Design and development of recirculation system for increasing Efficiency of solid oxide fuel cell (SOFC)

Kurzfassung

This project aimed to design and develop an anode off-gas recirculation system for a Solid Oxide Fuel Cell (SOFC) test rig, together with a high-temperature piping system and an emulator system for gas flow rate control. The existing SOFC test rig at the Department of Mechanical and Aerospace Engineering, King Mongkut's University of Technology North Bangkok, lacked a functional recirculation loop and had several outstanding limitations in temperature control and safety management. This project therefore undertook to address these gaps through system design, computational modelling, component selection, and physical testing. The recirculation system was designed to return hydrogen-rich anode exhaust gas back to the stack inlet for reuse, with the objective of improving fuel utilization efficiency and reducing thermal energy loss. A double-pipe counter-flow heat exchanger was sized to recover thermal energy from the anode exhaust at 750 °C to pre-heat the incoming reactant gas. The piping system was redesigned using Grade 316/316L stainless steel tubing and Swagelok fittings, selected for resistance to hydrogen corrosion and suitability for operating pressures up to 0.3 MPa at temperatures up to 800 °C, in accordance with ASME B31.12 and ASME B36.19M standards. An emulator system was developed using an ESP32 microcontroller, mass flow controller, stepper motor-driven proportional valves, and Modbus RS485 communication, to enable automated control and measurement of the anode inlet gas flow rate. The energy efficiency of the SOFC system with anode off-gas recirculation was evaluated using MATLAB simulation under two control strategies. In Case 1, the total hydrogen molar flow rate entering the stack was held constant while the fresh feed was adjusted with the recirculation ratio R. In Case 2, the fresh hydrogen supply from the cylinder was fixed and the stack current was allowed to vary with R. Parametric sweeps were performed over fuel utilization values of U = 0.6 to 0.9 and recirculation ratios of R = 0.0 to 0.8. The results showed that electrical efficiency generally increased with R for low-to-moderate fuel utilization levels. However, at U = 0.9, a steam dilution effect caused by the accumulation of product water in the recycled exhaust depressed the Nernst open-circuit voltage, resulting in a plateau or decline in efficiency beyond a critical recirculation ratio. In Case 2, U = 0.8 was identified as a preferable operating point when combined with moderate recirculation (R is approximately 0.6), yielding an electrical efficiency of approximately 56.9%. The simulation model employed an ohmic-only polarization approach and did not account for activation or concentration overpotentials, parasitic loads, or thermal gradients. During physical testing, a thermocouple failure was identified as the root cause of pipe damage sustained by the existing furnace from a prior project. The faulty thermocouple was replaced, and the furnace was subsequently verified to maintain temperature control at 750 °C, with a measurement discrepancy of approximately 4 °C relative to a reference instrument. Procurement of certain piping components and completion of the LabVIEW-based measurement circuit remained outstanding at the time of reporting. Full experimental validation of the recirculation system on the SOFC test rig is recommended as a priority for future work.