Holistic optimization of a fuel cell propulsion system, for electric aircraft applications

Abstract

Current aircraft propulsion technologies have undergone significant improvement, resulting in a reduction in emissions. In order to achieve a significant reduction in emissions, however, new advanced propulsion systems need to be developed, such as fuel cells. Hydrogen fuel cell propulsion systems have only water vapour emissions, with no carbon dioxide or nitrogen oxide emissions. A fuel cell propulsion system consists of a stack where the electrochemical reaction converts chemical energy stored in the fuel into electrical power. The three subsystems necessary for the stack to function are: the air subsystem that provides air to the stack, the hydrogen subsystem that provides hydrogen, and the cooling subsystem for thermal management of the stack. The performance of the stack, being sensitive to its reactant conditions, requires careful control such that during flight, constant power is provided to the propeller shaft and other power-consuming components. These components include the air compressor and the coolant pump, and their additional power consumption is referred to as parasitic power. The operating point of the stack and its subsystems must therefore be carefully determined.

In this thesis, a design and sizing methodology accounting for off-design performance of components and system-level power demand was applied. The aim was to determine design parameters that allow optimization of total system mass and parasitic power. Rather than optimizing individual components for maximum performance, they are designed for overall system performance. A steady-state system model was developed using component-level performance parameters. This allows each component to be represented by simplified behaviour parameters, enabling system-level analysis without requiring full geometric design details at early stages.

The methodology considers multiple flight conditions representing different operational phases. System performance varies significantly across these conditions, requiring balanced design choices across subsystems. A trade-off exists between efficiency, mass flow requirements, and thermal management constraints, which strongly influences system sizing and performance.

Several system configurations were evaluated using a parametric optimization approach. The results show that component interactions strongly influence overall system performance, and that optimal design choices arise from system-level trade-offs rather than isolated component optimization. In particular, thermal management requirements and compressor power demand play a dominant role in determining feasible configurations.

The results further indicate that fuel cell systems for aircraft applications require different design priorities compared to other applications, due to strong coupling between thermal loads, air supply requirements, and system mass. The study demonstrates the importance of integrated system-level optimization for the design of hydrogen fuel cell propulsion systems in aviation and highlights key trade-offs that must be considered in future development.