Recent inventories on the climate effects of aviation show that non-CO2 effects are responsible for about two-thirds of the total radiative forcing, and that the radiation effect of long-lived contrails makes the greatest contribution to the non-CO2 effects. Future developments towards more environmentally friendly air transport must therefore also take contrail formation into account in new propulsion concepts.
A propulsion concept, especially for short distances, is energy generation with fuel cells. These have the advantage that only water vapour is produced as exhaust gas. This means that all environmental and climate impacts due to CO2, nitrogen oxides, soot and other aerosols are eliminated. However, contrail formation is still possible and its effect on the climate must therefore be investigated.
The theory of contrail formation for fuel cell propulsion developed at the PA now shows that due to the relatively high emission of water vapour at a moderate exhaust gas temperature, the formation of contrails relative to kerosene propulsion is significantly increased; at winterly temperatures, there can even be contrails on the ground and the formation of contrails is unavoidable even at moderate altitudes, where it is not yet possible with kerosene propulsion.
This sounds daunting at first, but it's actually not a problem at all. The additional contrails of fuel cell aircraft are short-lived and therefore not climate-effective. Only where long-lasting climate-effective contrails are produced with kerosene driven engines, such contrails are also possible with fuel cells. However, the climate effect of a single contrail is lower with pure hydrogen operation than with kerosene, as only water vapour and no soot is emitted. There are no condensation nuclei from soot. This reduces the radiative effects and the life duration of the contrails.
From a climate point of view, flying with fuel cells is therefore to be welcomed.
Contrails over the island Juist (CC BY-NC-ND 3.0 Klaus Gierens (DLR))
Reference: Gierens, K., 2021: Theory of Contrail Formation for Fuel Cells. Aerospace, 8, 164. doi:10.3390/aerospace8060164.