Dynamic Modelling and Controls of an Air Supply System for In-Flight Proton Exchange Membrane Fuel Cells (PEM-FC)

Abstract

To cover the increasing demand of on-board electrical power and for further reduction of emissions, the conventional auxiliary power unit (APU) may be replaced by a fuel cell system with an expected efficiency increase of 25% to 50% when compared to start-of-the-art GT-APU. The main components of an in-flight FC system are a compressor-turbine unit, a kerosene reformer, and the fuel cell. Polymer exchange membrane fuel cells (PEM-FC) may be favored because of their currently advanced level of development, their high power density and the available liquid water in the cathode-off gases which can be used as service water on-board. Transient requirements may have significant impact on system design and operating range and will therefore be addressed in this paper. During in-flight operation, air has to be compressed from the ambient to a pressure near standard conditions, which allows the application of state-of-the-art PEM-FC and ensures a constant power density independent from the operating altitude. A centrifugal compressor is chosen for pressurization of the system and is powered by a radial turbine, which allows autonomous operation at cruising altitude without external power. For off-design operation and transients, electric support from the PEM-FC is necessary, see [1]. The radial turbine itself is run by the hot exhaust gases from a post-combustor using the remaining energy in the cathode off-gases. A thorough trade-off between suitable compressor techniques for the air supply system was carried out in [1]. Turbomachinery revealed to be favourable for the PEM air supply system due to their low specific weight and high efficiency. The air supply system resembles the turbocharger for a combustion engine (Fig. 1), which represents a good starting point for a successful integration into the flight environment and further development due to known technology. Based on a turbomachinery design which satisfies the system requirements, the dynamic behavior of the air supply system is analyzed when coupled to the PEM fuel cell. The main focus is on the detection of sensitive system parameters causing system response delay or critical operating conditions. The present paper suggests system features, turbomachinery design parameters and controller types which achieve inherent stability and fast response of the air supply system throughout the entire flight envelope.