Patrick Meyer

A major challenge for aircraft fuel cell propulsion systems is to ensure that the air properties on the cathode side remain within a narrow, suitable envelope throughout the flight. The components must maintain almost constant temperature, pressure and humidity levels under widely varying ambient conditions. The choice of components must take into account the aviation-specific requirements for weight and waste heat. In this numerical study, we investigate a novel cathode air supply system for a hydrogen fuel cell propulsion system which replaces the state-of-the-art electrical components used to drive the compressor in the cathode air supply system with a hydrogen-fuelled micro gas turbine. Previous studies have shown the potential of waste heat and overall cathode gas path size reduction but the off-design performance of such system is yet to be investigated. Hence, based on realistic regional aircraft flight missions and realistic atmospheric conditions, we investigate the off-design performance of the propulsion system. Therefore, a constant mass flow algorithm along cathode and gas turbine gas paths is developed and presented. Next, earth observation data are used to determine realistic boundary conditions and air contamination. Based on these data, the possible contaminant ingestion of the fuel cell is evaluated to allow for future sizing of filters for robust operation. Furthermore, the effects of realistic ambient conditions on the thermodynamic cycle yield important information about necessary revisions of the cycle design point.

Hydrogen fuel-cell-powered all-electric aircraft are promising for decarbonizing short-range aviation, but the substantial low-temperature waste heat demands a compact thermal management system (TMS). This study presents a methodological framework for the integrated co-design of the TMS and powertrain using multi-objective optimization and holistic mission-level analysis to identify optimal TMS designs and operating strategies. Changes in TMS net drag translate into changes in required aircraft thrust, while changes in powertrain, TMS, and fuel mass affect the available payload under a constant maximum take-off mass assumption. This iterative process yields performance metrics across TMS cooling architectures (parallel or series), heat exchanger mass-drag characteristics, coolant temperature targets (50, 70, or 90 °C), and installation objectives (minimizing mass or ram-air duct length). The optimal design is a parallel cooling architecture that balances mass-specific heat rejection of 4.77 kW kg⁻¹ at hot-day take-off with drag-specific heat rejection of 1.29 kW N⁻¹ at standard-day cruise. A reduction in coolant temperature at standard-day missions entails no significant performance penalties and could improve the efficiency of electrical components. A shorter ram-air duct significantly decreases the available payload by 630 kg but may facilitate nacelle integration. The findings underscore that holistic TMS-powertrain co-design and optimization is essential for rigorous design of sustainable all-electric aircraft.

The development of novel propulsion systems is essential to achieve the zero-emissions goals in aviation. One promising approach is the electrification of aircraft engines using hydrogen-based polymer electrolyte membrane fuel cells (PEMFC). In addition to the fuel cell stack, the propulsion system includes several subsystems, which determine the mass and volume and thus the feasibility of the architecture. A key subsystem is the cathode air supply, which preconditions the air for efficient and reliable operation. The compressor work required to pressurize the air has a significant impact on the power requirements, efficiency, and mass of the overall system. In addition, the operating range of the compressor influences the possible operating strategy of the fuel cell system. Another crucial subsystem of PEMFC-aircraft is the thermal management system, which manages the heat rejection of all heat sources. The paper is organized into two parts. In this part, Part I, a design approach for the air supply system and its components is presented. The objective is to apply the design method to a reference medium-range aircraft with a variable number of cathode air supply systems. This is an important decision point that influences both the design of the individual components of the cathode air supply system and the aircraft design, and thus the performance during the entire flight mission. The design points and boundary conditions are derived from an overall system simulation. Based on this, the compressor, turbine, and thermal management system are designed for the identified design points.

The use of polymer electrolyte membrane fuel cells (PEMFC) to generate propulsion power for future aircraft represents a promising approach to reducing carbon emissions caused by the aviation sector. The design of a PEMFC system requires a detailed consideration of the individual subsystems, such as the air supply and thermal management system (TMS). These subsystems have a significant impact on performance, parasitic power requirements, and system mass. For this reason, a comprehensive system analysis is required to evaluate the effects on aircraft performance for the entire flight mission. The first objective of this part, Part II, is the integration of the components designed previously into the overall system calculation to cover interdependencies. For this purpose, the off-design performance of the air supply compressor, turbine, and thermal management is considered for a flight mission analysis on a standard and hot day scenario. Thereby, the effects on the propulsion power demand of the aircraft due to component masses, parasitic powers, and additional drag caused by the ram air heat exchanger are taken into account. The overall design process is iterative, as updated aircraft power demand affects the boundary conditions for the component design. The second objective is to evaluate the influences and sensitivities of different operating strategies, power densities, as well as heat exchanger pressure losses on key system parameters such as fuel consumption, waste heat, and system mass. In addition, a weighted mission-specific efficiency is defined in order to facilitate a fast, yet application-oriented assessment of turbo component designs.

The design point (DP) selection of a novel gas turbine cathode air supply system (GTCS) for a fuel cell-based aircraft propulsion system is investigated. Previous studies were based solely on on-design considerations regarding the DP selection which led to substantial violations of turbine inlet temperature and component limits during subsequent mission analysis. Therefore, the need for an in-depth analysis taking into account the off-design performance is obvious. In this study, we take into consideration a broad design parameter space of the GTCS based on fuel cell design operating pressure, air excess ratio and turbine inlet temperature. For these design parameters, on-design calculations are executed in a full factorial design of experiments for a single-spool GTCS. Then, off-design analyses of each DP are done in crucial missions such as the reference, design, maximum power, hot day and maximum power hot day. Based on the off design results, the design parameter space is narrowed down to a design envelope from which a final design point is chosen. Next, the operating envelope of this design is investigated in more detail to gain further insight into the feasible combinations of power settings and corresponding operating pressure of the fuel cell stack. Finally, the process is repeated for a 2-spool design. Design points were identified for both a single-spool and a 2-spool system. The latter however yields improvements regarding fuel consumption (-1.8%) due to an improved GTCS efficiency (+4.9%) and component limits such as the surge margin of the low and high pressure compressor.

The development of novel propulsion systems is essential to achieve the zero-emissions goals in aviation. One promising approach is the electrification of aircraft engines using hydrogen-based polymer electrolyte membrane fuel cells (PEMFC). In addition to the fuel cell stack, the propulsion system includes several subsystems, which determine the mass and volume and thus the feasibility of the architecture. A key subsystem is the cathode air supply, which preconditions the air for efficient and reliable operation. The compressor work required to pressurize the air has a significant impact on the power requirements, efficiency, and mass of the overall system. In addition, the operating range of the compressor influences the possible operating strategy of the fuel cell system. Another crucial subsystem of PEMFC-aircraft is the thermal management system, which manages the heat rejection of all heat sources in the fuel cell system. The paper is organized in two parts. In this part, Part I, a design approach for the air supply system and its components is presented. The objective is to apply the design method to a reference medium-range aircraft with a variable number of cathode air supply systems. This is an important decision point that influences both the design of the individual components of the cathode air supply system and the aircraft design, and thus the performance during the entire flight mission. The design points and boundary conditions are derived from an overall system simulation. Based on this, the compressor, turbine, and thermal management system are designed for the identified design points. In the companion paper, Part II, the compressor and turbine performance maps, as well as the key characteristics of the thermal management design are embedded into the overall system simulation tool in order to evaluate the entire flight mission and cover the impact of component design on the aircraft performance.

The use of polymer electrolyte membrane fuel cells (PEMFC) to generate propulsion power for future aircraft represents a promising approach to reduce carbon emissions caused by the aviation sector. The design of a PEMFC system requires a detailed consideration of the individual subsystems, such as the air supply and thermal management system. These subsystems have a significant impact on performance, parasitic power requirements, and system mass. For this reason, a comprehensive system analysis is required to evaluate the effects on aircraft performance for the entire flight mission. In Part I of this two-part paper, an initial design workflow is presented, focusing on the design point selection and the design of the turbo components and thermal management. The first objective of this part, Part II, is the integration of the components designed previously into the overall system calculation to cover interdependencies. For this purpose, the off-design performance of the air supply compressor, turbine, and thermal management is considered for a flight mission analysis on a standard and hot day scenario. Thereby, the effects on the propulsion power demand of the aircraft due to component masses, parasitic powers, and additional drag caused by the ram air heat exchanger are taken into account. The overall design process is iterative, as updated aircraft power demand affects the boundary conditions for the component design. The second objective of this study is to evaluate the influences and sensitivities of different operating strategies, power densities, as well as heat exchanger pressure losses on key system parameters such as fuel consumption, waste heat, and system mass. In addition, a weighted mission-specific efficiency is defined in order to facilitate a fast, yet application-oriented assessment of turbo component designs.

This paper investigates the cathode air supply system of a medium-range fuel cell-powered aircraft. The main focus is on determining a suitable system architecture. Based on the literature, promising humidification and cooling strategies for cathode gas preconditioning are identified and evaluated. System configurations with membrane and spray humidification as well as intercooling between compressor stages and wet compression are calculated using a thermodynamic cycle calculation model. By analysing the design parameter spaces of the cathode air supply systems, the different configurations are compared in terms of their influence on key system parameters such as specific fuel and parasitic power consumption or heat exchanger size to find the most promising architecture. Further, the operating limits of the systems at different operating pressures and temperatures are determined. The second area of focus is the identification of a suitable system design point. It is discussed that the most critical operating points of the individual components are located at different flight conditions. Therefore, off-design performance is taken into account in order to design the entire system. A model based on the constant mass flow method is introduced to calculate steady state operating points for the system using membrane humidification. It can be seen that the turbo components have their design point at top of climb operating point, while the heat exchanger has to be designed for the maximum waste heat at take-off and the humidifier for the most critical operating point in terms of membrane water content at cruise.