Hydrogen Storage Minimisation under Industrial Flexibility Constraints: A Techno-economic Analysis of Off-grid Green Ammonia Production

Electrifying ammonia production using renewable energy (RE) and water electrolysis is a critical step in the worldwide transition from fossil fuels to alternative energy sources. However, the common requirement that the ammonia reactor operate at a steady production level harms the system’s economic feasibility due to the large hydrogen and battery storage required to overcome RE variability. In this study, we examine the sensitivity of the plant storage capacity requirement to the flexibility of the ammonia reactor. We examine two aspects of ammonia reactor flexibility: ramping rate flexibility and the range of operation (turndown flexibility). We develop a storage dispatch and ammonia reactor scheduling optimization, which computes the minimum storage requirement given a RE generation profile and set of reactor flexibility parameters. We optimize across a sweep of flexibility parameters for two locations in the United States. We find that turndown flexibility is the most important, while ramping flexibility has little effect on the overall storage requirement. Further, we see that seasonal variability in the RE generation profile is the primary driver of high storage capacity requirement. We find that with a turndown flexibility of 60% of the ammonia plants rated capacity, which is understood to be achievable with existing ammonia reactor technology, the storage capacity was reduced by 84 % in one of the locations we examined, which resulted in a 22% decrease in the levelized cost of ammonia with pipe-based hydrogen storage.