Study and modelling of a fuel cell air supply system with turbocharger for a regional hybrid electric aircraft

The evolution and accumulation of operational experience in the field of fuel cells are opening new perspectives for the use of these technologies in continuous power generation in aviation. In this prospect, the pursuit for high power density and system efficiency becomes essential. This thesis contributes to the initial stage of investigating and modelling auxiliary elements within a fuel cell power system (FCPS), which is intended to become an integral part of the powertrain of a regional hybrid more electric aircraft. Starting from a previous thesis work conducted at the Polytechnic of Turin, where a model of air supply system for the aircraft was developed using a simple layout with the compressor powered by an electric motor, it emerged that the compressor system is the most significantly energy-impacting component among the auxiliary subsystems of the FCPS, powered directly by the fuel cell itself. The air supply system is a crucial element in the fuel cell's interaction with its environment and has a significant impact on the performance, stability, and control of the fuel cell power system. Particularly in aviation, where high compression ratios are required, the design of this component is one of the main challenges in ensuring the overall effectiveness of the FCPS. Therefore, in some applications, harnessing the energy contained in the air exiting the fuel cell stack via an expander can become essential. The primary aim of this thesis was to conduct a feasibility study, perform a preliminary design, and model an air supply system with a layout that provides for the recovery of the enthalpy energy of the gases leaving the stack. After an accurate analysis of the state of the art regarding air supply system issues, energy recovery system integration and all the possible architectures used in the literature, two architectures widely adopted in the automotive sector were evaluated: the electric turbocharger type architecture and the serial booster type architecture. Both architectures involve the use of a turbocharger with a radial turbine and compressor, similar to those used in internal combustion engines, making them robust choices suitable for aviation requirements. Subsequently, a preliminary analysis of the system's performance was conducted in order to determine the optimal operating pressure for the fuel cell stack. Next, the static sizing of the main components of the air supply system was performed, in particular the turbocharger, the process of which was described in detail. Through the dimensioning, it was possible to make a preliminary assessment of the masses, moments of inertia and power density of the architectures, as well as their impact on the overall efficiency of the system. Based on these results, the best architecture was selected for the case under consideration. For the chosen architecture, a dynamic model was implemented using the Simulink platform. The turbomachines were modelled using the performance maps previously developed in static sizing. Using the dynamic model, it was possible to simulate the entire flight mission and evaluate the system's response to current disturbances at the fuel cell stack. A control system was devised to regulate the stack inlet air flow and operating pressure in order to optimise the overall system performance and ensure that the compressor operates safely with promising results.