114 While UAV operations conducted BVLOS are not yet routine around the world, the regulatory framework is developing rapidly, stimulating demand for electric models with greater endurance than batteries can provide for missions such as infrastructure inspection, border patrol and large area surveys, writes Peter Donaldson. The choice of power plant is now one of engine-driven generator or fuel-cell system. Both provide higher energy density than even the best lithium-ion batteries can manage, but each one has its pros and cons. Fuel cells are almost silent in operation, producing virtually no vibration, and their local emissions are limited to water vapour. They are also significantly more efficient than engines as they convert 40-60% of the energy available from the fuel into electricity, compared with 20-30% for the best internal combustion generator sets. Hydrogen production, transport and storage systems are still evolving, and they are relatively complex and expensive. The infrastructure is sparse, making logistics difficult. While fuel cells are coming down in price, the upfront costs are still comparatively high. Further, although fuel cells tend to be lighter than their equivalent generator sets, the added weight of hydrogen tanks can negate this advantage. The almost universal availability of hydrocarbon fuels is a big logistic plus for generator sets, making refuelling in remote or less developed areas straightforward. This could be a crucial advantage for long-range BVLOS operations where aircraft might not be able to return to a central hub very often. Obviously, ICE genset is far more mature as a technology than aviation fuel cells, with all the benefits of reliability, ease of servicing and ruggedness that maturity brings. ICE-driven generators typically have higher power-to-weight ratios than fuel-cell systems, enabling them to support heavy payloads and deliver more energy for longer, which could be decisive in larger vehicles carrying relatively high payloads for long missions. The disadvantages of enginedriven generators boil down to noise, vibration and emissions, making them less acceptable in environmentally sensitive areas. While they are very well understood, and simpler to service and maintain than fuel cells, they generally need more frequent maintenance, negating some of their logistic advantages. This could be decisive for operations where minimal downtime and service interruption are crucial. Further, the lower fuel efficiency of an ICE genset not only leads to the burning of more fuel over time, but it can also mean heavy loads and more frequent interruptions for refuelling, which are not desirable for longer endurance missions. This state of play is likely to make the choice between these two types of power plant highly mission-dependent, with fuel cells prevailing when environmental sensitivity is high, and long endurance is crucial in areas where hydrogen fuel is easily available. However, the genset will probably win out for operations with high power demands carried out in remote, rugged areas where compressed hydrogen is all but unobtainable. However, as environmental rules tighten and fuel-cell technology matures, power plants based on it are likely to gain the overall advantage. Until then, conversion kits of both types for large electric UAVs could become very sought after. October/November 2024 | Uncrewed Systems Technology PS | Powerplant options for electric BVLOS UAVs Now, here’s a thing This… is likely to make the choice between these two types of power plant highly mission-dependent …fuel cells prevailing when environmental sensitivity is high
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