Effect Of Inlet Relative Humidity Research Articles

A numerical analysis of the effect of layer-scale and microscopic parameters of membrane electrode assembly in proton exchange fuel cells under two-phase conditions

A multiphysics, multiphase, multiscale model of the membrane electrode assembly (MEA) of a polymer electrolyte membrane fuel cell (PEMFC) is presented. The model accounts for varying local effective transport that arise from assembly compression of the gas diffusion layer (GDL) and MEA microstructure, providing a more realistic description of heterogeneities. The model is validated against previous experimental data in terms of polarization curve, effect of inlet relative humidity on performance, and rib/channel saturation distribution. Then, a parametric analysis of layer-scale (compression and GDL thickness) and microscopic (pore radius and defect volume fraction) parameters is presented. The results show that performance can be improved by reducing compression under the rib (provided that electrical contact resistances are negligible), tailoring GDL thickness at intermediate values around 200μm, designing macroporous layer (MPL) and catalyst layer (CL) with pore sizes between 50–100nm and 0.5–1μm, respectively, and reducing large defect volume fractions in MPLs and CLs. To achieve these goals, an integral design of MEAs is necessary to minimize electrical contact resistances, cracks and defects, reduce inhomogeneous compression and produce highly porous multiscale structures with large continuum pore sizes at multiple scales.

Investigating the Influence of Inlet Relative Humidity on Polymer Electrolyte Membrane Fuel Cell Performance by Visualizing 4-D Water Distributions in Gas Diffusion Layers

The catastrophic effects of atmospheric greenhouse gases and the depletion of non-renewable resources has led to the urgency to develop clean, sustainable energy technologies to meet increasing energy demands. However, the intermittent nature of current renewable energy technologies warrants the acquisition of on-demand renewable energy through either energy production or storage methods. Polymer electrolyte membrane fuel cells (PEMFCs) are promising candidates for this task, as they utilize the most abundant element on Earth, hydrogen, to produce high amounts of power under rapid changes in load with little to no greenhouse gas emissions (1). Therefore, PEMFCs have great potential to help offset the negative impact of atmospheric pollution due to excessive carbon emissions. However, liquid water management issues associated with high power output of the fuel cell typically leads to reduced performance and durability of the fuel cell, and thereby hinders global implementation of these devices (2). To minimize these losses and improve GDL material designs, an understanding of the relationship between product liquid water distributions in the cathode GDL and transport properties of PEMFCs under varying operating conditions is highly valuable. Previous works have characterized the effect of operating temperature on liquid water pathways and distributions in GDLs by visualizing operando PEMFCs with 3D imaging techniques such as X-ray computed tomography (CT) (3). The high-speed, high-resolution capabilities of these imaging techniques enable the visualization of dynamic pore-scale activity to elucidate transport mechanisms in the GDL.In this work, the effect of inlet relative humidity on the formation and distribution of liquid water pathways in cathode GDLs is investigated by imaging a PEMFC operando with synchrotron X-ray CT at high spatial resolution, enabling the resolution of water in the individual pores of the GDL. The contribution of a microporous layer (MPL) is also explored by imaging a cell with an MPL and without. Imaging is conducted on a specialized cell designed to facilitate continuous rotation about the CT stage, enabling fast acquisition of consecutive scans to achieve high temporal resolution useful for visualizing the dynamic development of preferential water pathways. Additionally, electrochemical impedance spectroscopy was performed to quantify mass transport losses. The sequence of CT images was utilized to capture the dynamics of liquid water development as well as stabilized water distributions. Visualizing the reconstructed images shows that as current increases, water production increases, and the development of water pathways to breakthrough at the flow field interface is observed. Additionally, results show that an increase in relative humidity led to a significant increase in cathode GDL water saturation.The contribution of this study to understanding transport mechanisms in GDL materials is significant to the characterization and optimal design of materials for improved PEMFC performance. Ultimately, the goal of this work is to accelerate the worldwide adoption of PEMFCs as a sustainable, reliable solution to replace conventional carbon-emitting energy sources.1. Alaswad et al., J. Hydrog. Energy, 41, (2016)2. Nagai et al., J. Power Sources, 435, (2019)3. D. Shum et al., Electrochem. Acta, 256, (2017)

Investigation of Inlet Gas Relative Humidity on Performance Characteristics of PEMFC Operating at Elevated Temperature

Raising the operating temperature is considered to be an effective method to improve the output performance of proton exchange membrane fuel cells (PEMFCs). In this paper, the effects of inlet relative humidity in the anode (RHa) and cathode (RHc) on the polarization curve and impedance spectra of a single rotating serpentine PEMFC were investigated by experimental method at the operating temperature of 90 °C. It was found that the output performance is the smallest in the high RH case (RHa90%/RHc90%) due to the largest mass transfer resistance. However, the ohmic resistance in the dry case (RHa50%/RHc50%) is the highest, and it shows better output performance at more than 1.0 A/cm2 because of the lowest mass transfer resistance. The impact of the changes in the RHa value on the polarization curve is more apparent than that of the RHc changes at high current density. The largest power density can be attained and the efficiency can reach 24.4% when the RHa is 90% and RHc is 50%.

Entropy generation analysis based on a three-dimensional agglomerate model of an anion exchange membrane fuel cell

A three-dimensional agglomerate numerical model has been used to analyse entropy generation in an anion exchange membrane fuel cell. The effects of inlet relative humidity, platinum loading, carbon loading and ionomer volume fraction on entropy generation have been analysed in the present study. The reversible and irreversible heat have been identified as the main sources of entropy production for all the parameters tested. Even though the production of entropy due to Ohmic loss occurs due to both electronic and ionic potentials, the production of entropy as a result of the first is insignificant in comparison to the latter due to the fact that the electronic conductivity is significantly higher in comparison to the ionic diffusivity coefficient. A strong effect of the back diffusion of water has been found on the entropy production due to Ohmic loss, as the hydration of the membrane directly affects the anion transport through the membrane. Water condensation at the anode catalyst layer and gas diffusion layer has been observed in a few cases, giving rise to the entropy generation due to latent heat. Nevertheless, this has been observed to be 1 to 2 orders of magnitude lower in comparison to the entropy generated due to reversible, irreversible and Ohmic heats.

Three-dimensional agglomerate model of an anion exchange membrane fuel cell using air at the cathode – A parametric study

A three-dimensional agglomerate model of an anion exchange membrane fuel cell has been developed and used to perform a parametric analysis of the effects of inlet relative humidity, ionomer water uptake, platinum loading, carbon content and ionomer volume fraction on the overall fuel cell performance. Improved cell performance has been obtained when the anode relative humidity was higher compared to the cathode resulting in a significant amount of water back diffusion, as well as higher oxygen partial pressure at the cathode enhancing the oxygen mass transport. Increasing the membrane water content positively affects the overall performance of the fuel cell because of the improvement of ionic conductivity. An increase in platinum loading has been found to have a positive impact on the fuel cell performance. Carbon loading influences the thickness of the catalyst layer, directly affecting concentration and Ohmic losses in the catalyst layer. An increase in the ionomer volume fraction enhances the transportation of ions and also the diffusion of membrane water through the membrane. A decrease in the volume fraction of ionomer in the catalyst layer leads to a reduction in the membrane water content and ion diffusion rate, thus deteriorating the overall performance of the fuel cell.

Numerical Simulation of Transient Response of Inlet Relative Humidity for High Temperature PEM Fuel Cells with Material Properties

High temperature proton exchange membrane fuel cells (HT-PEMFCs) have been drawing much attention due to their easy water management and other advantages. A three-dimensional non-isothermal transient model of HT-PEMFCs with phosphoric acid doped polybenzimidazole (PBI) membrane is developed in this study. The inlet relative humidity (RH) is considered for the membrane conductivity in the model. The effect of inlet RH on the transient response of the cell is discussed and the results show that the increase of inlet RH had positive effect on cell performance but negative effect on transient response.

Numerical studies on liquid water flooding in gas channels used in polymer electrolyte fuel cells

Water management plays an important role in the development of low-temperature polymer electrolyte fuel cells (PEFCs). The lack of a macroscopic gas channel (GC) flooding model constrains the current predictions of PEFC modeling under severe flooding situations. In this work, we have extended our previously developed one-dimensional channel flooding model for including phase change and liquid water flooding on the anode side. Several case studies are conducted to explore the effects of inlet relative humidity, net water transfer coefficient and GC shape on water film flooding in the GCs on both sides with the assumption that, both GCs are free from water droplets for simplicity. We demonstrate that the present model is capable of capturing key phenomenon of liquid water flooding in GCs. Our primary results show that gas drag force prevents liquid water accumulation in GCs, which dominates the liquid water removal. Both inlet relative humidity and stoichiometry can influence the liquid water flooding in GCs dramatically. At last, it is shown that increasing the upper half corner angle of the GC can mitigate the film water flooding in GCs.

Airside characteristics of heat, mass transfer and pressure drop for heat exchangers of tube-in hydrophilic coating wavy fin under dehumidifying conditions

The airside heat, mass and momentum transfer characteristics of seven wavy fin-and-tube heat exchangers with hydrophilic coating under dehumidifying conditions were experimented. The test inlet air dry bulb temperatures were 20, 27 and 35oC, the inlet relative humidity were 50%, 60%, 70% and 80%, and the air velocity were 0.5, 1.0, 2.0, 3.0 and 4.0ms−1. The test results indicate that both the Colburn jm factor and the Colburn jh factor decrease with the increase of fin pitch, and this phenomenon becomes more and more pronounced as Reynolds number decreases. The friction factor is very sensitive to the change of fin pitch, and the friction factor shows a cross-over phenomenon as fin pitch changes. The Colburn jh factor decreases and the Colburn jm factor increases when the number of tube rows increases, while the friction performance is insensitive to the change of the number of tube rows. The effects of inlet relative humidity on the heat transfer and friction performance can be omitted, but the Colburn jm factor decreases with the increase of the inlet relative humidity. The predictive ability of the available state-of-the-art heat transfer and pressure drop correlations was evaluated with the experiment data of the present study. The new heat, mass and momentum transfer correlations were proposed to describe the present test results according to the multiple linear regression technique. The mean deviations of the proposed jh, jm and f correlations are 6.3%, 8.9% and 7.9%, respectively. Comparing to published data reduction method, the process line on psychrometric chart of fin-and-tube heat exchanger for partially wet conditions and more accurate overall heat transfer coefficient equation are put forward in this paper.

Effects of inlet relative humidity and inlet temperature on the performance of counterflow wet cooling tower based on exergy analysis

The purpose of this study is to analyze the influence of the ambient temperature and humidity on the performance of a counterflow wet cooling tower according to the second law, exergy analysis, of thermodynamics. First, the properties of water and air flow through the tower were predicted and validated by the experiment. Exergy analysis then has been carried out for investigating the cooling tower performance with various inlet air conditions, relative humidity and dry bulb temperature, while the water side condition is kept constant. According to the analysis in this paper, the similar result in terms of required dry air flow rate, exergy change of water and that of air, exergy destruction and second law efficiency were obtained for the various inlet air conditions. The exergy change of water Δ x w is higher than that of air Δ x air, since Δ x w is the available energy of water to supply to air throughout the tower while Δ x air is the available energy of air to recover or utilize that supplied by water. It reveals that Δ x air is dominated by the exergy change of air due to evaporative heat transfer. In addition, it gives a clearer explanation of the cooling tower performance and gives clear trends for optimization.

Optimization of polymer electrolyte fuel cell cathode catalyst layers via direct numerical simulation modeling

The cathode catalyst layer (CL), due to sluggish oxygen reduction reaction and several transport losses therein, plays an important role in the overall performance of polymer electrolyte fuel cells (PEFCs). The relative volume fractions of the constituent phases, i.e. the electronic, electrolyte and void phases, of the cathode CL need to be selected appropriately in order to achieve an optimal balance between oxygen diffusion and proton conduction. In this work, the influence of electrolyte and void phase fractions of the cathode CL on the cell performance is investigated based on a pore-level description of species and charge transport through a random CL microstructure via the direct numerical simulation (DNS) model. Additionally, the effects of inlet relative humidity and net water transport from the anode on the cathode performance have been studied which indicate the interdependence between the CL composition and the cell operating conditions. The results indicate that the low humidity operation benefits the performance by enhancing the oxygen transport especially under high current densities. Finally, the DNS model predicts the volume fractions of 0.4 and 0.26 for the void and electrolyte phases, respectively, as the optimal composition of the catalyst layer for the best performance.

A comparison of the airside performance of the fin-and-tube heat exchangers in wet conditions; with and without hydrophilic coating

An experimental study was performed on compact fin-and-tube heat exchangers in wet conditions. Airside performance for both hydrophilic coated and un-coated surface is examined. It is found that the effect of inlet relative humidity on the heat transfer performance is small. For un-coated surfaces, the effect of inlet relative humidity has a pronounced effect on pressure drops. It is likely that this phenomenon is related to the condensate flow pattern along the fin surface. The heat transfer performance for the hydrophilic coating surface is lower than the corresponding un-coated surface tested at the same wet condition. Further, the degradation of heat transfer performance may be up to 20% for fin pitches of 1.2 mm. The pressure drops for the hydrophilic coated surface are also lower than the corresponding un-coated surfaces. A maximum 40% reduction is observed for plain fin geometry. The effect of inlet condition on frictional performance is more pronounced in the enhanced slit geometry.

Heat and momentum transfer for compact louvered fin-and-tube heat exchangers in wet conditions

The air side performances of two louvered fin surfaces under dehumidifying conditions were examined in this study. The test results indicated that the effect of fin pitch on the heat transfer performance is comparatively small, and the friction factors increase significantly with fin pitch for fully wet conditions. The effect of inlet relative humidity on the sensible heat transfer performance is negligible. However, there is a detectable effect of inlet condition on the friction factors for type (I) louver due to condensate retention. A correlation of heat, momentum, and mass transfer was proposed to correlate the present database with a mean deviation of 5.94%, 6.1%, and 7.89%, respectively.