This book contains a model-based analysis and optimisation of the Haber-Bosch designs for power-to-ammonia. The work presented helps to answer which Haber-Bosch design is suitable for the flexible operation and production by varying process variables: operational pressure, process feed temperature, process feed composition and feed flow rate. Among several Haber-Bosch design possibilities, two synthesis loops and five three-bed autothermal synthesis reactor systems are considered. The synthesis loops vary in terms of ammonia separation unit allocation, i.e. after and before a synthesis reactor system, and the reactor system configurations differ in inter-stage cooling methods. Besides, the operation and production flexibilities, design variants are also compared in terms of yield. All the Haber-Bosch designs allow operation for a large load range span via multi-variable optimisation and therefore are in principle suitable for power-to-ammonia.