u SuMo 2 Sh 2 o 1 jc 2FSpecies conservation equation uCi Deff Ci Si Energy conservation equation CpuT D T ST u Su K ST Hc - Gc ( I Icr ) a - Ieff8ICAER 2013ST I a Icr4F Ha Ga 6F 9. Voltage and current density relation Xch ja aia ref ref Xch aF exp a RT Xo jc aic ref ref Xo cF exp c RT I Icr 1 Xo aic ref ref tc Xo cF exp c RT Average current densityI jadz Effective diffusion coefficient of porous layer Deff D 1.5 Methanol flux in the membrane and crossover current density m D m chdC chI m m Nch 9dzICAER 2013FIcr 6 FNchEc E 0 a c IRm 10. Water crossover icell dC Nw m Dw eff ,m nd F dx Methanol crossover Nch m ch icell D m ch Cch ac / m F tdNet water generation Nw Nm 2 Nch wm icell 2F Nw Nm w Nmco w Now10ICAER 2013 icell Nw ( 1) 6F 11. Assumptions and Boundary Conditions Steady state, non-isothermal and single phase flow conditions Mass fraction and velocity of methanol at the channel inlet is given as inlet condition.Ambient pressure condition is given as outlet condition at the channel outlet Methanol at the cathode catalyst layer is completely oxidized 11ICAER 2013 12. Temperature distribution ( C) The graphite plate temperature is maintained at 60 C and the methanol is sent at 27 C. The methanol solution temperature is raised to 57 C when it passes through the flow field plate. The methanol solution at high temperature is sent to the methanol tank and circulated back to the fuel cell. This helps in improving the cell performance. 12ICAER 2013 13. Methanol distribution (mass fraction)Mass fraction of methanol indicates the cell current density distribution Double channel serpentine takes a turn and has long channel length which helps in methanol diffusion under the rib. Methanol distribution in anode catalyst layer controls the cell performance which can be controlled by flow field design and operating conditions such as cell temperature and methanol concentration. 13ICAER 2013 14. Effect of methanol concentration on cell performanceThe difference between experimental data and predicted data is 0.2 to 0.3V and it is due to cathode potential is not taken into account for predicting cell voltage. 14ICAER 2013 15. Effect of methanol concentration on methanol crossover and water crossoverHigher the methanol crossover leads to high mixed potential. This affects the cell performance and fuel utilization efficiencyNet water transfer coefficient decreases from 55 to 32 as the current density increases. 15ICAER 2013 16. At 1M, 50% of water generation is due to methanol crossover at low current density and it decreases with increasing current density. As the methanol concentration decreases, water concentration is more in methanol solution and it leads to reduction in methanol crossover and increase in water crossover. 16ICAER 2013 17. Conclusions A three dimensional non-isothermal model is developed for anodeside of DMFC. The model results are compared with experimental data. Methanol and temperature distribution in anode are found. The methanol concentration doesnt have significant impact on netwater generation. Even though the methanol crossover is high at 1M, FUE is 57% at230mA/cm2. 17ICAER 2013 18. References 1.2.3.4.5.18Jiabin Ge, Hongtan Liu, 2006, A Three-Dimensional Mathematical Model for Liquid-Fed Direct Methanol Fuel Cells, International Journal of Power Sources 160: 413421. Marcos Vera, 2007, A Single-phase Model for Liquid-feed DMFCs with Non-Tafel kinetics, International Journal of Power Sources 171: 763 777 Li, X.Y., Yang, W.W., He, Y.L., Zhao, T.S., Qu, Z.G., Effect of Anode Microporous Layer on Species Crossover through the Membrane of the Liquid-Feed Direct Methanol Fuel Cells, International Journal of Applied ThermalEngineering,doi:10.1016/j.applthermaleng.2011.10.051. Yang, W.W., Zhao, T.S., Xu, C., 2007, Three-dimensional Two-phase Mass Transport Model for Direct Methanol Fuel Cells, International Journal of Electrochimica Acta 52: 61256140. Nobuyoshi Nakagawa, Mohammad Ali Abdelkareem, Kazuya Sekimoto, 2006, Control of Methanol Transport and Separation in a DMFC with a Porous Support International Journal of Power Sources 160: 105115.ICAER 2013 19. 19ICAER 2013 20. Experimental Results20ICAER 2013