Characterizing SOFC Stack Electrochemical Performance at Elevated Pressure

Abstract

Pressurized operation of high temperature fuel cell stacks is of increasing interest for a variety of applications, including high efficiency, hybrid solid-oxide fuel cell power generation systems. In this presentation, we report on our efforts to characterize the electrochemical performance of 1-kWeand 5-kWe Ceres Power SteelCell solid-oxide fuel cell stacks, with a focus on elevated pressure operation in the range of 2-5 barg. Increased pressure operation requires particular attention to pressure differentials across both the membrane electrode assembly and between the stack and surrounding atmosphere. The effort is part of a five-year U.S. DoE ARPA-E INTEGRATE program to combine the high efficiency of solid-oxide fuel cells with the high efficiency balance-of-plant equipment to demonstrate a 70%-efficient distributed power generator. In this architecture, the highly efficient solid-oxide fuel cells (SOFCs) are placed upstream of an internal combustion engine, where the anode tail gas, consisting of partially converted steam-reformed natural gas, is further utilized by the engine to produce electricity. The stacks are operated between 2-5 barg to maximize overall system efficiency and to increase SOFC power density.Few studies have been reported on SOFC stack operation at elevated pressures. The team at the Colorado School of Mines has designed and operated the pressurized stack test stand shown in the figure to characterize electrochemical performance of various kW-scale SOFC stack test modules developed by Ceres Power, Ltd. Minimizing pressure differentials across the anode and cathode during elevated-pressure operation is of primary concern. Excessive pressure on one side of the cell can cause substantial mechanical stress to the metal-supported membrane-electrode assemblies. Pressure differentials in this work are maintained below the manufacturer’s specifications of +/- 25 mbar at operating pressures of 0-5 barg. In addition to performance measurements at elevated pressures, studies have also been conducted on diluted fuel streams representative of high anode tail-gas recycle and various amounts of internal reforming. The stacks show steady, stable performance over all conditions tested.A fuel feed is produced with mass flow controllers that represents partially, steam-reformed natural gas with anode tail-gas recycle. A natural gas fuel processor will be incorporated into the stand at a later stage of the research program. Reactant gas feeds are preheated in multiple zones to properly control stack temperatures. The insulated stack test module is housed inside of a pressure vessel that has individual pressure control in order to minimize the pressure differentials between the internal flow channels of the stack and the surrounding environment. Exhaust gases exit the pressure vessel where heat exchangers are utilized to partially cool the 600°C gases before reaching back-pressure regulators.Stack operation is challenging because of the combination of a tight thermal operating window and the long system response time to changes in inlet conditions. These time constants become even larger at low air flow rates. Lower air flow (while maintaining required oxygen utilization) is desired in order to reduce the power required by the balance of plant, especially when operating at higher pressures. In order to mitigate this, a model predictive controller is being implemented. A dynamic model of the fuel cell was first tuned and validated against experimental transient response data. Then the resulting model was linearized at multiple operating points. The linearizations were then utilized within a gain-scheduled moving-horizon predictor that can take past input and output data, along with a proposed future input, and predict the future output response. The predictor performance has been validated, and then inserted into a model-predicative controller. Optimization algorithms select the input temperatures, flow rates, and electric current required to quickly reach a desired operating condition while meeting all operational constraints, including thermal constraints.In this talk, we will present our findings on the electrochemical performance of 1 and 5-kWe Ceres Power SteelCell stack test modules and the effects of pressure, fuel composition, anode recycle, and degree of internal reforming on stack performance. Results influence the design and net efficiency of the hybrid system. The trajectory of the research will be discussed. Figure 1