Flexible design and operation of off-grid green ammonia systems with gravity energy storage under long-term renewable power uncertainty

The conventional ammonia production process heavily depends on fossil fuels, making it urgent to redesign the synthesis process to reduce greenhouse gas emissions and address the challenges of depleting resources. Off-grid ammonia synthesis powered by renewable energy offers a feasible pathway to producing carbon-free ammonia. However, a significant challenge for off-grid green ammonia plants is ensuring the reliable operation of the relatively inflexible ammonia synthesis units under intermittent and unpredictable wind and photovoltaic power conditions. To address this challenge, this study proposes a novel off-grid green ammonia system and a discrete multi-stable flexible control strategy for ammonia synthesis. For the first time, gravity energy storage is integrated into a large-scale green ammonia project to ensure a continuous power supply to the ammonia synthesis reactor under limited flexible operation. The optimal design and operation of the proposed system are modeled as a mixed-integer nonlinear problem, which is reformulated into a linear version through piecewise linearization. Additionally, the stochastic nature of renewable energy generation is fully considered. A scenario generation framework based on Copula function theory and Markov stochastic processes is developed to accommodate the long-term simulation needs of chemical production. The effectiveness of the proposed method is validated through a case study. A sensitivity evaluation is also carried out to analyze the impact of wind and photovoltaic power configurations, load adjustment periods, and system component costs on system revenue and module capacity. This study provides new insights into the system configuration and flexible operation of green ammonia systems and is expected to guide the construction and operation of practical green ammonia plants.