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

In this paper, we investigate the operating range of a turbocharged 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 consists of an electrically assisted turbocharger, an aftercooler, and a membrane humidifier. We use a conceptual design tool with physics-based component models for system design and steady-state off-design calculations. Our use case is a fuel cell propulsion system for a 50-seat regional aircraft. We examine the influence of flight altitude and fuel cell operating parameters on the system operating range and efficiency. The results show that the system power output is constrained by the compressor speed at high altitudes and the aftercooler cooling capacity at low altitudes. Furthermore, the attainable inlet gas humidification limits high-altitude part load operation. This limit can be overcome by reducing the operating temperature or by increasing the operating pressure. We investigate the influence of pressure control mechanisms on the operating range and efficiency, namely a fuel cell bypass, a variable geometry turbine and a backpressure valve. Our results indicate that the bypass is most effective in extending the operating range. In conclusion, we show the challenges associated with fuel cell systems at high altitudes and examine technical solution options. We further present a generalized approach to evaluate the performance of air supply systems for fuel cell propulsion, applicable to a wide range of applications beyond those studied in this paper.