This paper presents an overview of the most relevant fuel cell types and identifies the most promising options for application in propulsion systems for commercial electrified aviation. The general design, operating principles and main characteristics of polymer electrolyte membrane, alkaline, direct methanol, phosphoric acid, molten carbonate and solid oxide fuel cells are described. Evaluation criteria are derived from aviation-specific requirements for the application of fuel cells in electrified aircraft. Based on these criteria, the presented fuel cell types are evaluated by means of a weighted point rating. The results of this evaluation reveal the high potential for application of solid oxide, low-temperature and high-temperature polymer electrolyte membrane fuel cells. Design challenges of all fuel cell types are being emphasised, for instance, concerning cold start, cooling and supply of pressurised air.

This paper presents an overview of the most relevant fuel cell types and identifies the most promising options for application in propulsion systems for commercial electrified aviation. The general design, operating principles and main characteristics of polymer electrolyte membrane, alkaline, direct methanol, phosphoric acid, molten carbonate and solid oxide fuel cells are described. Evaluation criteria are derived from aviation-specific requirements for the application of fuel cells in electrified aircraft. Based on these criteria, the presented fuel cell types are evaluated by means of a weighted point rating. The results of this evaluation reveal the high potential for application of solid oxide, low temperature and high temperature polymer electrolyte membrane fuel cells. Design challenges of all fuel cell types are being emphasised, for instance, concerning cold start, cooling and supply of pressurised air.

This paper identifies potential weaknesses and challenges of hydrogen fuel cell systems as main energy provider for electrified aircraft propulsion and presents potential solutions. The general design, operating principles and main characteristics of hydrogen-fuelled low-temperature polymer electrolyte membrane fuel cell systems (PEMFCSs) are described. The safety assessment process in aviation according to Aerospace Recommended Practices ARP4754A and selected methods according to ARP4761 are introduced. The functions of fuel cell systems in electrified aircraft powertrains are analysed in functional structure trees on aircraft, powertrain and fuel cell system level. By means of a Functional Hazard Analysis (FHA), potential malfunctions and their effects are investigated and their severity is evaluated. Critical failure modes are identified and requirements for acceptable failure probabilities are stipulated. Solution options to mitigate failure effects are stated. The results of the mentioned analyses reveal design challenges associated with the application of fuel cell systems in electrified aircraft propulsion, for instance, concerning functional independence as well as solutions for cold start conditions, heat transfer and lightweight design. Furthermore, safety challenges arise due to the utilisation of cryogenic hydrogen as fuel and the high amount of electric energy.

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