Counter-flow Operation Research Articles

Study of internal performance of commercial-size fuel cell stack with 3D multi-physical model and high resolution current mapping

The inhomogeneous spatial reaction distributions over the large-scale active area of proton exchange membrane fuel cell stack are critical to the system energy efficiency and lifetime for the automotive applications. In this work, a commercial-size fuel cell stack of 406 cm2 active area is designed with asymmetric reactants flow fields for fuel cell vehicles. To understand the internal performance of the fuel cell, a segmented device with 396 segments is well designed based on the multi-layered printed circuit board technology for high resolution current mapping of the presented stack. Validated by the segmented fuel cell measurements, a 3D multi-physical large-scale model is developed to analyze the detailed distributions of current density, water content and temperature inside the fuel cell stack under different operating conditions. In the counter-flow operation of hydrogen and air, the lowest current density consistently locates around anode inlet while the mid portion of the active area near cathode inlet performs the best. Increasing the air stoichiometric ratio significantly improves the uniformity of current density distribution and the overall performance. The local flow field structures of Cross-flow configurations apparently enhance the in-plane temperature uniformity than the Parallel-flow ones. With the combination of current mapping and coupled modeling, both the distribution trends and local details of internal performance could be analyzed comprehensively to bridge the gap between the stack design and energy efficiency performance.

Two-Dimensional simulation of purge processes for dead-ended H2/O2 proton exchange membrane fuel cell

ABSTRACT Proton exchange membrane fuel cells (PEMFCs) with the dead-ended anode (DEA) and dead-ended cathode (DEC) can achieve high utilization of hydrogen and oxygen but also cause the accumulation of liquid water in the cell and performance degradation. In this study, a two-dimensional multiphase transient PEMFC model is developed to investigate the purge strategy of the PEMFC with DEA and DEC. This model is well validated with the experimental data. The simulation results show that the voltage increases to a certain extent at the beginning of the dead-ended operation and then decreases gradually. During the purging period, the voltage reaches a peak value and then decreases. Therefore, the optimal purge duration is defined as the purge stops when the voltage reaches the maximum value, and the optimal purge duration decreases with the increase of scavenging velocity and increases with the increase of voltage drop rate, however, the energy efficiency under the optimal purge duration is not the highest. Besides, compared with the purging anode and cathode operation, the peak value of the purging cathode operation decreases gradually. Compared with the counter-flow operation, the co-flow operation leads to earlier purging moment, larger ohmic resistance, and voltage decay rate. The excessive accumulation of liquid water leads to a more poor uniformity of the local current density distribution. The purging cathode and counter-flow operation is the most ideal purge strategy during short-time operation, due to the highest energy efficiency about 52.3%, the second-highest output performance, and the second-longest purge interval about 180.01 s.

Concentrator photovoltaic thermal management using a new design of double-layer microchannel heat sink

The concentrator photovoltaic (CPV) systems commonly endure high cell temperatures, which affect their overall performance. Besides, the high cell temperature could lead to physical damages within the whole system due to the induced thermal stresses. Therefore, an efficient cooling is mandatory to achieve a higher net output power from the CPV and safe operation. The current work's main purpose is to investigate the integration of double-layered microchannel heat sink (DL-MCHS) with CPV cell as thermal management device. A three-dimensional (3D) model is presented to investigate the performance of a CPV cooled by the DL-MCHS to figure out the ability to handle the effective heat dissipation under different terrestrial conditions. The parallel flow (PF) and counter flow (CF) cooling orientations were studied. Ethanol coolant is used to cool the CPV under different concentration ratios (CR) of 5, 10, 15, and 20 suns. The results show that the temperatures remarkably decreases with the inlet flowrate is increased. The counter flow operation (CF) achieved the best temperature uniformity index (Tuni) when the coolant mass flowrate (V̇) ranged between 200 and 1200 ml/hr. Especially at CR 5 suns, the temperature uniformity index, Tuni, enhanced to 99.87% at 1200 ml/hr.

An exergy analysis of a concentric tube heat exchanger using hBN-water nanofluids

This study investigated the effects of using nanofluids prepared with hexagonal boron nitride (hBN) nanoparticles on the thermal performance and pressure drop of a concentric tube heat exchanger. Experiments were carried out with water-hBN nanofluids for stable, dispersed, 0.01%, 0.1% and 1% volume concentrations, at different flow rates and Reynolds numbers under parallel and counter-flow conditions. When the experimental results were examined, the exergy loss was higher in the parallel-flow heat exchanger compared to the counter-flow heat exchanger. The highest exergy loss value was obtained for distilled water at the highest Reynolds number (Re = 8,700) for parallel flow operation at 170 W. The exergy loss of water at the highest Reynolds number (Re = 8,700) for parallel flow operation increased by 15.7%, 24.8% and 49.8% for hBN-water concentrations of 0.01%, 0.1% and 1%, respectively. Exergy loss of water at the highest Reynold number (Re = 8,700) for counter flow operation increased by 11.3%, 17.3% and 29.2% for hBN-water concentrations of 0.01%, 0.1% and 1%, respectively. When evaluating the exergy analysis of the system, exergy losses due to pressure drops were negligible for both flows (parallel and counter).

An exergy analysis of a concentric tube heat exchanger using hBN-water nanofluids

This study investigated the effects of using nanofluids prepared with hexagonal boron nitride (hBN) nanoparticles on the thermal performance and pressure drop of a concentric tube heat exchanger. Experiments were carried out with water-hBN nanofluids for stable, dispersed, 0.01%, 0.1% and 1% volume concentrations, at different flow rates and Reynolds numbers under parallel and counter-flow conditions. When the experimental results were examined, the exergy loss was higher in the parallel-flow heat exchanger compared to the counter-flow heat exchanger. The highest exergy loss value was obtained for distilled water at the highest Reynolds number (Re = 8,700) for parallel flow operation at 170 W. The exergy loss of water at the highest Reynolds number (Re = 8,700) for parallel flow operation increased by 15.7%, 24.8% and 49.8% for hBN-water concentrations of 0.01%, 0.1% and 1%, respectively. Exergy loss of water at the highest Reynold number (Re = 8,700) for counter flow operation increased by 11.3%, 17.3% and 29.2% for hBN-water concentrations of 0.01%, 0.1% and 1%, respectively. When evaluating the exergy analysis of the system, exergy losses due to pressure drops were negligible for both flows (parallel and counter).

A Mathematical Model toward Real-Time Monitoring of Automotive PEM Fuel Cells

A computationally efficient model toward real-time monitoring of automotive polymer electrolyte membrane (PEM) fuel cell stacks is developed. Computational efficiency is achieved by spatio-temporal decoupling of the problem, developing a new reduced-order model for water balance across the membrane electrode assembly (MEA), and defining a new variable for cathode catalyst utilization that captures the trade-off between proton and mass transport limitations without additional computational cost. Together, these considerations result in the model calculations to be carried out more than an order of magnitude faster than real time. Moreover, a new iterative scheme allows for simulation of counter-flow operation and makes the model flexible for different flow configurations. The proposed model is validated with a wide range of experimental performance measurements from two different fuel cells. Finally, simulation case studies are presented to demonstrate the prediction capabilities of the model.

Study of internal multi-parameter distributions of proton exchange membrane fuel cell with segmented cell device and coupled three-dimensional model

Understanding multi-parameter distributions inside the proton exchange membrane fuel cell is critical for the stack design and operation optimization. In this work, in situ measurement and three-dimensional model of internal parameter distributions for fuel cell are studied both experimentally and numerically. A segmented cell device based on printed circuit board with embedded sensors is designed to detect local current, relative humidity and temperature in the single cell simultaneously. Meanwhile, a two-phase flow multi-physical fuel cell model validated by the in situ measurement is built to analyze various internal performances. Quantitative impact of convective gas flow on internal parameter distribution is analyzed to reveal the mechanism of water and heat balance for counter-flow operation. The results show that air flow rate is critical to the parameters distributions including current, relative humidity, reactants concentrations and temperature, but hydrogen flow rate effect is neglected. With parametric sweep modeling, the anode and cathode relative humidity distribution profiles could be respectively described by a linear-approximate piece-wise function, which is helpful to quickly predict and evaluate the internal water content distributions for various fuel cell designs and operations.

A Literature Review on Corrugated Plate Heat Exchanger

Abstract The main motto of all the modifications in heat transfer equipment is to save energy and capital investments by reducing cost. The heat exchanger with minimum cost and at low pumping power with high rate of heat transfer is considered to be more efficient. In the current study the focus is made on researcher's development in experimental and analysis of heat exchanger with corrugated shape and chevron angle plate heat exchanger which includes design, experimental outcomes with different fluids like air, water, glycerin, nano fluids and also numerical analysis. The heat exchanger of corrugated type has a large surface area for heat transfer per unit volume, which leads to a high thermal performance with counter-flow operation. Heat exchangers with corrugated shape are mostly used in different engineering areas.

Physical modeling of polymer-electrolyte membrane fuel cells: Understanding water management and impedance spectra

A transient 2D physical continuum-level model for analyzing polymer electrolyte membrane fuel cell (PEMFC) performance is developed and implemented into the new numerical framework NEOPARD-X. The model incorporates non-isothermal, compositional multiphase flow in both electrodes coupled to transport of water, protons and dissolved gaseous species in the polymer electrolyte membrane (PEM). Ionic and electrical charge transport is considered and a detailed model for the oxygen reduction reaction (ORR) combined with models for platinum oxide formation and oxygen transport in the ionomer thin-films of the catalyst layers (CLs) is applied. The model is validated by performance curves and impedance spectroscopic experiments, performed under various operating conditions, with a single set of parameters and used to study water management in co- and counter-flow operation. Based on electrochemical impedance spectra (EIS) simulations, the physical processes which govern the PEMFC performance are analyzed in detail. It is concluded that the contribution of diffusion through the porous electrodes to the overall cell impedance is minor, but concentration gradients along the channel have a strong impact. Inductive phenomena at low frequencies are identified from physics-based modeling. Induction is caused by humidity dependent ionomer properties and platinum oxide formation on the catalyst surface.

Thermal and fluid flow first-principles numerical design of an enhanced double pipe heat exchanger

This investigation is concerned with the thermal and fluid flow design of a double-pipe heat exchanger in which the wall of the inner pipe is helically corrugated. In this arrangement, the inner tube is geometrically unique since it is comprised of alternating ridges and grooves. By means of the continuous helical twist, the pipe wall creates a swirl flow component superimposed on an axial flow within both fluid streams. Steady numerical simulations were used as the design tool. Results were obtained for both parallel and counter flow, and for both constant- and variable-fluid property models. Two thermal-hydraulic models were employed. In one, the heat exchanger was analyzed as a single end-to-end entity. In the other, a spatially periodic model was created to obtain generally applicable thermal resistance results. Comparisons with simple smooth-walled double-pipe exchangers showed roughly a factor of three increases in heat transfer for the helically corrugated double-pipe heat exchangers. The corresponding comparison of the pressure drops revealed the corrugated double-pipe heat exchangers had anywhere from a factor of two to four larger pressure drop. These comparisons correspond to the two compared cases having the same perimeters of the participating pipes. Counter-flow operation yielded higher rates of heat transfer, but to an extent that depends on whether the fluid properties are temperature-dependent or constant. Reynolds numbers for the investigated cases ranged from 420 to 2000.

Improved performance and stability of direct borohydride fuel cells (DBFCs) with porous polypyrrole support

This work focused on the development of a method to fabricate a multilayer macro/mesoporous polypyrrole (PPPr) support to provide a high surface area for catalyst deposition in a direct borohydride fuel cell (DBFC) using sodium borohydride as a fuel and hydrogen peroxide as an oxidant. The multilayer macroporous structure was constructed through simultaneous electropolymerisation of two-monomer mixture (i.e. pyrrole and methylene blue) on an NH4Cl salt template; poly(methylene blue) and NH4Cl were later selectively removed from the composite film as verified by FTIR. The SEM and TEM results demonstrated that a uniform distribution of polypyrrole particles was obtained with an apparent macroporous structure in a varied range of 18–65 µm which consequently provided a well-dispersed and well–distributed Pt deposition. It was also apparent that the PPPr had a higher BET surface area by the introduction of macro/mesoporous structure as compared to the bulk polypyrrole sample (BPPr). In addition, the PPPr was highly electrically conducting with the surface conductivity of 9.6 ± 0.8 × 101 S cm−1 and electrochemically active surface area of 5.4 cm2, which were increased to 42.5 ± 0.9 × 101 S cm−1 and 30.0 cm2 respectively with the presence of Pt catalyst within the PPPr-Pt sample. The application of PPPr-Pt to DBFC was shown to have an excellent electrochemical stability and an improved power density of 2-fold and 3.6-fold higher than the BPPr and commercial Vulcan XC-72 carbon black respectively. By increasing fuel concentration to 0.3 M and using counter flow operation, the DBFC using PPPr-Pt catalyst exhibited a maximum power density of 139 mW cm−2.

Selection of suitable operating conditions for planar anode-supported direct-internal-reforming solid-oxide fuel cell

A numerical model was implemented to analyze the thermodynamic performance of the co- and counter-flow operations of an anode-supported direct internal reforming (DIR) planar solid oxide fuel cell (SOFC). This developed model was validated by comparing with experimental and simulated results taken from the literature. The model is capable of capturing the detailed distribution of the local temperatures, species concentrations, current density, and polarization losses in streamwise direction. Energy and exergy concepts were used to evaluate the DIR-SOFC performance under co- and counter-flow operations. The study indicates the energy and exergy efficiencies of DIR-SOFC performance under co-flow operation are more sensitive to the increase of current density than that under counter-flow operation. Particular attention was paid to cell temperature profiles to avoid mechanical failure due to high thermal stresses. The result shows that the material constraints need to be considered as well as the energy and rational efficiencies in evaluating the performance of SOFC. The preferred flow configuration can be changed depending on the cell geometry and operation conditions if we consider the material constraints.

Experimental analysis of spatio-temporal behavior of anodic dead-end mode operated polymer electrolyte fuel cell

During the anodic dead-end mode operation of fuel cells, the inert gases (nitrogen and water) present in the cathode side gas channel permeate to the anode side and accumulate in the anode gas channel. The inert gas accumulation in the anode decreases the fuel cell performance by impeding the access of hydrogen to the catalyst. The performance of fuel cell under potentiostatic dead-end mode operation is shown to have three distinct regions viz. time lag region, transient current region and a steady state current region. A current distribution measurement setup is used to capture the evolution of the current distribution as a function of time and space. Co- and counter-flow operations of dead-end mode confirm the propagation of inert gas from the dead-end of anode channel to the inlet of anode. Experiments with different oxidants, oxygen and air, under dead-end mode confirm that nitrogen which permeates from cathode to anode causes the performance drop of the fuel cell. For different starting current densities of 0.15 A cm −2, 0.3 A cm −2 and 0.6 A cm −2 the inert gas occupies 35%, 45% and 57%, respectively of anode channel volume at the end of 60 min of dead-end mode operation.

In Situ PEM Fuel Cell Water Measurements

Efficient PEM (Polymer Electrolyte Membrane) fuel cell performance requires effective water management. To achieve a deeper understanding of water transport and performance issues associated with water management, we have conducted in situ water examinations to help understand the effects of components and operations. High Frequency Resistance (HFR), AC Impedance and Neutron imaging were used to measure water content in operating fuel cells. High resolution neutron radiography was used to image fuel cells during a variety of conditions. The effect of specific operating conditions, including flow direction (co-flow or counter-flow) was examined. Counter-flow operation was found to result in higher water content than co-flow operation, which correlates to lower membrane resistivity. A variety of cells were used to quantify the membrane water in situ during exposure to saturated gases, during fuel cell operation, and during hydrogen pump operation. The quantitative results show lower membrane water content than previous results suggested.

Recycle Effect on Double‐Pass Concentric Circular Mass Exchangers under Uniform Wall Fluxes

AbstractA study on improvement in device performance of a double‐pass concentric circular mass exchanger with uniform wall fluxes and external refluxes was conducted under counterflow operations. The linear superposition of an asymptotic solution and a homogeneous solution with the use of an orthogonal expansion technique for solving such a conjugated Graetz problem is formulated theoretically and the analytical predictions are applied to investigate a substantial masstransfer improvement. A mathematical expression for the average Sherwood numbers and the outlet dimensionless concentration is given. Comparisons of analytical results are made with the single‐pass operations of the same working dimensions (without permeable barriers inserted and without recycle). Considerable device improvement in mass transfer rate is obtained by introducing the recycle‐effect concept in designing such double‐pass devices. The effect of permeable barriers on the mass‐transfer efficiency improvement as well as on the power consumption increment has also been delineated.

High Resolution Neutron Radiography Imaging of Operating PEM Fuel Cells: Effect of Flow Configuration and Gravity on Water Distribution

High resolution neutron radiography was used to image polymer electrolyte membrane fuel cells (PEMFCs) during operation under a variety of conditions. The effect of specific operating conditions, including flow direction (co-flow or counter-flow), orientation of the PEMFC with respect to gravity, and simulated anode recycle loop, were examined. Counter-flow operation was found to result in higher water content than co-flow operation, with more uniformly distributed water. Orientation with respect to gravity was also significant, with net water transport observed from the higher side of the cell to the lower side. The higher anode flow rate resulting from the simulated recycle loop facilitated convective removal of liquid water from the anode flow field, leading to lower overall levels of water in the PEMFC.

Water flux in membrane fuel cell humidifiers: Flow rate and channel location effects

Abstract A straight, single channel membrane humidifier was constructed to measure temperature and moisture profiles along both the donor and receiver channels. A persulfonic Nafion membrane was used as the water exchange medium. We report on results obtained with single-phase vapour-to-vapour, counter flow operation. First, the heat loss to the surroundings was quantified and found to affect the overall performance significantly. Second, the results from varying flow rates indicate that lower flow rates lead to higher outlet dew point values of the receiver stream which can be related to longer residence times. It was also found that moisture transfer is more strongly influenced by the flow rate through the receiver side than the donor side. Finally, five-point dew point profiles for both donor and receiver sides are reported for various temperature conditions. No stream wise variation in moisture flux was observed, and the average flux value increased from 3.3 × 10−5 kg s−1 m−2 at 30 °C to 2.0 × 10−4 kg s−1 m−2 at 70 °C under fully humidified donor-side inlet conditions.

Investigation of Locally Resolved SOFC Characteristics along the Flow Path

Solid oxide fuel cells (SOFCs) are characterized locally in a segmented cell arrangement. The tests were performed in counter flow operation for hydrogen concentrations from 2 % to 100 % to identify concentration limitations and to optimize fuel utilization. Cell characterisations were obtained for different cell concepts by voltage/current density U(i) and temperature measurements as well as gas chromatography measurements at 16 distinct points across the cell. The results show a substantial variation of current density and voltage distribution along the flow path with varying hydrogen content and fuel utilization. Especially at high fuel utilizations approaching 100% a dramatic dependence of power density along the flow path occurs which is associated with increasing degradation. The observation of inhomogeneous temperature distribution emphasizes the importance of a thermal management adapted to the cell operation characteristics, particularly when the stack itself only has a low thermal mass.

SOFC characteristics along the flow path

Solid oxide fuel cells (SOFCs) in metallic housings were integrally and locally characterised. The tests were performed in counter flow operation for hydrogen concentrations from 2% to 100%, to identify concentration limitations and to optimise fuel utilisation. Cell characterisations were performed by spatially resolved electrochemical impedance spectroscopy (EIS), current density/voltage ( i– V) and temperature measurements as well as gas chromatography measurements at 16 distinct points across the cell. The results show a substantial variation of current density and voltage distribution along the flow path with varying hydrogen content and fuel utilisation. The fuel utilisation was calculated from the local current densities and compared to the values measured by gas chromatography. Both sets of results showed good agreement. At low hydrogen inlet concentrations the voltage at the fuel outlet drops to values that might be harmful for the stability of the anode since reoxidation of nickel can occur. The impedances obtained by local EIS did not show an overall coherent dependency on the hydrogen concentration. EIS under load revealed two distinct domains: in the range of hydrogen concentrations of 2–10% H 2 the impedance decreased significantly with increasing hydrogen content whereas at higher hydrogen contents the impedance was hardly affected. This indicates significant concentration and diffusion overpotential at low hydrogen concentrations. The local data showed differing behaviour in the middle of the cell compared to the fuel outlet. Leakage at the sealing could be identified as a possible reason. As an additional method of investigation, the voltage drop over the contact resistance of the cathode side was measured. Temperature measurements show that local temperatures differ significantly depending on the load applied to the cell. This observation emphasizes the importance of a thermal management adapted to the characteristics on operation conditions of the cells, particularly when the stack itself has only a low mass.

Anode-supported intermediate temperature direct internal reforming solid oxide fuel cell. I: model-based steady-state performance

Mathematical modelling is an essential tool for the design of solid oxide fuel cells (SOFCs). The present paper aims to report on the development of a dynamic anode-supported intermediate temperature direct internal reforming planar solid oxide fuel cell stack model, that allows for both co-flow and counter-flow operation. The developed model consists of mass and energy balances, and an electrochemical model that relates the fuel and air gas composition and temperature to voltage, current density, and other relevant fuel cell variables. The electrochemical performance of the cell is analysed for several temperatures and fuel utilisations, by means of the voltage and power density versus current density curves. The steady-state performance of the cell and the impact of changes in fuel and air inlet temperatures, fuel utilisation, average current density, and flow configuration are studied. For a co-flow SOFC operating on a 10% pre-reformed methane fuel mixture with 75% fuel utilisation, inlet fuel and air temperatures of 1023 K, average current density of 0.5 A cm −2 , and an air ratio of 8.5, an output voltage of 0.66 V with a power density of 0.33 W cm −2 and a fuel efficiency of 47%, are predicted. It was found that cathode activation overpotentials represent the major source of voltage loss, followed by anode activation overpotentials and ohmic losses. For the same operating conditions, SOFC operation under counter-flow of the fuel and air gas streams has been shown to lead to steep temperature gradients and uneven current density distributions.