Cell Design Parameters Research Articles

Dynamic Model for Understanding Spatial Temperature and Species Distributions in Internal-Reforming Solid Oxide Fuel Cells

Direct internal reformation of methane in solid oxide fuel cells (SOFCs) leads to two major performance and longevity challenges: thermal stresses in the cell due to large temperature gradients and coke formation on the anode. A simplified quasi-two-dimensional direct internal reformation SOFC (DIR-SOFC) dynamic model was developed for investigation of the effects of various parameters and assumptions on the temperature gradients across the cell. The model consists of 64 nodes, each containing four control volumes: the positive electrode, electrolyte, negative electrode (PEN), interconnect, anode gas, and cathode gas. Within each node the corresponding conservation and chemical and electrochemical reaction rate equations are solved. The model simulates the counter-flow configuration since previous research (Achenbach, 1994, “Three-Dimensional and Time-Dependent Simulation of a Planar Solid Oxide Fuel Cell Stack,” J. Power Sources, 49(1), p. 333) has shown this configuration to yield the smallest temperature differentials for DIR-SOFCs. Steady state simulations revealed several results where the temperature difference across the cell was considerably affected by operating conditions and cell design parameters. Increasing the performance of the cell through modifications to the electrochemical model to simulate modern cell performance produced significant changes in the cell temperature differential. Improved cell performance led to a maximum increase in the temperature differential across the cell of 31 K. An increase in the interconnect thickness from 3.5 to 4.5 mm was shown to reduce the PEN temperature difference about 50 K. Variation of other physical parameters such as the thermal conductivity of the interconnect and the rib width also showed significant effects on the temperature distribution. The sensitivity of temperature distribution to heat losses was also studied, showing a considerable effect near the fuel and air inlets. Increased heat transfer from the cell edges resulted in severe temperature gradients approaching 160 K/cm. The dynamic capability of the spatially resolved dynamic model was also demonstrated for a 45% power increase perturbation while maintaining constant fuel and air utilizations.

Electrochemical Atomic Layer Deposition (E-ALD) of Palladium Nanofilms by Surface Limited Redox Replacement (SLRR), with EDTA Complexation

Atomic-scale control in the formation of Pd thin films is being developed using electrochemical atomic layer deposition (E-ALD) via surface limited redox replacement (SLRR). Pd has unique hydrogen storage properties. To study hydrogen storage capacity, hydrogen charging and discharging kinetics and its catalytic properties at the nanoscale will require films with well-defined thickness and structure. SLRR is the use of underpotential deposition (UPD) to form a sacrificial atomic layer of a less noble metal, such as Cu or Pb, and to exchange it at open circuit potential (OCP) for a more noble metal (Pd) via galvanic displacement. The deposits were grown using an automated electrochemical flow cell system which allowed sequential variation of solutions and potentials. Electron probe microanalysis (EPMA) revealed excess growth at the flow cell ingress, suggesting that the SLRR mechanism involved electron transfer from substrate to Pd2+ ions, rather than direct electron exchange from sacrificial metal atom(s) to Pd2+ ions. Ethylenediaminetetraacetic acid (EDTA) was used to slow the galvanic displacement by complexing the Pd2+, in an attempt to form more uniform Pd deposits. The resulting films were more homogeneous and displayed the expected Pd voltammetry in H2SO4. The charge for UPD remained constant from cycle to cycle, indicating no roughening of the surface. Ways of optimizing complexing agent properties, as well as the flow cell design and deposition parameters are discussed.

Technological Aspects and Simulation of Efficient Solar Cells based on In1-xGax N Thin Films, Deposited by Molecular Beam Epitaxy

Photovoltaic properties of n-In1-xGaxN/p-Si,Ge (IGN) heterostructures have been evaluated by device simulation and are compared with the performance of c-Si homojunction solar cells. Best achievable cell performance for tandem cells under AM1.5 illumination conditions were 18% for p-Ge and up to 27% for n- InGaN/p-Si contacts, achievable under optimum cell design, material quality and operation parameters. It was shown that the quality of the deposited semiconductor films (IGN) determines the solar cell efficiency. Some critical aspects of the IGN growth, as effects of substrate temperature and V/III ratio, using MBE technique were investigated. Possible routes to overcome crucial problems in growth of high quality IGN films are discussed.

Adsorbing colloidal flotation removing metals ions in a modified jet cell

The removal of Cu 2+, Zn 2+ and Ni 2+ ions from synthetic wastewater with a modified Jameson cell (MJC) was studied using adsorption colloidal flotation (ACF). A colloidal dispersion of Fe (OH) 3 (formed in situ from FeCl 3) at pH 11 was used as an adsorbent colloid to ensure full adsorption and precipitation. The precipitates were flocculated with polyacrylamide and hydrophobised with sodium oleate and pine oil as a frother during the flotation stage. In the modified jet cell, the downcomer was sealed at the bottom with a diffuser, and the re-flotation of detached flocs and the probability of bubbles/particles capture was enhanced, which improved the recovery rate. As a result, the modified Jameson cell was more efficient (higher loaded carrier recoveries) than the conventional jet cell (CJC) in removing heavy metals ions. The physico-chemical characteristics, cell design and operating parameters were studied, and the removal efficiency was evaluated by monitoring the final concentration of ions in the treated effluent. The results indicated that the removal efficiencies of the MJC were approximately 95% and 98% for dilute (Cu 2+, Zn 2+ and Ni 2+ concentration of 2 mg/L) and concentrated wastewater (25 mg/L of each ion), respectively. The optimal parameters included a Fe +3/ion ratio of 0.5 and a minimum air flow-rate/feed flow-rate ratio of 0.18. The results are discussed in terms of the physical and physico-chemical parameters, and the findings suggested that the proposed flotation technique has great potential for the treatment of wastewater.

Design, fabrication, and characterization of a planar, silicon-based, monolithically integrated micro laminar flow fuel cell with a bridge-shaped microchannel cross-section

We report the fabrication of a planar, silicon-based, monolithically integrated micro laminar flow fuel cell (μLFFC) using standard MEMS and IC-compatible fabrication technologies. The μLFFC operates with acid supported solutions of formic acid and potassium permanganate, as a fuel and oxidant respectively. The micro-fuel cell design features two in-plane anodic and cathodic microchannels connected via a bridge to confine the diffusive liquid–liquid interface away from the electrode areas and to minimize crossover. Palladium high-active-surface-area catalyst was selectively integrated into the anodic microchannel by electrodeposition, whereas no catalyst was required in the cathodic microchannel. A three-dimensional (3D) diffusion-convection model was developed to study the behavior of the diffusion zone and to extract appropriate cell-design parameters and operating conditions. Experimentally, we observed peak power densities as high as 26mWcm−2 when operating single cells at a flow rate of 60μLmin−1 at room temperature. The miniature membraneless fuel cell design presented herein offers potential for on-chip power generation, which has long been prohibited by integration complexities associated with the membrane.

Theoretical Analysis of Stresses in a Lithium Ion Cell

A mathematical model to simulate the generation of mechanical stress during the discharge process in a dual porous insertion electrode cell sandwich comprised of lithium cobalt oxide and carbon is presented. The model attributes stress buildup within intercalation electrodes to two different aspects: changes in the lattice volume due to intercalation and phase transformation during the charge/discharge process. The model is used to predict the influence of cell design parameters such as thickness, porosity, and particle size of the electrodes on the magnitude of stress generation. The model developed in this study can be used to understand the mechanical degradation in a porous electrode during an intercalation/deintercalation process, and the use of this model results in an improved design for battery electrodes that are mechanically durable over an extended period of operation.

Sliver solar cells for concentrator PV systems with concentration ratio below 50

AbstractThis paper links together two different yet complimentary technologies: concentrator photovoltaics (CPVs) and Sliver technology. Recent research and development and commercialisation efforts in concentrator technologies have centred on high‐concentration systems, encouraged by the availability of high‐efficiency, multi‐junction III‐V cells. In contrast, little attention has been paid to the potential of systems with low‐to‐medium levels of concentration. Arguably, this is due to the absence of any suitable, low‐cost concentrator cells, readily available at a commercial scale. Sliver technology is a candidate for the supply of commercial low‐cost cells suitable for systems with concentration ratios in the range of 5–50. This can be achieved via judicious choice of cell design parameters and with only minor changes to the fabrication process suitable for 1‐sun Sliver cells. Device modelling is used to show that Sliver cells are suitable for illumination intensities up to 5 W/cm2, with unavoidable emitter resistance limiting performance for higher intensities. The best cells manufactured for operation at low and medium concentration had efficiencies of 18·8% at 9 suns (above 18·6% between 5 and 15 suns) and 18·4% at 37 suns (above 18·2% between 30 and 50 suns), respectively. Incorporation of sidewall texturing and SiN anti‐reflection coatings would yield efficiencies exceeding 20% for similar cells. Concentrator Sliver cells can be produced to almost any length and are perfectly bifacial, features which add significantly to their attractiveness to concentrator system designers. The availability of cheap concentrator Sliver cells could provide opportunities for new, low‐cost concentrator systems, which in turn has the potential to provide a pathway to low‐cost solar electricity. Copyright © 2009 John Wiley & Sons, Ltd.

Impact of Unit Cell Design and Operating Parameters on Membrane Durability in PEMFCs

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Transport, sensitivity, and dimensional optimization studies of a tubular Solid Oxide Fuel Cell

In this paper, the impact of the design and operating parameters of a tubular Solid Oxide Fuel Cell (SOFC) is studied using a well-validated steady-state model. The profiles of species concentrations and pressure in the flow channels when the cell terminal voltage is changed are studied. In addition, both the axial and radial profiles of the species concentrations inside the electrodes are presented. The model is also used to study the effects of the parameters that can significantly influence the design criteria of an anode-supported tubular SOFC. The effects of the flowrate of H 2 , inlet pressure, and the cell temperature on the power output from the cell are studied. Cell characteristic parameters such as the porosity of the electrodes, effective diffusivity of the species, and the rate of the electrochemical reactions are varied and their impact on the cell performance is observed. The influence of the cell design parameters such as the thickness of the electrodes and the electrolyte on the steady-state polarization curve are also studied. Finally, a dimensional study is presented. In the dimensional study, the radius of the anode flow channel, the length of the cell, and the annulus size are varied keeping the solid volume and the total cell volume constant. This study shows that it is possible to design guidelines for the optimum performance of the cell.

Modeling and optimizing the performance of a passive direct methanol fuel cell

Passive direct methanol fuel cells (DMFCs) are promising energy sources for portable electronic devices. Different from DMFCs with active fuel feeding systems, passive DMFCs with nearly stagnant fuel and air tend to bear comparatively less power densities. In the aspect of cell performance optimization, there could be significant differences in cell design parameters between active and passive DMFCs. A numerical model that could simulate methanol permeation and the pertinent mixed potential effect in a DMFC was used to help seek for possibilities of optimizing the cell performance of a passive DMFC by studying impacts from variations of cell design. The subjects studied include catalysis of the anode and the cathode, membrane thickness, membrane conductivity, and methanol concentration. In contrast to general understandings on a DMFC with active fuel and reactant gas, our simulation results for a passive DMFC used in this study indicated that the catalysis of the cathode appeared to be the most important parameter. The maximum power density was predicted to improve by 38% with the thickness of the cathodic catalyst layer doubled and by 36% with the catalyst loading doubled. The improvement on cell performance would multiply if we simultaneously adopted the most optimal parameters during the simulation study.

Main and interaction effects of PEM fuel cell design parameters

In this work, a two-dimensional model is used to analyze the main and interaction effects of five design factors, at three levels in a polymer electrolyte membrane (PEM) fuel cell. The model used in this study is a detailed two-dimensional steady-state model, solved using a finite element partial differential equation solver. The factors considered are channel width, shoulder width, gas distribution electrode (GDE) thickness, GDE conductivity and GDE porosity. A full factorial design is used to minimize statistical errors and study interactions accurately. The model used is a two-dimensional, across-the-channel model. The model is run at both the inlet and exit concentrations for fuel and oxidant, allowing the study of interaction effects over a range of operating conditions. The analysis is conducted for operating potentials of 0.7 and 0.6 V and a range of current densities. The strongest interaction effects are found to exist between channel size and GDE conductivity, while the weakest interaction effects are between GDE thickness and GDE porosity.

Photovoltaic properties and technological aspects of In1−xGaxN/Si, Ge (0<x<0.6) heterojunction solar cells

Photovoltaic properties of n-In1−xGaxN/p-Si, Ge (IGN) heterostructures, covering the compositional range 0<x<0.6, have been evaluated by 1d device simulation, and are compared with the performance of c-Si homojunction thin film cells. Film morphology and physical properties were characterized by high-resolution transmission electron microscopy (TEM), secondary ion mass spectrometry (SIMS) and photoluminescence (PL). Best achievable cell performances under AM1.5 illumination conditions were 18% for p-Ge, and up to 27% for n-IGN/p-Si contacts, achievable under optimum cell design, materials and operation parameters. Pure InN bottom layers, exhibiting an intrinsic band gap of 0.7 eV, reveal a reduced efficiency of 2.5%. The cell efficiency is strongly affected by film quality, accounted for by variation of electron affinity, majority carrier mobility, minority carrier lifetime, film thickness and doping levels. The morphology of thin IGN and InN films deposited onto silicon and sapphire substrate material revealed granular growth, along with a high density of grain boundaries. TEM resolved the formation of a very thin homogeneous silicon nitride interlayer on silicon substrates. The electrically isolating layer almost completely suppresses the photovoltaic effect. Depth profiling of InN films deposited onto sapphire substrates by SIMS analysis indicated oxygen as the dominant material contamination. It accounts, among other effects, for a gradually increasing band gap throughout the film structure. Observed large photoluminescence broadening effects, and related short minority carrier lifetimes are most likely related to high levels of oxygen contamination and concentration of grain boundaries. Possible routes to overcome these problems are discussed.

Parameter Optimization on Combined Gas Turbine-Fuel Cell Power Plants

A thermodynamic model for a gas turbine-fuel cell hybrid is created and described in the paper. The effects of gas turbine design parameters such as compressor pressure ratio, compressor efficiency, turbine efficiency, and mass flow are considered. The model allows to simulate the effects of fuel cell design parameters such as operating temperature, pressure, fuel utilization, and current density on the cycle efficiency. This paper discusses, based on a parametric study, optimum design parameters for a hybrid gas turbine. Because it is desirable to use existing gas turbine designs for the hybrids, the requirements for this hybridization are considered. Based on performance data for a typical 1600hp industrial single shaft gas turbine, a model to predict the off-design performance is developed. In the paper, two complementary studies are performed: The first study attempts to determine the range of cycle parameters that will lead to a reasonable cycle efficiency. Next, an existing gas turbine, that fits into the previously established range of parameters, will be studied in more detail. Conclusions from this paper include the feasibility of using existing gas turbine designs for the proposed cycle.

Optimization of the Operating Parameters of a Proton Exchange Membrane Fuel Cell for Maximum Power Density

The performance of fuel cells can be significantly improved by using optimum operating conditions that maximize the power density subject to constraints. Despite its significance, relatively scant work is reported in the open literature on the model-assisted optimization of fuel cells. In this paper, a methodology for model-based optimization is presented by considering a one-dimensional nonisothermal description of a fuel cell operating on reformate feed. The numerical model is coupled with a continuous search simulated annealing optimization scheme to determine the optimum solutions for selected process constraints. Optimization results are presented over a range of fuel cell design parameters to assess the effects of membrane thickness, electrode thickness, constraint values, and CO concentration on the optimum operating conditions.

Incorporation of voltage degradation into a generalised steady state electrochemical model for a PEM fuel cell

Currently there has been very little reliability or end-of-life analysis conducted for polymer electrolyte membrane fuel cell (PEM) stacks, and detailed designs of PEM systems are still in a rapid evolutionary stage. Voltage degradation as a fuel cell ages is a widely observed phenomenon and results in a significant reduction in the electrical power produced by the stack. Little systematic information has been reported, however, and this phenomenon has not been included in electrochemical models. An earlier work described the development of the generalised steady state electrochemical model (GSSEM) which accepts as input the values of the operating variables (anode and cathode feed gas pressure and compositions, cell temperature and current density), and cell design parameters such as the active area and Nafion membrane thickness. This work will introduce new terms to the model to account for membrane electrode assembly (MEA) ageing, which is a factor in the durability of the stack. One term is based on the concept that the water-carrying capacity (a principal factor in membrane resistance) of the membrane deteriorates with time-in-service. A second term involves the apparent catalytic rate constants associated with the reactions on the anode and cathode side, and the changes in catalytic activity or active site density due to catalyst degradation. A third term deals with the decrease in the rate of mass transfer within the MEA. The resulting model is largely mechanistic, with most terms being derived from theory or including coefficients that have a theoretical basis, but includes empirical parameters to deal with the changing performance. Changes in the polarisation curve predicted by the generalised steady state electrochemical degradation model (GSSEDM) are demonstrated from the data for the performance of typical PEM fuel cell hardware.

Design, Analyses, and Fabrication Procedure of Amtec Cell, Test Assembly, and Radioisotope Power System for Outer-Planet Missions

This paper describes the results of OSC's department of energy (DOE)-sponsored alkali metal thermal-to-electric conversion (AMTEC) generator studies. The paper was prepared in response to an invitation from the International Astronautic Federation for the presentation of a 1-h keynote lecture on our AMTEC studies at the Space Nuclear Power and Propulsion Systems session of the 48th International Astronautical Congress. The paper covers a broad range of topics, which explains its unusual length. After presenting the background of those studies and the operational principles of the AMTEC converters, it describes a novel methodology for the coupled solution of the interdependent thermal, electrical, and fluid flow differential and integral equations governing their performance, and the application of that procedure to OSC's recommended cell design. As explained in the paper, that design was the result of parametric analyses of 35 cell design variations to determine the individual effects of 27 design parameters and various operating parameters on the cell's power output, efficiency, and critical temperatures. The analysis revealed a method of substantially improving the cell's performance by enhancing the internal reflectivity of the cell wall, and identified a specific cell wall composition and coating to achieve that goal. The paper presents a set of OSC-recommended cell design parameters for the ultimate long-service flight generators, as well as a set of parameters for an interim cell design for planned short-term ground tests. The primary difference between them is that the ultimate design employs refractory metal components to enhance compatibility with high-temperature sodium vapor, while the interim design is based on stainless steel, nickel, and Haynes-25, which are expected to be adequate for initial short-term tests, but not for long-duration missions at high operating temperatures. The paper also presents a detailed fabrication sequence for the recommended interim cell design, which is being implemented by AMPS, Inc., for a DOE-sponsored technology development program. It then describes the extension of that analytical procedure to a variety of OSC-designed radioisotope-heated generators employing the recommended cell design, with particular attention to the thermal insulation between the outside of the 16 cells and the inside of the generator housing. The studies found that the performance of the generator is optimized by employing a hybrid insulation system, in which the space between the cells is filled with fibrous Min-K insulation, and the generator walls are lined with tapered (i.e., graded-length) multifoil insulation. The paper then examines the performance of the OSC generator designs for various fuel loadings, output voltages, and mission phases, and assesses their ability to meet the stipulated temperature constraints and power demands of the Europa Orbiter and Pluto Express missions under consideration by NASA. Finally, it presents an OSC-recommended design and fabrication procedure of an electrically heated four-cell test assembly with hybrid insulation to simulate the prototypic 16-cell generator. That test assembly will be analyzed by OSC, built by AMPS, and tested by the Air Force Phillips Laboratory (AFPL), to check the validity of the analytical predictions. The reason for basing the recommended test assembly on the interim rather than the ultimate refractory metal cell design is to avoid the delays required to develop the new technology for the latter.

Multi-electrode zirconia electrolyte amperometric sensors

The concept of multi-component gas sensors based on stabilized zirconia amperometric cells has been proposed and tested using cells with two working electrodes. Variations of the cell structure, geometric parameters, materials of working electrodes and working regimes are carried out for optimal adaptation to the particular gas analytic application. Experiments have been conducted in gas mixtures containing nitrogen, oxygen, nitrogen oxide and some combustible gases. Optimization of cell design and working parameters has had a result in a good separation of signals from the measured gas components at the particular electrode. Cross-effects from different electrochemically active gas components have been studied as well. These effects are negligible in measurement of oxygen and combustible components. Cross-effects from H 2O and CO 2 have been found at large potential difference applied to the working electrodes. These effects are eliminated by optimal selection of electrode potentials and temperatures.

High-Performance Radial AMTEC Cell Design for Ultra-High-Power Solar AMTEC Systems

Solar thermal Alkali-Metal-Thermal-to-Electric-Conversion (AMTEC) power systems potentially have several important advantages over current solar photovoltaic power systems in ultra-high-power spacecraft applications for medium-earth orbit (MEO) and geosynchronous orbit (GEO) missions. This work presents key aspects of radial AMTEC cell design to achieve high cell performance in solar AMTEC systems delivering larger than 50 kW(e) with AMTEC cell conversion efficiency larger than 22 percent. A new design parameter methodology is demonstrated establishing optimum design parameters in radial cell design to satisfy high-power mission requirements. Specific temperature- and pressure-dependent relationships define critical dependencies between key cell design parameters, particularly the impact of parasitic thermal losses on Beta Alumina Solid Electrolyte (BASE) area requirements, voltage, BASE tube number, and system power production for maximum power-per-BASE-area and optimum efficiency conditions. High-level system tradeoffs are demonstrated using the design parameter methodology to establish high-power radial cell design requirements and determine optimum radial AMTEC designs. [S0199-6231(00)00102-7]

Engineering of Solid Oxide Fuel Cells with Ceria‐Based Electrolytes

Solid oxide fuel cells that contain mixed ionic‐electronic conductors (MIECs) as electrolytes exhibit maximum efficiency at high ionic current density due to a small electronic leakage current. At low ionic current density the electronic current increases due to partial reduction of the electrolyte and efficiency decreases. This leads to restrictions in the optimum cell operating range and cell design. Provided the necessary materials and cell design parameters are known, the optimum cell design, especially the electrolyte thickness, can be predicted. The optimization of fuel cells with a mixed‐conducting ceria electrolyte is outlined. The cell optimization considered in this study includes operating range (V‐I), MIEC thickness, and the electrochemical performance of the electrodes. Defect chemical considerations predict an optimum in MIEC thickness depending on the operating temperature.

DESIGN OF LANDFILL DAILY CELLS TO REDUCE COVER SOIL USE

Daily cell design for a landfill is analysed from the viewpoint of minimizing the use of daily cover soil. The landfill's daily cell is idealized to be in the shape of a parallelopiped. To minimize cover soil use and for fixed values of the working face length, volume of refuse, and slope of compaction, the optimal ratio of length to height for the daily cell is found to be the slope gradient (horizontal to vertical). For this optimum ratio, an equation is provided relating the soil-to-refuse ratio to daily cell design parameters. For situations away from the optimum length-to-height ratio, an equation is provided to estimate the excess soil required. These equations can help designers evaluate tradeoffs when a designer attempts to minimize cover soil use.