Transient operation and controller analysis of a fuel cell based powertrain for aviation

Kurzfassung

In commercial aviation the necessity to reduce greenhouse emissions is a strong inducement for technical innovations. A promising key technology on the path to emission-free aviation is electric propulsion in combination with polymer-electrolyte membrane fuel cells (PEMFC). In contrast to most ground-based applications the operation of airborne fuel cell systems is facing some challenges. On the one hand, these are the varying environmental conditions as for example air temperature and pressure. On the other hand, the energy system needs to be able to provide sufficient electric power to the propulsion even in fast transients. As battery storage at present comes with a large weight penalty it is favorable to provide the energy solely by fuel cells. In this study it is investigated how these preconditions affect the transient operation of the fuel cell system in an aircraft and which implications arise on the implementation of the fuel cell system controls. For this purpose, a control-oriented model of a fuel cell system with focus on stack, power conditioning and air supply is used. Subsequently, the fuel cell system is coupled with a simple electric motor and propeller model providing the boundary conditions induced by the propulsion unit. The overall system is sized to propel a Cessna 172 airplane which also sets the conditions for a generic flight missions used for a dynamic simulation of the powertrain. The results show that in the chosen scenario the fuel cell based powertrain is able to satisfactorily provide the required propulsion even in the transients. Furthermore, it can be seen that because of the strong coupling between cathode pressure and air mass flow a centralized control is necessary. In addition the study revealed a tradeoff between power delivery and oxygen excess ratio which in turn is crucial for stack lifetime and needs to be considered when tuning controllers for the powertrain.