Operating Range of Turbocharged Air Supply Systems for Fuel Cell Aircraft Propulsion

Abstract In this paper, we investigate the operating range of a turbocharged polymer electrolyte membrane (PEM) fuel cell system for regional aircraft propulsion using conceptual design methods. Specifically, we focus on the operating limitations imposed by the air supply system. The air supply system under investigation consists of an electrically assisted turbocharger, a compressor aftercooler, and a membrane humidifier. A conceptual design tool with physics-based component models is used to design the air supply system and to calculate its steady-state off-design behavior. Our use case is a fuel cell propulsion system for a 50-seat regional aircraft. We examine the influence of the flight altitude and the fuel cell operating parameters on the system operating range and efficiency. The results show that the maximum fuel cell system power output is constrained by several component limitations throughout the flight mission, among them the maximum compressor spool speed at high altitudes and the aftercooler cooling capacity at low altitudes. In addition, the results show that high altitude part load operation is limited by the attainable inlet gas humidification. This limit can be overcome by reducing the fuel cell operating temperature or by increasing the fuel cell operating pressure. We investigate the influence of different pressure control mechanisms on the operating range and fuel cell system efficiency, namely, a fuel cell bypass, a variable geometry turbine and a backpressure valve. Our results indicate that the operating range can be most effectively extended using a fuel cell bypass. In conclusion, we show the challenges associated with fuel cell air supply systems at high altitude and examine technical solution options with a focus on part-load behavior from an aircraft engine performance perspective. We further present a generalized approach to evaluate the performance of air supply systems for fuel cell propulsion that is applicable to a wide range of system architectures and applications beyond those studied in this paper.