Exergy-Based Performance Analysis and Optimization of Autonomous Fuel Cell-Based Energy Systems With Solid Oxide and Proton Exchange Membrane Fuel Cells Fed by Biogas of Variable Composition

Abstract

This study presents a unified thermodynamic and exergy-based assessment of autonomous fuel cell energy systems operating on biogas with varying compositions. Two alternative configurations—a solid oxide fuel cell (SOFC)-based system and a proton exchange membrane fuel cell (PEMFC)-based system—were evaluated using a common balance-of-plant framework to enable a consistent and meaningful comparison. Biogas mixtures with CH₄/CO₂ ratios of 50/50 and 90/10 were considered. The developed model incorporates detailed mass, energy, and exergy balances for all major system components, including the fuel-processing unit, heat exchangers, afterburner, and fuel cell stack. A deterministic parametric analysis was performed to investigate the effects of reformer temperature, afterburner temperature, and biogas methane content on overall system performance. The results indicate net electrical efficiencies ranging from approximately 48%–55% and overall exergy efficiencies between 73% and 81%, depending on the fuel cell technology and fuel composition considered. The largest exergy destruction rates were observed in the fuel-processing subsystem and the electrochemical conversion unit. Furthermore, methane-rich biogas was found to enhance thermodynamic performance by improving both energy conversion efficiency and exergy utilization. The proposed unified modeling framework enables the direct identification and comparison of configuration-dependent performance characteristics and irreversibility patterns. Overall, the findings demonstrate that autonomous fuel cell-based systems can provide highly efficient off-grid electricity generation when fuel composition and operating conditions are appropriately selected.