Design and Performance of a Hybrid-Electric Fuel Cell Flight Demonstration Concept

Abstract

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 Study on the Use of Solid-Oxide Fuel Cells for Increased Power Generation on Small Aircraft

“…However, the linear trend of power from scaling the engine would seem to eventually lose out to fuel cells at a additional output around 100 to 200 kW. This also happens to be near the FUELEAP SOFC X-57-F power of 120 kW [1].…”

“…Because this initial test contained relatively few degrees of freedom (Table 3), we implemented a full-factorial experiment and scripted the execution. The factor inputs were derived based on the 120 kW FUELEAP Design and Analysis Cycle "DAC-1" and "DAC-2" architectures [1]. The expectation was that the efficiencies would be similar but the power to weight ratio (specific power) would be less for lower-power systems.…”

“…By choosing a value for steady-state power output and also considering an extra 10% power for battery charging and peak loads, the expected weight of the fuel cell could be approximated by W SOFC = P out /P sp S O F C , where W SOFC is the fuel cell weight, P out is the total system power output, and P sp S O F C is the mass-specific power. Based on the FUELEAP DAC designs [1], we used a simple linear interpolation of the total averaged system mass density, less batteries, to determine the expected SOFC volume. To account for packing inefficiencies at small scales, a 10% packing factor was applied to the SOFC system volume in addition to the packing already included in the FUELEAP design.…”

“…The initial study used incremental packing efficiencies to account for additional volume in each component of the SOFC APU system. However, because the scaled SOFC volume was derived from the FUELEAP DAC models [1], SOFC packing efficiency was already accounted for. Rather than attempt to account for these values separately, we decided to treat the system as an integrated unit and applied an overall packing efficiency of 90% to the total volume.…”

“…"Hybrid-electric" aircraft architectures that leverage multiple or alternative power sources are also becoming more popular with rising concerns over efficiency. For example, NASA's Fostering Ultra Efficient, Low-Emitting Aviation Power (FUELEAP) project investigated a 120kW hybrid-electric power system architecture, with the majority of the net electrical power provided by solid oxide fuel cells (SOFCs) using reformed hydrocarbon fuel as the fuel source [1]. 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.…”