Efficiency optimization of power-to-ammonia systems based on pressurized solid oxide electrolysis

Solid oxide electrolysis cells (SOECs) enable efficient hydrogen production by utilizing waste heat for steam evaporation, providing strong synergies with exothermic ammonia synthesis. However, there is a need for efficient systems designs to couple these processes for renewable ammonia production. Here, the influence of operating pressure and system configuration on power-to-ammonia (PtA) efficiency is investigated using an experimentally validated model of electrolyte-supported cell (ESC)-based stacks. Main findings were: (1) PtA efficiencies increase with SOEC pressurization in systems without sweep air, reaching 72% at 8 bar, while sweep air systems peak at ∼69% at 2 bar due to air compression demand at elevated pressures. (2) Part-load operation in modular plants reduces efficiency, dropping to 45% at 10% load because of module hot standby consumption. (3) Waste heat from the Haber–Bosch process combined with SOEC off-gas recovery, meets steam demand in most cases, with up to 62% supplied by the SOEC itself. (4) At high pressure without sweep air, electrical steam generation is required due to reduced off-gas heat and higher boiling points. In such a case, increasing the recirculation ratio can increase system efficiency by reducing evaporator load.