A 20 kW solid oxide electrolysis cell system with hydrogen recirculation: operational performance and thermodynamic evaluation

To support the growing industrial emphasis on performance of Solid Oxide Electrolysis Cell (SOEC) systems for commercialization, the Korea Institute of Machinery and Materials (KIMM) developed a 20 kW-SOEC system with hydrogen recirculation and conducted a comprehensive thermodynamic assessment of the validated operational performance data. The results demonstrate stable operation for over 3,000 h, with the stack temperature maintained between 730–750 °C and a steam conversion ratio of 32–35%. The overall system electrical efficiency remained above 80% (41.4 kWh/kgH₂), whereas the SOEC stacks themselves consumed 35.9 kWh/kgH₂. To provide deeper insight into system performance, comprehensive energy and exergy analyses were performed to quantify heat losses and assess thermodynamic irreversibility during the two experiment stages. The energy analysis revealed substantial thermal losses, primarily due to unrecovered latent heat from gas cooling, followed by losses from the stack hot box. In Stage II of the experiment, the thermal insulation of the hot boxes was improved, resulting in a 5.9% reduction in total heat loss and a 1.4% increase in overall system efficiency. The exergy analysis indicated that thermodynamic irreversibility primarily occurred in the SOEC stacks and furnace heaters, accounting for 39.5% and 38.6%, respectively, of the total system exergy destruction in Stage II. The overall system exergy efficiency increased from 76.7% in Stage I to 78.0% in Stage II, confirming that reduced heat losses from the hot boxes contribute to improved system performance.