Waste-heat-integrated renewable ammonia production: Thermodynamic limits, scale effects, and technoeconomic optimisation

Renewable-powered green ammonia production is constrained by substantial waste-heat losses, strong nonlinear subsystem interactions, and scale-dependent economic penalties that limit its efficiency and viability. This study quantifies how integrating vapour absorption refrigeration cycle–based waste-heat recovery reshapes the thermodynamic, economic, and environmental optima of green ammonia production systems across plant capacities ranging from 10 to 50 MW. Process modelling and data-driven optimisation show that ammonia production is governed primarily by electrical input, whereas refrigeration capacity, revenue generation, and greenhouse-gas mitigation depend on the interplay between recoverable waste heat and refrigeration-cycle performance. When operated under capacity-specific optimal conditions, waste-heat recovery enables more than a 200 % increase in total system-level avoided greenhouse-gas emissions between 10 and 50 MW, relative to a system without waste-heat recovery. Energy and exergy efficiencies remain nearly constant, varying by less than 3 %. The analysis further identifies a pronounced scale effect in the levelised cost of ammonia. Increasing plant capacity reduces the LCOA by approximately 40 % up to 25 MW, beyond which the addition of parallel production units introduces a temporary cost penalty. Continued scaling offsets this penalty, yielding a further approximately 45 % reduction at 50 MW. These results demonstrate that optimal system performance cannot be inferred from monotonic scaling assumptions; rather, it depends on discrete capacity transitions and waste-heat utilisation strategies. A machine-learning surrogate coupled with multi-objective optimisation enables rapid identification of these optimal operating regimes while preserving thermodynamic consistency. Overall, the results establish waste-heat recovery as a scale-sensitive design lever for achieving economically viable and low-carbon green ammonia production.