Performance Evaluation of a Thermally Integrated Fuel Cell System in the Presence of Uncertainties

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, hydrodesulphurizer (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. INTRODUCTION Integrated fuel cell and fuel reforming (FCFR) systems provide highly efficient and versatile solutions for mobile and stationary power applications. An FCFR system, investigated in our previous work [1], was formed by adding a catalytic burner (CB) 1 ded From: http://proceedings.asmedigitalcollection.asme.org/ on 02/18/2016 Te in the outlet of a proton exchange membrane (PEM) fuel cell (FC). The CB utilizes the excess hydrogen in the FC to preheat the inlets of the catalytic partial oxidizer (CPOX) reformer. A dynamic model of the resulting thermally integrated FCFR system was developed and used to analyze the transient behavior and to identify the key parameters that affect its load following capabilities. A schematic of the system is given in Figure 1, where the corresponding dynamic states included in the model are also shown. Feedback control design and closed loop analysis, also performed in [1], resulted in improving the system performance and in mitigating hydrogen starvation and reactor overheat issues during transients. The feedback controller uses the measurements of the CPOX and CB temperatures and yields the time optimal actuator control signals. In order to implement the state feedback design, an observer (i.e., a model of the system) is incorporated to provide the state information for the feedback controller. Since the model of the system is embedded in the controller, any modeling error could affect the closed loop operation and may lead to performance deterioration and even system instability. In addition, other uncertainties, such as component aging, are inevitable and will lead to change in the system characteristics. Thus, it is important to evaluate the robustness of the feedback scheme for different uncertainty scenarios. In this paper, several uncertainty sources are considered and their effects are analyzed. First, the robustness of the control scheme is validated against clogging of the CPOX and the hydro-desulfurizer (HDS) reactors. Clogging is a common phenomenon due to carbon deposition caused by aging or occasional overheating. Clogging increases the resistance of the reCopyright c © 2006 by ASME rms of Use: http://www.asme.org/about-asme/terms-of-use D mix mix wrox wrox PCH 4 Pair PH 2 an PH 2 cpox P T an P AN MIX CPOX WROX (WGS+PROX) CB cb T cb m air