Abstract
Multi-stack proton exchange membrane (PEM) water electrolysis systems are increasingly deployed to improve the scalability and flexibility of green hydrogen production. However, sharing balance-of-plant equipment introduces coupling between stacks, and differences in stack performance increase the complexity of plantwide operation. In particular, non-identical efficiencies and safety constraints, such as hydrogen-to-oxygen (HTO) ratio limits, can render single-stack or equal-power-sharing control strategies suboptimal. In this work, the steady-state optimal operating regime of a two-stack PEM electrolysis system is characterized using a plantwide optimization approach and active constraint mapping over a range of system power loads. Performance differences between the stacks are represented through variations in Faraday efficiency to emulate simplified degradation. For identical stacks, the system behaves similarly to a single large electrolyzer, where equal power distribution is optimal, and the active constraint regions closely resemble those of a single-stack system. As the stack performance differences increase, the optimal power distribution becomes asymmetric, with the more efficient stack preferentially loaded. However, HTO safety constraints in the degraded stack may limit the utilization of the more efficient stack and introduce additional active constraint regions, resulting in more complex operating regimes.