Unmanned Aerial Vehicles: The Possibility of Using Fuel Cell and Battery Technologies As the Prime Mover

Pourrahmani, Hossein

The adverse impacts of fossil fuels on the environment have guided the authorities to set the official 100% phase out target for sales or registrations of new internal combustion engines (ICEs) that use fossil fuels as the prime mover. Batteries and fuel cells have proved to be an appropriate alternative for the ICEs to be used in cars, trucks, and buses. Although there have been limited number of studies to use batteries as the prime mover of small size planes, the possibility of using the integrated system of battery and fuel cell is yet to be investigated in the airplane industry.

Due to the infancy of using batteries and fuel cells in airplane sector, the studies are limited to the domain of Unmanned Aerial Vehicles (UAVs). UAVs are designed to fulfill long and high-altitude flight missions. The flight duration and flight altitude have been proved to be improved using the proton exchange membrane fuel cells (PEMFCs) as the prime-mover of the UAVs. However, the low acceleration of the PEMFCs has triggered the idea of integrated PEMFCs and Lithium-Ion (Li-ion) battery. Thus, the integrated system can improve the low range of batteries by maintaining high acceleration and fast re-fueling time.

Among different types of fuel cells, PEMFCs are established to be the best option for mobility applications, and the well-known Nickel Manganese Cobalt (NMC), and Lithium Titanite Oxide (LTO) Li-Ion batteries are considered as the most efficient types. LTO enjoys from the highest performance, lifespan, and safety by higher costs, whereas NMC provides acceptable performance with low costs and high specific energy.

Published results have indicated that PEMFC can be utilized in an UAV, reducing the overall weight and leading to the longest flight endurance. However, better acceleration and simpler configurations can be achieved by Li-Ion battery packs, although the practical implementation is still limited by the actual chemistry. So far, it is believed that fuel cells can be suitable for cruise flight while batteries are more appropriate for other flight modes.

In addition to the possibility of the integrated battery and fuel cell system to improve the overall performance of the UAVs, the integration of the turbochargers to the PEMFCs is believed to facilitate the transition from engines to more fuel cell/battery technologies. To reach the maximum thrust and minimum fuel consumption rate in an UAV, a hybrid propulsion system of a PEMFC and a single stage turbocharged engines is able to provide the maximum thrust, the minimum fuel consumption rate, and the thermal efficiency of 34.58 mg/s, 69.56N, and 62.41%, respectively.

This study evaluates the possibility of integrating the fuel cell technology to the batteries as an alternative for fossil fuel-based combustion engines for the small sized UAV application. To further improve the performance of the UAV, a turbocharger is also integrated to reach higher thrusts and lower fuel consumption rates. The suggested integrated system, which is shown in Fig. 1., is designed to provided 6.8 kW power for a UAV with TWR (Thrust to Weight Ratio) of 0.4 and weight of 17.7kg. PEMFC will be used as the selected type of fuel cell while NMC is the considered type of Li-Ion battery. The turbocharger is also a hydrogen fueled ICE with the compression ratio of 11.15. The goal of this study is the performance evaluation of the current integrated system in an optimized condition considering the size, weight, hydrogen consumption, thrust, and overall efficiency as the output parameters.