| Summary: | Solar fuel generation from thermochemical H<sub>2</sub>O or CO<sub>2</sub> splitting is a promising and attractive approach for harvesting fuel without CO<sub>2</sub> emissions. Yet, low conversion and high reaction temperature restrict its application. One method of increasing conversion at a lower temperature is to implement oxygen permeable membranes (OPM) into a membrane reactor configuration. This allows for the selective separation of generated oxygen and causes a forward shift in the equilibrium of H<sub>2</sub>O or CO<sub>2</sub> splitting reactions. In this research, solar-driven fuel production via H<sub>2</sub>O or CO<sub>2</sub> splitting with an OPM reactor is modeled in isothermal operation, with an emphasis on the calculation of the theoretical thermodynamic efficiency of the system. In addition to the energy required for the high temperature of the reaction, the energy required for maintaining low oxygen permeate pressure for oxygen removal has a large influence on the overall thermodynamic efficiency. The theoretical first-law thermodynamic efficiency is calculated using separation exergy, an electrochemical O<sub>2</sub> pump, and a vacuum pump, which shows a maximum efficiency of 63.8%, 61.7%, and 8.00% for H<sub>2</sub>O splitting, respectively, and 63.6%, 61.5%, and 16.7% for CO<sub>2</sub> splitting, respectively, in a temperature range of 800 <b>°</b>C to 2000 °C. The theoretical second-law thermodynamic efficiency is 55.7% and 65.7% for both H<sub>2</sub>O splitting and CO<sub>2</sub> splitting at 2000 °C. An efficient O<sub>2</sub> separation method is extremely crucial to achieve high thermodynamic efficiency, especially in the separation efficiency range of 0–20% and in relatively low reaction temperatures. This research is also applicable in other isothermal H<sub>2</sub>O or CO<sub>2</sub> splitting systems (e.g., chemical cycling) due to similar thermodynamics.
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