Comas Haynes

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.

Characterizing heat transfer within a commercial-grade tubular solid oxide fuel cell for enhanced thermal management

A thermal transport model has been developed for analyzing heat transfer and improving thermal management within tubular solid oxide fuel cells (TSOFCs). The model was constructed via a proven electrochemical model and well-established heat transfer correlations. Its predictions compare favorably with other published data. Air temperatures consistently approach that of the fuel cell. This is primarily due to the high operating temperature of the cell (1000°C), the moderate magnitudes of radiation and airflow, and cell geometry. The required inlet air temperature (for thermally steady-state operation) has linear dependence on operating voltage and fuel utilization. Inlet air temperature has an inverse proportionality with respect to air stoichiometric number (i.e., inverse equivalence ratio). The current standard for airflow within TSOFCs was found to be excessive in consideration of the regenerative preheat effect within the supply pipes that feed air to the cell. Thermal management of simple TSOFC systems could be enhanced if commonly used air stoichiometric numbers were decreased.

‘Design for power’ of a commercial grade tubular solid oxide fuel cell

Abstract Fuel cell systems must not only be thermodynamically efficient, but cost competitive as well. Since power plant capital costs are typically measured per unit of rated power (i.e.,

Simulating process settings for unslaved SOFC response to increases in load demand

/kW), one means of attempting an economically attractive product is to “design for power”. A proven model of a commercial grade tubular solid oxide fuel cell (TSOFC) is presented and verified. The model was used to reveal subtle aspects of enhancing power generation. Internal reformation, while beneficial to system size and integration, hinders power generation. Increasing the electrode thicknesses of the TSOFC design can be beneficial to power generation. Finally, decreasing fuel utilization, at a prescribed operating voltage, is also favorable for power generation. These latter trends are explained in view of the common assertions that thinner electrodes and increased fuel utilizations are the means of fuel cell operation enhancement.

Clarifying reversible efficiency misconceptions of high temperature fuel cells in relation to reversible heat engines

Abstract A common approach to reliable load-following is to “slave” fuel cell response to the reactants supply subsystem (e.g. fuel processor). A change-in-power demand may be time-sensitive, however, and slaving the fuel cell response to that of the slower balance-of-plant components may not be practical. A model has been constructed to simulate the unrestrained load-following characteristics of a commercial-grade fuel cell. Analyses have shown that initial conditions exist which facilitate timely responses to increases in load demand, among them lower fuel utilization, larger operating voltage and optimized cell potential reduction.

System-interaction analyses of solid-oxide fuel cell (SOFC) power-conditioning system

Abstract High temperature fuel cells are promising energy conversion devices. Optimal design and analysis require a thorough understanding of their second law limitations. Fuel cells do not produce work from thermal energy as do heat engines. This has led to the provocative statement that fuel cells are ‘non-Carnot limited’. This label easily, yet erroneously, connotes that an ideal fuel cell is superior to an externally reversible heat engine. Clarity is achieved by analyzing the corresponding systems of these technologies. Conventional reversible fuel cell efficiency is also addressed, and a modified relation is developed. It accounts for the needed coupling of high temperature fuel cells with reversible heat engines, in order for maximum work to be produced.

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

Effects of ripple-current dynamics and load transients on the performance and reliability of solid-oxide fuel cell (SOFC) is investigated in this paper. A brief description of the proposed system-modeling and simulation methodologies is presented. The effect of harmonics in the current, drawn from the SOFC by a power electronics subsystem (PES), on the temperature, and fuel utilization of the SOFC is studied. Impacts of modulation strategies on the transient response, flow parameters and current density are studied. The impact of the magnitude of SOFC output current and microcrack density and material degradation is demonstrated.

Enhancing the Performance Evaluation and Process Design of a Commercial-Grade Solid Oxide Fuel Cell via Exergy Concepts

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 Dynamic Bulk SOFC Model Used in a Hybrid Turbine Controls Test Facility

Fuel cell technology is a promising means of energy conversion. As the technology matures, process design and analysis are gaining importance. The conventional measures of fuel cell performance (i.e., gross real and voltage efficiencies) are limited indices-of-merit. Contemporary second law concepts (availability/exergy, irreversibility, exergetic efficiency) have been used to enhance fuel cell evaluation. A previously modeled solid oxide fuel cell has been analyzed using both conventional measures and the contemporary thermodynamic measures. Various cell irreversibilities were quantified, and their impact on cell inefficiency was better understood. Exergetic efficiency is more comprehensive than the conventional indices-of- performance. This parameter includes thermal irreversibilities, considers the value of effluent exergy, and has a consistent formulation. Usage of exergetic efficiency led to process design discoveries different from the trends observed in conjunction with the conventional efficiency measures. The decision variables analyzed were operating pressure, air stoichiometric number (inverse equivalence ratio), operating voltage and fuel utilization.

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

The paper will describe the dynamic model of a pressurized Solid Oxide Fuel Cell (SOFC). The model is required to run on a digital controller in real-time so that its predicted thermal output may be used to control the fuel valve of an experimental hybrid system that is being used to investigate control methods for fuel cell / gas turbine hybrid systems. The model presented is a bulk model in order to best guarantee real-time operation. The equations and parameters used in the model are presented. The effects of pressure are partially considered and a V-I and P-I curve are plotted in order to show the general fuel cell model behavior. Also presented are dynamic comparisons to a 1-D model. The integration of the model with the experimental system is shown as well.Copyright