this technology. These drawbacks have thus far limited market penetration in all but the smallest vehicle classes. Current approaches to airborne electricity generation revolve around storage of the energy in batteries onboard the aircraft, which greatly limits range and endurance, or generation of electricity by combustion of stored hydrocarbon fuel as part of a hybrid-electric architecture, which suffers from mass and efficiency challenges.Borer [1] outlined these approaches and recommended pursuit of an additional option to the portfolio of solutions available to store energy for airborne electric propulsion. Borer proposed the use of a hybrid-electric solution that generates electricity primarily from a Solid Oxide Fuel Cell (SOFC) fed from an onboard fuel reformer, which converts heavier hydrocarbons into products suitable to be consumed by the SOFC. To be viable, this power system needs to operate at a specific power level of at least 300 W/kg and an efficiency of 60%, referenced to the lower heating value (LHV) of the stored fuel. These characteristics represent significant advances in the state-of-the-art for hybrid-electric SOFC architectures -a recent National Academies report indicated that such architectures are capable of a specific power of 100 W/kg and an efficiency of 30-40% [2]. More recent research suggests that higher specific power and efficiency levels are achievable [3].This paper is one of a number of concurrent papers related to NASA's Fostering Ultra-Efficient, Low-Emitting Aviation Power (FUELEAP) project. The authors present a detailed application of a hybrid-electric, heavy-fuel, SOFC power system developed under FUELEAP. Specifically, this paper describes the integration of a 120 kW power system in place of the battery system used on NASA's X-57 "Maxwell" distributed electric propulsion (DEP) technology flight demonstrator. Section II provides background on FUELEAP and the X-57 program. Section III describes the integration of the hybrid-electric SOFC power architecture onto the X-57 and provides performance estimates for two different design spirals, as well as comparison to gasoline-fueled and battery-powered alternatives. Finally, Section IV summarizes the results of this research. Other companion papers include: a more detailed description of the power system design [4], electrical integration of this power system into the X-57 [5], thermal cycle testing of SOFC hardware to assess suitability for aviation operations [6], system safety analysis of the FUELEAP X-57 demonstrator concept [7], and the use of model-based systems engineering frameworks to aid in the development of future demonstrator concepts [8].
A growing demand for higher-power electrical equipment on aircraft is driving the exploration of alternative electrical power generation devices. "Hybrid-electric" aircraft architectures that leverage multiple or alternative power sources are also becoming more popular with rising concerns over efficiency. Fuel cells are particularly interesting as auxiliary or secondary electrical power generation devices, due to their relative efficiency and longevity, and could be used as a "power pod" for vehicles without enough on-board electrical generation capability. This paper examines the feasibility and potential value of using an externally-mounted, hybridelectric, solid-oxide fuel cell (SOFC) power system on a Class IV unmanned aerial vehicle (UAV) for the purpose of power generation in excess of the baseline electrical load. Moreover, the paper will demonstrate that a SOFC auxiliary power unit (APU) provides significant value to an aircraft by producing a relatively large amount of additional electrical power for a relatively low initial investment when compared to producing additional power by scaling the engine.
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