Climate impact of fuel cell induced liquid water droplet emissions

Abstract

Air traffic is expected to continue growing exponentially in the coming years. Associated kerosene-based aircraft emissions (e.g. CO2, water vapor, NOx, soot, sulfate) affect the global climate in several ways, leading to a positive climate impact based on radiative forcings. To make aviation more sustainable various alternative propulsion systems are currently under development. One promising concept is the introduction of fuel cell powered aircraft for short-haul flights. Fuel cells produce water which could be released to the atmosphere either in the form of water vapor and/or liquid droplets. We study the evolution of the liquid droplets, the number of droplets freezing and the radiative impact of the ice crystals. Multiple box model studies have been performed to evaluate under which conditions emitted droplets are able to freeze and persist in the plume. For this purpose, the fraction of droplets that freeze is calculated dependent on the thermodynamic evolution of the plume, which is again dependent on the plume dilution, and dependent on the growth of the droplets. We estimate the freezing of droplets and the survival of ice crystals within the vortex phase in the climate model ECHAM5-CCMod. The emission of a large number of small droplets may prohibit further formation of droplets but may also lead to low fractions of droplets freezing and to a large ice crystal loss during the vortex phase. First results indicate that the freezing of droplets emitted from fuel cells in the main air traffic areas is only common in the winter months and that freezing displays a large regional variability. We compare ice crystal numbers within the young plume and the resulting climate impact when varying liquid water emissions and the sizes of the emitted droplets.