Electrical propulsion systems for civil transportation aircraft

Hinz, Arne; de Doncker, Rik W. (Thesis advisor); Stumpf, Eike (Thesis advisor)

1. Auflage. - Aachen : E.ON Energy Research Center, RWTH Aachen University (2022)
Book, Dissertation / PhD Thesis

In: E.ON Energy Research Center 109. Ausgabe : PGS, Power Generation and Storage Systems
Page(s)/Article-Nr.: 1 Online-Ressource : Illustrationen, Diagramme

Dissertation, RWTH Aachen University, 2022


Motivation, Goal and Task of the Dissertation: International regulations increasingly constrain, among others, aviation´s greenhouse gas and pollutant emissions. Ultimately, the aviation industry has to achieve climate-neutrality. To accomplish this goal, a variety of options are under investigation. This includes aircraft operation on synthetic fuels, batteries and hydrogen together with enhancing jet engine technology as well as hybrid and full electric propulsion schemes. With focus on electrical drivetrains, this thesis aims to contribute to the implementation of climate-neutral propulsion systems in civil transportation aircraft. Thereby, the propulsion system is examined on system level to account for the comprehensive interdependence of the drivetrain components in addition to the effects of external requirements. Recent approaches to electrify civil transportation aircraft apply separated power and thrust generation. In case of large airplanes, this necessitates high-voltage on board grids for power transfer. Nevertheless, heavy wiring increases the electric propulsion system´s weight. Further, high-voltage requirements exacerbate the sensitivity to cosmic ray effects while there are limited advantages in powertrain design despite lighter cables. Hence, superconductive components are proposed for electric propulsion systems in aviation to mitigate the voltage requirements. However, these components suffer from the low mechanical strength of superconductors and heavy cooling systems. Major Scientific Contributions: In reaction to the challenges in the state of the art, a modular electrical engine concept is introduced. It enhances electrical propulsion systems for aviation by integrating power and thrust generation into a concentrated entity. The application of a modular propulsion scheme further reduces the voltage requirements. Moreover, this concept splits up single large propulsion power requirements into numerous smaller drivetrain modules, which jointly provide the demanded mechanical power. Prior to describing the development of the electrical engine concept, the thesis provides an overview of the environmental conditions of aviation operation. Relevant regulatory and test procedures complete the set of constraints to be considered for electrical engine design. Further, a review of energy sources determines the most suitable option for electrical propulsion systems in aviation. This includes effects that the adoption of the respective energy source imposes beyond the aviation sector. Recent turboprop and jet engines serve as reference to assess electrical engine design. Therefore, present performance together with future prospects of conventional propulsion systems are the benchmarks for electrical engine operation. The electrical engine concept splits into two design options. The direct drive engine concept connects the electrical machine directly to the propeller. This approach is best suited for superconductive electrical machines, which can provide high power density while operating on low rotational speeds. Aside, the geared drive engine concept places a gearbox between electrical machine and propeller. This decouples the speed of the electrical machine from the propeller speed. Hence, high-speed electrical machines become applicable enabling power dense designs without superconductors. In this context, a detailed analysis of switched reluctance machines reveals the potential of this machine type for application in power dense electrical drivetrains in aviation. To enable the comparison of size, weight and operation performance with conventional engines, models of the electrical engine´s main components provide appropriate details. Thereby, the power electronic system design is adjusted to operate in high altitudes whereas the protection system is aligned with certification requirements. Fuel cells are combined with a turbocharger to interface with surrounding atmosphere. In addition, a cooling system is developed to examine the impact of fuel preheating and operation temperatures. This set of component models allows investigating the effects of different flight situations and environmental conditions on electrical engine operation. The evaluation of the obtained electrical engines bases on retrofit of given aircraft. Thereby, a number of electrical engines replaces the conventional engine while neglecting aerodynamic effects. This enables direct comparison of conventional with electrical engines. Further, aircraft specific variables as cruise velocity and altitude are incorporated into electrical engine design to enhance model accuracy. In focus are the ATR72-600, A320-200 and the A350-900, as these represent aircraft operated under the CS-25 regulation of the EASA. The result of the electrical engine assessment provides details about necessary component improvements to obtain electrical engine performance equal to conventional engines. The consideration of the complete propulsion system demonstrates that the advantages of superconductive electrical machines compared to conventional high-speed machines are limited, even when a catalyst-driven para-to-ortho hydrogen conversion is included to reduce cooling demand on cryogenic temperature levels. Instead, mainly the fuel cell and cooling system require improvements to enable electrical engine designs competitive with conventional aircraft propulsion systems.