Exergy Efficiency Analysis of a Power-to-Methane System Coupling Water Electrolysis and Sabatier Reaction

Abstract

The seasonal and intermittent features result in the renewable power curtailment. Seasonal power storage is required to utilize the curtailed renewable power. Power-to-methane (PtM) systems coupling solid oxide electrolysis cell (SOEC) and Sabatier reactors offers an efficient and promising pathway to integrate the renewable power with the existing natural gas network. SOEC operates at a higher temperature and yet produce hydrogen or syngas with a controllable H2/CO ratio with higher efficiency compared to low temperature electrolyzers. Besides, with the coupling of the SOEC and the Sabatier reactor, methane can be synthesized in one reactor from renewable energy. In this study, we firstly compared the power-to-hydrogen processes using the alkaline electrolysis cell (AEC), the proton exchange membrane electrolysis cell (PEMEC) and the SOEC. Exergy efficiency analysis is also carried out to distinguish the qualities of the heat, electricity and gaseous fuels. The results show the PtM system using SOEC is more efficient than those using the other electrolysis technologies at high current density due to lower polarization and better heat coupling. For SOEC, H2O/CO2 co-electrolysis coupling with methanation reactor exhibits a slightly lower exergy efficiency at low current density than H2O electrolysis coupling Sabatier reactor, but exceeds at high current density. At the H2O flow rate of 0.036 mol s-1 m-2 and H/C ratio of 5.5, a peak exergy efficiency of 75.66% is achieved at the current density of -5500 A m-2.