COMPUTATIONAL FLUID DYNAMICS MODEL OF HIGH PERFORMANCE PROTON EXCHANGE MEMBRANE FUEL CELL WITHOUT EXTERNAL HUMIDIFICATION Željko Penga, Frano Barbir Faculty of electrical engineering, mechanical engineering and naval architecture, Ruđera Boškovića 32, 21000 Split, Croatia, +385 (21) 305-958, zpenga@fesb.hr Abstract: Proton exchange membrane fuel cell (PEMFC) performance depends on the amount of water present inside the ionomer membrane, since the protonic conductivity of the membrane is proportional to the membrane water content. The mass balance of electro-chemical reactions inside an operating cell shows that sufficient amount of water is generated during PEMFC operation inside the cathode catalyst layer to keep the membrane well hydrated, without the need for external humidification. However, most of the commercially available PEMFC stacks use external humidification units on a regular basis. The usage of external humidification results in higher specific volume of the stack, higher complexity and maintenance requirements, narrow operating range and consequently higher system costs, hindering the potential of PEMFCs in replacing the internal combustion engine in automotive applications. High performance operation without the need for external humidification presented in this work is based, and proven experimentally, on establishing a non-uniform temperature flow field for PEMFC. This novel concept demonstrates numerous advantages over the common approach where isothermal temperature field is considered, without compromising the performance, while the flooding of the channels and gas-diffusion layers is reduced, and at the same time enables high performance operation for a wide range of operating parameters. The non-uniform temperature field required for such operation is determined using the Mollier h- chart, by calculating the generated amount of water per channel length and determining the required temperature profile which will result in close to 100% relative humidity along the entire flow field. The developed 3D computational fluid dynamics model takes into consideration different physical processes inside an operating cell and shows very good agreement with the experimental results from previous studies [1,2] for different operating parameters. Keywords: Self-humidified PEMFC, Computational fluid dynamics, Non-uniform temperature field 1. INTRODUCTION Proton exchange membrane fuel cell stacks are a promising solution for replacing the internal combustion engines in automotive applications. Most of the automotive companies are developing, or have already developed a vehicle powered by PEMFCs. The World Green Car Award in 2016 is awarded to Toyota Mirai, which employs a high-end PEMFC stack of 370 single cells [3]. However, the main drawbacks of commercial fuel cell stack application, besides limited commercial availability of hydrogen refueling stations at the moment, are the price and complexity of the fuel cell system. Such system can be seen in Fig. 1., and the objective of this paper is development of a fuel cell system without external humidifiers or hydrogen recirculation pump. Once such operation is attained, the initial price of the system could be reduced by 30-40% and the system complexity would be significantly reduced. 1
Figure 1. Commercial PEMFC system components: 1 mass flow controller; 2 external humidifier; 3 insulation and line heating from humidifier unit to the cell; 4 PEMFC; 5 insulation of hydrogen recirculation loop; 6 hydrogen recirculation pump; 7 deionized water tank and temperature control; 8 deionized water circulation pump; 9 heat exchanger. The performance of the fuel cell is highly dependent on the water content of the ionomer membrane. This is due to the fact that high membrane protonic conductivity can be achieved only if the membrane is saturated with water. If the membrane water content is low, it results in higher ohmic losses, i.e. lower operating electric potential for a fixed operating electrical current. The commercial systems solve this problem by employing external humidifiers. The external humidifiers saturate the reactant gases with water vapor at a certain temperature, which is usually kept at a value close to the operating temperature of the cell. The reactants enter the cell at high relative humidity and the membrane is well hydrated from the anode and cathode side. Consequently, the membrane water content is high, as well as the operating electric potential. But, since the fuel cell produces water during the operation, the generated water inside the cell is unable to evaporate because the reactants are already saturated with water vapor. The result of excess liquid water generation inside the cell causes blockage of the pores of the gas-diffusion layers. Since the reactants are unable to pass through liquid water, the result is reduced electrochemically active surface area of the membrane. If the operating current density is high, more water is being generated, and the blockage of the reactant channels can occur. This phenomenon is called flooding of the cell. The result of flooding is a catastrophic decrease in PEMFC performance and must be avoided during the operation at all times. The potential of flooding occurrence is the main reason for the design of reactant channels with greater cross-sections, and the requirement for limiting the cell operation only for a certain range of operating electrical currents. Greater cross-sections of the channels result in higher material costs of the stack, higher specific weight and lower specific power of the stack. Since the operating cell produces water and heat during the electrochemical reactions of hydrogen and oxygen on the triple-phase boundaries inside the cathode catalyst layer, and the generated water is sufficient to humidify the reactants and keep the membrane water content 2
high, there should be no need for external humidification. The mentioned concept is already implemented to a limited extent commercially [3], although it requires a hydrogen recirculation pump to establish high relative humidity along the entire flow field. The requirement of the hydrogen recirculation pump in such commercial systems is necessary because the operating temperature of the cell in [3] is kept quite uniform, even though a slight temperature gradient of ca. 5 K is observed. The novelty of this work is in establishing a non-uniform temperature flow field for an operating cell by means of control of the mass flow rate of the coolant liquid. In this way, both heat and water generated during the operation of the cell are being used. The result is a high performance fuel cell, without the requirement for external humidification or hydrogen recirculation pump, which will consequently result in a very simple, robust and low-cost PEMFC system. 2. METHODOLOGY In order to study different physical processes inside an operating cell, a steady-state CFD PEMFC model has been developed in a previous study [1]. The CFD model was calibrated and validated with experimental data from Tolj et al. [2] with very good agreement of every parameter being measured. However, the previous study was developed for a segmented single cell where the temperature of each segment was controlled by Peltier thermo-elements. This approach had limited practicality, and the goal of this work is in establishing a required temperature profile with mass-flow rate control of the coolant liquid deionized water. 2.1. Domains Geometry of the CFD model is implemented from single cell geometry from work of Tolj et al. [2], with added coolant channels. The cross section of the fuel cell domains is shown in Fig. 2. The complete geometry represents the cross section shown in Fig. 2 extruded in length of 200 mm. Figure 2. Numerical domains, cross-section view: 1 bipolar plate/end plate; 2 reactant channel; 3 coolant channel; 4 gas-diffusion layer; 5 catalyst layer; 6 proton exchange membrane. 2.2. Mesh In order to enable study of different physical processes inside PEMFC, a grid dependency study was carried out for various mesh configurations by varying mesh type and number of finite volumes. Since the geometry is quite simple, and has large difference in scale size (edge length span from 10 m catalyst layer thickness, to 0.2 m axial length of PEMFC), it was most convenient to use a structured hexahedral mesh, i.e. Cartesian grid. The total number of finite volumes was 320,000. The cross section of the channels was divided into four sub-divisions, 3
while the gas-diffusion layers, catalysts and the membrane were divided into four sub-divisions along their thickness. 3. GOVERNING EQUATIONS AND BOUNDARY CONDITIONS Numerical calculations were carried out using ANSYS Fluent 16.2 PEM Fuel Cell Module. The operating parameters used in the setup are shown in Table 1. The parameters used for the model calibration were open-circuit voltage of 0.91 V, implemented from work of Tolj et al. [2], and the cathode reference current density, calibration was carried out for the operating current density of 500 macm -2. The resulting value for cathode reference current density used during the simulations was 0.0155 Am -2. The remaining material parameters are kept on the default values in PEM Fuel Cell Module, since further data was not available from the experiments. Table 1. Operating parameters Parameter Values Cathode stream abs. pressure 1.01325 Anode stream abs. pressure 1.01325 Cathode stream inlet temperature 30 C Anode stream inlet temperature 25 C Coolant inlet temperature 30 C Fuel cell temperature 60 C o coolant induced Relative humidity of cathode stream inlet 75% Relative humidity of anode stream inlet dry Current density 500 m m 2 Stoichiometry, cathode 2.15 Stoichiometry, anode 1.2 Channel length 200 mm Channel width at anode and cathode 1 mm Channel height at anode and cathode 1 mm Membrane thickness (Nafion 212) 0.05 mm Catalyst layer thickness 0.01 mm Platinum loading 0.5 mg m 2 Gas-diffusion layer thickness 0.38 mm Effective area 2000 mm Membrane dry density 2 g m 3 Membrane molecular weight 1100 g mol The governing equations are based on mass, momentum, energy and species conservations including appropriate sink and source terms. The type of analysis used was steady-state and single-phase, including equations for Joule heating, reaction heating, electrochemistry sources, Butler-Volmer rate and membrane water transport. Water transport equations consist of an equation for electro-osmotic drag coefficient by Springer et al. [4] and diffusion equations by Motupally et al. [5] for different membrane water content intervals. The membrane water content equation used in the software was adopted from Zawodzinski et al. [6]. The governing equations of Fluent PEM fuel cell module are based on energy, mass, momentum and species conservation including appropriate sink and source terms. For steady-state simulations, time dependent parameters are omitted from the equations. The resulting equations are: 4
3.1. Continuity ( ) (1) where represents source term for continuity equation. This term is only applicable for the triple-phase boundary (catalyst) regions. Inside the gas channels, gas-diffusion layers and the membrane, the source term is set to zero. 3.2. Momentum ( ) ( ) ( ) ( (2) ) Where represents the source term for momentum equation and applies only for the porous medium. For other domains, is set to zero. 3.3. Species ( ) ( ) (3) where the index represents different species oxygen, hydrogen and water vapor. The term represents the source and sink term for the species inside the catalyst layers and accounts for the reactant consumption in anode and cathode catalyst layers and water generation inside the cathode catalyst layer. In other domains, the term value is set to zero. 3.4. Energy ( ) ( ) (4) where represents the heat source term for the energy equation. The heat source term applies only for the cathode catalyst layer, for other domains it is set to zero. 3.5. Water transport through the membrane Electro-osmotic drag represents the water flux from the anode to the cathode side, and is defined by expression (5) where represents electro-osmotic drag coefficient, and represents membrane water content, defined as ( ) (6) ( ) ( ) (7) Where represents water activity, defined as (8) Where represents water vapor pressure, and represents water vapor saturation pressure. Since the model is single phase, the Leverett function term value is set to zero. Back-diffusion flux is defined as (9) where and represent density and the equivalent weight of the dry membrane, respectively. The term represents membrane water diffusivity, defined as (10) ( ) ( ) ( ) ( ) ( ) ( ) (11) 5
Energy and the Environment 2016 3.6. Domain boundary conditions The domain type for all domains is defined as fluid bodies, except for current collectors which are defined as solid bodies. In work of Tolj et al. [2] the current collector material was stainless steel SS316L, therefore this material was selected in this study as well. 3.7. Inlet boundary conditions The mass flow inlet boundary condition is defined for anode and cathode inlets. The anode inlet hyd ogen tempe tu e w s set to 25 C, with el tive humidity of 0%. C thode inlet i tempe tu e w s set to 30 C, with el tive humidity of 75%. The node mixtu e of g ses consisted of hydrogen and water vapor (while the mass fraction of water vapor was set to zero on the anode inlet), the cathode mixture of gases consisted of nitrogen, oxygen and water vapor. The mass flow rate of the coolant is prescribed to result in a temperature profile close to the water vapor saturation profile. 3.8. Outlet boundary conditions On the anode and cathode reactant channel outlets, the pressure outlet boundary condition is applied with a zero gauge pressure and backflow total temperature set according to the temperature field setup. 3.9. Wall boundary conditions Non-slip boundary conditions were applied to the walls. The temperature was fixed on terminals of the anode and cathode current collectors for isothermal temperature field study. For nonuniform temperature field case, the walls are defined as adiabatic surfaces, and the resulting temperature profile is defined by the coolant mass flow rate. The electric potential on the anode terminals was set to zero. On the cathode terminals different values of electric potential are defined in order to extract the polarization curve. The remaining surfaces are defined as walls by default. 3.10. Prescribing a desired temperature profile Prescribing a desired temperature profile is done by controlling the mass flow rate of the coolant liquid flowing in the co-flow direction with the cathode air. The mass flow rate of the coolant is determined by expression (12) where represents generated heat and water phase change enthalpy, represents specific heat, and represents the desired temperature difference between the outlet and inlet of the coolant liquid. The generated heat is defined by a heat source term inside the software by expression (13) where is heat released from electrochemical reaction of hydrogen and oxygen on the triple-phase boundaries inside the cathode catalyst layer, while represent anode/cathode over-potential, transition current, is the total electrical current, are ohmic losses, and is the phase change enthalpy of water evaporation/condensation. The term is neglected during the calculations, since the CFD model is single-phase. 6
4. RESULTS AND DISCUSSION Since the CFD model has already been validated in a previous study [1] with experimental results from Tolj et al. [2], the results of the simulations in this work are only shown for the coolant induced temperature field. The induced temperature field is shown for the cross section planes along the single cell length in Fig. 3. The resulting temperature profile is quite smooth and more appropriate than step-like temperature profile induced by Peltier thermo-elements on a segmented cell in the previous study [1]. Figure 3. Temperature contours on the cross-section planes along the single cell length with denoted flow directions of the reactant gases and the coolant liquid. In a previous study, the temperature profile was induced by segmenting the cell linearly into five equidistant segments. On each of the segments, the temperature was kept at a prescribed value with Peltier thermo-elements. Such approach resulted in an approximation of the water vapor saturation profile, but the concept could only be used on a single cell in a laboratory and resulted in a quite complex experimental setup. The segmented temperature profile on the current collector terminals is shown in Fig. 4. along with the desired temperature profile, i.e. water vapor saturation profile, extracted from Mollier h- chart. The temperature profile from the CFD analysis carried out in this work shows very good agreement with the desired temperature profile and shows the potential of application of the concept on a stack level. Figure 4. Comparisons of temperature profiles In order to have better insight in the potential for practical application of the concept and to extract the polarization curve, simulations have been carried out for different operating current 7
densities. In Fig. 5, a few interesting details can be noticed. At lower current densities, the resulting polarization curve is very close to the polarization curve for isothermal case from work of Tolj et al. [2] t ope ting tempe tu e of 60 C. This is due to a very low heat generation of the cell, since the cell has active area of only 20 cm 2 and the mass flow rate of the coolant would have to be extremely low to establish the desired temperature profile. However, in most practical applications, the current densities are higher, and since the membrane degradation is more prominent at higher operating electric potential, the low current density region is of minor importance. At the operating current density of 500 macm -2, the resulting polarization curve point is virtually identical to the operating point from segmented non-uniform temperature profile from Tolj et al. [2], since the simulated temperature profile established with the coolant is very close to the experimental one. At higher current densities, the mass flow rate of the coolant was increased to result in a very similar temperature profile to the original one. In the work of Tolj et al. [2], the temperature profile on the current collector terminals was fixed at all times. This resulted in the occurrence of higher temperature inside the cell, than the temperature on the current collector terminals, which is elaborated in detail in a previous study [1]. By prescribing the desired temperature field with coolant, this is not the case, since the coolant channels are very close to the reactant channels. This results in higher performance of the cell on higher current densities. Since the water generated inside the cell during operation is used for reactant humidification, the mass transport losses are significantly reduced. Figure 5. Comparisons of polarization curves 5. CONCLUSIONS The concept of prescribing a desired temperature profile which will result in high performance of the PEMFC without external humidification or the requirement for hydrogen recirculation pump has been proved analytically. The concept is very simple, and the usage of a standardized PEMFC system component the coolant loop, as shown earlier in Fig. 1. The desired temperature profile, i.e. temperature gradient, is prescribed by mass flow rate control of the coolant fluid, which required one thermocouple to measure the outlet temperature of the coolant with a feedback for the regulation valve which is used for mass flow rate control. Besides better performance of the cell, lower initial cost by 30-40%, and simplicity, this concept also results in wider operating range of the fuel cell. Since the generated water inside the cell is used for 8
reactant humidification, there is very little chance that the flooding will occur. The experimental setup for further investigation of the concept is currently under development. 5. LIST OF SYMBOLS water activity CFD Computational fluid dynamics specific heat, Jkg -1 K -1 gas diffusion coefficient membrane water diffusivity gravitational acceleration, ms -2 enthalpy, source term for enthalpy of water phase change, Wm -3 electrochemical heat source, Wm -3 electrical current, A back diffusion flux, kgm -3 s -1 thermal conductivity, Wm -1 K -1 equivalent weight of the dry membrane, gmol -1 molar mass of water, gmol -1 mass flow rate, kgs -1 electro-osmotic drag coefficient pressure, Nm -2 PEMFC proton exchange membrane fuel cell water vapor saturation pressure, Nm -2 partial pressure of water vapor, Nm -2 heat flux, W overpotential anode, cathode, V ohmic resistance, liquid water saturation - source term for heat, Wm -3 source term for species, kgm -3 s -1 source term for continuity equation, kgm -3 s -1 source term for momentum equation, kgm -3 s -1 source term for energy equation, Wm -3 SS316L austenitic chromium- nickel steel containing molybdenum (stainless steel) temperature, K velocity vector species mass fraction Greek symbols difference transition current anode, cathode, Am -3 membrane water content dynamic viscosity, kgm -1 s -1 density, kg m -3 dry membrane density, kgm -3 absolute humidity, kg 2 kg i 9
ACKNOWLEDGEMENTS The research leading to these results has received funding from the Croatian Science Foundation project IP-11-2013-8700 W te nd e t M n gement nd Du ility of PEM Fuel Cells. Ž. Penga also acknowledges support he has received from EU FP7 Programme through Fuel Cells and Hydrogen Joint Undertaking unde g nt g eement n 325275 (P oje t S PP IRE). REFERENCES [1] Peng, Ž., Tolj, I., B i, F.: Computational fluid dynamics study of PEM fuel cell performance for isothermal and non-uniform temperature boundary conditions, International Journal of Hydrogen Energy (In-Press) [2] Tolj, I., Bezm linović, D., B i, F.: Maintaining desired level of relative humidity through a fuel cell with spatially variable heat removal, International Journal of Hydrogen Energy, Volume 36 (2011), Issue 20, 13105-13113 [3] Konno N., Mizuno S., Nakaji H., Ishikawa Y. Development of Compact and High- Performance Fuel Cell Stack. SAE Int. J. Alt. Power 2015;4(1):123-129. [4] Zawodzinski TA, Springer TE, Davey J, Jestel R, Lopez C, Valerio J, Gottesfeld S. A comparative-study of water-uptake by and transport through ionomeric fuel-cell membranes. J Electrochem Soc 1993;140(7):1981-85. [5] Motupally S, Becker AJ, Weidner JW. Diffusion of water through Nafion 115 membranes. J Electrochem Soc 2000;147:3171-7. [6] Zawodzinski TA, Springer TE, Davey J, Jestel R, Lopez C, Valerio J, Gottesfeld S. A comparative-study of water-uptake by and transport through ionomeric fuel-cell membranes. J Electrochem Soc 1993;140(7):1981-85. 10