Applications – Transportation | Aviation: Fuel cells

Abstract

Traditional kerosene combustion based propulsion contributes to climate change through direct engine emissions of CO2, water vapor, SOx, NOx and soot, and by the formation in the aircraft plume or the atmosphere of additional climate forcers like condensation trails (contrails) or soot and sulfate-based aerosols. The pure CO2 footprint of fuels depends mainly on the type and origin of the fuel, whereas the non-CO2 emissions depend on the way of oxidation (combustion, fuel cell) as well as on the type of fuel. Hydrogen and Sustainable aviation fuel (SAF) have the potential to reduce their net CO2 footprint strongly, as well as some non-CO2 climate forcers. Fuel cells offer major benefits in electrical efficiency and emissions for hybrid-electric aircrafts. Low temperature Proton Exchange Membrane (PEM) fuel cells are today by far the most used type of PEM fuel cells with a comparably high TRL for automotive applications. High electrical efficiencies are achieved based on Hydrogen as fuel and power densities of 3–4kWel/kg on a stack level are reported for automotive applications. State of the art Solid Oxide Fuel Cell (SOFC) systems were mainly developed for stationary applications, are based on a planar cell geometry and with only a minor focus on gravimetric power density. The benefits of the SOFC are mainly a convenient cooling concept, highest achievable efficiencies in combination with a gas turbine and fuel versatility. Even a comparably simple integrated gas turbine process promises electrical efficiencies of around 60–70% in a hybrid configuration with a SOFC. Recent progress in additive manufacturing technologies for ceramic and metallic materials are in this context a major enabler for lightweight SOFCs. The energy density of lightweight liq. H2-PEM, liq. H2-SOFC and Propane-SOFC are estimated to be gravimetrically 20 times higher and volumetrically 10–20 times higher than today's battery packages. The Solid Oxide, Proton Conducting Ceramic and High Temperature PEM fuel cells are because of their benefits in terms of efficiency or integration compared to the LT-PEM fuel cells, very interesting fuel cell types for aviation applications. Nevertheless, fundamental research in terms of manufacturing (SOFC/grav. Power density), material stability (PCC/cathode stability) and electrochemistry (HT-PEM/cathode overpotential) is required to make these fuel cell types accessible for the aviation sector.