Fuel cell propulsion with effective contrail management can drastically reduce the climate impact of aviation. There are challenges, i.a. as fuel cell systems are comparatively heavy, and hydrogen still rather expensive. Possibly high energy efficiency is therefore important to implement this technology. Converting hydrogen exergy to work aboard the aircraft could (1) supply auxiliary useful energy and (2) absorb parts of fuel cell system waste heat already within the aircraft. Utilising liquid hydrogen exergy enables a number of synergetic effects all contributing to decrease fuel consumption if exergy utilisation system complexity is tolerated and the system is sufficiently light.
