M.R. von Spakovsky

Single domain PEMFC model based on agglomerate catalyst geometry

Abstract A steady two-dimensional computational model for a proton exchange membrane (PEM) fuel cell is presented. The model accounts for species transport, electrochemical kinetics, energy transport, current distribution, and water uptake and release in the catalyst layer. The governing differential equations are solved over a single computational domain, which consists of a gas channel, gas diffusion layer, and catalyst layer for both the anode and cathode sides of the cell as well as the solid polymer membrane. The model for the catalyst regions is based on an agglomerate geometry, which requires water species to exist in both dissolved and gaseous forms simultaneously. Data related to catalyst morphology, which was required by the model, was obtained via a microscopic analysis of a commercially available membrane electrode assembly (MEA). The coupled set of differential equations is solved with the commercial computational fluid dynamics (CFD) solver, CFDesign™, and is readily adaptable with respect to geometry and material property definitions. The results show that fuel cell performance is highly dependent on catalyst structure, specifically the relative volume fractions of gas pores and polymer membrane contained within the active region as well as the geometry of the individual agglomerates.

Solid-oxide-fuel-cell performance and durability: resolution of the effects of power-conditioning systems and application loads

We describe methodologies for comprehensive and reduced-order modeling of solid-oxide-fuel-cell (SOFC) power-conditioning system (PCS) at the subsystem/component and system levels to resolve the interactions among SOFC, balance-of-plant subsystem, and power-electronics subsystem (PES) and application loads (ALs). Using these models, we analyze the impacts of electrical-feedback effects (e.g., ripple-current dynamics and load transients) on the performance and reliability of the SOFC. Subsequently, we investigate the effects of harmonics in the current, drawn from the SOFC by a PES, on the temperature and fuel utilization of the SOFC. We explore the impacts of inverter space-vector modulation strategies on the transient response, flow parameters, and current density of the SOFC during load transients and demonstrate how these two traditionally known superior modulation/control methodologies may in fact have a negative effect on the performance and durability of the SOFC unless carefully implemented. Further, we resolve the impacts of the current drawn by the PES from the SOFC, on its microcrack density and electrode/electrolyte degradation. The comprehensive analytical models and interaction-analysis methodologies and the results provided in this paper lead to an improved understanding, and may yield realizations of cost-effective, reliable, and optimal PESs, in particular, and SOFC PCSs, in general.

Benefits and design challenges of adaptive structures for morphing aircraft

The purpose of this paper is to discuss the future of adaptive structures leading towards the concept of a fully morphing aircraft configuration. First, examples are shown to illustrate the potential system-level mission benefits of morphing wing geometry. The challenges of design integration are discussed along with the question of how to address the optimisation of such a system. This leads to a suggestion that non-traditional methods need to be developed. It is suggested that an integrated approach to defining the work to be done and the energy to be used is the solution. This approach is introduced and then some challenges are examined in more detail. First, concepts of mechanisation are discussed as ways to achieve optimum geometries. Then there are discussions of non-linearities that could be important. Finally, the flight control design challenge is considered in terms of the rate of change of the morphing geometry. The paper concludes with recommendations for future work.

Fuel cell systems and system modeling and analysis perspectives for fuel cell development

Abstract Within the context of mounting pressures on existing resources and the environment, fuel cell systems can and probably will play a major role. From a second law standpoint, their potential for effectively contributing to solutions, which deal with these pressures is great. This is true for fuel cells as standalone systems and even more so as systems working in concert with more conventional energy conversion processes. In order to put these systems in perspective, a brief summary of existing processes for energy conversion is presented as are projections for future improvements. The principal types of fuel cell systems are then discussed. This is followed by a brief discussion of their second law role followed by some perspectives on the modeling and analysis needs for future fuel cell system developments.

Effects of Battery Buffering on the Post-Load-Transient Performance of a PSOFC

For a planar solid-oxide fuel cell (PSOFC)-based power system, the differences in the response times of the PSOFC stack, the power electronics subsystem (PES), and the balance-of-plant subsystem (BOPS) cause low-reactant conditions near the PSOFC electrodes during load transients. Because the BOPS cannot instantaneously provide enough fuel to the PSOFC, the load transients have a detrimental effect on the performance and life of the fuel cell. To alleviate the degrading effects of load transients on PSOFC stacks, the effectiveness of the energy buffering is investigated.

Investigation of system and component performance and interaction issues for solid-oxide fuel cell based auxiliary power units responding to changes in application load

SOFC stacks respond quickly to changes in load while the balance of plant subsystem (BOPS) responds in times several orders of magnitude higher. This dichotomy diminishes the reliability and performance of SOFC electrodes with increasing load as do current and voltage ripples which result from particular power electronics subsystem (PES) topologies and operation. These ripples and the difference in transient response between the electrical-electrochemical components for the PES and stack subsystem (SS) and those for the chemical-thermal-mechanical components of the BOPS must be approached in a way which makes operation of the entire system not only feasible but ensures that efficiency and power density, fuel utilization, fuel conversion, and system response is optimal at all load conditions. Thus, a need exists for the development of transient component- and system-level models of SOFC-power conditioning systems (i.e. coupled BOPS, SS, and PES) and the development of methodologies for optimizing subsystem responses and for investigating system-interaction issues, which reduce the lifetime and efficiency of a SOFC. A preliminary set of chemical, thermal, electrochemical, electrical, and mechanical models based on the first principles and validated with experimental data were developed and implemented using a number of different platforms. These models were then integrated in such a way as to permit component, subsystem, and system analyses; the development of control strategies; and the synthesis/design and operational optimization of a SOFC based auxiliary power unit (APU) and its components both for steady state and transient operation in transportation and stationary applications. Some pertinent results of these efforts are presented below.

A study of various energy- and exergy-based optimisation metrics for the design of high performance aircraft systems

This paper shows the advantages of applying exergy-based analysis and optimisation methods to the synthesis/design and operation of aircraft systems. In particular, an Advanced Aircraft Fighter (AAF) with three subsystems: a Propulsion Subsystem (PS), an Environmental Control Subsystem (ECS), and an Airframe Subsystem – Aerodynamics (AFS-A) is used to illustrate these advantages. Thermodynamic (both energy and exergy based), aerodynamic, geometric, and physical models of the components comprising the subsystems are developed and their interactions defined. Off-design performance is considered as well and is used in the analysis and optimisation of system synthesis/design and operation as the aircraft is flown over an entire mission. An exergy-based parametric study of the PS and its components is first presented in order to show the type of detailed information on internal system losses which an exergy analysis can provide and an energy analysis by its very nature is unable to provide. This is followed by a series of constrained, system synthesis/design optimisations based on five different objective functions, which define energy-based and exergy-based measures of performance. The former involve minimising the gross takeoff weight or maximising the thrust efficiency while the latter involve minimising the rates of exergy destruction plus the rate of exergy fuel loss (with and without AFS-A losses) or maximising the thermodynamic effectiveness. A first set of optimisations involving four of the objectives (two energy-based and two exergy-based) are performed with only PS and ECS degrees of freedom. Losses for the AFS-A are not incorporated into the two exergy-based objectives. The results show that as expected all four objectives globally produce the same optimum vehicle. A second set of optimisations is then performed with AFS-A degrees of freedom and again with two energy- and exergy-based objectives. However, this time one of the exergy-based objectives incorporates AFS-A losses directly into the objective. The results are that with this latter objective, a significantly better optimum vehicle is produced. Thus, an exergy-based approach is not only able to pinpoint where the greatest inefficiencies in the system occur but appears at least in this case to produce a superior optimum vehicle as well by accounting for irreversibility losses in subsystems (e.g., the AFS-A) only indirectly tied to fuel usage.

Development of a Comprehensive Simulation Platform to Investigate System Interactions Among Solid-Oxide Fuel Cell, Power-Conditioning Systems, and Application Loads

SOFC stacks respond quickly to changes in load and exhibit high part- and full-load efficiencies (due to rapid electrochemistry), which is not true for the balance of plant (BOP), where load-following time constants are several orders of magnitude higher. This dichotomy diminishes the reliability and performance of the electrode with increasing demand of load. Because these unwanted phenomena are not well understood, the manufacturers of SOFC use conservative schemes to control stack responses to load variations, which limit the applicability of SOFC systems from a cost standpoint. Thus, a need exists for the synthesis of component- and system-level models of SOFC power-conditioning systems and the development of methodologies for investigating the system-interaction issues (which reduce the lifetime and efficiency of a SOFC) and optimizing the responses of each subsystem. Equally important are “multiresolution” finite-element modeling and simulation studies that can predict the impact of changes in system-level variables (e.g., current ripple and load-transients) on the local current densities, voltages, and temperature (these parameters are very difficult or cumbersome, if not impossible to obtain) within a SOFC cell. Towards that end, this paper presents a design methodology (with illustrations) for a simulation tool that will enable comprehensive analyses of above (critical) issues.Copyright

3D Quantum thermodynamic description of the non-equilibrium behavior of an unbounded system at an atomistic level

Quantum thermodynamics (QT) provides a general framework for the description of non-equilibrium phenomena at any level, particularly the atomistic one. This theory and its dynamical postulate are used here to model the storage of hydrogen on and in a carbon nanotube. The tube is placed at the center of a tank with a volume of 250 nm3. The thermodynamic system of interest is the hydrogen, which is assumed isolated and having boundaries that coincide with the walls of the tank and the carbon nanotube. The hydrogen is initially prepared in a state far from stable equilibrium (i.e., with the hydrogen molecules probabilistically near one of the outer tank walls) after which the system is allowed to relax (evolve) to a state of stable equilibrium. To predict this evolution in state, the so-called energy eigenvalue problem, which entails a many-body problem that for dilute and moderately dense gases can be modeled using virial expansion theory, is first solved for the geometry involved. The energy eigenvalues and eigenstates of the system found are then used by the nonlinear Beretta equation of motion of QT to determine the evolution of the thermodynamic state of the system as well as the 3D spatial distributions of the hydrogen molecules in time. The simulation results provide a quantification of the entropy generated due to irreversibilities at an atomistic level and show in detail the trajectory of the thermodynamic state of the system as the hydrogen molecules, which are initially arranged to be far from the carbon nanotube, spread out in the system and eventually become probabilistically more concentrated near the carbon atoms, which make up the nanotube.

Modeling the System and Component Performance Interactions of a SOFC Based APU for Changes in Application Load: Transient Response and Control Strategy

Solid-Oxide-Fuel-Cell (SOFC) stacks respond in seconds to changes in load while the balance of plant subsystem (BOPS) responds in times several orders of magnitude higher. This dichotomy diminishes the reliability and performance of SOFC electrodes with changes in load. In the same manner current and voltage ripples which result from particular power electronic subsystem (PES) topologies and operation produce a negative effect on the SOFC stack subsystem (SS) performance. The difference in transient response among the sub-systems must be approached in a way which makes operation of the entire system not only feasible but ensures that efficiency and power density, fuel utilization, fuel conversion, and system response are optimal at all load conditions. Thus, a need exists for the development of transient component- and system-level models of SOFC based auxiliary power units (APUs), i.e. coupled BOPS, SS, and PES, and the development of methodologies for optimizing subsystem responses and for investigating system-interaction issues. In fact the transient process occurring in a SOFC based APU should be systematically treated during the entire creative process of synthesis, design, and operational control, leading in its most general sense to a dynamic optimization problem. This entails finding an optimal system/component synthesis/design, taking into account on- and off-design operation, which in turn entails finding an optimal control strategy and control profile for each sub-system/component and control variable. Such an optimization minimizes an appropriate objective function while satisfying all system constraints. A preliminary set of chemical, thermal, electrochemical, electrical, and mechanical models based on first principles and validated with experimental data have been developed and implemented using a number of different platforms. These models have been integrated in order to be able to perform component, subsystem, and system analyses as well as develop optimal syntheses/designs and control strategies for transportation and stationary SOFC based APUs. Some pertinent results of these efforts are presented here.© 2004 ASME