Control concept for solid oxide electrolysis reactors to promote highly transient operation in modular plants

Solid Oxide Electrolysis Cells (SOECs) offer the lowest specific electrical energy demand among electrolysis technologies, making them highly suitable for large-scale hydrogen production, where electricity accounts for 70 %–85 % of the levelized cost of hydrogen. To comply with guarantees of origin for green hydrogen, SOEC systems must operate reliably in power-following mode with fluctuating renewable energy sources (RES). However, transient operation induces thermal gradients within SOEC stacks, accelerating degradation and increasing the risk of premature failure.

This study proposes a dynamic control concept that enables rapid power modulation with limited thermal stress, based on an experimentally validated multi-stack SOEC reactor model. A large-scale SOEC plant is considered, consisting of multiple modules, each comprising a multi-stack reactor and independent balance-of-plant components. The module-level power control employs a PI controller, augmented with model-based current slew-rate limit correlations and feed-forward step changes between hot standby and thermoneutral operation. For a moderate thermal gradient limit of ±5 K min^-1, optimised control parameters enables transitions from hot standby to 80 % nominal power in 35 s and to 100 % in 3 min – approximately six times faster than conventional linear current ramps. The control concept is further applied to a modular SOEC plant under a real wind park power profile. Two key factors influencing power-following capability are identified: the number of modules and the lower power limit of an individual module’s operating range ($P_{\mathrm{mod,low}}$). The proposed control concept improves power-following capability by reducing power mismatch by 45 % and significantly decreases the required module count, enhancing both system efficiency and scalability.