Comprehensive Dynamic Modelling and Control Strategy Development for Operational Flexibility of a Solid Oxide Electrolyser System

A solid oxide electrolyser coupled to photovoltaic (PV) power operates through production, hot standby and night states. A Modelica model of the balance of plant of the 25 kW PROMETEO pilot is presented, with ten control loops and a hysteresis-based state machine dispatched by the PV power of a grid-connected plant. The stack model, validated on 5 kW data from the FBK laboratories, is scaled up to 25 kW. The constant-voltage, temperature-tracking strategy is simulated over clear-sky, intermittent-cloud and satellite-derived PV days. At 1.32 V/cell, the stack heat-balance point, the sweep air rests at its minimum and the current is tracked through the inlet temperatures alone, giving 14.1 kg of hydrogen over two days at 44 kWh/kg, 65% covered by the PV. The feasible voltage window is narrow: below 1.30 V/cell the stack exceeds 780 °C and above 1.34 V/cell the current becomes steam-limited, requiring a current or steam protection. The strategy is robust to the stack thermal capacity, the PV field size and a ±20% change in the area-specific resistance, which shifts the operating temperature by about 20 K. The sweep-air loop must be disabled outside production; otherwise, blower and heaters saturate and the stack cools.