Novel two-phase based cooling systems for fuel cell electric aircraft

Hydrogen fuel cell powered propulsion is a promising option for shifting air travel to non-fossil fuel alternatives. However megawatt-scale fuel cells require large heat exchange surfaces for cooling, creating new challenges for efficient aircraft design. New, bespoke fuel cell cooling systems for aviation are required. This paper presents a novel cooling system for a fuel cell electric aircraft based on two-phase cooling with the natural refrigerant methanol. Three novel cooling systems are presented and evaluated by means of system simulation. The two-phase cooling systems comprise a pumped two-phase cooling circuit which is extended by a high-temperature refrigeration cycle. The effects inside the ram air channel and additional drags on the aircraft resulting from the cooling system as well as the influence of the Meredith-Ram-Jet-Effect are analyzed. The novel two-phase based cooling systems are compared by means of system simulation. The comparison utilizes steady-state operating points as well as an example of a dynamic flight profile. Additionally, the usage of a variable air intake flap is discussed and evaluated. The variable intake flap ensures sufficient fuel cell cooling for high loads at take-off, and efficient cooling during cruise operation. The resulting best system architecture is a pumped two-phase cooling circuit extended by a high-temperature refrigeration cycle with coupled heat exchangers. This system ensures a critical fuel cell temperature for high heat production is not exceeded in take-off, and that a high cooling system efficiency is achieved during cruise operation while maintaining the 
fuel cell below 80◦C. This system is evaluated with the dynamic flight profile and the influence of the Meredith Ram-Jet-Effect is discussed. The thermodynamic process of the air inside the ram air channel is shown used to and describe how the Meredith-Ram-Jet-Effect reduces the cooling system drag. The core drag can be reduced by 65% during cruise operation.