Favorable Start-Up behavior of polymer electrolyte membrane water electrolyzers.

• Proposition of heat-up efficiency to evaluate start-up procedures energetically. • Thermal parameters for electrolysis systems with up to 1 MW el are presented. • Efficiency when only using heating is always lower than partial-load heat-up. • Introduction of a new dimensionless heat-up number for electrolyzers. • Dimensionless heat-up number enables general estimation of time constants. Dynamically-operated water electrolyzers enable the production of green hydrogen for cross-sector applications while simultaneously stabilizing power grids. In this study, the start-up phase of polymer electrolyte membrane (PEM) water electrolyzers is investigated in the context of intermittent renewable energy sources. During the start-up of the electrolysis system, the temperature increases, which directly influences hydrogen production efficiency. Experiments on a 100 kW el electrolyzer, combined with simulations of electrolyzers with up to 1 MW el , were used to analyze the start-up phase and assess its implications for operators and system designers. It is shown that part-load start-up at intermediate cell voltages of 1.80 V yields the highest efficiencies of 74.0 % LHV compared to heat-up using resistive electrical heating elements, which reaches maximum efficiencies of 60.9 % LHV. The results further indicate that large-scale electrolyzers with electrical heaters may serve as flexible sinks in electrical grids for durations of up to 15 min. [ABSTRACT FROM AUTHOR]