Exergy analysis of an integrated solid oxide electrolysis cell-methanation reactor for renewable energy storage

Renewable power intermittency requires storage for load matching. A system combining a solid oxide electrolysis cell (SOEC) and a methanation reactor (MR) could be an efficient way to convert excess electricity into methane, which can be integrated with the existing natural-gas network. In this paper, a comprehensive exergy analysis is performed for three methane production systems: (i) water electrolysis + Sabatier reactor (SR, CO2 MR), (ii) H2O/CO2 co-electrolysis + MR, and (iii) a single SOEC-MR reactor, is performed. First, we find that in the case of the water electrolysis + SR system, upon replacing the low-temperature electrolysis cell with SOEC, the exergy efficiency is dramatically increased by 11% points of percentage at current densities higher that 8000 Am-2, owing to lower electricity consumption. Second, the type of SOEC, operating mode, and operating conditions are optimized for this system. Results show that H2O/CO2 co-electrolysis + MR performs more efficiently than water electrolysis + SR at high current density, especially when using an intermediate-temperature SOEC. The optimal H/C ratio and temperature are found to be 10.54 and 650 degrees C, respectively. A pressurized intermediate-temperature SOEC enables the system to achieve better thermal integration and improves the exergy efficiency to over 77.43% at 6 bar. Finally, the single SOEC-MR reactor with a spatial temperature gradient has the potential to improve the exergy efficiency to 81.34% while utilizing a compact system.