T–S diagram efficiency analysis of two-step thermochemical cycles for solar water splitting under various process conditions

• Representation of two-step thermochemical cycles for water splitting in comprehensive T–S diagrams.
• Influence of process conditions (pressure, temperature, conversion) on the thermodynamic efficiency is shown.
• Identification of entropy requirements of redox materials when aiming at certain process conditions.
• Results show efficiency trends which provide valuable insight for conceptual process layout.

Temperature–entropy diagrams combined with a pinch point analysis are introduced, providing a vivid and detailed tool to analyse two-step thermochemical water-splitting processes. The impacts of different temperature and pressure conditions, as well as different water conversion rates, are studied. Further, requirements regarding the entropy change in the redox material to reach desired process conditions are presented. This paper provides insights on how future research shall be oriented to achieve optimal conditions for this challenging but also very promising class of reactions.