Abstract
Green hydrogen and green ammonia are leading carbon-free energy carriers for decarbonizing hard-to-abate sectors. Conventional evaluation based on round-trip efficiency (RTE) consistently favors hydrogen but fail to account for safety. This study introduces Hazard-Adjusted Round-Trip Efficiency (HA-RTE) metric, which integrates energy efficiency with quantified safety penalties derived from a normalized Extended Inherent Safety Index. Eight grid-to-grid Power-to-X-to-Power pathways are assessed by pairing green hydrogen and green ammonia with gas turbines, internal combustion engines, solid oxide fuel cells, and alkaline fuel cells. Hydrogen pathways exhibit higher baseline RTE, reaching 34.10% in H2-SOFC systems; however, they are associated with elevated chemical and process hazards, resulting in substantial efficiency penalties. After hazard adjustment, ammonia pathways retain more usable performance, with NH3-SOFC achieving the highest HA-RTE at 20.12%, surpassing H2-SOFC at 18.08%. Sensitivity analyses confirm that increasing chemical or process hazard weighting and maximum penalty severity disproportionately penalize hydrogen pathways. The results demonstrate that HA-RTE provides a more realistic and decision-relevant comparison of energy carriers than RTE alone and highlights ammonia as a safer and more robust option for large-scale energy deployment.
