Prevention of Contrail Formation in Hydrogen Fuel Cell Aircraft ^†

Contrail emissions are aviation’s largest non-CO₂ contribution to global climate change. According to the Schmidt–Appleman criterion, potential future aircraft propulsion systems may enhance contrail formation relative to conventional engines through three mechanisms: (1) increased overall efficiency, (2) the use of hydrogen as fuel, and (3) external cooling in low-temperature fuel cell propulsion systems, which is the most critical factor. This paper presents the thermodynamic background and a system concept for contrail prevention applicable to conventional gas turbines, hydrogen combustion, and fuel cell propulsion systems. First, it is shown that fuel cell propulsion and hydrogen combustion exhibit equivalent thermodynamic contrail propensity when fuel cell exhaust is mixed with cooling air, analogous to core–bypass mixing in a conventional turbofan engines. Second, contrail mitigation via controlled condensation of exhaust water vapor is analyzed. It is demonstrated that the required cooling for LT-PEM fuel cell systems is 3–5 times lower than for turbofan engines, due to the already extensive thermal management in fuel cells. Since contrail avoidance is only necessary in ice supersaturated regions, a control scheme is proposed that limits condensation to the minimum required amount of water, thereby significantly reducing the overall drag impact. Avoiding contrail formation could provide a substantial climate benefit for future propulsion architectures.