Diego F. Rancruel

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.

A Decomposition Strategy Applied to the Optimal Synthesis/Design and Operation of an Advanced Fighter Aircraft System: A Comparison With and Without Airframe Degrees of Freedom

A decomposition methodology based on the concept of “thermoeconomic isolation” applied to the synthesis/design and operational optimization of an advanced tactical fighter aircraft is the focus of this research. Conceptual, time, and physical decomposition were used to solve the system-level as well as unit-level optimization problems. The total system was decomposed into five sub-systems as follows: propulsion sub-system (PS), environmental control sub-system (ECS), fuel loop sub-system (FLS), vapor compression and PAO loops sub-system (VC/PAOS), and airframe sub-system (AFS) of which the AFS is a non-energy based sub-system. A number of different configurations for each sub-system were considered. The most promising set of candidate configurations, based on both an energy integration analysis and aerodynamic performance, were developed and detailed thermodynamic, geometric, physical, and aerodynamic models at both design and off-design were formulated and implemented. A decomposition strategy called Iterative Local-Global Optimization (ILGO) developed by Munoz and von Spakovsky (2000b,c) was then applied to the synthesis/design and operational optimization of the advanced tactical fighter aircraft. This decomposition strategy is the first to successfully closely approach the theoretical condition of “thermoeconomic isolation” when applied to highly complex, highly dynamic non-liner systems. This contrasts with past attempts to approach this condition, all of which were applied to very simple systems under very special and restricted conditions such as those requiring linearity in the models and strictly local decision variables. This is a significant advance in decomposition and has now been successfully applied to a number of highly complex and dynamic transportation and stationary systems. This paper presents the detailed results from one such application, which additionally considers a non-energy based sub-system (AFS).© 2003 ASME

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.

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

Advanced Fighter Aircraft Sub-Systems Optimal Synthesis/Design and Operation: Airframe Integration Using a Thermoeconomic Approach

A decomposition methodology based on the concept of “thermoeconomic isolation” and applied to the synthesis/design and operational optimization of an advanced tactical fighter aircraft is presented in this paper. Conceptual, time, and physical decomposition are used. The physical decomposition strategy employed, called Iterative Local-Global Optimization (ILGO), was developed by Munoz and von Spakovsky and has been applied to a number of complex stationary and transportation applications. This decomposition strategy is the first to successfully closely approach the theoretical condition of “thermoeconomic isolation” when applied to highly complex, highly dynamic, non-linear systems. The total system is decomposed into eight sub-systems, five of which have degrees of freedom (a total of 493). These five are the airframe sub-system (AFS), the propulsion sub-system (PS), the environmental control sub-system (ECS), the fuel loop subsystem (FLS), and the vapor compression and PAO loops subsystem (VC/PAOS). The three without degrees of freedom are the equipment group (EGS), the permanent payload (PPS) and the expendable payload (EPS). For the ones with degrees of freedom, detailed aerodynamic, geometric, thermodynamic, and physical models at both design and off-design were formulated and implemented. The highly complex problem of integrating the synthesis/design of the airframe sub-system with the PS and the thermal driven sub-systems (ECS, FLS, and VC/PAOS) is solved using the novel decomposition approach mentioned above. The most promising set of aircraft sub-system configurations based on both aerodynamic performance and energy integration analysis are evaluated for all mission stages including the transients segments. The optimal configuration (synthesis) and geometry (design) of the airframe sub-system is determined simultaneously with that for the total aircraft system, i.e. with that for all of the other sub-systems. Results for this system and its sub-systems are presented below.

A Multidiscipline and Multi-rate Modeling Framework for Planar Solid-oxide-fuel-cell based Power-Conditioning System for Vehicular APU

A numerical modeling framework for planar solid-oxide fuel cell (PSOFC) based vehicular auxiliary power unit (APU) is developed. The power-conditioning system (PCS) model comprises the comprehensive transient models of PSOFC, balance-of-plant and power-electronics subsystems (BOPS and PES, respectively) and application load (AL). It can be used for resolving the interactions among PSOFC, BOPS, PES and AL, control design and system optimization and studying fuel-cell durability. The PCS model has several key properties including: (i) it can simultaneously predict spatial as well as temporal dynamics; (ii) it has two levels of abstraction: comprehensive (for detailed dynamics) and reduced-order (for fast simulation); and (iii) the fast-simulation model can be implemented completely in Simulink/Matlab environment, thereby significantly reducing the cost as well as time and provides the avenue for real-time simulation and integration with vehicular power-train models employing the widely used ADVISOR. The computational overhead and accuracy of the fast-simulation and comprehensive models are compared. Significant savings in time compared to using the former were obtained, without compromising accuracy.

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

Investigation of Control Strategy Development Using an Integrated Model of a SOFC Based APU Under Transient Conditions

A typical approach to the synthesis/design optimization of energy systems is to only use steady state operation and high efficiency (or low total life cycle cost) at full load as the basis for the synthesis/design. Transient operation as reflected by changes in power demand, shut-down, and start-up are left as secondary tasks to be solved by system and control engineers once the synthesis/design is fixed. However, start-up and shut-down may be events that happen quite often and, thus, may be quite important in the creative process of developing the system. This is especially true for small power units used in transportation applications or for domestic energy supplies, where the load demand changes frequently and peaks in load of short duration are common. The duration of start-up is, of course, a major factor which must be considered since rapid system response is an important factor in determining the feasibility of solid oxide fuel cell (SOFC) based auxiliary power units (APUs). Start-up and shut-down may also significantly affect the life span of the system due to thermal stresses on all system components. Therefore, a proper balance must be struck between a fast response and the costs of owning and operating the system so that start-up or any other transient process can be accomplished in as short a time as possible yet with a minimum in fuel consumption. In this research work we have been studying the effects of control laws and strategies and transients on system performance. The results presented in this paper are based on a set of transient models developed and implemented for the components of a 5 kW net power SOFC based APU and for the high-fidelity system which results from their integration. The simulation results given below are for two different start-up approaches: one with steam recirculation and component preheating and the second without either. These start-up simulations were performed for fixed values of a number of system-level parameters (e.g., fuel utilization, steam to methane ratio, etc.) and were used to generate sufficient information to permit the development of appropriate control strategies for this critical operating point. These strategies are based on a balance between fuel consumption and response time. In addition, energy buffering hardware was added to the system configuration in order to minimize the effect of transients on fuel cell stack performance and lifetime.Copyright