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RESEARCH PAPERS

J. Fuel Cell Sci. Technol. 2005;3(2):89-98. doi:10.1115/1.2173662.

The internal reforming and electrochemical reactions appear in the porous anode layer, and may lead to inhomogeneous temperature and gas species distributions according to the reaction kinetics. In this study, a fully three-dimensional calculation method has been further developed to simulate and analyze chemically reacting transport processes in a thick anode duct. The composite duct investigated consists of a porous anode, the fuel flow duct and solid current connector. Momentum and heat transport together with gas species equations have been solved by coupled source terms and variable thermophysical/transport properties of the fuel gas mixture. Furthermore, the heat transfer due to the fuel gas diffusion is implemented into the energy balance based on multicomponent diffusion models. The fuel cell conditions such as the combined thermal boundary conditions on solid walls, mass balances (generation and consumption) associated with the various reactions, and gas permeation between the porous anode and flow duct are applied in the analysis. Simulation results show that the internal reforming and the electrochemical reactions, and cell operating conditions are significant for species distribution, fuel gas transport and heat transfer in the subdomains of the anode.

Commentary by Dr. Valentin Fuster
J. Fuel Cell Sci. Technol. 2005;3(2):99-110. doi:10.1115/1.2173663.

A control-oriented mathematical model of a polymer electrolyte membrane (PEM) fuel cell stack is developed and experimentally verified. The model predicts the bulk fuel cell transient temperature and voltage as a function of the current drawn and the inlet coolant conditions. The model enables thermal control synthesis and optimization and can be used for estimating the transient system performance. Unlike other existing thermal models, it includes the gas supply system, which is assumed to be capable of controlling perfectly the air and hydrogen flows. The fuel cell voltage is calculated quasistatically. Measurement data of a 1.25kW, 24-cell fuel cell stack with an integrated membrane-type humidification section is used to identify the system parameters and to validate the performance of the simulation model. The predicted thermal response is verified during typical variations in load, coolant flow, and coolant temperature. A first-law control volume analysis is performed to separate the relevant from the negligible contributions to the thermal dynamics and to determine the sensitivity of the energy balance to sensor errors and system parameter deviations.

Commentary by Dr. Valentin Fuster
J. Fuel Cell Sci. Technol. 2005;3(2):111-118. doi:10.1115/1.2173665.

The fuel cell system and fuel cell gas turbine hybrid system represent an emerging technology for power generation because of its higher energy conversion efficiency, extremely low environmental pollution, and potential use of some renewable energy sources as fuels. Depending upon the type and size of applications, from domestic heating to industrial cogeneration, there are different types of fuel cell technologies to be employed. The fuel cells considered in this paper are mainly the molten carbonate (MCFC) and the solid oxide (SOFC) fuel cells, while a brief overview is provided about the proton exchange membrane (PEMFC). In all these systems, heat exchangers play an important and critical role in the thermal management of the fuel cell itself and the boundary components, such as the fuel reformer (when methane or natural gas is used), the air preheating, and the fuel cell cooling. In this paper, the impact of heat exchangers on the performance of SOFC, MCFC gas turbine hybrid systems and PEMFC systems is investigated. Several options in terms of cycle layout and heat exchanger technology are discussed from the on-design, off-design and control perspectives. A general overview of the main issues related to heat exchangers performance, cost and durability is presented and the most promising configurations identified.

Commentary by Dr. Valentin Fuster
J. Fuel Cell Sci. Technol. 2005;3(2):119-124. doi:10.1115/1.2173666.

Durability of the proton exchange membrane (PEM) is a major technical barrier to the commercial viability of polymer electrolyte membrane fuel cells (PEMFC) for stationary and transportation applications. In order to reach Department of Energy objectives for automotive PEMFCs, an operating design lifetime of at least 5000h over a broad temperature range is required. Reaching these lifetimes is an extremely difficult technical challenge. Though good progress has been made in recent years, there are still issues that need to be addressed to assure successful, economically viable, long-term operation of PEM fuel cells. Fuel cell lifetime is currently limited by gradual degradation of both the chemical and hygro-thermomechanical properties of the membranes. Eventually the system fails due to a critical reduction of the voltage or mechanical damage. However, the hygro-thermomechanical loading of the membranes and how this effects the lifetime of the fuel cell is not understood. The long-term objective of the research is to establish a fundamental understanding of the mechanical processes in degradation and how they influence the lifetime of PEMFCs based on perfluorosulfuric acid membrane. In this paper, we discuss the finite element models developed to investigate the in situ stresses in polymer membranes.

Commentary by Dr. Valentin Fuster
J. Fuel Cell Sci. Technol. 2005;3(2):125-130. doi:10.1115/1.2173667.

The electrochemical performance of La0.65Sr0.3MnO3-type (LSM) anode-supported single cells, produced by alternative production processes, has been investigated at intermediate temperatures. In particular, three different variations of the production route were investigated in more detail: (1) the use of nonground LSM powder for the cathode current collector layer, (2) the use of noncalcined and nonground YSZ powder for the cathode functional layer, and (3) the use of tape casting versus warm pressing as the production process for anode substrates. Results from electrochemical measurements performed between 700 and 900°C with H2 (3vol%H2O) as fuel gas and air as the oxidant showed that performance increased with increasing grain size of the outer cathode current collector layer: the highest performance was achieved with nonground LSM powder. Furthermore, performance was not adversely influenced by the use of noncalcined and nonground YSZ for the cathode functional layer. Also the use of anode substrates with a thickness of about 0.7mm and produced by tape casting, instead of those with a thickness of about 1.5mm and applied by warm pressing, did not detrimentally affect the electrochemical performance of this type of SOFC.

Commentary by Dr. Valentin Fuster
J. Fuel Cell Sci. Technol. 2005;3(2):131-136. doi:10.1115/1.2173668.

A silicon-based micro direct methanol fuel cell (μDMFC) for portable applications has been fabricated and its electrochemical characterization carried out. A membrane-electrode assembly (MEA) was specially fabricated to mitigate methanol crossover. The cell with active area of 1.625cm2 demonstrated a maximum power density of 50mWcm2 at 60°C. Since the silicon wafer is too fragile to compress for sealing, and a thicker layer of gold has to be coated on the silicon wafer to reduce contact resistance, further development of micro DMFCs for high power application was carried out using stainless steel as bipolar plate in which flow channels were fabricated by photochemical etching technology. The maximum power density of the micro DMFC reaches 62.5mWcm2 at 40°C and 100mWcm2 at 60°C with atmospheric pressure. An 8-cell air-breathing DMFC stack has been developed. Mass transport phenomena such as water transport and oxygen transport were investigated. By using a water management technique, cathode flooding was avoided in our air-breathing DMFC stack. Furthermore, it was found that oxygen transport in the air-breathing cathode is still very efficient. The DMFC stack produced a maximum output power of 1.33W at 2.21V at room temperature, corresponding to a power density of 33.3mWcm2. A passive DMFC using pure methanol was demonstrated with steady-state output power of 2025mWcm2 over more than 10h without heat management.

Commentary by Dr. Valentin Fuster
J. Fuel Cell Sci. Technol. 2005;3(2):137-143. doi:10.1115/1.2173669.

This paper presents the electricity and hydrogen co-production concept, a methodology for the study of SOFC hydrogen co-production, and simulation results that address the impact of reformer placement in the cycle on system performance. The methodology is based on detailed thermodynamic and electrochemical analyses of the systems. A comparison is made between six specific cycle configurations, which use fuel cell heat to drive hydrogen production in a reformer using both external and internal reforming options. SOFC plant performance has been evaluated on the basis of methane fuel utilization efficiency and each component of the plant has been evaluated on the basis of second law efficiency. The analyses show that in all cases the exergy losses (irreversibilities) in the combustion chamber are the most significant losses in the cycle. Furthermore, for the same power output, the internal reformation option has the higher electrical efficiency and produces more hydrogen per unit of natural gas supplied. Electrical efficiency of the proposed cycles ranges from 41 to 44%, while overall efficiency (based on combined electricity and hydrogen products) ranges from 45 to 80%. The internal reforming case (steam-to-carbon ratio of 3.0) had the highest overall and electrical efficiency (80 and 45% respectively), but lower second law efficiency (61%), indicating potential for cycle improvements.

Commentary by Dr. Valentin Fuster
J. Fuel Cell Sci. Technol. 2005;3(2):144-154. doi:10.1115/1.2174063.

A two-dimensional dynamic model was created for a Siemens Westinghouse type tubular solid oxide fuel cell (SOFC). This SOFC model was integrated with simulation modules for other system components (e.g., reformer, combustion chamber, and dissipater) to comprise a system model that can simulate an integrated 25kw SOFC system located at the University of California, Irvine. A comparison of steady-state model results to data suggests that the integrated model can well predict actual system power performance to within 3%, and temperature to within 5%. In addition, the model predictions well characterize observed voltage and temperature transients that are representative of tubular SOFC system performance. The characteristic voltage transient due to changes in SOFC hydrogen concentration has a time scale that is shown to be on the order of seconds while the characteristic temperature transient is on the order of hours. Voltage transients due to hydrogen concentration change are investigated in detail. Particularly, the results reinforce the importance of maintaining fuel utilization during transient operation. The model is shown to be a useful tool for investigating the impacts of component response characteristics on overall system dynamic performance. Current-based flow control (CBFC), a control strategy of changing the fuel flow rate in proportion to the fuel cell current is tested and shown to be highly effective. The results further demonstrate the impact of fuel flow delay that may result from slow dynamic responses of control valves, and that such flow delays impose major limitations on the system transient response capability.

Commentary by Dr. Valentin Fuster
J. Fuel Cell Sci. Technol. 2006;3(2):155-164. doi:10.1115/1.2174064.

Energy conversion today is subject to high thermodynamic losses. About 50% to 90% of the exergy of primary fuels is lost during conversion into power or heat. The fast increasing world energy demand makes a further increase of conversion efficiencies inevitable. The substantial thermodynamic losses (exergy losses of 20% to 30%) of thermal fuel conversion will limit future improvements of power plant efficiencies. Electrochemical conversion of fuel enables fuel conversion with minimum losses. Various fuel cell systems have been investigated at the Delft University of Technology during the past 20 years. It appeared that exergy analyses can be very helpful in understanding the extent and causes of thermodynamic losses in fuel cell systems. More than 50% of the losses in high temperature fuel cell (molten carbonate fuel cell and solid oxide fuel cell) systems can be caused by heat transfer. Therefore system optimization must focus on reducing the need for heat transfer as well as improving the conditions for the unavoidable heat transfer. Various options for reducing the need for heat transfer are discussed in this paper. High temperature fuel cells, eventually integrated into gas turbine processes, can replace the combustion process in future power plants. High temperature fuel cells will be necessary to obtain conversion efficiencies up to 80% in the case of large scale electricity production in the future. The introduction of fuel cells is considered to be a first step in the integration of electrochemical conversion in future energy conversion systems.

Commentary by Dr. Valentin Fuster
J. Fuel Cell Sci. Technol. 2005;3(2):165-174. doi:10.1115/1.2174065.

This article aims to develop an entropy based method of systematically improving efficiency of fuel cells. Entropy production of both electrochemical and thermofluid irreversibilities is formulated based on the Second Law. Ohmic, concentration, and activation irreversibilities occur within the electrodes, while thermal and friction irreversibilities occur within the fuel channel. These irreversibilities reduce the overall cell efficiency by generating voltage losses. Unlike past studies, this article considers fuel channel irreversibilities within the total entropy production, for both solid oxide fuel cells (SOFCs) and proton exchange membrane fuel cells (PEMFCs). Predicted results of entropy production are shown at varying operating temperatures, surface resistances, and channel configurations. Numerical predictions are compared successfully against past measured data of voltage profiles, thereby providing useful validation of the entropy based formulation. The Second Law stipulates the maximum theoretical capability of energy conversion within the fuel cell. Unlike past methods characterizing voltage losses through overpotential or polarization curves, the entropy based method provides a useful alternative and systematic procedure for reducing voltage losses.

Commentary by Dr. Valentin Fuster
J. Fuel Cell Sci. Technol. 2006;3(2):175-179. doi:10.1115/1.2174066.

Oxygen permeable ceramics based on mixed conductors are attracting much attention for use in partial oxidation of hydrocarbons as a novel technique for syngas and pure hydrogen production. This paper describes the preparation and oxygen permeation properties including the methane reforming property of a novel member of oxygen permeable ceramics. The materials used are solid solutions of (La0.5Ba0.3Sr0.2)(FexIn1x)O3δ. The single phase of perovskite-type (La0.5Ba0.3Sr0.2)(FexIn1x)O3δ is obtained in the range of x=0.4 to 0.9. The highest oxygen flux densities of 2.2 and 11μmolcm2s (membrane thickness, L=0.2mm) are attained for (La0.5Ba0.3Sr0.2)(FexIn1x)O3δ(x=0.6) at 1000°C under He/air and CH4/air gradients, respectively. The electrical conductivity of (La0.5Ba0.3Sr0.2)(Fe0.6In0.4)O3δ is dominated by p-type conduction having a slope of 14 under the high P(O2) region. The oxide-ion conductivity of the same sample is estimated to be 0.05Scm at 800°C. Even though the oxygen flux density slightly decreases with increasing time, high CO selectivity of 90% is kept for 100h. The oxygen flux density of the solid solution is also discussed in the context of surface exchange kinetics.

Commentary by Dr. Valentin Fuster
J. Fuel Cell Sci. Technol. 2005;3(2):180-187. doi:10.1115/1.2174067.

This paper presents the bubble transport phenomenon at the anode of a micro-direct methanol fuel cell (μDMFC) from a mesoscopic viewpoint. Carbon dioxide bubbles generated at the anode may block part of the catalyst/diffusion layer and also the flow channels that cause the μDMFC malfunction. Lattice-Boltzmann simulations were performed in this paper to simulate the two-phase flow in a microchannel with an orifice which emulates the bubble dynamics in a simplified porous diffusion layer and in the flow channel. A two-dimensional, nine-velocity model was established. The buoyancy force, the liquid-gas surface tension, and the fluid-solid wall interaction force were considered and they were treated as source terms in the momentum equation. Simulation results and parametric studies show that the pore size, the fluid stream flow rate, the bubble surface tension, and the hydrophilic effect between the fluid and the solid wall play the major roles in the bubble dynamics. Larger pore size, higher methanol stream flow rate, and greater hydrophilicity are preferred for bubble removal at the anode diffusion layer and also the flow channels of the μDMFC.

Commentary by Dr. Valentin Fuster
J. Fuel Cell Sci. Technol. 2005;3(2):188-194. doi:10.1115/1.2174068.

Aimed at improving the maximum available power density in a planar-type solid oxide fuel cell, an analytical model is proposed in this work to find the optimum size of a current collector that collects the current from a specific active area of the electrode-electrolyte layer. Distributed three-dimensional current collectors in gas delivery field are designated to allow a larger area of the electrode-electrolyte layer to be active for electrochemical reaction compared to conventional designs that gas channels are separated by current collectors. It has been found that the optimal operating temperature of a planar-type solid oxide fuel cell might be around 850°C, if the sizes of the distributed current collectors and their control areas are optimized. Decreasing the size of both the current collector and its control area is advantageous in achieving a higher power density. Studies also show that the optimal sizes of the current collector and the current collection area investigated at 850°C and zero concentration polarization are applicable to situations of different operating temperatures, and different concentration polarizations. The optimization results of the sizes of current collectors and their control areas are relatively sensitive to the contact resistance between the current collectors and the electrodes of the fuel cell. Results of great significance are provided in the analysis, which will help designers to account for the variation of contact resistance in optimization designing of a bipolar plate of fuel cells.

Commentary by Dr. Valentin Fuster
J. Fuel Cell Sci. Technol. 2005;3(2):195-201. doi:10.1115/1.2174069.

The increasing demand on primary energy and the increasing concern on climatic change demand immediately a sustainable development, but still there remain open questions regarding its technical realization. The second law of thermodynamics is a very simple but efficient way to define the principle design rules of sustainable technologies in minimizing the irreversible entropy production. The ideal, but real process chain is defined by a still reversible structure or logic of the process chain—the reversible reference process chain—but consisting of real components with an irreversible entropy production on a certain level. It can easily be shown for energy conversion and for transportation that hybridization in general can indeed be a measure to meet the reversible process chain and to minimize the entropy flow to the environment. Fuel cells are principal reversible converters of chemical energy and thus a key element within hybridization. Depending on application, combined heat and power process (CHP) may be a hybridization step or only a slight improvement. There is a fundamental difference in heating a house or in supplying an endothermic chemical reaction with reaction entropy. The use of heat recovery and isolation is a necessary measure to minimize the entropy flow to the environment and can be described by a reversible reference process as well. The application of reversible reference process chains shows that hybrid systems with fuel cells are a technical feasibility to approach very closely the thermodynamic potential. This development differs from the past where the technical possibilities of materials and manufacturing limited the technology to meet reversibility and thus sustainability.

Commentary by Dr. Valentin Fuster
J. Fuel Cell Sci. Technol. 2005;3(2):202-207. doi:10.1115/1.2174070.

The detailed dynamic characteristics of direct methanol fuel cells need to be known if they are used for transportable power sources. The dynamic response of a direct methanol fuel cell to variable loading conditions, the effect of cell temperature and oxygen flow rate on the cell response, and the cell response to continuously varying cell temperatures were examined experimentally. The results revealed that the cell responds rapidly to variable current cycles and to continuously varying cell temperatures. The increasing rate of gradual loading significantly influences the dynamic behavior. The effects of cell temperature and oxygen flow rate on the cell dynamic responses are considerable, but the cell voltage differences over the range of cell temperatures and oxygen flow rates are small for gradual loading. The cell response value to cell temperature during decreasing temperature is lower than that during increasing temperature.

Commentary by Dr. Valentin Fuster
J. Fuel Cell Sci. Technol. 2005;3(2):208-212. doi:10.1115/1.2174071.

In the present paper we report the NiO solubility in a variety of alkali carbonate compositions in the presence of MgO, CaCO3, BaCO3, and La2O3 additions under different conditions of gas composition and temperature. Two types of binary eutectic melts were chosen, namely, the standard electrolyte (62-38) (Li-K)2CO3 and (52–48)% (Li-Na)2CO3 mixtures. The ternary eutectic melt chosen was (43.5-31.5-25.0)mol%(Li-Na-K)2CO3. The observed reduction of NiO solubility induced by the additives can be explained in terms of a higher basic character of all the modified electrolytes, although the NiO dissolution still follows an acidic dissolution mechanism in all the conditions under study.

Commentary by Dr. Valentin Fuster
J. Fuel Cell Sci. Technol. 2005;3(2):213-219. doi:10.1115/1.2179435.

An experimental study is under way to assess the performance of solid-oxide cells operating in the steam electrolysis mode for hydrogen production over a temperature range of 800900°C. Results presented in this paper were obtained from a ten-cell planar electrolysis stack, with an active area of 64cm2 per cell. The electrolysis cells are electrolyte supported, with scandia-stabilized zirconia electrolytes (140μm thick), nickel-cermet steam/hydrogen electrodes, and manganite air-side electrodes. The metallic interconnect plates are fabricated from ferritic stainless steel. The experiments were performed over a range of steam inlet mole fractions (0.1–0.6), gas flow rates (10004000sccm), and current densities (00.38Acm2). Steam consumption rates associated with electrolysis were measured directly using inlet and outlet dewpoint instrumentation. Cell operating potentials and cell current were varied using a programmable power supply. Hydrogen production rates up to 100Nlh were demonstrated. Values of area-specific resistance and stack internal temperatures are presented as a function of current density. Stack performance is shown to be dependent on inlet steam flow rate.

Commentary by Dr. Valentin Fuster

TECHNICAL BRIEF

J. Fuel Cell Sci. Technol. 2005;3(2):220-223. doi:10.1115/1.2174072.

A semiempirical equation was used to represent the performance characteristics of a 20-cell proton exchange membrane electrolyzer stack. The coefficients of the equation are the exchange current densities and membrane conductivity. These coefficients were determined using experimental data and a nonlinear curve fitting method. The anode exchange current density was found to be 1.65×108Acm2, the cathode exchange current density 0.09Acm2, and the membrane conductivity 0.075Scm1. External programmable power supplies were used to obtain the (IV) characteristic curve of a commercial proton exchange membrane electrolyzer. Stack current, voltage, and system temperature were monitored while 1A current steps were applied to the electrolyzer stack.

Commentary by Dr. Valentin Fuster

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