An examination of hydrogen fuel cells and lithium-ion batteries for electric vertical take-off and landing (EVTOL) aircraft

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Ng, W.
Datta, A.
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The primary drawback of electric vertical take-off and landing (eVTOL) aircraft is their poor range and endurance with practical payloads. The objective of this paper is to examine the use of hydrogen fuel cells to overcome this drawback through simulation and hardware testing. The paper 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. 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 paper also demonstrates fuel cell and battery power sharing capabilities in an unregulated parallel configuration as well as in a regulated circuit. A regulating architecture was developed that achieved a reduction in power plant weight. Finally, the paper outlines weight models of motors, batteries, and fuel cells needed for eVTOL sizing, and carries out sizing analysis for three progressively longer on-demand urban air taxi missions. The objective aircraft was sized to carry a minimum of 400 lb payload for an on-demand air taxi-like mission with 5 min hover and 15-60 min cruise at 150 mph. This revealed that for ranges within 75 mi, an all-electric tilting proprotor configuration is feasible with current technology if high C-rate batteries are available. Either a battery-only or fuel cell and battery hybrid power plant is ideal, depending on the range of the mission. In particular, a 5700 lb gross take-off weight aircraft with disk loading of 11 lb/ft2 could be sized using a hybrid power plant with fuel cells and 10C batteries to carry a payload of 430 lb for a 75 mi (inter-city) mission. A smaller aircraft of 4000 lb gross weight and a disk loading of 27 lb/ft2 could be sized using a 6C battery only power plant to carry a payload of 490 lb for a shorter 38 mi (intra-city) mission. Research priorities would depend on target mission duration and range. For any mission beyond 40. miles (or 15 minutes at 150 mph) fuel cells appear to be a compelling candidate. Based simply on performance numbers (cutting-edge numbers proven at a component level but not in flight), ease of re-fueling, high w% hydrogen storage due to the short duration of eVTOL missions, and lack of a compressor due to low-altitude missions, fuel cells appear to far surpass any realistic future projections of Li-ion energy levels. However, for missions less than40 miles, improving battery energy density is the priority. All mission lengths require improved battery power density to 6-10 C for 150 Wh/kg batteries.