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This study provides a direct comparison of hydrogen crossover in PEM (Nafion 117) and alkaline water electrolysis (Zirfon) at a temperature of 60 • C applying state-of-the-art separating unit materials. To this end, occurring crossover mechanisms are described first, before experimental data of the anodic hydrogen content are shown in dependence of current density, system pressure and process management strategy. The results suggest that permeation in PEM electrolyzers is mainly governed by diffusion due to a supersaturated concentration of dissolved hydrogen within the catalyst layer, showing a share of 98% of the total permeation flux at 1 A cm −2 and atmospheric pressure. Permeation in alkaline electrolyzers also exhibits a significant influence of supersaturation, but the overall crossover is mainly influenced by mixing the electrolyte cycles, which makes up a share of 90% at 0.7 A cm −2 and 1 bar. Generally it becomes evident that hydrogen permeation across the separating unit is more than one order of magnitude smaller in alkaline electrolysis, which is mainly a consequence of the significantly lower hydrogen solubility in concentrated KOH electrolyte. Finally, this study concludes with an assessment of the impact of separating unit thickness and provides mitigation strategies to reduce hydrogen crossover.

Ambitious targets to reduce emissions caused by aviation in the light of an expected ongoing rise of the air transport demand in the future drive the research of propulsion systems with lower CO 2 emissions. Regional hybrid electric aircraft (HEA) powered by conventional gas turbines and battery powered electric motors are investigated to test hybrid propulsion operation strategies. Especially the role of the battery within environmentally friendly concepts with significantly reduced carbon footprint is analyzed. Thus, a new simulation approach for HEA is introduced. The main findings underline the importance of choosing the right power-to-energy-ratio of a battery according to the flight mission. The gravimetric energy and power density of the electric storages determine the technologically feasibility of hybrid concepts. Cost competitive HEA configurations are found, but do not promise the targeted CO 2 emission savings, when the well-to-wheel system is regarded with its actual costs. Sensitivity studies are used to determine external levers that favor the profitability of HEA.

The thermodynamic and operational boundaries to store electrical energy chemically are evaluated in this contribution. Methanol is considered as a candidate for chemical energy storage. The production of methanol from exhaust CO2 could be one way to recyle CO2 and lower the global CO2 emissions. Energetic analysis reveals that exergy losses are most severe in the parts of the system when electrical energy is converted to chemical (electrolysis) and when chemical energy is converted to electrical (power generation). In methanol production, the exergetic efficiency is 83.1%, when the chemical exergy of hydrogen and methanol, the exergy of the power input and the released heat are taken into consideration. The exergetic efficiency of the overall energy conversion-storage system including methanol as storage medium was evaluated to be between 16.2 and 20.0% depending on the applied conversion technology. Methanol is suitable not only as stationary energy storage, but it could also be used as fuel for transportation. The energy storage system with hydrogen as storage medium shows higher exergetic efficiency than the methanol route. However, the storage of hydrogen is clearly more complex and cost-intensive.