To make an all-electric aircraft possible, both high power densities and efficiencies are needed. However, particularly high demands are also placed on the thermal management system. Often, the electric motor and cooling system are considered without co-optimization. Particularly in the case of electric motors with conductors directly cooled by a liquid, there is great potential for optimization, since the temperature-dependent Joule losses determine the largest part of the losses. This publication shows the main influencing parameters for the electric motor and cooling system: coolant speed and winding temperature. In addition, the influence of the cooling system control during a flight mission is demonstrated and its potential in mass reduction is quantified. It could be shown that with a low utilized electric motor the maximum winding temperature of 130 $°$C is beneficial, the cooling system should work in almost all operation points in its sized operation and the mass of the heat exchanger and pump is negligible compared to the mass of the electric motor and energy storage.

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 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.

Fully electric propulsion systems integrating hydrogen-powered fuel cells and batteries are promising options to reduce the overall climate impact of regional aircraft. However, the increase in low-temperature heat sources aboard the aircraft calls for advanced thermal management system solutions. To address this challenge, this study presents a sizing methodology for ram air heat exchangers in the nacelle-integrated cooling loop of an all-electric regional aircraft based on the ATR-72 platform. Different discretization schemes are compared to identify an optimal sizing method. The results highlight the simplicity and efficiency of the 0D ε-NTU model. Geometric design variables are optimized with respect to drag and mass during a hot-day take-off. The resulting Pareto front reveals a tendency for low airflow outlet temperatures and large diffuser area ratios to result in lightweight designs but in turn, induce high drag and require a large installation space. Comparative analyses of specific optimal ram air duct designs and equivalent skin heat exchangers demonstrate the potential of a second heat sink over a flight mission. The limited heat transfer area of the skin heat exchanger proves insufficient for hot-day take-off and climb but offers advantages during cruise and descent thanks to the reduced drag.

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.