Effect of flow regime of circulating water on a proton exchange membrane electrolyzer

The flow characteristics of circulating water in a proton exchange membrane (PEM) electrolyzer were experimentally evaluated using a small cell and two-phase flow theory. Results revealed that when a two-phase flow of circulating water at the anode is either slug or annular, then mass transport of the water for the anode reaction is degraded, and that the concentration overvoltage increases at higher current density compared to that when the flow is bubbly. In a serpentine-dual flow field, when both phases of the two-phase flow are assumed laminar, then the increase in pressure drop caused by the increase in gas production can be explained relatively well using the Lockhart–Martinelli method with the Chisholm parameter. The optimal flow rate of circulating water was also discussed based on mass balance analysis.     

Numerical studies on rib & channel dimension of flow-field on PEMFC performance

Distributions in reactant species concentration in a PEMFC cause distributions in local current density, temperature and water over the area of a PEMFC. These can lead to effects such as flooding or drying of the membrane and cause stresses in different regions of the fuel cell. Changing flow-field configuration, including channel path length, width, or height to distribute the gas more evenly, is one method of minimizing these stresses. This work numerically investigated how serpentine flow-fields with different channel/rib's cross section areas affect performance and species distributions for both automotive and stationary conditions. Further, the influence of flow direction to performance and its distribution was also reported. The prediction revealed that for stationary condition, narrower channel with wider rib spacing gives higher performance but opposite results when automotive condition is applied.     

Passive control of liquid water storage and distribution in a PEFC through flow-field design

Liquid water stored in the diffusion media (DM) in a polymer electrolyte fuel cell (PEFC) can dramatically impact steady and transient performance, degradation, and heat transfer. In this study, seven different flow-field designs, with landing-to-channel (L:C) ratio from 1:3 to 2:1, were investigated at dry and fully humidified conditions, using neutron imaging. The results revealed the impact of flow-field geometry on stored liquid overhead is significant. In some cases, the stored water content in the cell can be nearly double that of another design, despite similar performances at low to medium current density. In general, a smaller L:C ratio reduces flooding losses and minimizes the stored water content. Additionally, the channel–DM interface plays a key role. For the same L:C ratio, a reduced number of channel–DM interfaces was shown to reduce flooding and stored liquid water content at steady state. This also suggests that using proper flow-field design can decrease the parasitic power consumption and the stored water content in the cell without any sacrifice from the cell performance. For dryer operating conditions, however, membrane dehydration becomes a dominant effect and a high landing-to-channel ratio flow-field is higher performing.     

Investigations on high performance proton exchange membrane water electrolyzer

In order to improve proton exchange membrane water electrolyzer (PEMWE) performance, some factors related to the processes of preparing the Membrane Electrode Assemblies (MEAs), such as iridium (Ir) electrocatalyst loading and Nafion^® content at the anode, thicknesses of proton exchange membrane and gas diffusion layers (GDLs), were examined. In addition, a home-made supported Ir/titanium carbide (Ir/TiC, 20% Ir by weight) was developed for the anode. With best commercial Ir catalyst loading of 1.5 mg cm⁻² Ir at the anode, the cell's current densities of 1346 mA cm⁻², 1820 mA cm⁻² and 2250 mA cm⁻² were achieved at the cell potentials of 1.80 V, 1.90 V and 2.00 V, respectively. A PEMWE with 0.3 mg cm⁻² Ir loading of Ir/TiC anode catalyst was comparatively stable and gave current densities of 840 mA cm⁻², 1130 mA cm⁻² and 1463 mA cm⁻² at the cell potentials of 1.80 V, 1.90 V and 2.00 V, respectively. Based on catalysis efficiency of Amperes per milligram of Ir, the Ir/TiC catalyst is found to be more active than unsupported Ir catalyst.     

Comparative performance investigation of different gas flow configurations for a planar solid oxide electrolyzer cell

The performances of the solid oxide electrolyzer cells (SOECs) are closely tied to the designs of gas flow configurations. This paper performed a numerical comparative investigation on a planar SOEC with the co-flow, counter-flow, and cross-flow configurations. The experimental measurements for I-V curve were conducted and compared to the simulations for model validation. Based on the 3-dimensional numerical simulations, the distribution characteristics of the species mass fractions, temperature, current density, Nernst potential, and activation polarizations for variant gas flow configurations were analyzed and compared in detail. The intrinsic relationships and mutual effects between these parameters were examined. The simulation results show that the operating temperature gradient of the counter-flow configuration is smaller than that of co-flow and cross-flow, which is favorable for the durability of the cells. The distributions of the current density and activation polarizations in the case of cross-flow configuration appear in checkerboard characteristic. Compared to the co-flow and counter-flow, the cross-flow configuration obtains the best performance under the same boundary conditions as it produces the most hydrogen under the same boundary conditions.     

Effect of mixed heat-resistances on the optimal configuration and performance of a heat-engine cycle

The finite-time thermodynamic performance of a generalized Carnot-cycle, under the condition of mixed heat-resistances, is studied. The optimal configuration and the fundamental optimal relation between power and efficiency of the cycle are derived. The results provide some guidance for the design of practical engines.     

Influence of PEMFC gas flow configuration on performance and water distribution studied by SANS: Evidence of the effect of gravity

A non intrusive method based on small angle neutron scattering (SANS) has been developed to determine the water concentration profile through the thickness of Nafion^® 117 membrane during fuel cell operation. This technique was used to study the effect of gas flow configuration, co- or counter-flow, on water repartition within the fuel cell both within and outside the membrane. As it has been reported previously in the literature the counter-flow configuration gives better performance than co-flow but more surprisingly we evidence a significant difference in performance between symmetric configurations either in co- or counter-flow. Indeed, for a given current density, cell voltage is higher when the cathode inlet is at the bottom of the cell. We demonstrate that the gravity retains liquid water within the cell which leads to a better membrane hydration. Moreover, we have been able to correlate the average water content within the membrane with the performance and especially with the voltage drop resulting from the membrane resistance.     

Investigation of flow boiling in horizontal tubes: Part I—A new diabatic two-phase flow pattern map

Several important modifications to the flow pattern map of Kattan-Thome-Favrat [J. Heat Transfer 120(1) (1998) 140-147] made, resulting in a significantly new version of the map. Based on the dynamic void fraction measurements described in [Int. J. Multiphase Flow 30 (2004) 125-137], the stratified-wavy region has been subdivided into three subzones: slug, slug/stratified-wavy and stratified-wavy. Furthermore, annular-to-dryout and dryout-to-mist flow transition curves have been added and integrated into the new flow pattern map, identified by distinct trends of the heat transfer coefficient as a function of vapor quality and by flow pattern observations to determine (and then predict) the inception and completion of dryout in horizontal tubes.     

Effects of flow-field and mixture inhomogeneities on the ignition dynamics in continuous flow reactors

The objective of this study is to characterize effects of turbulence and flow-field inhomogeneities on the mixing and ignition-dynamics in flow reactors. Specific focus is on investigating the ignition characteristics of hydrogen-containing fuels at gas-turbine-relevant operating conditions. Two different model formulations are developed to describe the mixing, induction, and subsequent ignition and combustion. Utilizing these models, parametric studies are performed in a generic flow reactor configuration that is representative of commonly employed facilities. Diagnostics is developed to quantify the ignition dynamics. Results show that in the case of an initially homogeneous mixture, the ignition process is fairly insensitive to the underlying flow-field. However, by considering inhomogeneities in temperature and mixture composition it is shown that the ignition process exhibits a more pronounced sensitivity to temperature perturbations, and the ignition delay is only weakly sensitive to initial equivalence ratio perturbations. Simulation results show that temperature fluctuations of less than 10% of the mean temperature are sufficient to significantly affect the ignition-onset. Results from this parametric study identify the need for quantitative measurements of temperature and composition to better characterize flow reactor facilities. A time-scale analysis is performed to characterize competing physical processes that are associated with turbulent mixing, autoignition, and flame propagation. Qualitative comparisons with experimental data suggest the possibility for deflagrative ignition modes that can occur at low temperature operating conditions.     

Assisted water management in a PEMFC with a modified flow field and its effect on performance

This work designed and tested innovative flow channels in order to improve water management in a polymer electrolyte membrane fuel cell (PEMFC). The design employed slanted channels with an angle of 20° in a flow plate to collect the liquid water that permeated from the gas diffusion layers. The effects of orientations of the slanted channels in up-slanted and down-slanted directions and relative humidity levels on the cell performance were investigated. The experimental results showed that modifying the anode flow field using down-slanted channels provided higher cell performance. Water concentration at the gas diffusion layer is reduced resulting in more back diffusion of water from the cathode to anode, thus inducing membrane hydration and improving the conductivity. Promotion of water removal by applying down-slanted channels in the cathode side did not improve the performance. This work has demonstrated that channel cross-section design alone could improve the PEM fuel cell performance. The anode down-slanted cell indeed improved the performances at extremely wet condition and the power was equally good as that without modified flow channel at less wet condition.     

Development and performance analysis of a new solar tower and high temperature steam electrolyzer hybrid integrated plant

In this article, an extensive thermodynamic performance assessment for the useful products from the solar tower and high-temperature steam electrolyzer assisted multigeneration system is performed, and also its sustainability index is also investigated. The system under study is considered for multi-purposes such as power, heating, cooling, drying productions, and also hydrogen generation and liquefaction. In this combined plant occurs of seven sub-systems; the solar tower, gas turbine cycle, high temperature steam electrolyzer, dryer process, heat pump, and absorption cooling system with single effect. In addition, the energy and exergy performance, irreversibility and sustainability index of multigeneration system are examined according to several factors, such as environment temperature, gas turbine input pressure, solar radiation and pinch point temperature of HRSG. Results of thermodynamic and sustainability assessments show that the total energetic and exergetic efficiency of suggested paper are calculated as 60.14%, 58.37%, respectively. The solar tower sub-system has the highest irreversibility with 18775 kW among the multigeneration system constituents. Solar radiation and pinch point temperature of HRSG are the most critical determinants affecting the system energetic and exergetic performances, and also hydrogen production rate. In addition, it has been concluded that, the sustainability index of multigeneration suggested study has changed between 2.2 and 3.05.     

Numerical simulation of horizontal tube bundle falling film flow pattern transformation

It is important to study the falling-film pattern of a horizontal tube bundle in order to set up a heat and mass transfer model accurately. The falling-film pattern of a horizontal tube bundle is simulated in this paper. The technique is based on computational flow dynamics (CFD) for the two-phase flow of gas and water. The experimental results were used to validate the mathematical model. It indicates that the simulation results accord with experimental data well. The simulated results show that the flow pattern varies with different flow rates. Under the different flow rates, it observes the droplet, droplet-columnar, columnar, columnar-sheet and sheet flow patterns. The critical value is 0.0125 kg/s between droplet and columnar, and the critical value is 0.02 kg/s between columnar and sheet.     

Analysis of water transport in a high pressure PEM electrolyzer

This paper analyzes, through experimental data and a transport model, the water transported through the membrane under different operating conditions in a on a Proton Exchange Membrane (PEM) electrolyzer operating with a high-pressure gradient across the membrane from the cathode (high-pressure) side to the anode (nearly ambient-pressure) side. The phenomena involved in this movement are described and analyzed, with a focus on the electro-osmotic drag coefficient, n_eo. We have observed that the behavior of the hydraulic percolation determines the results obtained for the electro-osmotic drag, while the contribution of the water diffusion is negligible. In general, the cathode pressure significantly reduces the water transport (a positive effect). Also, operation at lower current density reduces the net electro-osmotic drag coefficient, n_g; therefore, the best operation strategy for obtaining dried hydrogen at the cathode is to impose high cathode pressure and low current density.     

Effect of flow geometry on anode-supported single chamber SOFCs arrayed as V-shape

The flow geometry of a single chamber solid oxide fuel cell was studied in order to reduce the effect of cell distance in the anode-facing-cathode stack. V-shape and converse V-shape configuration micro stacks consisting of two anode-supported yttria-stabilized zirconia membrane fuel cells were fabricated. The flow geometry reduced the effect of cell distance and improved the uniformity of performance of single cells in both stacks. The converse V-shape configuration was beneficial for maintaining the heat and the gases, in particular at low flow rates; the performance of the converse V-shape stack was better than that of the V-shape stack.     

Numerical investigation of flow field configuration and contact resistance for PEM fuel cell performance

A steady-state three-dimensional non-isothermal computational fluid dynamics (CFD) model of a proton exchange membrane fuel cell is presented. Conservation of mass, momentum, species, energy, and charge, as well as electrochemical kinetics are considered. In this model, the effect of interfacial contact resistance is also included. The numerical solution is based on a finite-volume method. In this study the effects of flow channel dimensions on the cell performance are investigated. Simulation results indicate that increasing the channel width will improve the limiting current density. However, it is observed that an optimum shoulder size of the flow channels exists for which the cell performance is the highest. Polarization curves are obtained for different operating conditions which, in general, compare favorably with the corresponding experimental data. Such a CFD model can be used as a tool in the development and optimization of PEM fuel cells.     

The effect of flow distributors on the liquid water distribution and performance of a PEM fuel cell

Water management is one of the important factors which determine the performance of a Proton Exchange Membrane (PEM) fuel cell using hydrogen as fuel. For developing efficient water management systems, it is important to know the potential locations of formation and the nature of distribution of liquid water in the fuel cell. In the present study a PEM fuel cell with three different types of flow distributors are modeled and numerically simulated to find out the water formation and distribution characteristics. The model is validated by comparing the simulated polarization curve to experimental data. It is found that the type of flow distributor used plays a major role in determining the distribution of liquid water in the cell. A parallel flow distributor exhibits poor water removal capabilities whereas a serpentine flow distributor exhibits better water removal. A mixed flow distributor is found to give better water distribution characteristics compared to the parallel and serpentine distributors. Further the effect of liquid water formation and distribution on the species transport, temperature distribution and current generation are also investigated.     

Conceptual design and configuration performance analyses of polygenerating high temperature fuel cells

The use of a high temperature fuel cell (HTFC) to continuously and simultaneously polygenerate hydrogen in combination with electricity and heat represents a promising technology as a source of fuel for fuel cell vehicles. Different configurations of polygenerating HTFC, including different designs with internal and external reforming are options to polygenerate electricity, hydrogen and heat. The current study analyzes and compares six different configurations based on solid oxide technology. Efficiency results based upon the Supplemental Input Method demonstrate that internal reforming configurations achieve higher performance than when hydrogen product is produced in an external reformer. The overall efficiency and the efficiency in the generation of each product are used as the basis for comparison.     

Numerical analysis of the influence of the channel cross-section aspect ratio on the performance of a PEM fuel cell with serpentine flow field design

This work numerically investigates the influence of the channel cross-section aspect ratio (defined as the ratio height/width) on the performance of a PEM fuel cell with serpentine flow field (SFF) design. The local current densities, velocity distributions, liquid water concentration in the membrane, hydrogen and oxygen concentrations and temperature were analyzed in the PEM fuel cell for 10 different aspect ratios, varying between 0.07 and 15, to understand the channel cross-section aspect ratio effect. The area of the channel cross section (1.06 mm²) and the total effective reactive area of the PEM fuel cell (256 mm²) were maintained constant in all cases. The obtained results show that at low operating voltages the cell performance is independent of the channel cross-section aspect ratio. At high operating voltages, where the influence of mass transporting velocity is predominant, as the channel cross-section aspect ratio increases the cell performance is improved. The models with high aspect ratio show, in general, more uniform current distributions, with the higher maximum and minimum intensity values, temperature distributions with smaller gradients and a superior contain of water in the membrane, which allows to obtain a higher performance. From these models the 10/06 and 12/05 aspect ratio present the best combination of variables, as shown by their polarization curves.     

Effect of flow fields on the performance of micro-direct methanol fuel cells

This study evaluated the performance of micro-direct methanol fuel cells (DMFCs) with four kinds of flow fields fabricated on silicon wafers by microelectromechanical system (MEMS) technology. The flow fields and membrane electrolyte assembly (MEA) of 2.25 cm² active area were assembled to micro-DMFCs. These micro-DMFCs yielded the maximum power densities ranged from 11 to 23 mW cm⁻² for the methanol solution concentrations of 1 M, 2 M, 3 M, 4 M and 5 M at the temperature of 20 ± 1 °C. The maximum power densities implied that under the ambient temperature and low flow rate of methanol solution, performance of micro-DMFCs with different flow fields was sorted as: double-channel serpentine (DSFF) > single-channel serpentine (SSFF) > mixed multichannel serpentine with wide channels (MMFW) > mixed multichannel serpentine with narrow channels (MMFN) flow field. Increasing the flow rate of methanol solution from 0.0503 to 0.1128 ml min⁻¹, performance of all micro-DMFCs was improved. Further increasing the rate to 0.3479 from 0.1128 ml min⁻¹, the maximum power densities of micro-DMFCs with MMFW and MMFN increased, however, those of micro-DMFCs with SSFF and DSFF decreased. When the electric load was changed, the micro-DMFC with SSFF took a longer time to reach a stable power output than other micro-DMFCs.     

Numerical investigation of the performance of symmetric flow distributors as flow channels for PEM fuel cells

This work reports on the performance of a single PEM fuel cell using symmetric flow patterns as gas delivery channels. Three flow patterns, two symmetric and one serpentine, are taken from the literature on cooling of electronics and they are implemented in a computational model as gas flow channels in the anode and cathode side of a PEMFC. A commercial CFD code was used to solve the physics involved in a fuel cell namely: the flow field, the mass conservation, the energy conservation, the species transport, and the electric/ionic fields under the assumptions of steady state and single phase. An important feature of the current modeling efforts is the analysis of the main irreversibilities at different current densities showing the main energy dissipation phenomena in each cell design. Also, the hydraulic performance of the flow patterns was studied by evaluating the pressure drop and pumping power. The first part of this work reveals the advantages of using a serpentine pattern over the base symmetric distributors. The second part is an optimization of the symmetric patterns using the entropy minimization criteria. Such an optimization led to the creation of a flow structure that promotes an improved performance from the point of view of power generation, uniformity of current density, and low pumping power.