This work investigates a combined internal combustion engine and solid oxide fuel cell (SOFC) hybrid powertrain for unmanned aerial vehicles (UAV). UAVs are increasingly used in large agriculture for crop management and water resource visual inspection, and in militarized applications, as they allow for safer, unmanned reconnaissance missions. The limited flight time of UAVs, as a result of the traditional lithium polymer batteries used for power, has restricted the widespread implementation of the UAV technology.
A hybrid power train, utilizing energy dense liquid fuel, provides the capability of powering a UAV for longer duration missions. The hybrid power train consists of a small internal combustion engine that acts as a partial oxidation fuel reformer, simultaneously producing mechanical shaft power. The 0.3 in3 piston engine is a typical air cooled, glow engine utilizing a 60/40 percent (by volume) mixture of methanol and nitromethane, respectively. The syngas generated by the combustion engine can then be utilized by a tubular SOFC stack to generate electrical energy for the UAV flight systems. The SOFC system operating on combustion exhaust from the engine produced a maximum of ~650 mW/cm2, while the engine was continually producing ~750 W of mechanical shaft power.
In case of an engine failure, the liquid fuel may be directly utilized by the SOFC system to maintain power generation. Additionally, the engine may be manually shutdown to provide silent onboard power generation. In testing, a tubular SOFC provided with direct liquid 60/40 methanol/nitromethane fuel was capable of producing above 550 mW/cm2 for maximum power. The SOFC system was able to operate continuously under direct liquid fueling for 4 hours without degradation.
The power produced by the proposed hybrid powertrain is expected to be sufficient to power a 15 kg UAV for long endurance missions lasting in the range of 200-500% of current recorded UAV flight duration.