Hydrogen fuel cell and battery hybrid architecture for range extension of electric VTOL (eVTOL) aircraft

The objective of this paper is to study the impact of combining hydrogen fuel cells with lithium-ion batteries through an ideal power sharing architecture to mitigate the poor range and endurance of battery powered electric vertical takeoff and landing (eVTOL) aircraft. The benefits of combining the two sources is first demonstrated by a conceptual sizing of an electric tiltrotor for an urban air taxi mission of 75 mi cruise and 5 min hover. It is shown that an aircraft of 5000-6000 lb gross weight can carry a practical payload of 500 lb (2-3 seat) with present levels of battery specific energy (150 Wh/kg) if only a battery-fuel cell hybrid powerplant is used, combined in an ideal power sharing manner, as long as high burst C-rate batteries are available (4-10 C) for a limited duration (2.5 min). A powerplant using batteries alone can carry less than half the payload; fuel cells alone can not lift off the ground. The operation of such a parallel system is explained using systematic hardware testing and modeling and simulation. The concepts of unregulated and regulated power sharing architectures are described. A regulated architecture that can implement ideal power sharing is built-up in a step-by-step manner. It is found only two switches and three DC-to-DC converters are necessary, and if placed appropriately, are sufficient to achieve the desired power flow. The power system model is validated with test data and used to gain fundamental understanding of the power sharing architecture.