An Experimental and Analytical Investigation of Hydrogen Fuel Cells for Electric Vertical Take-Off and Landing (eVTOL) Aircraft

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Ng, Wanyi
Datta, Anubhav
The objective of this thesis is a comprehensive investigation of hydrogen fuel cells for electric vertical take-off and landing (eVTOL) aircraft. The primary drawback of battery powered eVTOL aircraft is their poor range and endurance with practical payloads. This work uses simulation and hardware testing to examine the potential of hydrogen fuel cells to overcome this drawback. The thesis develops steady state and transient models of fuel cells and batteries, and validates the models experimentally. An equivalent circuit network model was able to capture the waveforms and magnitudes of voltage as a function of current. Temperature and humidity corrections were also included. Examination of the results revealed that the transient behavior of batteries and fuel stacks are significant primarily shortly after startup of the fuel stack and at the limiting ranges of high and low power; for a nominal operating power and barring faults, steady state models were adequate. This work then demonstrates fuel cell and battery power sharing in regulated and unregulated parallel configurations. It details the development of a regulated architecture, which controls power sharing, to achieve a reduction in power plant weight. Finally, the thesis outlines weight models of motors, batteries, and fuel cells needed for eVTOL sizing, and carries out sizing analysis for on-demand urban air taxi missions of three different distances -- 50, 75, and 150~mi of cruise and 5~min total hover time. This revealed that for ranges within 75 mi, a light weight (5000-6000~lb gross weight) all-electric tilting proprotor configuration achieves a practical payload (500~lb or more) with current levels of battery specific energy (150~Wh/kg) if high burst C-rate batteries are available (4-10~C for 2.5~min). Either a battery-only or battery-fuel cell (B-FC) hybrid power plant is ideal depending on the range of the mission: For inter-city ranges (beyond approximately 50~mi), the mission is impossible with batteries alone, and fuel cells are a key enabling technology; a VTOL aircraft with a B-FC hybrid powerplant, an aircraft with 6200~lb gross take-off weight, 10~lb/ft$^2$ disk loading, and 10~C batteries, could be sized to carry a payload of 500~lb for a range of 75~mi. For this inter-city range, the research priority centers of fuel cells, as they appear to far surpass future projections of Li-ion battery energy levels based on performance numbers (at a component level), high weight fraction of hydrogen storage due to the short duration of eVTOL missions, and lack of a compressor due to low-altitude missions, with the added benefit of ease of re-fueling. However, for an intra-city mission (within approximately 50~mi), the B-FC combination provides no advantage over a battery-only powerplant; a VTOL aircraft with a battery-only powerplant with the same weight and disk loading as before, and 4~C batteries, can carry a payload of 800~lb for a range of 50~mi. For this mission range, improving battery energy density is the priority.