Experimental and theoretical evaluation of a 60 kW PEM electrolysis system for flexible dynamic operation

Electrolysis systems based on low temperature PEM cells have fast ramp rate and load-following capability that makes them suitable to provide services to the power grid, whose reliability is hindered by the increasing deployment of dynamic and non-controllable renewable energy sources. Flexible operation of the complete electrolysis unit is influenced by system design and auxiliaries: this work provides an insight on the dynamics of integrated components, aiming at the enhancement of the flexible operation of an industrial-scale PEM elec-trolysis system. A numerical dynamic model of a complete electrolysis unit is implemented with the software Simulink, combining customized dynamic models of the main balance of plant components. Mass and energy balances are solved during variable load operation, while PI-type and on-off controllers are included for system operation control. The electrolysis stack performance depends on semi-empirical polarization curves, reflecting voltage dependance on temperature, pressure, and current density. The solved transients of the device provide original indications for increasing the dynamic performance of the system.A 60 kWe commercial unit is tested to validate the model, assessing its nominal and partial load performance. The experimental data confirm the capability of the system to work at partial load without relevant deviations in the stack temperature and pressure. Despite the lower specific consumption of the stack at partial load, the system shows an increase in the average net specific consumptions when the load decreases toward its minimum (from 67 kWhe/kgH2 above 1 A/cm2 to 140 kWhe/kgH2 at 0.3 A/cm2).The simulation results and the experimental data are in good agreement, confirming the prediction capacities of the model with a good accuracy. Among the different investigated operation strategies, the strongest reduction in average net specific consumption at partial load is given by the control of water flowrate to the stack (110 kWhe/kgH2 at 0.3 A/cm2) or of the hydrogen dryer regeneration (70 kWhe/kgH2 at 0.3 A/cm2). These are the suggested points to be investigated for further improvements of fast-changing load systems.