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Transportation Systems

2006;():1-12. doi:10.1115/FUELCELL2006-97003.

The present work considers a two-fluid mathematical model for the gas-liquid flow in PEM fuel cells. One fluid represents the continuous gas phase flow through the layers of the fuel cell. For this fluid, the governing equations of momentum, energy, mass continuity and species mass fractions, are considered with additional inter-fluid exchange source terms. The second fluid represents the dispersed liquid phase that is formed from the condensed water vapor inside the layers of the PEM fuel cell. For this fluid only the momentum and mass continuity equations need to be included, as no electrochemical reactions are essentially possible. The dispersed fluid is made up of small droplets in the gas channel. The mean droplet diameter can be computed from a balance equation of the forces acting on the emerging liquid water from the pores of the GDL into the gas channels. The droplet diameters are found to range between 150 and 170 microns for the present PEM fuel cell at 0.8 V. In the present work, the full momentum conservation equations are invoked, in the layers of the fuel cell, for the two fluids. The resulting governing equations for u, v, T and species mass fractions together with the electric potential and mass continuity equations are solved iteratively, using a modified SIMPLE algorithm, for the two fluids. One solution domain is superimposed over all the layers of the fuel cell. Special care is devoted to the electric potential, ‘Poisson-type’, equation boundary condition to prevent any escape of protons through the two gas diffusion layers and simultaneously insuring a non-singular matrix of finite-difference coefficients. The obtained two-fluid and single-phase numerical simulations are compared with the corresponding experimental and numerical data available in the literature. The 2-fluid model shows that the blocking effect of the liquid phase starts to dominate, for cell voltage less than 0.65 V; in this case, the flowing 2-phase flow produces faster drop in cell voltage as the loading electric current increases. This phenomenon was partially hindered by previous LHF model results and essentially completely bypassed by the single-phase simulations.

Commentary by Dr. Valentin Fuster
2006;():13-24. doi:10.1115/FUELCELL2006-97008.

A zero-dimensional, dynamic model of a PEM fuel cell with an autothermal methane reformer section is generated. In the model, reformer consists of three reactors for the autothermal steam reforming, the water gas shift, and the selective oxidation. Physical approaches are favored over empirical modeling equations in order to describe the relevant transport phenomena. Thus, in spite of the low order modeling, only physical parameters are required for the input. The aim of the model is the calculation of the cell voltage for a given set of geometrical and thermodynamic data including the current density. The fuel cell water management is modeled with equations for the water transport through the membrane by electro-osmotic drag and diffusion as well as the membrane humidity. Flooding due to liquid water content and dehydration of the membrane is simulated dynamically with this model. These critical conditions are identified by the cell voltage. Due to pressure feedback, dynamic simulations show partial pressure fluctuation in the reformer reactors caused by load changes of the fuel cell.

Commentary by Dr. Valentin Fuster
2006;():25-30. doi:10.1115/FUELCELL2006-97016.

This paper observes phenomena related to water production behavior inside a fuel cell and analyzes the effect on the current and temperature distribution across the reaction area. A fuel cell permitting direct observation of the phenomena in the cell, 2-D temperature measurements in the cathode channels, and local current density measurements on the anode side was manufactured. The experimental results showed the production and flow of liquid water in the cell, and there were good correlations among the distributions of current density, temperature, and water amounts in the channels. The behavior of current, voltage, water distribution, and pressure differences in the cathode channels were used to hypothesize about the possibility of gas paths deep in the gas diffusion layer in the flooded condition and a positive feedback mechanism in the drying-out condition.

Commentary by Dr. Valentin Fuster
2006;():31-43. doi:10.1115/FUELCELL2006-97020.

Storing hydrogen gas under pressure to provide an energy source for fuel cells or internal combustion engines is a real issue that must be addressed. Diatomic hydrogen does not occur naturally and therefore, must be made through electrolysis, methane reforming or some other process. From the production of pure hydrogen to the final end user, the entire cycle must be considered. Once formed, hydrogen will need to be compressed to a storage container. A hydrogen based transportation system will be both an economic and engineering challenge.

Topics: Hydrogen , Storage
Commentary by Dr. Valentin Fuster
2006;():45-53. doi:10.1115/FUELCELL2006-97021.

PEM fuel cells are promising candidate as most environmentally friendly power source for transport and stationary cogeneration applications due to its high efficiency, high power density, fast startup and system robustness. But the PEM fuel cell is still too expensive for widespread commercialization. Bipolar plate is one of the most important and costliest components of PEM fuel cells and accounts to more than 80% of the weight and 30% of the total cost in a fuel cell stack. To reduce the cost and weight of fuel cell stacks and at the same time meeting several technical requirements for mass production, a prototype of low-cost stamped bipolar plates made of stainless steel 316 sheets has been introduced in this paper. Base on micro sheet forming process simulation experiments, the influence of some key dimensions of the flow channel to the formability of the stamped polar plate is also detailedly studied. Micro-forming simulation results show that relative punch radius r/t (punch radius r, sheet thickness t) and the ration of the width of coolant channel to channel depth w/h (width of coolant channel w, channel depth h) are import factors that decide the final formability of the whole polar plate. Large r/t is recommended for compact flow channel design and larger w/t is recommended for safer forming process.

Commentary by Dr. Valentin Fuster
2006;():55-61. doi:10.1115/FUELCELL2006-97027.

A computational model is developed for a PEM unit cell capable of describing the reactions that occur in the cell in understoich conditions. Such conditions can occur as a result of reactant supply system failure (blockage, leaks, system control, etc.). The model applies to cells with straight channels, and mass transport in the MEA (membrane crossover as well as transport through the GDE) in the channel cross-plane is described only in an average sense assuming linear diffusive mechanisms. Several other major assumptions are made, the most significant being that the cell is always at saturated conditions and is taken to be isothermal. Several electrochemical and mass transport coefficients are not available in the literature and “best guesses” are taken. The results of the model are not yet validated experimentally. However, it is the first model proposed that captures these phenomena in a comprehensive way at the local level and also couples the phenomena through channel flow. Cathode understoich results show the expected Hydrogen evolution at the cathode. Anode understoich results show anode Carbon oxidation. An interesting third run is shown where at low currents, a partial anode understoich condition occurs where the cell voltage remains positive, but the anode is starved of Hydrogen near outlet. Carbon corrosion at the cathode occurs in this case.

Commentary by Dr. Valentin Fuster
2006;():63-79. doi:10.1115/FUELCELL2006-97030.

Proton Exchange Membrane (PEM) fuel cell system performance can be significantly improved with suitable control strategies. Control appropriate models of the fuel cell stack and balance of plant are presented along with current control research. Fuel cell stack models are zero dimensional and range from simple empirical stack polarization curves to complex dynamic models of mass flow rates, pressures, temperatures, and voltages. Balance of plant models are also zero dimensional and can be used individually to build a complete system around a stack. Models of this type are presented for the air compressor, air blower, manifolds, reactant humidification, fuel recirculation, air cooling, and stack cooling. Current control work is surveyed with regard to feedforward, feedback, observers, optimization, model prediction, rule based, neural networks, and fuzzy methods. The most promising fuel cell stack model is evaluated. Additionally, improvements to the balance of plant models are recommended. Finally, future control work is explored with a desire for system control that leads to greater output power.

Commentary by Dr. Valentin Fuster
2006;():81-87. doi:10.1115/FUELCELL2006-97032.

A numerical model that employs the finite-element method and a fully-coupled implicit solution scheme via Newton’s technique is presented for simulating the performance of polymer-electrolyte-membrane (PEM) fuel cells. With our model, solved are the multi-dimensional momentum, mass & species, and charge conservation equations that govern, respectively, pressure-gradient driven flows along the gas flow channels (GFCs) and within the gas diffusion layers (GDLs), species transport along GFCs and within GDLs, and proton and water transport within the membrane as well as the ButlerVolmer constitutive equations describing the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR). For simplicity, the present version of our model considers PEM fuel cell operation as isothermal and water present as vapor, and treats the anode and cathode catalyst layers as respective interfaces at which HOR and ORR take place. With our numerical approach, all governing equations are solved simultaneously and quadratic convergence is ensured due to the use of Newton’s method with an analytical Jacobian. To demonstrate the utility of our computational approach, computed predictions of velocity field, contours of hydrodynamic pressure and molar concentrations of hydrogen, oxygen and water species, and current distribution and polarization (or I-V) curves from a two-dimensional case study of a simplified PEM fuel cell are presented. To help assess the validity of our PEM fuel cell model, measurements of current distribution and polarization curves were performed using a segmented PEM fuel cell, and the resultant experimental data as well as that from the literature are compared with computed predictions.

Commentary by Dr. Valentin Fuster
2006;():89-92. doi:10.1115/FUELCELL2006-97033.

The fuel cell is one of promising environment-friendly energy sources for the next generation. The bipolar plate is a major component of the PEM fuel cell stack, which takes a large portion of stack cost. In this study, as alternative materials for bipolar plate of PEM fuel cells, graphite composites were fabricated by compression molding. Graphite particles mixed with epoxy resin were used as the main substance to provide electric conductivity. To achieve desired electrical properties, specimens were made with different mixing ratio, processing pressure and temperature and tested. To increase mechanical strength, one or two layers of woven carbon fabric were added to the original graphite and resin composite. Thus, the composite material was consisted of three phases: graphite particles, epoxy resin, and carbon fabric. By increasing mixing ratio of graphite, fabricated pressure and process temperature, electric conductivity of the composite were improved. The results of tensile test showed that the tensile strength of the two-phase graphite composite was about 4 MPa, and that of three-phase composite was increased to 57 MPa. As surface properties, contact angle and surface roughness were tested. Contact angles were higher than 100°. The average surface roughness was 0.96 μm.

Commentary by Dr. Valentin Fuster
2006;():93-96. doi:10.1115/FUELCELL2006-97040.

A novel method of using a liquid phase oxidizer in fuel cell applications has been discovered by researchers at UC Davis. This paper outlines potential implications for improving heat transfer and catalytic activity with this method. Experimental data have been collected and the results show that the proposed method of using liquid phase oxidizer does indeed allow operation of PEM fuel cell systems. Data indicate an improvement in overvoltage at low current but also clearly indicate a severely limited concentration polarization region with non-regenerated fluid. The preliminary data indicate the physical feasibility of the method but also show that more research and development is required.

Commentary by Dr. Valentin Fuster
2006;():97-104. doi:10.1115/FUELCELL2006-97041.

To achieve optimal performance with minimal parasitic losses and degradation, the relationship between water removal parameters such as flow rate and the diffusion media (DM) surface properties must be clearly identified. An extensive experimental study of the influence of controllable engineering parameters, including surface PTFE (Teflon™) coverage (ranging from 5% to 20% of wt.) and operational air flow rate, on liquid droplet deformation at the interface of the DM and the gas flow channel was performed. A new visualization technique was developed to better understand the droplet mechanisms with enhanced optical access of both side and top views of the flow channel of a simulated H2 PEFC. A telecentric lens and 5 mm by 5 mm prisms embedded in the flow channel side walls were used for the first time to measure droplet receding and advancing surface angles in an enclosed flow channel. The influence of channel air flow rate and emerging droplet size on droplet characteristics with varying PTFE content in the DM was investigated to identify the conditions under which the droplet tends toward an unstable state. The results indicate that operational conditions, droplet height, chord length, and level of surface hydrophobicity of the DM directly affect the droplet instability. At high flow rates, the surface hydrophobicity of the DM enhances the efficacy of droplet removal, and helps to avoid local channel flooding, however at low flow rates, regardless of the amount of PTFE content, droplet instability (and removal) is unaffected by the DM surface PTFE content.

Commentary by Dr. Valentin Fuster
2006;():105-110. doi:10.1115/FUELCELL2006-97059.

In this paper, R & D on the electrocatalysts and the proton conductive membranes for proton exchange membrane fuel cells in our group is presented. It is shown that both the electrocatalysts and the proton conductive membranes have attained an enhanced performance.

Commentary by Dr. Valentin Fuster
2006;():111-116. doi:10.1115/FUELCELL2006-97060.

PtRuIr/C catalyst was prepared via a microwave-irradiated polyol plus annealing (MIPA) synthesis strategy. Tests by CO stripping voltammetry and in the single PEM fuel cells showed a greatly high CO-tolerant performance of the catalyst. The catalyst was characterized by a series of techniques, such as TEM, XRD, EDS, XPS and gas chromatography, etc., and the data were discussed with respect to the PtRu/C catalyst prepared following the same procedure. On the basis of the characterizations, the performance-structure relationship was explored and a speculative mechanism was supposed.

Commentary by Dr. Valentin Fuster
2006;():117-126. doi:10.1115/FUELCELL2006-97062.

A thermal model of the Proton Exchange Membrane Fuel Cell (PEMFC) was developed to investigate the performance of a large active area fuel cell with the water cooling thermal management system. The model includes three sub-models: water transport model, electrochemical reaction model and heat transfer model. The water transport model calculates water distribution and the electric resistance of the membrane electrolyte. The electrochemical reaction model for the agglomerate structure cathode catalyst layer predicts the cathode overpotentials including mass transport limitation effect at high current density region. Two-dimensional heat transfer model incorporated with coolant and gas channels predicts the temperature distribution within the fuel cell. By integrating those sub-models, local electric resistance and overpotentials depending on the water and temperature distribution can be predicted. The model was calibrated with published experimental data and sensitivity studies were performed. The effects of the inlet gas temperature and humidity on the fuel cell performance were explored. In addition, the effect of the temperature distribution, and accordingly the electric resistance distribution within the fuel cell depending on the coolant temperature and flowrate was investigated. The results shows that the change in the local electric resistance due to temperature distribution eventually causes fuel cell power decrease and it is also concluded that the coolant temperature and flowrate should be controlled properly depending on the operating conditions in order to minimize the temperature distribution while maximizing power output of the fuel cell.

Commentary by Dr. Valentin Fuster
2006;():127-132. doi:10.1115/FUELCELL2006-97063.

The over-potentials in a fuel cell due to ohmic losses and concentration polarization can be reduced if the gas delivery field and the current collection system are well designed. To obtain such a substantial understanding for designing the gas delivery and current collection system, this study proposed a model to theoretically analyze the current collection process, and finally a method and tool of optimization for scales of gas channels and current collection ribs is presented. The analysis found that small current collectors and collection area is advantageous for getting high power density in both PEMFCs and SOFCs.

Commentary by Dr. Valentin Fuster
2006;():133-150. doi:10.1115/FUELCELL2006-97067.

Recent trends and advances in hydrogen/air Proton Exchange Membrane Fuel Cells (PEMFC) are incorporated into a dynamic control oriented model. This type of model is important for development of control systems for PEMFC powered transportation where unpredictable and widely varying changes in power demand can be expected. Self humidification and low pressure operation are the two major changes to past systems. As a result, a high pressure air compressor, air cooler, and inlet gas humidifiers are no longer required. Also, the likelihood of cathode flooding is reduced. The overall fuel cell model consists of four basic sub-models: anode, cathode, fuel cell body, and cooling. Additionally, the oxidant supply blower, cooling pump, and cooling fan are explicitly incorporated. Mass and energy conservation are applied to each using a lumped parameter control volume approach. Empirical modeling is minimized as much as possible, however it is necessary for model manageability in a control context. Interactions between each subsystem and balance of plant components are clearly defined. The overall model is capable of capturing the transient behavior of the flows, pressures, and temperatures as well as net output power. The influence of the charge double layer effect on transient performance is also explored. Numerical simulations of the system are presented which illustrate the usefulness of the model. Finally, future control work is described.

Commentary by Dr. Valentin Fuster
2006;():151-159. doi:10.1115/FUELCELL2006-97075.

Predicting the water dynamics and estimating humidity and flooding conditions in a low-temperature fuel cell are critical for robust operation and long life. Previous work by McKay et al [1] shows that the fuel cell anode, cathode, and membrane water dynamics and gaseous species concentrations can be accurately modeled by discretizing the partial differential equations that describe mass transport into three segments. Avoiding sensitivities associated with over-parameterization, and allowing for the real-time computations necessary for embedded controllers, requires in-depth investigation of the model order. In this paper the model from [1] is formulated into a bond graph representation. The objective is to establish the necessary model order for the fuel cell model using the Model Order Reduction Algorithm (MORA) [2], where an energy-based metric termed the Activity is used to quantify the contribution of each element of the model. Activity is a scalar quantity that is determined from the generalized effort and flow through each element of the model. We show the degree of model order reduction and provide a guideline for appropriate discretization.

Commentary by Dr. Valentin Fuster
2006;():161-172. doi:10.1115/FUELCELL2006-97077.

A three-dimensional steady-state electrochemical mathematic model is established where the mass, fluid and thermal transport processes are considered as well as the electrochemical reaction. The influences of the parameters of interest, which include the porosity, the permeability, and the thickness of the gas diffusion layer and the inlet gas stoichiometric flow rate, on the performance of fuel cells are identified. By applying the Powell algorithm, multiple optimum parameters are obtained from the optimization solution with respect to the objective function, which is defined as the maximum potential of the electrolyte fluid phase at the membrane/cathode interface with a typical value of the cell voltage. By comparing the optimized oxygen mole fraction and the local current density distribution with the reference case, the results shown in the paper provide useful tools for a better design of fuel cells.

Commentary by Dr. Valentin Fuster
2006;():173-179. doi:10.1115/FUELCELL2006-97086.

This paper reports on the study of gas diffusion media (GDM) intrusion into reactant gas channels and its effect on the performance of the proton exchange membrane (PEM) fuel cell. The PEM fuel cell under consideration consists of a membrane electrode assembly (MEA) sandwiched between two layers of gas diffusion media commonly made of carbon paper or cloth. The GDM/MEA/GDM assembly is then compressed between two adjacent bi-polar plates. In this configuration, the compression pressure is transmitted under the lands of the reactant gas flow-field onto GDMs on which the portion over the channels remain unsupported. Because of the relatively low bending and compressive stiffness, it is found that GDMs can easily intrude into the reactant gas channels. The direct consequence of GDM intrusion is the pressure drop increase in the reactant gases in the intruded channels. This is further compounded by cell-to-cell or channel-to-channel variation in GDM thickness and mechanical properties, which results in non-uniform reactant gas flow distribution and ultimately negatively impacts the fuel cell performance. In this study, we have developed a GDM intrusion model based on the finite element method (FEM. We have also devised an experimental setup to measure the GDM intrusion, in which we found good agreement between the model prediction and experimental measurement. Combining the FEM based intrusion model and a flow redistribution model we have investigated the effect of GDM channel intrusion on the reactant flow distribution and the impact on the fuel cell performance. It is found that a 20% reduction of reactant flow can be induced with a 5% additional blockage in channels by GDM intrusion. Based on the findings from the current study, we attribute the significant performance variation in a 30-cell fuel cell stack to the variation in reactant flow induced by the variation in GDM intrusion. The results from the analytical study and fuel cell testing both suggested that the product variations in GDM would need to be significantly reduced and the stiffness of the GDM would need to be increased if the PEM fuel cells of high power density were to be used reliably at a relatively low stoichiometry.

Commentary by Dr. Valentin Fuster
2006;():181-187. doi:10.1115/FUELCELL2006-97094.

Mechanical fracture of Nafion® membrane limits the life of PEM FC stacks. This is likely a result of gradual strength degradation and mechanical stress/strain transients induced by the cycling relative humidity (RH). Mechanical properties of Nafion® membrane strongly depend on water content. The objectives of the authors’ work are (1) to understand the fundamental mechanical behavior of an ionomer membrane, i.e., Nafion® , as a function of RH and (2) to develop physically meaningful models to perform stress/strain analysis of membrane electrode assemblies under RH and temperature variations. To characterize the mechanical response of an ionomer as a function of temperature and relative humidity, an environment chamber capable of generating temperatures from 25 to 100 degrees Centigrade and relative humidities from 5 to 85 percent was designed and built. An electromechanical membrane test (load) frame was mounted as an integral part of the system. An optical strain measurement device was used to record axial extension and lateral contraction of the membrane specimens without contact. Extensive mechanical tests on a commercial ionomer membrane were conducted under carefully controlled hydration and temperature. Fully nonlinear, fully anisotropic elasto-plastic constitutive representation of this ionomer material was obtained as function of temperature and RH. Water content significantly affects the elastic modulus of the membranes. Experimental data show that the elastic modulus of the membrane continuously increases up to about twice the original value during dry out. Such has been taken into account in order to accurately model the stress/strain history of the membrane during dry-out. The collected experiment data were represented in material constitutive models for use in a finite element code, ABAQUS. A 3-layer membrane electrode assembly (MEA) structure has been modeled to observe stress/strain distribution during RH and T cycling. Non-uniform electrode/membrane interfaces have been modeled as well as uniform sections to see the effects of geometric irregularities on the extreme values of stress and strain.

Commentary by Dr. Valentin Fuster
2006;():189-194. doi:10.1115/FUELCELL2006-97096.

Through-the-thickness flaws or “pinholes” in proton exchange membranes (PEM) can lead to gas crossover, reducing fuel cell efficiency, accelerating degradation, and raising safety issues. The multi-physics process that causes these flaws is not fully understood, but stress state, environmental exposure, and cyclic operation may all be contributing factors. Fracture mechanics has proven to be useful in characterizing degradation of many materials, including polymers subjected to environmental challenges. Although unclear if pinhole formation can be successfully characterized and predicted from a fracture perspective, this study continues our prior work to characterize PEMs in such a manner. Because of the lack of constraint, thin films often exhibit very high fracture energies and large plastic zones, features that are not consistent with observations of PEM failures. In an effort to obtain the fracture energy with very little dissipation, knife-slitting tests were conducted to reduce the crack tip plasticity. With modifications made to the systems used by Wang and Gent (1994) and by Dillard et al (2005), a slitter that maintains a constant tearing angle during the slitting process was developed. While fracture energies on the order of 104 J/m2 were measured with double edge notched test samples, and on the order of 103 J/m2 were measured with trouser tear samples, the knife slit test resulted in fracture energies as low as several hundred J/m2 . An environmental chamber was used to enclose the slitting process so experiments at elevated temperature and moisture levels could be conducted. The relevance of these fracture energies to observed PEM failures in operating fuel cells is not fully understood. Nonetheless, the ability to obtain fracture energies approaching the intrinsic fracture energy of these ductile membranes is believed to be useful in studying what appear to be more brittle fracture modes that have been observed in PEMs.

Commentary by Dr. Valentin Fuster
2006;():195-202. doi:10.1115/FUELCELL2006-97104.

Thermal and water management is critical to fuel cell performance. It has been shown that gas diffusion layer (GDL) can impose the mass transport limit; for example, it can block the reactant transport to active layer when flooding occurs at high current density conditions. Micro porous layer (MPL) in conjunction with backing layer (BL) has been used as a GDL material and was shown to be effective for water management. To study the transport processes in GDL and MPL modified GDL, an analytical solution is derived current study for calculation of two-phase, multicomponent transport in GDL. Two models were considered, the unsaturated flow model (UFM) and the separate flow model (SFM). Comparison of the calculated saturation level and oxygen mass fraction shows that UFM calculation can underestimate, as well as overestimate the saturation and oxygen concentration. The SFM was used to study the effects due to GDL property variations. The calculation shows that increase in liquid water transport in an MPL modified GDL is due to the abrupt change of liquid water flow rate when a step change in porosity or permeability is imposed. The calculation further shows that particle size of around 1 μm would be a good choice for MPL as it results in higher oxygen concentration at active layer and lower saturation in GDL.

Commentary by Dr. Valentin Fuster
2006;():203-210. doi:10.1115/FUELCELL2006-97109.

Hydrogen can be produced in a variety of methods including steam-reformation of hydrocarbon fuels. In past studies the quasi non-dimensional space velocity parameter (inverse residence time) has been shown to be insufficient in accurately predicting fuel conversion in hydrocarbon-steam reformation. Heat transfer limitations have been manifest with reactors of different geometries. In order to achieve ideal fuel conversion, the heat transfer limitations and the changes of these limitations with respect to geometry must be considered in the reactor design. In this investigation, axial and radial temperature profiles are presented from reactors of different aspect ratios while holding space velocity constant. Using both the temperature profile information as well as the traditional space velocity limitations one may be able to develop an optimal reactor design.

Commentary by Dr. Valentin Fuster
2006;():211-220. doi:10.1115/FUELCELL2006-97114.

Proton exchange membrane fuel cell (PEMFC) systems have been investigated as alternative power sources for the stationary and automotive applications. However, they do not appear on the market yet. One of the main reasons is the difficulty of the water/thermal management in the system. Several water recovery schemes have been proposed to overcome this difficulty and to simplify the system configuration. In this paper, the influence of the water recovery schemes on the water/thermal balances in the PEMFC system is investigated. One is a scheme without water phase change at the condenser and the humidifier. The other is a scheme with water phase change. As a result, at low operating pressure, the scheme with water phase change has a large heat dissipation from the system. This is because large water recovery is required in the condenser. With increasing operating pressure, the influence of the difference in the water recovery scheme is diminished from the thermal balance point of view. This is because the requirement of the water recovery becomes small. In the favorable operating range for the stack, the water/thermal balances in the system with water phase change is quite difficult because the heat loads in the intercooler and condenser are very large. However, the water recovery scheme without phase change can solve this difficulty. This scheme makes the component design simpler in this operating range.

Commentary by Dr. Valentin Fuster
2006;():221-231. doi:10.1115/FUELCELL2006-97122.

A tri-layer fuel cell includes separate flow channels for hydrogen and oxygen. One potential alternative flow channel design is the use of a Bi-polar plate that connects cathode of a tri-layer fuel cell to anode of the next tri-layer fuel cell in order to provide an efficient flow of current through the cells with reduced voltage loss. The design of the bipolar plates provides considerable engineering challenges. It requires being thin with good contact surfaces for the purpose reduced electrical resistances as well as efficient transport processes for the reactant gasses in micro-channels with reduced pressure drops. Fluid flow and heat and mass transport in gas flow channels plays an important role in the effective performance of the fuel cell. A bi-polar plate design with straight parallel channels is considered and flow field in gas flow channels are analyzed using computational fluid dynamic model. Results for pressure drop coefficient and heat transfer coefficients with varying flow Reynolds number are presented.

Topics: Gas flow , Fuel cells
Commentary by Dr. Valentin Fuster
2006;():233-241. doi:10.1115/FUELCELL2006-97124.

A Polymer Electrolyte Membrane (PEM) fuel cell stack requires elastomeric gaskets in each cell to keep the reactant gases within their respective regions. If any gasket degrades or fails, the reactant gases (O2 and H2 ) can leak overboard or mix with each other directly during operation or during standby, and affect the overall operation and performance of the fuel cell. The degradation of four commercial gasket materials was investigated in a simulated fuel cell environment in this study. In an effort towards predicting lifetime of fuel cells, two solutions and two temperatures were used in the short-term, accelerated aging tests. Bend-strip environment crack resistance tests were performed on samples with various bend angles. Weight loss was monitored and surface structure changes were examined using optical microscopy on the samples exposed to the simulated fuel cell environment for selected periods of time. Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectroscopy was employed to study surface chemistry of the gasket materials before and after exposure to the simulated fuel cell environment over time. Stress and strain analysis was conducted using finite element method (FEM) to quantify the stress/state in test samples. The test results reveal that two silicone materials were degraded significantly while the other two did not show much degradation up to 42 weeks exposure to the simulated fuel cell environment. Optical microscopy and ATR-FTIR spectroscopy analysis indicate that the surface chemistry altered gradually via mechanisms involving de-cross linking and chain scission in the backbone. From experimental and numerical results, it is concluded that there is an interaction between chemistry and stress that appears to accelerate the degradation of the gasket materials in fuel cell environment.

Topics: Gaskets , Fuel cells
Commentary by Dr. Valentin Fuster
2006;():243-252. doi:10.1115/FUELCELL2006-97127.

A parametric model of a proton exchange membrane fuel cell (PEMFC) operating with a polybenzimidazole (PBI) membrane is presented. The model is three dimensional and applicable for PEMFCs operating at intermediate temperatures (120–150 °C). It accounts for all transport and polarization phenomena, and the results compare well with published experimental data for equivalent operating conditions. Results for oxygen concentration and temperature variations are presented. The model predicts the oxygen depletion, which occurs in the catalyst area under the ribs, and which gives an indication of the catalyst utilization. Results also predict that for an output power density of 1 kW m−2 , a cell temperature rise of up to 30 K can be expected for typical laboratory operating conditions. Parametric analyses indicate that significant gain in fuel cell performance can be expected by increasing the conductivity of the PBI membrane. Further, results demonstrate that when the catalyst region is well utilized, increasing the catalyst activity results in only a small improvement in performance.

Commentary by Dr. Valentin Fuster
2006;():253-257. doi:10.1115/FUELCELL2006-97135.

The proton transfer mechanism is the fundamental principle of how the proton exchange membrane fuel cell (PEMFC) works. This paper develops a molecular dynamics technique to simulate the transfer mechanism of the hydrogen protons inside a Nafion 117 membrane. The realistic polymer structure of the Nafion is extremely huge and very complex, it is simplified to be a repeated structure with part of the major carbon-fluoride backbone and a side chain with radicals of SO3 − in this paper. Water molecules were assigned to distribute between side chains randomly. The simulation package of DLPOLY was employed as the platform. Simulation results show that the water molecules will cluster together due to the polarization characteristics, and the clusters are attracted by the side chain of the membrane electrolyte. Hydrogen protons are then transferred from one side chain to another through the water clusters. The migration process of the hydrogen protons within the membrane is a function of the water uptakes and many other factors. They are investigated to further improve the ionic conduction of the fuel cell membrane.

Commentary by Dr. Valentin Fuster
2006;():259-263. doi:10.1115/FUELCELL2006-97136.

This paper investigates the proton diffusion phenomenon between the anode catalyst and the electrode in an enzymatic bio-fuel cell. The bio-fuel cell uses enzymatic organism as the catalyst instead of the traditional noble metal, like platinum. The fuel is normally the glucose solution. The fuel cell is membrane-less and produces electricity from the reaction taken place in the organism. When the biochemical reaction occurs, the protons and electrons are released in the solution. The electrons are collected by the electrode plate and are transported to the cathode through an external circuit, while the protons migrate to the cathode by the way of diffusion. Unfortunately, protons are easy to dissipate in the solution because the enzyme is immersed in the neutral electrolyte. It is an important issue of how to collect the protons effectively. In order to investigate the diffusion process of the protons, a molecular dynamics simulation technique was developed. The simulation results track the transfer motion of the protons near the anode. The diffusivity was evaluated from the trajectory. The research concludes that the higher the glucose concentration, the better the proton diffusivity. The enzyme promotes the electrochemical reaction; however, it also plays an obstacle in the proton diffusion path.

Topics: Protons , Anodes , Biofuel
Commentary by Dr. Valentin Fuster
2006;():265-271. doi:10.1115/FUELCELL2006-97140.

As proton exchange membrane fuel cell technology advances, the need for hydrogen storage intensifies. Metal hydride alloys offer one potential solution. However, for metal hydride tanks to become a viable hydrogen storage option, the dynamic performance of different tank geometries and configurations must be evaluated. In an effort to relate tank performance to geometry and operating conditions, a dynamic, two-dimensional, multi-nodal metal hydride tank model has been created in Matlab-Simulink®. Following the original work of Mayer, Groll, and Supper and the more recent paper from Aldas, Mat, and Kaplan, this model employs first principle heat transfer and fluid flow mechanisms together with empirically derived reaction kinetics. Energy and mass balances are solved in cylindrical polar coordinates for a cylindrically shaped tank. The model tank temperature, heat release, and storage volume have been correlated to an actual metal hydride tank for static and transient adsorption and desorption processes. The dynamic model is found to accurately predict observed hardware performance characteristics portending a capability to well simulate the dynamic performance of more complex tank geometries and configurations. As an example, a cylindrical tank filled via an internal concentric axial tube is considered.

Commentary by Dr. Valentin Fuster
2006;():273-282. doi:10.1115/FUELCELL2006-97161.

Fuel cells are being considered increasingly as a viable alternative energy source for automobiles because of their clean and efficient power generation. Numerous technological concepts have been developed and compared in terms of safety, robust operation, fuel economy, and vehicle performance. However, several issues still exist and must be addressed to improve the viability of this emerging technology. Despite the relatively large number of models and prototypes, a model-based vehicle design capability with sufficient fidelity and efficiency is not yet available in the literature. In this article we present an analysis and design optimization model for fuel cell vehicles that can be applied to both hybrid and non-hybrid vehicles by integrating a fuel cell vehicle simulator with a physics-based fuel cell model. The integration is achieved via quasi-steady fuel cell performance maps, and provides the ability to modify the characteristics of fuel cell systems with sufficient accuracy (less than 5% error) and efficiency (98% computational time reduction on average). Thus, a vehicle can be optimized subject to constraints that include various performance metrics and design specifications so that the overall efficiency of the hybrid fuel cell vehicle can be improved by 14% without violating any constraints. The obtained optimal fuel cell system is also compared to other, not vehicle-related, fuel cell systems optimized for maximum power density or maximum efficiency. A tradeoff between power density and efficiency can be observed depending on the size of compressors. Typically, a larger compressor results in higher fuel cell power density at the cost of fuel cell efficiency because it operates in a wider current region. When optimizing the fuel cell system for maximum power density, we observe that the optimal compressor operates efficiently. When optimizing the fuel cell system to be used as a power source in a vehicle, the optimal compressor is smaller and less efficient than the one of the fuel cell system optimized for maximum power density. In spite of this compressor inefficiency, the fuel cell system is 9% more efficient on average. In addition, vehicle performance can be improved significantly because the fuel cell system is designed both for maximum power density and efficiency. For a more comprehensive understanding of the overall design tradeoffs, several constraints dealing with cost, weight, and packaging issues must be considered.

Commentary by Dr. Valentin Fuster
2006;():283-289. doi:10.1115/FUELCELL2006-97162.

The cold-start behavior and the effect of subzero temperatures on fuel cell performance were studied using a 25-cm2 PEMFC. The fuel cell system was housed in an environmental chamber that allowed the system to be subjected to temperatures ranging from sub-freezing to those encountered during normal operation. Fuel cell cold-start was investigated under a wide range of operating conditions. The cold-start measurements showed that the cell was capable of starting operation at −5 °C without irreversible performance loss when the cell was initially dry. The fuel cell was also able to operate at low environmental temperatures, down to −15 °C. However, irreversible performance losses were found if the cell cathode temperature fell below −5 °C during operation. Freezing of the water generated by fuel cell operation damaged fuel cell internal components. Several low temperature failure cases were investigated in PEM fuel cells that underwent sub-zero start and operation from −20 °C. Cell components were removed from the fuel cells and analyzed with scanning electron microscopy (SEM). Significant damage to the MEA and backing layer was observed in these components after operation below −5 °C. Catalyst layer delamination from both the membrane and the gas diffusion layer (GDL) was observed, as were cracks in the membrane, leading to hydrogen crossover. The membrane surface became rough and cracked and pinhole formation was observed in the membrane after operation at subzero temperatures. Some minor damage was observed to the backing layer coating Teflon and binder structure due to ice formation during operation.

Commentary by Dr. Valentin Fuster
2006;():291-300. doi:10.1115/FUELCELL2006-97164.

In this work, we focus on robustness analysis of an integrated fuel cell and fuel reforming (FCFR) system, which relies on a feedback controller to mitigate hydrogen starvation and temperature overshoot during load transitions. The fuel reformer is used to process natural gas into a hydrogen rich flow to be utilized in a proton exchange membrane fuel cell (PEM-FC). The feedback controller uses the catalytic burner (CB) and the catalytic partial oxidizer (CPOX) temperatures as measurements and adjusts the air and fuel actuator commands to assure fast load following and high steady state efficiency. Several uncertainty sources which can potentially lead to closed loop performance deterioration are considered, including CPOX clogging, hydro-desulphurizer (HDS) clogging, fuel uncertainty and CB parameter uncertainty. Steady state and transient performance are analyzed for the different uncertainty scenarios, for both open and closed loop operation (i.e., with and without feedback control). The robustness of load following and CPOX temperature regulation of the closed loop system (feedforward and feedback controlled) is established, while the open loop system (feedforward controlled) is shown to be vulnerable to all sources of uncertainties considered.

Commentary by Dr. Valentin Fuster
2006;():301-305. doi:10.1115/FUELCELL2006-97172.

Carbon nanotubes (CNTs) have attracted increasing attention because of their unique structural, mechanical, and electronic properties. Surface chemistry modifications are also useful and critical to manipulate the adsorptive properties of CNTs and develop their hydrogen storage potential. Therefore, the synthesis or identification of multi-wall carbon nanotubes (MWCNTs) and H2 storage capacity in MWCNTs were investigated. Experimentally, the MWCNTs were produced from the catalytic-assembly benzene-thermal routes by reduction of C6 Cl6 with metallic K or Na in the presence of Co/Ni catalyst precursors at 503–623 K. The diameters of K-MWCNTs and Na-MWCNTs ranged from 30–100 and 20–60 nm, respectively. The H2 storage capacity of MWCNTs improved by Pd or NaAlH4 ranged from 2.5–3.5 wt%. Extended X-ray absorption fine structural (EXAFS) spectra showed that the Pd or PdCl2 possess a Pd-Pd or Pd-Cl bond distance of 2.76 or 2.25 Å with a coordination number of 6 or 2, respectively. Therefore, Pd nanoparticles are well dispersed on MWCNTs, which may improve the H2 storage capacity significantly.

Commentary by Dr. Valentin Fuster
2006;():307-317. doi:10.1115/FUELCELL2006-97177.

We present here a calibrated and experimentally validated lumped parameter model of fuel cell polarization for a hydrogen fed multi-cell, low-pressure, proton exchange membrane (PEM) fuel cell stack. The experimental methodology devised for calibrating the model was completed on a 24 cell, 300 cm2 stack with GORE™ PRIMERA® Series 5620 membranes. The predicted cell voltage is a static function of current density, stack temperature, reactant partial pressures, and membrane water content. The maximum prediction error associated with the sensor resolutions used for the calibration is determined along with a discussion of the model sensitivity to physical variables. The expected standard deviation due to the cell-to-cell voltage variation is also modelled. In contrast to other voltage models that match the observed dynamic voltage behavior by adding unreasonably large double layer capacitor effects or by artificially adding dynamics to the voltage equation, we show that a static model can be used when combined with dynamically resolved variables. The developed static voltage model is then connected with a dynamic fuel cell system model that includes gas filling dynamics, diffusion and water dynamics and we demonstrate the ability of the static voltage equation to predict important transients such as reactant depletion and electrode flooding. It is shown that the model can qualitatively predict the observed stack voltage under various operating conditions including step changes in current, temperature variations, and anode purging.

Commentary by Dr. Valentin Fuster
2006;():319-328. doi:10.1115/FUELCELL2006-97182.

To integrate a fuel cell into a vehicle platform many subsystems must be engineered to support the electrical power production of the fuel cell plant. These subsystems include the control of fuel and air supply as well as managing thermal and water throughput in the fuel cell stack. For the fuel cell plant to operate at optimum performance, one must examine the individual components that make up the “balance of plant” of the fuel cell system. Specifically, the power used to run the system must be scrutinized with the power produced by the system. Knowing how individual balance of plant components perform is the first step in design and optimization studies, as well as automated control system development. To address these issues, this paper examines how balance of plant components and subsystems affect parasitic power consumption, fuel cell power production, membrane hydration, hydrogen usage, and water production.

Commentary by Dr. Valentin Fuster
2006;():329-331. doi:10.1115/FUELCELL2006-97192.

The superthin PEM (≤ 30 μm in thickness) can be used in CCMs (Catalyst coated membranes) and helpful to lower the cost of fuel cells. In this paper, the CCM based on Nafion NRE® 211 membrane (thickness ∼25 μm) was prepared and assembled into a single fuel cell. The activation time, the V-I curves and the voltage vs time plot were used to characterize the performance of CCMs under variuos hydrogen/air humidifying conditions at ambient pressure. The experimental results showed that the fuel cell with CCMs based on NRE® 211 membrane had a shorter activation time and higher performance under humidifying conditions compared to that based on nafion NRE® 212 membrane (thickness ∼50 μm). However, it’s important to remove water from anode in order to maintain a stable performance of fuel cell. Moreover, the performance of the single fuel cell using superthin membranes could be improved at a high current density under non-humidifying conditions.

Commentary by Dr. Valentin Fuster
2006;():333-338. doi:10.1115/FUELCELL2006-97207.

A kilo-Watt-class direct sodium-borohydride/hydrogen-peroxide (NaBH4 /H2 O2 ) fuel cell is fabricated and characterized in this research. Aqueous solution of NaBH4 is directly utilized at the fuel cell anode in place of gaseous hydrogen. Similarly H2 O2 /water solution is directly reduced at the cathode without being first converted through molecular oxygen. The direct utilization of fuel/oxidant results in much higher (35% higher) efficiency in energy utilization. The potential for a very high efficiency (over 80%) was fully validated in the experiments, because the use of H2 O2 overcomes the oxygen over-potential problem inherent to prior H2 /O2 fuel cell designs. Initial results indicate: 1) conversion efficiency over 50% at a practical current density of 200 mA/cm2 ; 2) power density over 0.5 W/cm2 , at 60 degree Celsius. The kilo-Watt-stack consists of 24 cells, generating an OCV (open circuit voltage) of ∼ 42 V. The design of the reactant manifold overcomes some issues unique to an all-liquid-reactant fuel cell. Such a technology is ideal for propulsion/power in air independent environment such as space and underwater.

Commentary by Dr. Valentin Fuster
2006;():339-346. doi:10.1115/FUELCELL2006-97209.

The performance of Cu-Ce-Al-oxide and Cu-Cr-Al-oxide catalysts of varying compositions prepared by co-precipitation method was evaluated for the PEM fuel cell grade hydrogen production via oxidative steam reforming of methanol (OSRM). The limitations of partial oxidation and steam reforming of methanol for the hydrogen production for PEM fuel cell could be overcome using OSRM and can be performed auto-thermally with idealized reaction stoichiomatry. Catalysts surface area and pore volume were determined using N2 adsorption-desorption method. The final elemental compositions were determined using atomic absorption spectroscopy. Crystalline phases of catalyst samples were determined by X-ray diffraction (XRD) technique. Temperature programmed reduction (TPR) demonstrated that the incorporation of Ce improved the copper reducibility significantly compared to Cr promoter. The OSRM was carried out in a fixed bed catalytic reactor. Reaction temperature, contact-time (W/F) and oxygen to methanol (O/M) molar ratio varied from 200–300°C, 3–21 kgcat s mol−1 and 0–0.5 respectively. The steam to methanol (S/M) molar ratio = 1.4 and pressure = 1 atm were kept constant. Catalyst Cu-Ce-Al:30-10-60 exhibited 100% methanol conversion and 152 mmol s−1 kgcat −1 hydrogen production rate at 300°C with carbon monoxide formation as low as 1300 ppm, which reduces the load on preferential oxidation of CO to CO2 (PROX) significantly before feeding the hydrogen rich stream to the PEM fuel cell as a feed. The higher catalytic performance of Ce containing catalysts was attributed to the improved Cu reducibility, higher surface area, and better copper dispersion. Reaction parameters were optimized in order to maximize the hydrogen production and to keep the CO formation as low as possible. The time-on-stream stability test showed that the Cu-Ce-Al-oxide catalysts subjected to a moderate deactivation compared to Cu-Cr-Al-oxide catalysts. The amount of carbon deposited onto the catalysts was determined using TG/DTA thermogravimetric analyzer. C1s spectra were obtained by surface analysis of post reaction catalysts using X-ray photoelectron spectroscopy (XPS) to investigate the nature of coke deposited.

Commentary by Dr. Valentin Fuster
2006;():347-351. doi:10.1115/FUELCELL2006-97215.

In order to increase efficiency of the fuel cell vehicle, it can be hybridized by using batteries or ultra-capacitors. A fuel cell vehicle model is developed and validated by comparing the simulation results with real vehicle operating results from the Hyundai Tucson fuel cell hybrid vehicle. And various types of hybridization structure are compared by simulation and the effect of component sizing is also studied. In the vehicle model, the component and controller models were developed to have modularity and integrated to have forward facing characteristics. Thus, the hybrid controller is designed and optimized by using the simulation. This paper also presents the fuel economy of the developed fuel cell hybrid vehicle when it is operated on the chassis dynamometer.

Commentary by Dr. Valentin Fuster
2006;():353-364. doi:10.1115/FUELCELL2006-97223.

The aim of the work is to perform a comparative study of the compression system for a polymer electrolyte membrane (PEM) fuel cell. This study wishes also to address some key-features on the suitability of different compressors with respect to the main system design issues (e.g. energy balance, system performance and control). According to the technologies currently considered as being able to meet fuel cell system requirements, sliding vane, twin-screw and centrifugal compressors have been studied. The analysis has been performed making use of a modeling approach based on both thermodynamic description of the plant and synthesis of experimental efficiency data. Altogether, the results indicate that the centrifugal compressor should be preferred due to its higher efficiency as compared to the other compressors analyzed. Nevertheless, specific applications (e.g automotive field) may redirect the selection of the optimal compression device towards the other typologies, because of the highly fluctuating power demands and the issues associated with the narrow centrifugal compressor operation range.

Commentary by Dr. Valentin Fuster
2006;():365-371. doi:10.1115/FUELCELL2006-97227.

Conventional reactors are large in size and thus have limitations on heat and mass transfer. To overcome these limitations, microreactors have been introduced. This study discusses the development of an integrated reaction and heat exchange approach to microreactor design that enhances reaction yields by allowing the reactant stream to follow optimal reactant temperature profiles. The study details the formulation of two-dimensional model for the integrated reaction and heat exchange reactor design, and applies these models to a parametric study of microreactor designs for the water gas shift (WGS) reaction. The parametric study investigates the sensitivities of design parameters for both the parallel-flow and counter-flow configurations and contributes to establishing general design guidelines for the micro-WGS reactor. The integrated approach achieved significantly higher catalyst utilization when compared to a conventional adiabatic reactor. The study also showed potentials of miniaturizing the reactor by reducing the wall thickness of the reactor without performance loss.

Topics: Heat , Modeling , Water
Commentary by Dr. Valentin Fuster
2006;():373-379. doi:10.1115/FUELCELL2006-97229.

We present calorimetric measurements on a single isothermal polymer electrolyte fuel cell, operated on dry hydrogen and oxygen at 50 °C. The measured heat production of the complete cell was decomposed into ohmic and non-ohmic heat effects. The results predicted the thermo-neutral potential of the cell within 7% error. The part of the heat production that originated mainly from the cathode overpotential, was analyzed in terms of standard overpotential theory, giving an exchange current density of the bulk cathode overpotential of 6×10−4 A/cm2 and a transfer factor of 0.27. (The cathode catalyst surface area was not determined.) This is the first time an overpotential of an electrode has been determined from its heat production.

Commentary by Dr. Valentin Fuster
2006;():381-390. doi:10.1115/FUELCELL2006-97233.

This paper describes a methodology for design and optimization of a polymer electrolyte membrane (PEM) fuel cell unmanned aerial vehicle (UAV). The focus of this paper is the optimization of the fuel cell propulsion system and hydrogen storage system for a baseline aircraft. Physics-based models, and experimentally-derived sub-system performance data are used to characterize the performance of each configuration within a design space. The results of aircraft synthesis and performance modeling routines are used to create response surface equations where tradeoffs among component specifications can be explored. Significant tradeoffs between fuel cell performance, hydrogen storage and aircraft aerodynamic and propulsion system design are presented. Validation and test results from a proof-of-concept fuel cell UAV propulsion system are presented. Validated models of the fuel cell and aircraft systems are used to predict the performance of fuel cell UAVs at the scale of the baseline aircraft.

Commentary by Dr. Valentin Fuster
2006;():391-399. doi:10.1115/FUELCELL2006-97234.

The flow field plate of a proton exchange membrane fuel cell (PEMFC) functions as electron conductor and provides the pathway for oxidant and fuel to reach the membrane electrode assembly (MEA). CFD-based simulation tools can be effective in designing and optimization of flow field plates as they cab fully account for the complexity and coupling of various transport phenomena as well as the 3-D geometry. The objective of this paper is to report on the development of such a simulation platform and on its application to investigate the impact of several geometric parameters on fuel cell performance and detailed distribution of transport processes. The simulation tool is built upon a commercial computational fluid dynamics (CFD) code, CFD-ACE+, along with supporting software and script codes to automate the design workflow. A 3-D, straight channel model with material properties and model parameters validated with experimental data is used as the baseline for the present study. The workflow includes automated grid generation, model setup and job execution. Parametric study is performed for geometric parameters including (1) Channel width versus land area width (2) Channel height (3) Channel pitch and length, as well as material parameters including (4) Porosity and (5) Electrical conductivity of the gas diffusion layer (GDL). Among these parameters, it is found that predicted cell performance is most sensitive to the channel/land width ratio and to the anisotropy of the GDL property. When isotropic properties are used for the GDL, the predicted cell performance decreases with increasing channel/land width ratio. This is because the current distribution in the MEA is dictated by electrical conduction through the GDL and increasing channel width causes current to peak underneath the land area, which in turn increases ohmic losses. When the in-plane electrical conductivity is reduced, the effect of mass transfer on the current distribution becomes comparable to electron transfer and the predicted trend line of cell performance shows an optimum value as a function of the channel/land width ratio. The CFD based design tool developed in the present work has the advantage of providing more reliable prediction than methods based on reduced dimensionality or simplified transport models.

Commentary by Dr. Valentin Fuster
2006;():401-409. doi:10.1115/FUELCELL2006-97235.

Near-uniform flow distribution in a fuel cell stack is essential to stack performance and overall system efficiency. The gradients induced by the non-uniformity of the flow within each of the unit cells also have a significant impact on stack durability. In typical configurations, the oxidant and fuel are fed into a stack through manifolds and then enter each unit cell through secondary inlet port. After flowing through the unit cells, the spent gases as well as possible liquid water then enter the outlet header to leave the stack. The objective of this paper is to develop a practical model to predict cell-to-cell flow distribution in a proton exchange membrane fuel cell (PEMFC) stack. The flow distribution is first simulated using a computational fluid dynamics (CFD) tool, CFD-ACE+, in a 3D computational domain for single-phase gas flows. The simulations use a domain encompassing the flow from the inlet header through an array of unit cells to the outlet header. The CFD simulations show that in the outlet header, the flow injected from the unit cells to the header changes the flow pattern considerably, which results in a reduced cross section area for the flow in the axial direction. A circulation zone is seen near the low velocity end of the header, which may potentially become a region where liquid water accumulates. Increasing static pressure along the flow direction is observed in the inlet header. The simulated results are validated and found to be in good agreement with experimentally measured pressures in a fuel cell stack. Based on the observations in the CFD simulations, a flow network model is developed to provide quick estimates of the flow distribution as a function of stack dimensions including header and unit cell geometry. In essence, the flow network model solves for the pressure at each junction of the unit cell and the header. Three fitting parameters are introduced to account for effects of surface roughness of the headers, reduced effective header area in the outlet header, and pressure drop in the unit cell. The flow network model is shown to capture the characteristics of pressure variation and flow distribution obtained in the CFD simulations. The flow network model can effectively match experimental data and be used as a fast tool for initial design of a PEMFC stack.

Commentary by Dr. Valentin Fuster
2006;():411-422. doi:10.1115/FUELCELL2006-97237.

Dynamic behavior and transient analysis are one of the most critical issues for high performance polymeric electrolyte membrane fuel cells. An improvement of performance can be achieved both with hardware modifications and with more sophisticated control strategies. To this regard, the availability of a reliable dynamic fuel cell model plays an important role in the design of fuel cell control and diagnostic system. This paper presents a non-linear, iso-thermal, zero-dimensional model of a pressurized PEM fuel cell system used for automotive applications. The model was derived from a detailed, iso-thermal, steady-state, dimensional model which explicitly calculated (and subsequently captured as a multi-D look-up table) the relationship between cathode and anode pressures and humidity and stack average current. Since in the electrochemical model the single cell performance depends on the membrane ionic resistance, which is strictly related to the membrane water content, a dynamic estimation of the membrane water diffusion has been considered. This takes into account the dependence of the cell voltage on the unsteady membrane water concentration. A similar approach still allows the development of a simple zero-dimensional dynamic model suitable for control system development and amenable to control-oriented humidity modelling.

Commentary by Dr. Valentin Fuster
2006;():423-429. doi:10.1115/FUELCELL2006-97238.

A quasi three-dimensional dynamic model of a proton exchange membrane fuel cell (PEMFC) has been developed and evaluated by comparison to experimental data. A single PEMFC cell is discretized into 245 control volumes in three dimensions to resolves local voltage response, current generation, species mole fractions, temperature, and membrane hydration spatially in the PEMFC. The model can further simulate transients in electrical load, inlet flow conditions, ambient conditions, and/or other parameters to provide insight into the local dynamic performance of a PEMFC. The quasi three-dimensional model has been validated against an experimental single cell. To compare the model, polarization constants were tuned to match one experimental operating point of the fuel cell. With this tuning, the model is shown to predict well the voltage current (V-I) behavior for the full range of cell operating current. Further, model comparison to an instantaneous increase in current indicates that the model can predict the transient electrochemical response of the PEMFC. This suggests such a model can be utilized for PEMFC system development, transient analysis of a PEMFC in general, as well as transient control design.

Commentary by Dr. Valentin Fuster
2006;():431-437. doi:10.1115/FUELCELL2006-97240.

Water management is a critical operation issue for achieving the highest possible performance of proton exchange membrane (PEM) fuel cells. Quantitative determination of water and species distribution is needed to understand the water management and reactant distribution effects. In this study, the measurement of water and oxygen distributions along cathode flow channels was carried out using gas chromatography (GC). Generally, it is difficult to measure water distribution where water concentration is too high. Here, the measurement of high levels of water saturation in cathode channels was performed according to fuel cell operating conditions. GC measurement was also carried out for flooding and non-flooding conditions. To compare the experimental results with computational results, the three-dimensional CFD simulation of a unit fuel cell was performed using es-pemfc, which is the PEM fuel cell module of commercial CFD code STAR-CD. For the entrance of flow channel that has relatively lower level of water content, the calculated results showed good agreement with measured results. However, some discrepancy between calculated and experimental results was still found for the flow channels near the cathode outlet. The study provides the necessity of the development and adoption of a comprehensive multidimensional PEM fuel cell models including two-phase flow and cathode flooding phenomena for the optimization of fuel cell performance.

Commentary by Dr. Valentin Fuster
2006;():439-447. doi:10.1115/FUELCELL2006-97249.

In Asia there are less private cars, but there is a high proportion of 2-stroke engines in scooters, motorcycles, auto-rickshaws (Tuk-Tuks), all running on petrol-oil mixtures with levels of hydrocarbon emissions (from partially burnt fuel and oil) well in excess of levels permitted in the USA and Europe. Worldwide Rickshaw/scooter/motorcycle type engine production is estimated at 17 million per year. According to National Transport Research Center (NTRC), the total population of registered (all types) motor vehicles in Pakistan in year 2000 was 4.224 million, out of which more than half of the population is (2.206 million) two wheelers or three wheelers (motorcycle/scooter/auto rickshaw). Almost all auto rickshaws have two stroke power packs and also 60% of motorcycle/scooters are of the same category. Pakistan is a very densely populated developing country, with very loose environment protection rules, which are practically unregulated due to large financial implications. This scenario leads to adverse air quality conditions especially in large cities of the country where the main contributory factors are vehicular traffic, that too, two stroke vehicles Industry, diesel-powered vehicles, and the omnipresent three-wheeled, two-stroke rickshaws all contribute to the extremely dirty air. Taxi/car use is increasing, but rickshaws have the advantage of being able to swarm through the congested car traffic in cities. This explains the over .6 million motorcycles/scooters/rickshaws currently in Pakistan, of which approximately 20% are two stroke Auto-rickshaws of 175 cc. Pakistan’s vehicle fleet has a growth rate of 8.0% (1990–99). The purpose of this study is to examine a particular application of fuel cell technology “The Auto Rickshaws”. They are small three-wheeled vehicles that can carry three people. Due to their small size and low price, rickshaws have traditionally been powered by high power density two-stroke internal combustion engines. Two-stroke engines produce a great deal of pollution and are an object of concern in many Asian countries. Severe pollution from two-stroke engines is a significant driver for cleaner technology. Thus, the target of this study is the Asian urban commuter, since a rickshaw is largely used in many Asian cities and contributes directly to air pollution in major crowded cities of Pakistan also. Countries like China, India, Bangladesh, Taiwan and Pakistan [1] are facing dramatic growth rates in two-stroke vehicle population as bicycle rickshaws are being replaced, so, low-powered but clean rickshaws would be a major step in providing mobility without compromising urban air quality.

Commentary by Dr. Valentin Fuster
2006;():449-452. doi:10.1115/FUELCELL2006-97265.

Catalytic partial oxidation (CPOX) of liquid fuels is an attractive option for producing a hydrogen-rich gas stream for fuel cell applications. However, the high sulfur content along with aromatic compounds present in liquid fuels may deactivate reforming catalysts. Deactivation of these catalysts by carbon deposition and sulfur poisoning is a key technical challenge. The relationship between catalyst supports and deactivation have been studied here for three catalysts (Rh/Ce0.5 Zr0.5 O2 , Pt/Ce0.5 Zr0.5 O2 , and Pt/Al2 O3 ) in a fixed bed catalytic reactor using a mixture of n-tetradecane, 1-methylnaphthalene, and dibenzothiophene to simulate logistic fuels. Carbon production during CPOX reforming was directly related to olefin formation. Olefins, which are known coke precursors, were observed on the Pt catalysts during CPOX of n-tetradecane with no sulfur (particularly from Pt/Al2 O3 ), but not on Rh/Ce0.5 Zr0.5 O2 . For the Rh/Ce0.5 Zr0.5 O2 , yields of H2 and CO dropped to a stationary level after the introduction of sulfur-containing feed (1000 ppm sulfur) or aromatic-containing feed (5 wt%), however, the catalyst activity was restored after removing the sulfur or aromatics from the feed. For the Pt catalysts, H2 and CO yields dropped continuously over time in the presence of sulfur or aromatics in feed. The superior performance of Rh/Ce0.5 Zr0.5 O2 can be attributed to the higher oxygen-ion conductivity of the Ce0.5 Zr0.5 O2 support as well as the activity of the Rh sites.

Topics: Catalysts , oxidation
Commentary by Dr. Valentin Fuster
2006;():453-461. doi:10.1115/FUELCELL2006-97271.

Liquid water formation and transport was investigated by direct experimental visualization in an operational transparent single-serpentine PEM fuel cell. We examined the effectiveness of various gas diffusion layer (GDL) materials in removing water away from the cathode and through the flow field over a range of operating conditions. Complete polarization curves as well as time evolution studies after step changes in current draw were obtained with simultaneous liquid water visualization within the transparent cell. At similar current density (i.e. water production rate), lower level of cathode flow field flooding indicated that liquid water had been trapped inside the GDL pores and catalyst layer, resulting in lower output voltage. No liquid water was observed in the anode flow field unless cathode GDLs had a microporous layer (MPL). MPL on the cathode side creates a pressure barrier for water produced at the catalyst layer. Water is pushed across the membrane to the anode side, resulting in anode flow field flooding close to the H2 exit.

Commentary by Dr. Valentin Fuster

Stationary Systems

2006;():463-465. doi:10.1115/FUELCELL2006-97009.

In the mid-1990’s, SARA developed and patented (Patent Number 6,200,697) a Direct Carbon-Air Fuel Cell (DCFC) which uses a molten hydroxide electrolyte in a cell design that is characteristic of what are commonly known as metal-air fuel cells. This technology forms the basis of the Direct Coal to Electricity Conversion system that is being developed at SARA with support from American Electric Power and the Electric Power Research Institute. The main feature of the cell which uses molten hydroxide electrolyte is the design simplicity in which the cathode is a simple iron container sparged with air. The drawback of this design however is chemical instability of the electrolyte due to its reaction with anode product CO2 resulting in hydroxide to carbonate conversion that lessens the cell performance and shorts the cell operation duration. Researchers at SARA are exploring various means to prevent or reduce the carbonate formation. One of the means is based on the use of high water content in the electrolyte that will shift the equilibrium of hydroxide to carbonate conversion to the left resulting in low CO32− ion concentration. Another means to prevent conversion of hydroxide melt into carbonate according to the literature [1–2] is based on the use of oxide additives such as SiO2 , As2 O3 , and MgO as well as oxyanions such as pyrophosphate and persulfate that decompose carbonate and therefore these compounds together with water might help in preventing conversion of hydroxides into carbonates. Unfortunately neither water content nor oxide additives exerted substantial reduction of carbonate formation at temperatures up to 650° C. Much higher temperatures are needed for these effects to be significant. Since the beginning of 2004, SARA has been performing experiments with a new generation of DCFC. The results of those experiments have permitted much longer term operation of the DCFC than was possible in earlier experiments. This has led SARA to a new cell configuration with a porous separator that separates electrolyte in the anode compartment (anolyte) from the electrolyte in the cathode compartment (catholyte) and prevents hydroxide to carbonate conversion in the catholyte. In this cell design the anolyte is carbonate melt whereas the catholyte is hydroxide melt. Consequently the electrochemical activity of carbon anodic dissolution is not as high as in hydroxide electrolyte, whereas the high activity of oxygen cathode and subsequently its simple design is retained. This new configuration has several advantages over the older cell configuration: (1) we can use particulate carbon directly in the cell, (2) the CO2 that is produced by the cell comes out in a form that can be easily sequestered, and (3) the electrolyte is stable for long term operation. The starting electrolyte in both cell compartments is a mixture of NaOH and LiOH (1:1 by mol). During the cell operation the anolyte is being converted to carbonate, the anode potential is getting less negative and at certain point it reaches the plateau. At this point gaseous CO2 starts leaving the cell and the electrolyte is stabilized showing no further changes during cell operation. No effects of CO32− ions on O2 cathode performance was observed over 500 h of operation indicating little or no CO32− transport through the separator. Time required converting hydroxide anolyte into carbonate one depends on the cell current, but the cell operation can also start with carbonate anolyte. In any case the amount of CO2 determined in anode off gas is proportional to the cell current indicating that CO2 is formed as a result of electrochemical reactions at the carbon anode.

Commentary by Dr. Valentin Fuster
2006;():467-476. doi:10.1115/FUELCELL2006-97013.

Performance analysis of the solid oxide fuel cell–micro gas turbine (SOFC–MGT) hybrid system has been made assuming the fuel to be methane-based artificial ones including H2 , CO, CO2 , H2 O or N2 of different concentration in preparation for the study of biomass fuelled SOFC–MGT hybrid system. This is based on the fact that the chemical composition of biomass fuel produced from different fuel production processes is diversified, i.e. in one case one chemical species rich in concentration and in another case another chemical species rich. In the analysis is used the multi–stage model for internal reforming SOFC module developed previously with some modification. With this model, studies cover not only the performance of the hybrid system but also the spatial distributions of temperature and concentration of some chemical species inside the module, namely in the cell stack and in the internal reformer.

Commentary by Dr. Valentin Fuster
2006;():477-484. doi:10.1115/FUELCELL2006-97036.

During shutdown and startup trips, the SOFC/GT hybrid system and its components must be protected from critical incidents such as anode oxygen exposure, excessive temperature gradients and carbon deposition. A further task is to minimise the need for auxiliary equipment and resources. The paper shortly presents a previously published detailed model of a pressurised SOFC hybrid cycle for the analysis of transients and control strategies. Shutdown and startup procedures are proposed based on the possibility to use the GT system for temperature control of the SOFC. Air flow and SOFC system inlet temperature are controlled by using a combustion device upstream the turbine, turbine exhaust gas throttling and a variable bypass around the recuperative heat exchanger. During startup, a small amount of hydrogen for the ignition phase of the SOFC is used. A considerable nitrogen demand for anode flushing has been detected, though it is uncertain how much is required to safely protect the anode from oxygen exposure. In the simulation, shutdown takes app. 2 hours and startup takes app. 5.5 hours. It is, however, uncertain how quick temperature variations the SOFC can withstand. A more flexible control strategy would allow the system to follow a low load immediately after startup initiation by utilizing the gas turbine.

Commentary by Dr. Valentin Fuster
2006;():485-497. doi:10.1115/FUELCELL2006-97037.

As supply of natural gas (NG) is limited, more attention is being given to operating fuel cells on syngas derived from gasification of feedstocks such as coal and biomass. Ammonia (NH3 ) is one of the problematic contaminants contained in syngas produced from these nitrogen containing feedstocks. NH3 can be easily oxidized to nitric oxide (NO) in a combustion process and thus if present in the anode exhaust gas would be problematic. The potential effects of NH3 (particularly at low levels) on fuel cell system performance have not been well studied. The former studies on NH3 have been limited to either the reforming process alone or testing the fuel cell at the cell level with NH3 containing gases. No studies have been accomplished on a fuel cell system level basis. Objectives of this work are to obtain a comprehensive understanding of fuel cell system performance on syngas containing NH3 using an integrated SOFC reformer system. Detailed analysis is conducted within the three major reacting components — indirect internal reformer, SOFC stack and combustion zone. Various simulation tools (etc., CHEMKIN, ASPEN, APSAT) are utilized for analysis. Results show that NH3 conversion (into N2 and H2 ) in the internal reformer is about 50% when temperature is 750° C. NH3 conversion (into N2 and H2 ) in the SOFC stack can affect NOx emissions significantly. More than 50% NH3 left from SOFC stack can convert into NOx in the combustion zone. Experimental study is also planned to validate the theoretical results.

Commentary by Dr. Valentin Fuster
2006;():499-501. doi:10.1115/FUELCELL2006-97050.

The HotModule®, a collaborative effort utilizing the Direct FuelCell® technology of FuelCell Energy, Inc. and the Hot Module® balance of plant design of MTU CFC Solutions GmbH, is a stationary sub-megawatt carbonate fuel cell power plant in the power range of 250 kWel . Its function and reliability has been verified meanwhile in many field test applications within a test program started in 2001. Whereas the focal points of the early field test plants were the demonstration of function and reliability, some of the following projects are aiming at the demonstration of a variety of fuels, as for example methanol and digester gas. In total 14 of the highly integrated HotModule systems were installed so far within this European field trial program until July 2005.

Commentary by Dr. Valentin Fuster
2006;():503-511. doi:10.1115/FUELCELL2006-97051.

A start-up test rig at TPG laboratory at the University of Genoa, Italy, has been designed and built for two main purposes: physically simulating different early start-up layout and procedures of high temperature fuel cell hybrid systems, and validating time-dependent hybrid system models based on TRANSEO software. Since start-up is a critical operating phase for high temperature fuel cell hybrid systems, and it may require specific modifications to the hybrid system layout, the start-up test rig is meant to be very flexible for testing several start-up layouts as well as the coupling of different turbomachines and stacks. Results for cold test, 700°C and 950°C start-up combustor outlet temperature tests are reported. Such results show the pressure and temperature quick rise during the early phase of start-up, which could represent an issue for the mechanical and thermal stress to the stack. A dynamic model of the test rig was built up and validated showing good agreement with the experimental results. This achievement was very useful to increase the confidence with predictive dynamic simulation tools during the start-up phase, where experimental data are hardly available and where the fuel cell materials may undergo risky thermal shocks.

Commentary by Dr. Valentin Fuster
2006;():513-521. doi:10.1115/FUELCELL2006-97053.

The electrical performance of solid oxide fuel cells (SOFC) has been traditionally characterized using isothermal cell tests and button cell tests. However, the evaluation of performance, operation, and structural integrity of cells in a typical SOFC stack are not only less amenable to confirmation through testing but are also significantly expensive than computational simulations. Computational models are invaluable in extending the measured isothermal cell test characteristics to predict both electrical performance and mechanical behavior of SOFCs in a stack under different operating conditions. The present investigation is part of an ongoing program of numerical developments and investigations to model the cell thermal and electrical characteristics in a stack environment. The ultimate objective is the development of an optimized cell geometry based on performance, structural integrity, and manufacturability. The flattened tubular high power density (HPD) cell, featuring five air channels fed by air feed tubes, was investigated. A CFD model of the HPD cell was developed using the commercial CFD software Fluent 6.2. A Fluent based SOFC model was used to simulate the electrochemical effects. The cathode, the anode, and the interconnection layers of the cell were resolved in the model and all modes of heat transfer, conduction, convection, and radiation were included. The results of the CFD model at isothermal conditions are presented and compared with experimentally measured isothermal cell V-J’s at 1000°C, 900°C, and 800°C. The model results agree well with the experimental data for cell temperatures of 1000°C and 900°C, after some tuning of exchange current density and tortuosity values. The agreement with the 800°C data however is not as good. The CFD model was then configured and analyzed with operating conditions typically encountered with a stack design that is currently under development. The resulting thermal, electrical, and flow fields are presented herein and discussed. It was found that the Fluent based SOFC model is a robust and effective tool for analyzing the complex and highly interactive three-dimensional electrical, thermal, and fluid flow fields, generally associated with the HPD cells. The computational time with the Fluent based model is however large in comparison with lumped-parameter approaches, mainly due to slow radiation convergence. Nevertheless, the comprehensive current density and thermal fields generated with the Fluent based model are necessary to enable a better prediction of thermal stresses within the cell, thereby permitting a more robust cell and module design.

Commentary by Dr. Valentin Fuster
2006;():523-528. doi:10.1115/FUELCELL2006-97057.

This paper studies the nonlinear behavior of a glass-ceramic seal used in planar solid oxide fuel cells (SOFCs). To this end, a viscoelastic damage model has been developed that can capture the nonlinear material response due to both progressive damage in the glass-ceramic material and viscous flow of the residual glass in this material. The model has been implemented in the MSC MARC finite element code, and its validation has been carried out using the experimental relaxation test data obtained for this material at 700°C, 750°C, and 800°C. Finally, it has been applied to the simulation of a SOFC stack under thermal cycling conditions. The areas of potential damage have been predicted.

Commentary by Dr. Valentin Fuster
2006;():529-535. doi:10.1115/FUELCELL2006-97061.

A major factor in global warming is CO2 emission from thermal power plants, which burn fossil fuels. One technology proposed to prevent global warming is CO2 recovery from combustion flue gas and the sequestration of CO2 underground or near the ocean bed. Solid oxide fuel cell (SOFC) can produce highly concentrated CO2 , because the reformed fuel gas reacts with oxygen electrochemically without being mixed with air in the SOFC. We therefore propose to operate multi-staged SOFCs with high utilization of reformed fuel to obtain highly concentrated CO2 . In this study, we estimated the performance of multi-staged SOFCs considering H2 diffusion and the combined cycle efficiency of a multistage SOFC / gas turbine / CO2 recovery power plant. The power generation efficiency of our CO2 recovery combined cycle is 68.5%, whereas the efficiency of a conventional SOFC/GT cycle with the CO2 recovery amine process is 57.8%.

Commentary by Dr. Valentin Fuster
2006;():537-546. doi:10.1115/FUELCELL2006-97072.

Combined Heat and Power (CHP) generation units based on intermediate temperature (600∼800°C) solid oxide fuel cell (SOFC) modules have been collaboratively developed by Mitsubishi Materials Corporation and The Kansai Electric Power Co., Inc. Currently, hydrocarbon fuel utilising units designed to produce modular power outputs up to 10 kWe-AC with overall efficiencies greater than 80% (HHV) are being tested. A unique seal-less stack concept is adopted to build SOFC modules accommodating multiple stacks incorporated of stainless steel separators and disk-type planar electrolyte-supported cells. In order to advance the current technology to achieve improved levels of efficiency and reliability, through design iterations, computational modelling tools are being heavily utilised. This contribution will describe the results of coupled computational fluid dynamics (CFD) analysis performed on our fourth-generation 1 kW class SOFC stack. A commercially available CFD code is employed for solving the governing equations for conservation of mass, momentum and energy. In addition, a local electrochemical reaction model is coupled to the rest of the transport processes that take place within the SOFC stack. It is found that the CFD based multi-physics model is capable of providing necessary and proper guidance for identifying problem areas in designing multi-cell SOFC stacks. The stack performance is also estimated by calibrating the computational model against data obtained by experimental measurements.

Commentary by Dr. Valentin Fuster
2006;():547-555. doi:10.1115/FUELCELL2006-97076.

This paper focuses on the transient behavior of a solid oxide fuel cell system used for stationary power production. Dynamic modelling is applied to identify the characteristic time scales of the system components when introducing a disturbance in operational parameters of the system. The information on the response of the system may be used to specify the control loops needed to manage the changes with respect to safe component operation. The commercial process modelling tool gPROMS is used to perform the system simulations. The component library of the tool is completed with dynamic models of a fuel cell stack and a prereformer. The other components are modelled for steady state operation. For the fuel cell a detailed dynamic model is obtained by writing the constitutive laws for heat transfer in the solid part of the cell and conservation of heat and mass in the air and fuel channels. Comprehensive representation of resistive cell losses, reaction kinetics for the reforming and heat conduction through the solid part of the cell is also included in the model. The prereformer is described as a dynamic pseudo-homogeneous one-dimensional tubular reactor accounting for methane steam reforming and water-gas shift reaction. The differences in the transient behavior of the system components and their interaction are investigated under load changes and feed disturbances.

Commentary by Dr. Valentin Fuster
2006;():557-564. doi:10.1115/FUELCELL2006-97084.

In conventional gas turbine systems combustion results in high exergy losses (∼30%) of fuel exergy input. Replacing the combustor with a high temperature fuel cell, like the Solid Oxide Fuel Cell (SOFC), will significantly reduce these exergy losses. As the SOFC electrochemically converts the natural gas, exergy losses are far lower (∼10%) compared to combustion. Natural gas entering a SOFC system has to be reformed first to hydrogen and carbon monoxide by steam reforming. Here it is chosen to use the heat generated by the fuel cell to drive the endothermic reforming reactions: internal reforming. The SOFC-GT system has the advantage that both fuel cell and gas turbine technology contribute to power production. In earlier work [1] several fuel cell system configurations with PEMFC, MCFC or SOFC, were analyzed studying the exergy flows. Here is focused on the SOFC-GT configuration, to get a detailed understanding of the exergy flows and losses through all individual components. Several configurations, combining the SOFC with the GT are possible. The selected operating conditions should prevent carbon deposition. Systems studies are performed to get more insight in the exergy losses in these combined systems. Exergy analysis facilitates the search for the high efficient SOFC-GT hybrid systems. Using exergy analysis, several useful configurations are found. Exergy losses are minimized by varying pressure ratio and turbine inlet temperature. Sensitivity studies, of equivalent cell resistance and fuel cell temperature, show that total system exergy efficiencies of more than 80% are conceivable, without using a bottoming cycle.

Commentary by Dr. Valentin Fuster
2006;():565-571. doi:10.1115/FUELCELL2006-97089.

Conversion of biomass in syngas by means of indirect gasification offers the option to improve the economic situation of any fuel cell systems due to lower costs for feedstock and higher power revenues in many European countries. The coupling of an indirect gasification of biomass and residues with highly efficient SOFC systems is therefore a promising technology for reaching economic feasibility of small decentralized combined heat and power production (CHP). The predicted efficiency of common high temperature fuel cell systems with integrated gasification of solid feedstock is usually significantly lower than the efficiency of fuel cells operated with hydrogen or methane. Additional system components like the gasifier, as well as the gas cleaning reduce this efficiency. Hence common fuel cell systems with integrated gasification of biomass will hardly reach electrical efficiencies above 30 percent. An extraordinary efficient combination is achieved in case that the fuel cells waste heat is used in an indirect gasification system. A simple combination of a SOFC and an allothermal gasifier enables then electrical efficiencies above 50%. But this systems requires an innovative cooling concept for the fuel cell stack. Another significant question is the influence of impurities on the fuel cells degradation. The European Research Project ‘BioCellus’ focuses on both questions — the influence of the biogenious syngas on the fuel cells and an innovative cooling concept based on liquid metal heat pipes. First experiments showed that in particular higher hydrocarbons — the so-called tars — do not have an significant influence on the performance of SOFC membranes. The innovative concept of the TopCycle comprises to heat an indirect gasifier with the exhaust heat of the fuel cell by means of liquid metal heat pipes. Internal cooling of the stack and the recirculation of waste heat increases the system efficiency significantly. This concept promises electrical efficiencies of above 50 percent even for small-scale systems without any combined processes.

Commentary by Dr. Valentin Fuster
2006;():573-583. doi:10.1115/FUELCELL2006-97095.

A system level analysis, inclusive of mass, is carried out for a cryogenic hydrogen fueled hybrid solid oxide fuel cell and bottoming gas turbine (SOFC/GT) power system. The system is designed to provide primary or secondary electrical power for an unmanned aerial vehicle (UAV) over a high altitude, long endurance mission. The net power level and altitude are parametrically varied to examine their effect on total system mass. Some of the more important technology parameters, including turbomachinery efficiencies and the SOFC area specific resistance, are also studied for their effect on total system mass. Finally, two different solid oxide cell designs are compared to show the importance of the individual solid oxide cell design on the overall system. We show that for long mission durations of 10 days or more, the fuel mass savings resulting from the high efficiency of an SOFC/GT system more than offset the larger powerplant mass resulting from the low specific power of the SOFC/GT system. These missions therefore favor high efficiency, low power density systems, characteristics typical of fuel cell systems in general.

Commentary by Dr. Valentin Fuster
2006;():585-594. doi:10.1115/FUELCELL2006-97107.

Effective control of cathode airflow in a direct fired solid oxide fuel cell gas turbine (SOFC/GT) hybrid power system is critical to thermal management of a fuel cell stack. Hybrid fuel cell turbine designs often incorporate the use of a valved hot air bypass in parallel with the cathode flow to divert a portion of the compressor effluent around the fuel cell system. The primary objective of this valve in the early development of hybrid power systems was to facilitate system startup. From a system controls perspective, the hot air bypass offers the means to balance and manipulate the level of airflow supplied to the fuel cell stack at a minimal efficiency penalty. Manipulation of this valve has a significant impact on stack performance and reliability, as well as cathodic exhaust airflow conditions. Since the turbine is directly coupled to the fuel cell subsystem through the cathode airflow, non-linear effects are propagated through the system components in response to any hot air bypass valve change. The effect of cathode flow transients on hybrid system performance has been evaluated though the manipulation of a hot air bypass valve on a hardware-based simulation facility designed and built by the U.S. Department of Energy, National Energy Technology Laboratory (NETL). A brief overview of this experimental facility is provided and has been described in more detail previously. Open loop experiments were conducted using the facility, where a perturbation was made to the hot air bypass flow and turbine speed was allowed to change in response. The impact of the transients to both fuel cell and turbine performance are discussed.

Commentary by Dr. Valentin Fuster
2006;():595-603. doi:10.1115/FUELCELL2006-97112.

Three-dimensional heat and mass transfer and electrochemical reaction in an anode-supported flat-tube solid oxide fuel cell (FT-SOFC) are studied. Transport and reaction phenomena mainly change in the streamwise direction. Exceptionally, hydrogen and water vapor have large concentration gradients also in the cross section perpendicular to the flow direction, because of the insufficient mass diffusion in the porous anode. Based on these results, we develop a simplified one-dimensional cell model. The distributions of temperature, current, and overpotential predicted by this model show good agreement with those obtained by the full three-dimensional simulation. We also investigate the effects of pore size, porosity and configuration of the anode on the cell performance. Extensive parametric studies reveal that, for a fixed three-phase boundary (TPB) length, rough material grains are preferable to obtain higher output voltage. In addition, when the cell has a thin anode with narrow ribs, drastic increase in the volumetric power density can be achieved with small voltage drop.

Commentary by Dr. Valentin Fuster
2006;():605-613. doi:10.1115/FUELCELL2006-97113.

The gasification of biomass wastes deriving from certain industrial processes is an interesting option for cogenerating heat and power. The utilization of the syngas in a high temperature fuel cell could lead to the improvement of electrical efficiency in comparison with traditional CHP plants. In this paper the performance of various Biomass Integrated Gasification Fuel Cell (BIGFC) plants are investigated. In particular an atmospheric down-draft gasifier has been considered for syngas production. The fuel cell used for power generation is a 250 kW solid oxide fuel cell, which has been simulated through a zero-dimensional steady-state model and integrated in Aspen Plus® software for evaluating the performance of the entire plant. Various system lay-outs have been investigated to analyze the effect on plant efficiency of the following parameters: (i) gasification air pre-heating; (ii) use of 90% pure oxygen for gasification; (iii) use of enriched air (55% O2 ) for gasification; (iv) recirculation of anodic gas flow; (v) installation of a SOFC/GT hybrid cycle for power production. BIGFC plants show an electrical efficiency in the range 20–27%, and a thermal efficiency of 39–59%. If a SOFC/GT hybrid cycle is installed electrical efficiency grows up to 39%.

Commentary by Dr. Valentin Fuster
2006;():615-619. doi:10.1115/FUELCELL2006-97120.

MTU’s HotModule is a High Temperature Molten Carbonate Fuel Cell System. It transfers the chemical energy of the fuel directly to electricity, heat and a useful depleted air with an electrical efficiency in the range of 42 to 52%. It convinces by minimal emissions of contaminants. The produced heat is given by the depleted air at a temperature level of 400 °C; this ensures a multi purpose and valuable utilization of the heat. The HotModule operated with natural gas is demonstrated meanwhile together with our partner Fuel Cell Energy Inc. in approximately 25 field trial plants and reached now a pre-commercial status. It is highly suitable for the utilization of hydrocarboneous gases, such as biogas, sewage gas, coal mine gas, of synthesis gases from thermal gasification processes of different waste material. Such gases are the most important renewable energy resources. In case of a consequent utilization of such gases for Combined Heat and Power Production a contribution of 12% to 15% of stationary consumable energy consumption can be reached. Even lean gases will be converted with high efficiency to electrical power and high exergetic heat. These characteristics recommend the HotModule for applications using the big potential of regenerative and secondary fuels with all their advantages in decentralized consumable energy supply, reduction of dependence on primary energy imports and reduction of greenhouse gas and other contaminants emission. MTU started recently a HotModule fed by methanol from waste material together with BEWAG in Berlin and many experimental work concerning applications with biogas and sewage gas has been performed with promising results. Due to the high electrical efficiency the HotModule saves about 1/3 of CO2 emission in comparison to conventional “prime movers”. If fuels are used, which are originated from renewable sources like biomass via fermentation or gasification, the balance of CO2 is zero within a suitable short period (in comparison to coal, natural gas and oil, where this period is some millions of years). The advantage of the Carbonate Fuel Cell HotModule is, that these fuel gases from the renewable sources can be used with the high performance and efficiency of the HotModule, even they are low caloric gases, which decline the electric efficiency of conventional prime movers significantly. The products of the HotModule are: • Electricity: DC for telecommunication and IT - AC to grid or to stand alone networks - Applications for uninterruptible power supply. • Premium Heat: Heat from HotModule is available in form of the depleted air at a high temperature. This high exergetic heat is valuable for steam production, industrial production processes as well as for many other processes e. g. in hospitals, in the food industry, in greenhouse farming. It can also be used in cascades of steam production for additional electricity generation via steam turbines, medium temperature processes like drying, cooking, and at the low temperature end for water heating and space heating and — may be — pool heating. • Cooling Power: Another important heat utilization is the production of cooling power for air conditioning and food storage facilities by thermal driven cooling systems, e. g. absorption chillers or steam injection chillers with the overlapping of the required energy amounts over the year: Cooling in summer, heating in winter. This leads to a thermal full power operation of the HotModule all over the year decreasing the pay back period of such equipment. • Fertilizing atmosphere: The depleted air consists of nitrogen, a small amount of oxygen, lots of water vapour and a substantial amount of CO2 (in the range of 5%vol). No contaminants, no toxic ingredients, no other loads. Mixed with fresh air, this depleted air is a most valuable atmosphere for greenhouse farming: Plants need the right temperature, the CO2-contents increase the growing rate of the plants (e. g. tomatoes need an average of 2%vol of CO2 in atmosphere for optimal growing; CO2-fertilizer) and the high water vapour content saves humidification water.

Topics: Fuels , Biomass , Fuel cells
Commentary by Dr. Valentin Fuster
2006;():621-622. doi:10.1115/FUELCELL2006-97146.

Solid oxide fuel cell (SOFC) works at high temperature and is normally used in stationary devices which are of wide interest in the world market. The most currently SOFC developers utilize yttria-stabilized zirconia (YSZ) as electrolyte, strontium-doped lanthanum manganite (LSM) as cathode and a Ni/YSZ cermet obtained from NiO/YSZ in situ reduction as anode. The electrode performance is directly influenced by powder grain sizes, homogeneity, purity, and amount of Ni. Although physical mixture is a simpler procedure it hardly gives homogeneous materials as suitable to SOFC applications. Alternative chemical methods are sol-gel, impregnation and those derived from Pechini route. The present work compares thermal stability and hydrogen reducibility of NiO/YSZ composites prepared by impregnation (I), Pechini (P) and physical mixture (PM) procedures.

Commentary by Dr. Valentin Fuster
2006;():623-627. doi:10.1115/FUELCELL2006-97147.

In the development of fuel cell and energy systems we can observe an increasing complexity, going from simple source-conversion-and product systems to single source multiproduct systems such as combined heat and power and so-called tri-generation systems in which for instance also chemicals can be produced simultaneously with electric power and heat. The logical next step would be to allow not only for multi products, but also for various sources supplied simultaneously to the system. This concept has been proposed by M. Geidl and was called an energy hub. In this paper examples are given of these MSMP systems with fuel cells. Examples are the integration of fuel cells in the existing natural gas infrastructure and in a possible future natural gas infrastructure in which hydrogen is mixed into the natural gas. Also a favorable example is given for the integration of wind energy and high temperature internal reforming fuel cells into a highly efficient system for the co production of hydrogen and electric power.

Commentary by Dr. Valentin Fuster
2006;():629-640. doi:10.1115/FUELCELL2006-97150.

A bottoming 275 kilowatt planar solid oxide fuel cell (SOFC) gas turbine (GT) hybrid system control approach has been conceptualized and designed. Based on previously published modeling techniques, a dynamic model is developed that captures the physics sufficient for dynamic simulation of all processes that affect the system with time scales greater than ten milliseconds. The dynamic model was used to make system design improvements to enable the system to operate dynamically over a wide range of power output (15 to 100% power). The wide range of operation was possible by burning supplementary fuel in the combustor and operating the turbine at variable speed for improved thermal management. The dynamic model was employed to design a control strategy for the system. Analyses of the relative gain array (RGA) of the system at several operating points gave insight into input/output (I/O) pairing for decentralized control. Particularly, the analyses indicate that for SOFC/GT hybrid plants that use voltage as a controlled variable it is beneficial to control system power by manipulating fuel cell current and to control fuel cell voltage by manipulating the anode fuel flowrate. To control the stack temperature during transient load changes, a cascade control structure is employed in which a fast inner loop that maintains the GT shaft speed receives its setpoint from a slower outer loop that maintains the stack temperature. Fuel can be added to the combustor to maintain the turbine inlet temperature for the lower operating power conditions. To maintain fuel utilization and to prevent fuel starvation in the fuel cell, fuel is supplied to the fuel cell proportionally to the stack current. In addition, voltage is used as an indicator of varying fuel concentrations allowing the fuel flow to be adjusted accordingly. Using voltage as a sensor is shown to be a potential solution to making SOFC systems robust to varying fuel compositions. The simulation tool proved effective for fuel cell/GT hybrid system control system development. The resulting SOFC/GT system control approach is shown to have transient load-following capability over a wide range of power, ambient temperature, and fuel concentration variations.

Commentary by Dr. Valentin Fuster
2006;():641-645. doi:10.1115/FUELCELL2006-97154.

Bi2 O3 doped scandia stabilized zirconia systems have shown promise for use as electrolytes in IT-SOFCs. Sintering properties, crystal phase transformation, microstructure, as well as electrical conductivity of the Bi2 O3 doped Sc2 O3 -ZrO2 systems were investigated. The effect of Bi2 O3 doping from 0 to 2.0mol%, and different sintering temperatures, on the properties and performance of the electrolyte were examined. The presence of Bi2 O3 aided the sintering process and better sintering for the doped system was achieved at lower temperatures. The rhombohedral phase in 10ScSZ was successfully stabilized to cubic phase at room temperature with a concentration of 1 mol% and 2 mol% Bi2 O3 sintered at 1100°C–1400°C. The achievement of cubic structure depends upon both the Bi2 O3 concentration and the sintering temperature. Higher electrical conductivity was achieved with Bi2 O3 doped Sc2 O3 -ZrO2 systems than 10ScSZ below 600°C. A maximum conductivity of 1.68 × 10−2 S/cm at 700°C was obtained for 2 mol% Bi2 O3 doped sample sintered at 1100°C.

Commentary by Dr. Valentin Fuster
2006;():647-652. doi:10.1115/FUELCELL2006-97158.

This paper discusses numerical analysis of heat and mass transfer characteristics in autothermal fuel reformer. Assuming local thermal equilibrium between bulk gas and surface of catalyst, one medium approach for energy equation is incorporated. Also, mass transfer between concentrations of bulk gas and near the surface of catalyst is neglected due to relatively low gas mixture velocity. For surface chemical reaction Langmuir-Hinshelwood reaction is incorporated when methane (CH4 ) is reformed to hydrogen-rich gases by autothermal reforming (ATR) reaction. Complete combustion, steam reforming, water gas shift and direct methane steam reforming reactions are included in the chemical reaction model. Under two operating conditions (O/C and S/C), ATR reactions are estimated from the numerical calculations. Mass, momentum, and energy equations are simultaneously calculated with chemical reactions. From the predicted results, we can estimate optimum operating conditions for high hydrogen yield.

Commentary by Dr. Valentin Fuster
2006;():653-658. doi:10.1115/FUELCELL2006-97159.

Reducing operation temperature of Solid Oxide Fuel Cell (SOFC) may provide many advantages for material selections of sealing, interconnects and Balance-Of-Plant (BOP). This study focus on the advanced performance of another cathode material and the performance according to structure change about Pr1−x Srx CoO3−δ compositions (PSCs, x = 0, 0.3, 0.5 and 0.7). High temperature XRD measurement and electrochemical impedance methods were used to study the characteristics of the material as a cathode material for Intermediate Temperature-operating Solid Oxide Fuel Cell (IT-SOFC) application. Lattice parameters and crystal structures of PSCs as well as Area Specific Resistance (ASR) of PSCs on solid oxide electrolytes are discussed at various x values (x = 0, 0.3, 0.5 and 0.7) and temperatures. One of various compositions of PSC showed 0.17 Ωcm2 of ASR on 10% Gd-doped cerium oxide at 700°C.

Commentary by Dr. Valentin Fuster
2006;():659-670. doi:10.1115/FUELCELL2006-97167.

The correct prediction of the temperature distribution is a prerequisite for the reliable determination of species and current distributions in any solid oxide fuel cell (SOFC) model. It is even more crucial if the model is intended for the analysis of thermo-mechanical stresses. This paper addresses the different mechanisms of heat generation and absorption in the fuel cell. Particular attention is paid to the heating associated with the oxidation of hydrogen, which is commonly assigned to the interface between electrolyte and anode in SOFC modeling. But for a detailed determination of the temperature profile in the fuel cell solid components the separate consideration of the cathodic and anodic half-reactions is required. A method for determining the specific entropy change of the half-reactions based on Seebeck-coefficient data is adopted from the literature and applied to the SOFC. In order to exemplarily demonstrate the contribution of the various heat sources to the overall heat generation as well as the influence of their location, a spatially discretized model of a tubular SOFC is used. Temperature profiles obtained with and without separate consideration of the electrode reactions are compared. The comparison shows that the spatially descretized reaction model is indeed necessary for the reliable assessment of temperature gradients in the ceramic SOFC components.

Commentary by Dr. Valentin Fuster
2006;():671-675. doi:10.1115/FUELCELL2006-97171.

Fuel processing is defined as conversion of any biomass, hydrocarbons or organics to a fuel gas reformate suitable for fuel cell (FC) anode reaction system. Rice husk is one of the potential organic sources of hydrogen and heat energy that can be generated from rice husk gasification processes. The high-purity hydrogen fed to the FC stack for power generation makes waste rice husk utilization system economically and environmentally attractive. Thus, the main objectives of this work were to develop a rice husk gasification process and the potential applications of high-purity hydrogen from syngas (CO and H2 ) on stationary power generator of FC system. In the lab-scale fixed-bed and bench-scale downdraft experimental approaches, gasification of rice husk was accompanied by a substantial production of syngas at 760–900 K. It was found that in addition to over 90% of syngas generation, approximately 7.17 × 105 kcal/hr of thermal energy was recovered and the cold gas efficiency was 78–86% when the gasifier was operated at O/C atomic ratios between 1.1 and 1.3. The product syngas can be further separated by pressure swing adsorption and Pd membrane purification units, which effectively purified and generated 99.999% pure hydrogen in an integrated FC Processor. Finally, cost or benefit analysis of a rice husk gasifier of 10-TPD (tons per day) was also performed to confirm the economic potential for such a recycling practice and determine if further development of stationary FC system would be warranted.

Commentary by Dr. Valentin Fuster
2006;():677-681. doi:10.1115/FUELCELL2006-97178.

The voltage loss in a single cell of a segmented-in-series flat-tube-type solid oxide fuel cell (SOFC) stack was evaluated. The stack exhibits a particular configuration, and the current flows along an in-plane direction in the anode and cathode; this differs from conventional planar-type SOFCs. Distributions of the current in the cell were studied in detail for many parameters such as the length of electrodes and conductivities of the cell components. Gas diffusion in a porous substrate and anode and the fuel utilization affect the concentration polarization; thus, they were also considered. By using the simulation results, appropriate configurations for the cell and stack were designed, and the simulation was validated by performance tests for a practical fuel cell stack. The stack exhibited a high performance of 0.76 V at 0.2 A/cm2 at Uf = 81.6% with dry H2 fuel, yielding a DC energy conversion efficiency of 53.6% HHV. Moreover, a bundle comprising 34 stacks was also fabricated, and excellent values of output voltage (342.5 V), output power (338.3 W), and electrical efficiency (49.5% HHV) at a current density of 0.2 A/cm2 were obtained.

Commentary by Dr. Valentin Fuster
2006;():683-689. doi:10.1115/FUELCELL2006-97183.

This paper presents the results of a study to evaluate the feasibility of deploying fuel cells in hydrocarbon producing facilities. For the majority of hydrocarbon production facilities, electric power is generated on-site, most often, by the combustion of some of the produced hydrocarbons. To optimize its performance, Shell is continuously looking at applying new technologies, which can increase the availability of her production facilities and/or reduced lifecycle costs and/or improve safety and environmental performance. Shell has identified fuel cell technology as being capable of delivering some of these benefits because of its potential to achieve high availability, reliability and fuel efficiency when compared to conventional technologies. An inventory has been made of the specific design specifications and the state-of-the-art of commercially available fuel cell systems. Most of the required capacities fall in the range of 1kW to 1 MW, which is compatible with state of the art fuel cell developments or it can be achieved in the near future. A software-screening tool has been constructed to evaluate the various options with respect to conventional technologies. The specific design specifications can vary from production site to site, but in general availability and low maintenance are two of the main criteria to be considered and most favorable for fuel cells. Depending on the specific requirements for a particular hydrocarbon production facility a polymer fuel cell, MCFC or SOFC system are considered suitable alternatives to conventional technology. The screening tool has been applied and evaluated in a case study of one of the unmanned production facilities of Shell. A 20 kW SOFC system was found to score higher than a commercially available gas engine of 25 kW on eight of the most important of several criteria. However, SOFC system lifecycle costs are still 15 to 20% higher due to the development costs needed for this ‘prototype’ SOFC system to make it suitable for use in hydrocarbon producing facility. When applied in more surface production facilities the SOFC system also becomes costs competitive with conventional technologies.

Commentary by Dr. Valentin Fuster
2006;():691-699. doi:10.1115/FUELCELL2006-97187.

This paper considers recent model results examining the transient performance of three common solid oxide fuel cell (SOFC) geometries (cross-flow, co-flow, and counter-flow) during load reduction events. Of particular note for large load decrease (e.g., shutdown) is the occurrence of reverse current over significant portions of the cell, starting from the moment of load loss up to the point where equilibrated conditions again provide positive current. This behavior results from the temperature gradients that exist in an SOFC stack. Also reported are test results from an experiment employing two separate button cells coupled together electrically (anode-to-anode and cathode-to-cathode) which are used to confirm the model predictions. The test results confirm the predictions of the model in that temperature gradients are a driver for current circulation within a cell. Also reported are test results of a button cell operated under reverse current to help begin to identify what effects such operation may have on fuel cell performance and durability.

Commentary by Dr. Valentin Fuster
2006;():701-709. doi:10.1115/FUELCELL2006-97188.

Toho Gas Co. Ltd. and Sumitomo Precision Products Co. Ltd. have been jointly developing a SOFC system using scandia-stabilized zirconia (ScSZ) electrolyte cells. Especially, we focused the scandia tetragonal zirconia polycrystalline (Sc-TZP) electrolyte, because the Sc-TZP electrolyte has good mechanical and electrical properties, therefore high reliability and power generation characteristics are expected. We have been developing the 1kW SOFC system using Sc-TZP electrolyte cells as proof of concept since 2002. The 1kW SOFC combined heat and power (CHP) system was installed in The 2005 World Exposition, Aichi, Japan (EXPO2005), and the system successfully operated during about six months. During the demonstration, some troubles caused by balance of plant (BOP) system and controlling system, and these experiences are useful to our system development. The target of our developing system is a small-scale commercial CHP application and target power range is below 10kW class. To apply such a small-scale commercial use, the rapid start up is very attractive for customers in Japanese market. In this study, we have been developing the rapid starting system. To shorten the start up time, reducing the volume of cell stack and strengthening the cell are developed in parallel. Because heating capacity is very affected factor to determine the start up time. To reduce the volume of cell stack, the improvement of cell performance is very attractive. For the electrolyte-supported type cell, the electrical conductivity of electrolyte material is very important factor on the cell performance. On the other hand, to realize the rapid start up system, the mechanical strength of electrolyte is also important factor, because in the rapid start up conditions, large temperature distribution may be easily occurred, and it leads the cell broken. The relation between electrical conductivity and mechanical strength is trade off in the electrolyte material, and then we focused the electrolyte in the range from 4mol% to 7mol%, and demonstrated that these materials have good combination of electrical and mechanical properties. To estimate the suitable composition, the mechanical strength of electrolyte from room temperature to 1073K that is the maximum operating temperature of our system were investigated. And piston on ring (POR) method was also investigated to estimate the strength of actual electrolyte sheets. Part of this work was performed as R & D program of New Energy and Industrial Technology Development Organization (NEDO).

Commentary by Dr. Valentin Fuster
2006;():711-717. doi:10.1115/FUELCELL2006-97200.

As one of the possible electrolyte materials for intermediate temperature SOFCs that works around 500 to 800°C, we focus on Gadolinia Doped Ceria (GDC). This ceramic material shows reasonable ion conductivity even at 600°C. It, however, is a mixed conductive material having non-negligible electronic conductivity. In this study, the fundamental performance of a simple planar SOFC with GDC electrolyte is numerically investigated. The effects of electrical leakage on the cell performance are the main focus of discussion. The electrolyte thickness is varied in a range from 50 to 200μm. Both air and fuel flows are assumed to be steady and laminar. Governing equations are the continuity, momentum, energy and mass transfer equations. They are solved numerically by the control volume method. It is found that the leakage of electricity becomes larger for the smaller electrolyte thickness cases, and is more prominent for an electrolyte thickness of less than 80μm. Under such conditions, output power and energy conversion efficiency decrease dramatically. On the other hand, energy conversion efficiency also decreases for an electrolyte thickness that is too large because of the increase of ohmic overpotential of the electrolyte. Consequently, there seems to be an adequate thickness for the electrolyte that gives preferable output power and energy conversion efficiency.

Commentary by Dr. Valentin Fuster
2006;():719-732. doi:10.1115/FUELCELL2006-97221.

Numerical modeling has helped the SOFC research for over a decade in which period the models grew in complexity and detail. Multi-dimensional detailed models such as FLUENT’s SOFC module calculate three dimensional distributions of velocity, temperature, concentration and electric potential inside all components of the fuel cell. Such models while being very helpful in understanding the processes inside the fuel cell may prove to be very expensive for transient simulations and simulations of multi-cell stacks. Hence reduced order modeling is still used for such applications. However, reduced order modeling entails reduction of detail and consequent loss in accuracy. In this paper a multi-dimensional SOFC code, FLUENT’s SOFC module, is compared with a reduced order pseudo three-dimensional model, DREAM SOFC. FLUENT’s SOFC module is a commercial solver built on the popular CFD solver FLUENT. DREAM SOFC is an in house code developed at Computational Fluid Dynamics and Applied Multi Physics (CFD&AMP) Center at West Virginia University. It is a combination of a one dimensional model for channels and three-dimensional models for the rest of the components in a SOFC. This approach avoids having to solve Navier-Stokes equations inside channels but still retains the three-dimensionality inside important components. Same test cases with similar conditions are simulated with these codes and results are compared with each other.

Commentary by Dr. Valentin Fuster
2006;():733-740. doi:10.1115/FUELCELL2006-97222.

The optimization process is in general an important issue to show the viability of solid oxide fuel cells (SOFCs) compared to traditional power sources. This optimization process can be done in a faster and cheaper way by making use of numerical simulations. In this study, three-dimensional, non-isothermal, steady state numerical simulations of planar solid oxide fuel cells (SOFC) are performed using the commercial FLUENT software. First, a detailed analysis of grid and iteration-dependent simulations is performed. This analysis predicts a 20% difference between a coarse and fine grid in the velocity magnitude in both anode and cathode gas flow channels, and in the y-component of current density. Then, the performance of a planar SOFC with changing channel aspect ratio is analyzed comparing their V-I curves and critical parameters like temperature, concentration, and current density distributions. The predictions show a 12 degrees difference in temperature at the fuel exhaust between low and high aspect ratio channel simulations. These results suggest that the channel aspect ratio is a significant parameter, worthwhile to be investigated.

Commentary by Dr. Valentin Fuster
2006;():741-750. doi:10.1115/FUELCELL2006-97224.

The successful widespread adoption of fuel cell systems is highly dependent upon the economics of the installation. This entails closely matching system capabilities with customer requirements. System sizing requires accurate predictions of building thermal and electrical loads. The TRNSYS-based building simulation model presented in this paper was developed to accurately integrate a fuel cell into the space heating, water heating, and cooling equipment in a building. The simulation tool determines water heating, space heating, and cooling loads for a single zone building on an hour-by-hour basis throughout the year using TMY2 weather data. It integrates empirical and theoretical state point models of the components of a fuel cell-based cogeneration and tri-generation system as well as baseline HVAC technologies. The key components include: hot water loops, stratified water tanks, boilers, furnaces, air conditioners, absorption chillers, space conditioning coils, heat rejection equipment, and ventilation controls. Various control options are incorporated to maintain setpoints, stage equipment, and limit power export. Renewable power systems such as PV and wind are also integrated into the model. The TRNSYS calculation engine iterates to find the state of the system for each hour. The simulation tool also includes post-processing capabilities to apply complex electric tariffs, organize annual simulation results, and manage multiple parametric runs. The tool has been developed to optimize the configuration of a fuel cell in a given building application and to complete numerous parametric runs to evaluate the economics of a system in different locations and building applications. This work was funded in part by the New York State Energy Research and Development Authority.

Commentary by Dr. Valentin Fuster
2006;():751-760. doi:10.1115/FUELCELL2006-97225.

Efficient and low polluting production of electricity and heat is an issue which cannot be postponed. Fuel cells, which convert the chemical energy stored in a fuel into electrical and thermal energy, are an efficient solution for such a problem. These devices rely on the combination of hydrogen and oxygen into water: oxygen is extracted from the air while hydrogen can be obtained from either fossil fuels or renewable sources. The use of biomass as hydrogen source in connection with fuel cells is an argument of particular interest, since high temperature gasification processes are actually utilized. Solid Oxide Fuel Cells (SOFC), working at high temperatures, have become therefore an interesting candidate to realize the internal reforming of the feed gas from a gasifier. The reforming reaction occurs at the anode of the SOFC, upstream and separated from the fuel cell reaction. The section of the anode where reforming occurs is adjacent to the section where electrochemical reaction occurs. So, heat produced by the electrochemical reaction can be transferred internally with minimal losses. Simulation models of the performance of SOFC stacks and biomass gasifiers are useful to visualize temperature, current and concentration distributions, which are difficult to measure by experimental techniques, allowing the definition of optimal choices in terms of geometries and operating conditions. In this work, an analysis of a SOFC coupled with a biomass gasifier is performed. The objective of this study is the identification of the main effects of the operating conditions on the fuel cell performance in terms of efficiency, and the distribution of the main electro-thermal-fluid-dynamics variables, namely current and temperature. A gasifier model has been implemented to calculate the equilibrium compositions using the Gibbs free energy minimization method. The obtained results are directly used to estimate the inlet gas composition for the SOFC. The SOFC has been modelled by a 3D approach (FLUENT), which solves the energy and mass transport and the internal reforming, coupled with a 0D electrolyte model which, starting from the local information in terms of gas composition, temperature and pressure, is able to predict the fuel cell performance in terms of electrical response and mass-energy fluxes. The whole model has been applied to the analysis of an integrated SOFC-gasifier system to address a planar SOFC response by varying the gasifier operating conditions and the global system performance.

Commentary by Dr. Valentin Fuster
2006;():761-765. doi:10.1115/FUELCELL2006-97250.

Bioethanol, obtained by biomass fermentation, could be an important hydrogen supplier as a renewable source. The availability of active, selective and stable catalyst for bioethanol steam reforming is a key point for the development of processes suitable to this purpose. In this work, the performance of different supported catalysts in the steam reforming of bioethanol at molten carbonate fuel cell (MCFC) operative condition has been focused and a decreasing activity has been related to the formation of carbon. Furthermore catalytic behaviour of a Ni supported catalyst has been tested under reforming condition both distillation industry’s waste and ethanol-water mixture. Results revealed that, superior alcohols (fusel oil) arising from the distillation process influence carbon formation and the presence of oxygen (ATR condition) preserves the catalyst activity which otherwise significantly deactivate mainly due to the carbon formation.

Commentary by Dr. Valentin Fuster
2006;():767-775. doi:10.1115/FUELCELL2006-97256.

The main objective of this paper is to examine the effects of transport geometry on the efficiency of an electrolyte-supported solid oxide fuel cell. A three-dimensional thermo-fluid-electrochemical model is developed to the influences of channel dimensions, rib width and electrolyte thickness on the temperature, mass transfer coefficients, species concentration, local current density and power density. Results demonstrate that decreasing the height of flow channels can significantly lower the average solid temperature and improve the cell efficiency due to higher heat/mass transfer coefficient between the channel wall and flow stream, and a shorter current path. However, this improvement is limited for the smallest channel. The cell with a thicker rib width and a thinner electrolyte layer has higher efficiency and lower average temperature. Numerical simulation will be expected to help optimize the design of a solid oxide fuel cell.

Commentary by Dr. Valentin Fuster
2006;():777-785. doi:10.1115/FUELCELL2006-97257.

The fuel cell industry is currently undergoing rapid development, and applications of fuel cell based power sources are diversifying. The advent of new and more sophisticated application areas and the expanding market necessitates development of efficient and robust fuel cell based power supplies that are reliable in their performance. These demands are answered not only by improved plant designs and innovations, but also by developing high-quality control algorithms. Quality and reliability of the complete system are ensured through extensive and varied testing. To this end an automated Hardware-in-the-Loop based control code verification and validation platform for the Delphi Solid Oxide Fuel Cell plant and control system has been developed. Verification activities are managed using the System Verification Manager tool. This paper outlines the application of this platform for safety and diagnostics verification and validation for a Solid Oxide Fuel Cell system.

Commentary by Dr. Valentin Fuster
2006;():787-792. doi:10.1115/FUELCELL2006-97260.

Reforming diesel, JP-8 (Jet Propellant 8 – standard U.S. military kerosene-based jet fuel), and other heavy hydrocarbon fuels is one option being investigated for providing H2 for distributed mobile and stationary fuel cell systems for military and civilian applications. Unlike natural gas, which is another hydrocarbon fuel being investigated, these fuels are high-boiling-point, multi-component liquids that contain high concentrations of refractory sulfur and aromatic compounds that can negatively impact the efficiency and operating lifetime of the fuel processor. Fuel injection, desulfurization, and carbon deposition are major issues that fuel processor developers must address when designing fuel processors for these fuels. The fuel injection system must prevent direct injection of liquid onto the catalyst surface and poor mixing of the fuel and oxidants (i.e., air and/or steam), both of which can result in excessively high temperatures that can damage or destroy the reforming catalyst. A highly efficient desulfurization process is required that can reduce the sulfur concentration to acceptable levels for both fuel processor and fuel cell catalysts without requiring large amounts of materials or complicated processes, and without generating excessive amounts of disposable waste. Highly active reforming catalysts and the proper choice of operating conditions (i.e., ratio of fuel to oxidant[s], temperature, residence time) are required for effective reforming of aromatic compounds to prevent carbon deposition on the reforming catalyst as well as in the fuel processor downstream of the reformer. In this paper, we will discuss how the chemical and physical properties of these fuels influence the design of the fuel processor focusing on the fuel injection system, the choice of desulfurization process, and the design and operation of the reformer. We will also discuss various approaches and design options for developing highly efficient fuel processors for reforming these fuels for both polymer electrolyte and solid oxide fuel cells.

Commentary by Dr. Valentin Fuster
2006;():793-819. doi:10.1115/FUELCELL2006-97262.

The influence of preparation techniques on the microstructure, grain-size and consequently on the electrical transport properties of the ABO3 structured materials used as electrode and electrolytes in all perovskite IT-SOFC were investigated. Nano-crystalline powders of La1-x Mx Ga1-y Ny O3±δ (M = Sr,; x = −0.10 to 0.15; N = Mg; y = −0.10 to 0.15) (LSGM) as electrolyte, porous La0.8 Sr0.2 Co0.8 Fe0.2 O3±δ (LSCF) or LaNi1-x Fex O3±δ (x = 0–0.5) (LNF) as cathode, La0.8 Sr0.2 Cr0.7 Mn0.3 O3±δ (LSCM) as anode and LaCrO3 or substituted LaCrO 3 as interconnect were synthesized by various wet chemical methods. The wet chemical methods like metal-carboxylate gel decomposition, hydroxide co-precipitation, sonochemical and regenerative sol-gel process followed by microwave sintering of the powders have been used. Microwave sintering parameters were optimized by varying sintering time, and temperature to achieve higher density of LSGM pellets. The phase pure systems were obtained at sintering duration of 30 min at 1200 °C. The XRD, HR-TEM, and SEM measurements revealed the average grain size of these perovskites was ∼ 22 nm range. The electrical conductivities of the compositions were measured by ac (5Hz–13MHz) and dc techniques. The conductivity of the sintered pellets was found to be ∼0.01–0.21 S/cm at 550–1000°C range for electrolyte and 1.5–100 S/cm at 25–1000°C for electrodes respectively. The effect of sonochemical, and regenerative sol-gel methods in processing large quantities of nano-crystalline perovskites with multi-element substitutions at A- and B-sites to achieve physico-chemical compatibility for fabricating zero emission all perovskite IT-SOFCs are reported in this paper.

Commentary by Dr. Valentin Fuster
2006;():821-825. doi:10.1115/FUELCELL2006-97268.

A load transient mitigation scheme is proposed in this paper for stand-alone solid oxide fuel cell (SOFC)-battery power systems. The scheme is based on low-pass filtering of load transients and fuel cell current control. The fuel cell is controlled in such a way to provide the steady-state load, while the battery will supply the transient load. The technique can be used to improve the output power quality of the overall system as well as the SOFC durability by mitigating the stresses on SOFC caused by the load transients. Simulation studies have been carried out to verify the proposed technique. Simulation results show the effectiveness of the proposed technique, which prevents the load transient to affect the fuel cell performance.

Commentary by Dr. Valentin Fuster
2006;():827-832. doi:10.1115/FUELCELL2006-97001.

The low-temperature Direct Propane Polymer Electrolyte Membrane Fuel Cell (DPFC) based on low-cost modified membranes was demonstrated for the first time. The propane is fed into the fuel cell directly without the need for reforming. A PBI membrane doped with acid and a Nafion 117 membrane modified or non-modified with silicotungstic acid were used as the polymer membranes. The anode was based on Pt, Pt-Ru or Pt/CrO3 electro catalysts and the cathode was based on a Pt electro catalyst. For non-optimized fuel cells based on H2SO4 doped PBI membranes and Pt/CrO3 anode, the open circuit potential was 1.0 Volt and the current density at 0.40 Volt was 118 mA.cm-2 at 95°C. For fuel cells based on Nafion 117 membranes modified with silicotungstic acid and on Pt/CrO3, the open-circuit voltage was 0.98 Volt and the current density at 0.40 Volt was 108 mA.cm-2 while fuel cells based on non-modified Nafion 117 membranes exhibited an open-circuit voltage of 0.8 Volt and the current density at 0.40 Volt was 42 mA.cm-2. It was also shown that propane fuel cells using anodes based on Pt-Ru/C anode (42 mW.cm-2) exhibit a similar maximum power density to that exhibited by fuel cells based on Pt-CrO3/C-anode (46 mW.cm-2), while DPFC using a Pt/C-based anode exhibited lower maximum power density (18 mW.cm-2) than fuel cells based on the Pt-CrO3/C anode (46 mW.cm-2).

Commentary by Dr. Valentin Fuster
2006;():833-842. doi:10.1115/FUELCELL2006-97010.

Proton exchange membrane fuel cells (PEMFCs) are expected to play a significant role in the next generation of energy systems and road vehicles for transportation. To achieve high performance, low cost and high reliability, significant attention is needed on detailed modeling and simulation of various physical processes in PEMFC unit-cells. Substantial effort is also required to reach proper water and thermal balances for PEMFC stacks and integrated energy systems. For modeling and analysis at the unit-cell and component level, typically CFD-based approaches might be appropriate. On the stack and system levels, methods like lumped parameter analysis and overall energy/mass balances are more suitable. This paper discusses various kinds of methods for modeling and analysis, and how these can be used as well as their applicability and limitations. The focus is placed on water management/two-phase flow regimes and characteristics of relevant models.

Commentary by Dr. Valentin Fuster
2006;():843-852. doi:10.1115/FUELCELL2006-97011.

The National Renewable Energy Laboratory (NREL) and Plug Power Inc. have been working together to develop fuel cell modeling processes to rapidly assess critical design parameters and evaluate the effects of variation on performance. This paper describes a methodology for investigating key design parameters affecting the thermal performance of a high temperature, polybenzimidazole (PBI)-based fuel cell stack. Nonuniform temperature distributions within the fuel cell stack may cause degraded performance, induce thermo-mechanical stresses, and be a source of reduced stack durability. The three-dimensional (3-D) model developed for this project includes coupled thermal/flow finite element analysis (FEA) of a multi-cell stack integrated with an electrochemical model to determine internal heat generation rates. Sensitivity and optimization algorithms were used to examine the design and derive the best choice of the design parameters. Initial results showed how classic design-of-experiment (DOE) techniques integrated with the model were used to define a response surface and perform sensitivity studies on heat generation rates, fluid flow, bipolar plate channel geometry, fluid properties, and plate thermal material properties. Probabilistic design methods were used to assess the robustness of the design in response to variations in load conditions. The thermal model was also used to develop an alternative coolant flow-path design that yields improved thermal performance. Results from this analysis were recently incorporated into the latest Plug Power coolant flow-path design. This paper presents an evaluation of the effect of variation on key design parameters such as coolant and gas flow rates and addresses uncertainty in material thermal properties.

Topics: Design , Fuel cells
Commentary by Dr. Valentin Fuster
2006;():853-861. doi:10.1115/FUELCELL2006-97039.

A dynamic model of a stationary PEM fuel cell system has been developed in Matlab-Simulink®. The system model accounts for the fuel processing system, PEM stack with coolant, humidifier with anode tail-gas oxidizer (ATO), and an enthalpy wheel for cathode air. For the fuel processing system four reactors were modeled: (1) an auto thermal reactor (ATR) (2) a high temperature shift (HTS) reactor, (3) a low temperature shift (LTS) reactor, and (4) a preferential oxidation (PROX) reactor. Chemical kinetics for ATR that describe steam reformation of methane and partial oxidation of methane were simultaneously solved to accurately predict the reaction dynamics. Chemical equilibrium of CO with H2 O was assumed at HTS and LTS reactor exits to calculate CO conversion corresponding to the temperature of each reactor. A quasi-two dimensional unit PEM cell was modeled with five control volumes for solving the dynamic species and mass conservation equations and seven control volumes to solve the dynamic energy balance and to capture the details of MEA behavior, such as water transport, which is critical to accurately determine polarization losses. The dynamic conservation equations, primary heat transfer equations and equations of state are solved in each bulk component and each component is linked together to represent the complete system. A comparison of steady-state model results to experimental data shows that the system model well predicts the actual system power and catalytic partial oxidation (CPO) temperature. Transient simulation of DC power is also well matched with the experimental results to within a few percent. The model predictions well characterized the observed dynamic CPO temperature, voltage, and temperature of stack coolant outlet observations that are representative of a generic PEM stationary fuel cell system performance. The model is shown to be a useful tool for investigating the effects of inlet conditions and for the development of control strategies for enhancing system performance.

Commentary by Dr. Valentin Fuster
2006;():863-869. doi:10.1115/FUELCELL2006-97103.

A general electrolyte model for calculation of the liquid electrolyte transport in fuel cells is presented. A 2-D formulation is used to describe the transport in an alkaline fuel cell. Numerical results were obtained by using commercial CFD software, in conjunction with the user defined functions that calculate the source terms of the transport equations. An order of magnitude analysis is conducted of the energy transport in the separator and electrode regions. The numerical calculation also examines the local primary current at the anode and cathode electrodes. The calculated current flux showed a higher value near the separator entrance and it decreased along the stream-wise direction. The non-uniformity of the local primary current is caused mainly by the species transport resistance between the electrodes instead of the temperature difference. The effects of four different electrolytes were also studied. The results suggested that the cell voltage differences were due to the competing effects of electrolyte conductance and species diffusion. Numerical calculation also captured the presence of shunt current. A net shunt current as high as 0.1 A/cm2 is calculated at the separator inlet and exit. Provisions to reduce shunt currents seem to be warranted for AFC operated at a condition similar to that examined in this paper.

Commentary by Dr. Valentin Fuster
2006;():871-877. doi:10.1115/FUELCELL2006-97106.

In typical Proton Exchange Membrane fuel cells, a compressed gasket provides a sealing barrier between cell and cooler bipolar plate interfaces. The gasket initially bears the entire bolt load, and its resisting reaction load depends on the cross-sectional shape of the gasket, bipolar plate’s groove depth, and the hyperelastic properties of the gasket material. A nonlinear, finite element analysis (FEA) model with various hyperelastic material models, large deformations, and contact was used to evaluate the load-gap curves. The deformed shapes and the distributions of stress, strain, and deflections are presented. Mooney-Rivlin and Arruda-Boyce hyperelastic material models were used, and a comparison of load-gap curves is shown. A process is presented that couples the computer-aided design geometry with the nonlinear FEA model that was used to determine the gasket’s cross-sectional shape, which achieves the desired reaction load for a given gap.

Commentary by Dr. Valentin Fuster
2006;():879-887. doi:10.1115/FUELCELL2006-97128.

A novel anode feed gas humidification method was investigated as part of an effort to reduce the mass, volume, and cost of the balance of plant for a commercial PEM fuel cell system. Ultrasonic fountain nebulization was utilized to ultrahumidify the anode feed gas for a PEM fuel cell. Ultrasonic nebulization ultrahumidification was found to increase the average voltage of the fuel cell by several percent, and reduce the amplitude of cyclic overvoltage. Most importantly, this humidification technique greatly increased the thermal fault tolerance of the PEM fuel cell; that is, this humidification technique allowed the PEM fuel cell to operate effectively at high temperatures without a need to increase the vapor pressure of the humidification water. In addition, this humidification technique shows potential to be used to increase the overall energy conversion efficiency of a PEM fuel cell system.

Commentary by Dr. Valentin Fuster
2006;():889-895. doi:10.1115/FUELCELL2006-97151.

In this paper the performance of a natural gas power system has been discussed. The power generation unit is composed by a fuel cell and a fuel processor integrated in a compact system. The hydrogen generator uses the steam reforming technology. A CO shift converter and a preferential oxidation reactor are used to minimize the CO concentration in the reformate gas. The hydrogen dilution in the reformate gas calls for a modification of the fuel cell feeding system. The dead-end mode is not practicable for the fuel cell operation, but it is necessary to open the anode flow channels. In order to increase the efficiency of the integrated system, the anode off gas mass flow is burned to supply heat for the reforming reaction. The fuel cell performance has been evaluated using the test bed of the University of Cassino. The experimental activity has been focused to evaluate the performance in different operating conditions. A semi-empirical model of the fuel cell has been employed to forecast the fuel cell behaviour with pure hydrogen and reformate gas feeding. The semi-empirical coefficients of the model have been fitted by using the experimental data. The target of the fuel cell modelling has been to develop a tool capable of predicting the performance in different operating conditions. The same tool can be used to identify the areas for design improvements.

Commentary by Dr. Valentin Fuster
2006;():897-905. doi:10.1115/FUELCELL2006-97174.

An efficient technique to fabricate metal-oxide/carbon composite nanotubes has been developed through a self-assembly processing that includes implantation of acidic groups and interaction between surface oxides and metal ions or hydration molecules. To functionalize carbon nanotubes, gaseous oxidation at 300 °C was firstly employed to build functional oxygen groups including carboxyl, carbonyl and hydroxyl group, on the ends or sidewalls of the nanotubes. It revealed that the oxidized nanotubes express a slight improvement of surface hydrophilicity, which was demonstrated by contact angle measurement. X-ray photoelectron spectroscope investigation indicated that the ratio of attached metal-oxide onto the oxidized nanotubes gradually increases with oxidation level, i.e., surface O/C atomic ratio. This evidence reflected that the surface oxides act as an adsorption center that strongly interacts with metal ions or hydration molecules in aqueous phase. Applying this method, SnO2 , RuO2 , NiO and PtRu nanoparticles having an average size of 5 nm were assembled on the oxidized carbon nanotubes.

Commentary by Dr. Valentin Fuster
2006;():907-910. doi:10.1115/FUELCELL2006-97193.

A large-scale polymer electrolyte membrane fuel cell (PEMFC) with novel interdigitated (or discontinuous) flow channel has been investigated experimentally. Interdigitated channel geometry has the advantages of effective water removal and higher reaction efficiency through forcing gas transport in the diffusion layer. In this study, multiple-Z type flow pattern has been adopted on the interdigitated channels. The active area of flow channel plate is 256 cm2 (16 cm × 16 cm). The channel width and depth are 1 mm and 0.8 mm respectively. The rib width is 1 mm. The performance of single PEM fuel cell with an interdigitated flow field is studied with appropriated operating conditions. The results demonstrated that the multiple-Z interdigitated flow channel has better performance compared with the conventional Z type by presented in the form of Current-Voltage (I-V) polarization curves. The pressure drop loss of multiple-Z interdigitated flow field increases about one time with the conventional one. The experimental results under the effects of gas humidification temperature and reactant gas flow rate, etc. have been comprehensively discussed in this work.

Commentary by Dr. Valentin Fuster
2006;():911-915. doi:10.1115/FUELCELL2006-97214.

In most PEM fuel cell MEA’s Nafion is used as electrolyte material due to its excellent proton conductivity at low temperatures. However, Nafion needs to be fully hydrated in order to conduct protons. This means that the cell temperature cannot surpass the boiling temperature of water and further this poses great challenges regarding water management in the cells. When operating fuel cell stacks on reformate gas, carbon monoxide (CO) content in the gas is unavoidable. The highest tolerable amount of CO is between 50–100 ppm with CO-tolerant catalysts. To achieve such low CO-concentration, extensive gas purification is necessary; typically shift reactors and preferential oxidation. The surface adsorption and desorption is strongly dependent upon the cell temperature. Higher temperature operation favors the CO-desorption and increases cell performance due to faster kinetics. High temperature polymer electrolyte fuel cells with PBI polymer electrolytes rather than Nafion can be operated at temperatures between 120–200°C. At such conditions, several percent CO in the gas is tolerable depending on the cell temperature. System complexity in the case of reformate operation is greatly reduced increasing the overall system performance since shift reactors and preferential oxidation can be left out. PBI-based MEA’s have proven long durability. The manufacturer PEMEAS have verified lifetimes above 25,000 hours. They are thus serious contenders to Nafion based fuel cell MEA’s. This paper provides a novel experimentally verified model of the CO sorption processes in PEM fuel cells with PBI membranes. The model uses a mechanistic approach to characterize the CO adsorption and desorption kinetics. A simplified model, describing cathode overpotential, was included to model the overall cell potential. Experimental tests were performed with CO-levels ranging from 0.1% to 10% and temperatures from 160–200°C. Both pure hydrogen as well as a reformate gas models were derived and the modeling results are in excellent agreement with the experiments.

Commentary by Dr. Valentin Fuster
2006;():917-924. doi:10.1115/FUELCELL2006-97217.

It is important to elucidate the transient characteristics of polymer electrolyte fuel cells (PEFC), especially when PEFC is applied to relatively small-scale power applications where it will be subjected to a wide range of loads, and may have frequent starts and stops. In addition, the water management problem, which is represented by flooding in cell and drying in proton exchange membrane (PEM), is another issue to address. The flooding is caused by liquid water accumulated in GDL and/or flow channel; the liquid water hinders mass transfer of gases to and from active layers; it can lead to rapid deterioration of cell performance. And the water management relates to the transient response of PEFC frequently. Based on these issues we wrote a numerical simulation program for unit-PEFC, which can simulate the successive events of vapor condensation, liquid saturation growth, corresponding to the dynamic change of cell voltage. We formulated mass, momentum and energy conservation equation with equivalent electric circuit; we discretized and numerically solved them. As for the gas/liquid two-phase flow formulation in GDL, we utilized multi-phase mixture (M2 ) model. As for the multi-component diffusion formulation, we utilized Stefan-Maxwell equation. Using the program, we simulated the transient response to rapid increase of load current. When the current density changed from 0.5 A/cm2 to 1.0 A/cm2 instantaneously, cell voltage (Vcell ) changed in the following manner. Just after the change of current, Vcell decreased instantaneously corresponding to IR resistance and decreased again in 10−1 s time-scale with the re-distribution of oxygen and with the charge of electric double layer capacitor. Then Vcell increased slightly in 101 s time-scale with PEM wetting. Finally, Vcell decreased in 102 ∼ 103 s time-scale with the development of liquid saturation in GDL.

Commentary by Dr. Valentin Fuster
2006;():925-933. doi:10.1115/FUELCELL2006-97243.

The National Institute of Standards and Technology (NIST), in conjunction with Virginia Tech, has developed a rating methodology for residential-scale stationary fuel cell systems. The methodology predicts the cumulative electrical production, thermal energy delivery, and fuel consumption on an annual basis. The annual performance is estimated by representing the entire year of climate and load data into representative winter, spring/fall, and summer days for six different U.S. climatic zones. It prescribes a minimal number of steady state and simulated use tests, which provide the necessary performance data for the calculation procedure that predicts the annual performance. The procedure accounts for the changes in performance resulting from changes in ambient temperature, electrical load, and, if the unit provides thermal as well as electrical power, thermal load. The rating methodology addresses four different types of fuel cell systems: grid-independent electrical load following, grid-connected constant power, grid-connected thermal load following, and grid-connected water heating. This paper will describe a partial validation of the rating methodology for a grid-connected thermal load following fuel cell system. The rating methodology was validated using measured data from tests that subjected the fuel cell system to domestic hot water and space heating thermal loads for each of the three representative days. The simplification of a full year’s load and climate data into three representative days was then validated by comparing the rating methodology predictions with the prediction of each hour over the full year in each of the six cities.

Topics: Fuel cells
Commentary by Dr. Valentin Fuster
2006;():935-941. doi:10.1115/FUELCELL2006-97245.

Background: The U.S. Army Engineer Research and Development Center, Construction Engineering Research Laboratory (ERDC-CERL) continues to manage The Department of Defense (DoD) Residential Proton Exchange Membrane (PEM) Fuel Cell Demonstration Project. This project was funded by the United States Congress for fiscal years 2001 through 2004. A fleet of 91 residential-scale PEM fuel cells, ranging in size from 1 to 5 kW, has been demonstrated at various U.S. DoD facilities around the world. Approach: The performance of the fuel cells has been monitored over a 12-month field demonstration period. A detailed analysis has been performed cataloging the component failures, investigating the mean time of the failures, and the mean time between failures. A discussion of the lifespan and failure modes of selected fuel cell components, based on component type, age, and usage will be provided. This analysis also addresses fuel cell stack life for both primary and back-up power systems. Several fuels were used throughout the demonstration, including natural gas, propane, and hydrogen. A distinction will be made on any variances in performance based on the input fuel stock. Summary: This analysis will provide an overview of the ERDC-CERL PEM demonstration fuel cell applications and the corresponding data from the field demonstrations. Special emphasis will be placed on the components, fuel cell stack life, and input fuel characteristics of the systems demonstrated.

Commentary by Dr. Valentin Fuster
2006;():943-949. doi:10.1115/FUELCELL2006-97252.

The overall characteristics of electric power grid in terms of continuity of the supply and energy quality are of outmost importance for both industrial and civil applications with special attention to the uninterruptible ones. Net congestion problems are becoming more and more frequent boosting the development of small energy generation systems with back-up function. In this field low temperature fuel cells are an interesting solution addressing both environmental and efficiency issues. In the present work the application of Polymer Electrolyte Fuel Cells (PEFC) for an Uninterruptible Power Supply (UPS) system (<1kWe ) is analysed by examining different possible technical solutions. This system is composed by a PEFC 1kWe stack, assisted by a set of battery and a supercapacitors pack, and using hydrogen stored into a metal hydride tank. Critical aspects as system start-up, response rapidity and autonomy are addressed to obtain an optimal configuration. Both numerical and experimental analysis have been carried out to characterize component behaviour. Once realized and tested, the system has proved to be able to work as UPS with an autonomy of 6.5 hours, only determined by hydrogen storage capability.

Commentary by Dr. Valentin Fuster
2006;():951-963. doi:10.1115/FUELCELL2006-97274.

Multi-walled carbon nanotubes (MWNTs) have been synthesized by the pyrolysis of acetylene using hydrogen decrepitated Mischmetal (Mm) based AB3 alloy hydride catalyst. MWNTs have been characterized by SEM, TEM, Raman and XRD studies. Pt-supported MWNTs (Pt/MWNTs) have been prepared by chemical reduction method using functionalized MWNTs. Composites of Pt/MWNTs and Pt/C have been used as electrocatalysts for oxygen reduction reaction in Proton Exchange Membrane Fuel Cell (PEMFC). Cathode catalyst with 50% Pt/MWNTs and 50% Pt/C gives the best performance because of the better dispersion and good accessibility of MWNTs support and the Pt electrocatalysts in the mixture for the oxygen reduction reaction in PEMFC. The paper emphasizes that Pt/C and Pt/MWNTs composites have good potential as catalyst support material in PEMFC.

Commentary by Dr. Valentin Fuster
2006;():965-971. doi:10.1115/FUELCELL2006-97276.

Hydrogen fuel cell technology is currently capable of providing adequate power for a wide range of stationary and mobile applications. Nonetheless, the sustainability of this technology rests upon the production of hydrogen from renewable resources. Among the techniques under current study, the chemical reforming of alcohols and other bio-hydrocarbon fuels, appears to offer great promise. In the so called autothermal reforming process, a suitable combination of total and partial oxidation supports hydrogen production from ethanol with no external addition of energy required. Furthermore, the autothermal reforming process conducted in a well insulated reactor, produces temperatures that promote additional hydrogen production through the endothermic steam reforming and the water-gas shift reactions, which may be catalyzed or uncatalyzed, with the added benefit of lowered carbon monoxide concentrations. In this study, an adiabatic ethanol reforming reactor was simulated assuming the reactants to be air (21% O2 and 79% N2 ) and ethanol (C2 H5 OH) and the products to be H2 O, CO2 , CO and H2 , with all constituents taken to be in the gaseous state. The air was introduced uniformly through a ring around the side of the reactor and the gaseous ethanol was injected into the center of one end, with products withdrawn from the center of the opposite end, to create an axisymmetric flow field. The gas flows within the reactor were assumed to be turbulent, and the chemical kinetics of a simple four reaction system was assumed to be controlled by turbulent mixing processes. Air and fuel flow rates into the reactor were varied to obtain six different levels of oxidation (air-fuel ratios) while maintaining the same total gaseous mass flow out of the reactor. The numerical results for the reacting flow show that hydrogen production is maximized when the air-fuel ratio on a mass basis is held at approximately 2.8. These findings are in qualitative agreement with observations from previous experimental studies.

Commentary by Dr. Valentin Fuster
2006;():973-976. doi:10.1115/FUELCELL2006-97278.

Based on innovative ceria-based composite (CBC) material advantages we have made strong efforts to make technical developments on scaling up material production, fabrication technologies on large cells and stack operated at low temperatures (300 to 600°C). Next generation materials for solid oxide fuel cells (SOFCs) have been developed based on abundant natural resources of the industrial grade mixed rare-earth carbonates named as LCP. Here we show the LCP-based materials used as functional electrolytes to achieve excellent fuel cell performances, 300–800 mWcm2 for low temperatures, exhibiting a great availability for industrialization and commercialization .

Commentary by Dr. Valentin Fuster
2006;():977-982. doi:10.1115/FUELCELL2006-97279.

We have made extensive efforts to develop various compatible electrode materials for the ceria-based composite (CBC) electrolytes, which have been, reported as most advanced LTSOFC electrolyte materials (Zhu, 2003). The electrode materials we have investigated can be classified as four categories: i) LSCCF (LaSrCoCaFeO) and BSCF perovskite oxides applied for our CBC electrolyte LTSOFCs; ii) LFN (LaFeO-based oxides, e.g. LaFe0.8 Ni0.2 O3 ) perovskite oxides; iii) lithiated oxides: e.g. LiNiOx, LiVOx or LiCuOx are typical cathode examples for the CBC LTSOFCs; iv) other mixed oxide systems, most common in a mixture of two-oxide phases, such CuOx-NiOx, CuO-ZnO etc. systems with or without lithiation are developed for the CBC systems, especially for direct alcohol LTSOFCs. These cathode materials used for the CBC electrolyte LTSOFCs have demonstrated excellent performances at 300–600°C, e.g. 1000 mWcm−2 was achieved at 580°C. The LTSOFCs can be operated with a wide range of fuels, e.g. hydrogen, methanol, ethanol etc with great potential for applications.

Commentary by Dr. Valentin Fuster

Portable Systems

2006;():983-991. doi:10.1115/FUELCELL2006-97019.

Liquid water transport inside PEM fuel cells is one of the key challenges for water management in a proton exchange membrane (PEM) fuel cell. Investigation of the air-water flow patterns inside fuel cell gas flow channels with porous transport layer (PTL) would provide valuable information that could be used in fuel cell design and optimization. This paper presents a numerical investigation of air-water flow across a PTL with a serpentine channel on PEM fuel cell cathode by use of a commercial Computational Fluid Dynamics (CFD) software package FLUENT. Detailed flow patterns with air-water across the porous media were investigated and discussed.

Commentary by Dr. Valentin Fuster
2006;():993-999. doi:10.1115/FUELCELL2006-97022.

The objective of this paper is to study the influences of water vapor concentration in membrane and flow channels under the different operation conditions. The studied flow channels by CFDRC code include serpentine and interdigitated flow channels which have different gas transportation mechanisms. At the same time, the computer code, based upon Okada’s one dimensional model, was built to predict the influences of the electro-osmosis effect, the foreign impurity cations and the water balance time on water concentration in the membrane by Fortran 90. Both of interdigitated and serpentine flow fields show that water vapor accumulates near the cathode outlet of the membrane. And, the serpentine flow field accumulates more water vapor than interdigitated flow field does. As the inlet water mass fraction below 10%, the drying out problem may happen to reduce current density. In addition, the foreign impurity cations may induce the stronger electro-osmosis effect and reduce the effect of water back diffusion; hence, cause the accumulating of water in the cathode. It needs more water balance time and decreases the membrane performance.

Commentary by Dr. Valentin Fuster
2006;():1001-1005. doi:10.1115/FUELCELL2006-97023.

In this paper we have successfully demonstrated a new micromachined fuel processing system including vaporizer, catalytic combustor and methanol steam reformer. This fuel processing system utilizes the thermal energy generated from the catalytic hydrogen combustion to heat up the entire system. For the first time, we have used carbon nanotubes as a supporting structure of Pt catalyst for combustion. The catalytic combustor could supply the energy to heat the reformer and maintain its working temperature. We have also developed a new coating method of reforming catalyst (Cu/ZnO/Al2 O3 ) and observed that adequate amount of hydrogen can be generated for PEMFC. We have successfully reported the feasibility of the proposed fuel processing system in each assembled component.