Ice crystal number parametrization for contrails generated by fuel cell propelled aircraft

Abstract

Current aircraft are producing a multitude of emissions. Apart from the greenhouse gas carbon dioxide (CO2), the exhausts of aircraft can contain nitrous oxides (NOx), soot and sulfur particles. In order to reduce the climate footprint of aviation, propulsive innovations are not only needed but inevitable. Thus, hydrogen propulsion, either via hydrogen combustion or through the utilization of fuel cell propulsion, has become a key technology. Although the usage of fuel cells might be mostly emission-free, they produce gaseous water in the exhaust. This water vapor, upon mixing with the cold ambient surrounding in the plume, can form water droplets and subsequently freeze into ice crystals. The ice crystals might persist in the atmosphere and can thus lead to climate-warming condensation trails. Consequently, the ice crystal number of fuel cell powered aircraft needs to be simulated in dependence on the respective exhaust conditions, to assess the attributed climate impact. Hence, the following thesis will initially derive the exhaust conditions for fuel cell propulsion systems. Through the subsequent use of a simulation model incorporating microphysics, exhaust parameters and ambient parameters can be utilized to simulate the initial formation of liquid water droplets and the ultimate freezing into ice crystals within the aircraft plume. The results show that increasing ambient and exhaust temperatures leads to the most substantial decline in ice crystal numbers. Additionally, the decrease in aerosol concentration, as well as the exhaust specific humidity, results in lower ice crystal numbers. Optimizing these parameters can consequently reduce the ice crystal formation and thus contrail formation in the plume by several orders of magnitude. The findings can guide fuel cell development and flight envelope planning to achieve more sustainable aviation in the future.