AN INVESTIGATION TO RESOLVE THE INTERACTION BETWEEN FUEL CELL, POWER CONDITIONING SYSTEM AND APPLICATION LOADS

Abstract

Solid-Oxide Fuel Cell (SOFC) stacks respond quickly to changes in load and exhibit high part- and full-load efficiencies due to its rapid electrochemistry. However, this is not true for the thermal, mechanical, and chemical balance-of-plant subsystem (BOPS), where load-following time constants are, typically, 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 (such as, delayed load-following to compensate for slow BOPS response or expensive inductor filtering) to control stack responses to load variations. This limits the applicability of SOFC systems for load-varying stationary and transportation applications 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, leading to optimal designs of power-conditioning electronics and optimal control strategies, which mitigate the electrical-feedback effects. Equally important are ''multiresolution'' finite-element modeling and simulation studies, which 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, for phase I of this project, sponsored by the U.S. DOE (NETL), we investigate the interactions among fuel cell, power-conditioning system, and application loads and their effects on SOFC reliability (durability) and performance. A number of methodologies have been used in Phase I to develop the steady-state and transient nonlinear models of the SOFC stack subsystem (SOFCSS), the power-electronics subsystem (PES), and the BOPS. Such an approach leads to robust and comprehensive electrical, electrochemical, thermodynamic, kinetic, chemical, and geometric models of the SOFSS, PES and application loads, and BOPS. A comprehensive methodology to resolve interactions among SOFCSS, PES and application loads and to investigate the impacts of the fast- and slow-scale dynamics of the power-conditioning system (PCS) on the SOFCSS has been developed by this team. Parametric studies on SOFCSS have been performed and the effects of current ripple and load transients on SOFC material properties are investigated. These results are used to gain insights into the long-term performance and reliability of the SOFCSS. Based on this analysis, a novel, efficient, and reliable PES for SOFC has been developed. Impacts of SOFC PCS control techniques on the transient responses, flow parameters, and current densities have also been studied and a novel nonlinear hybrid controller for single/parallel DC-DC converter has been developed.