Multi-scale modeling of protonic ceramic fuel cell stacks and systems

Abstract

Protonic ceramic fuel cells (PCFCs) are an emerging electrochemical ceramic technology that can be used to produce electricity from a variety of renewable and non-renewable fuels. In high temperature, ceramic fuel cells, the anode is typically impregnated with nickel, which facilitates in-situ hydrocarbon reforming, thereby allowing a more fuel-flexible fuel cell. However, the barium zirconate-based material sets used in PCFCs also exhibit profound coking and sulfur resistance that enables them to accept raw fuel feedstocks that range from hydrogen and methane to heavy hydrocarbons and alcohols – a characteristic that truly enables unprecedented fuel flexibility compared to solid oxide fuel cell (SOFC) technology, for example. PCFCs are further unique compared to other fuel cells because they are based upon mixed ionic-electronic conducting (MIEC) electrolytes; protons, oxide ions, and O-site polarons are all able to simultaneously migrate within the electrolyte membrane. Typical fuel cells like SOFCs, proton exchange membrane fuel cells (PEMFCs), and molten carbonate fuel cells (MCFCs) only exhibit single charge carrier mobility. The MIEC nature of the ceramic electrolyte complicates the modeling of the electrochemical characteristics of the PCFC. In this work, a semi-empirical electrochemical model of the PCFC is presented and validated against experimental data. This model is scaled up and coupled to dynamic conservation equations to capture stream-wise variation in gas concentration, temperature, and pressure as the fuel and air are consumed along the length of the fuel cell channel. The cell level model is scaled up to simulate the transient response of a fuel cell stack to changes in electric load. The PCFC stack model is incorporated into a complete, water-neutral standalone PCFC power generation system, wherein recuperative heat exchangers, a water recovery loop, and a fuel recycling loop are used to increase system performance and efficiency. Lastly, techno-economic analysis (TEA) is performed so that the PCFC system can be designed and optimized with minimized capital costs and levelized cost of electricity (LCOE). The PCFC system concept is projected to produce electric power at net efficiencies ranging from 50% to nearly 71% (LHV-basis), with LCOEs of 7.6 to 10.1 ¢ per kWh depending on system configuration and design point. The PCFC systems presented in this work have the potential to displace current power generators in the distributed generation marketplace because of the high efficiency operation and potentially low LCOE compared to the current technologies.