The effect of pressure and temperature on exergy efficiency in power-to-methane: A comparison of steam and co-electrolysis

• Co-electrolysis is more efficient than steam electrolysis for power-to-methane. • High pressure boosts methanation and cuts power demand in solid oxide reactors. • Methane yield of up to 59 % is achieved in solid oxide electrolysis reactors. • Co-electrolysis lowers water use and reduces compressor and evaporator power. Power-to-Methane (PtM), which converts renewable electricity into synthetic methane, provides a promising approach for long-term energy storage and industrial decarbonization. High temperature solid oxide electrolysis cell reactors (SOECs) are critical to PtM due to their high efficiency and synergy with the exothermic methanation process. Their ability to perform H 2 O/CO 2 co-electrolysis further enhances their process integration potential. However, co-electrolysis remains understudied, especially compared to steam electrolysis in industrial applications. This study develops a modelling framework to assess the thermodynamic potential of two PtM process configurations, comparing steam and co-electrolysis pathways. For a system with idealized components, the maximum achievable exergetic efficiency for different SOEC operating temperatures (750–850 °C) and pressures (1–23 bar) was determined. Results indicate that both systems perform best at the lowest temperatures, however co-electrolysis achieves higher system exergy efficiencies (85.0 % vs. 76.7 % in steam electrolysis). While steam electrolysis is more efficient at low pressures, co-electrolysis outperforms at high pressures due to increased in-situ methanation, reaching up to 59.2 % yield at 23 bar. This reduced cell voltage and specific power demand but also current density, revealing a reactor size-efficiency tradeoff for fixed methane output. Methane formation in co-electrolysis alters the internal recycle ratio needed to maintain the reducing gas inlet composition, which affects the isothermal operational voltage and reduces freshwater and specific power demand of compressors and evaporators. These findings demonstrate the potential of SOECs especially in co-electrolysis operation to achieve high efficiencies for PtM and its thermodynamic constraints at the system level.