Typical combustion temperatures result in the formation of oxides of nitrogen (NOx) due to dissociation of nitrogen molecules in ambient air, whereas the much-lower SOFC operating temperature will not foment such dissociation and NOx formation. As such, the hybrid SOFC-electric power generation scheme will result in effectively zero NOx emissions. This can be especially important as the hybrid SOFC-electric later moves into larger commercial aircraft as Auxiliary Power Units (APUs), since a substantial portion of surface NOx emissions at major airports are due to APU use on the ground. Furthermore, while this hybrid power system concept still utilizes hydrocarbon fuels, a given magnitude of savings in fuel consumption will effectively result in a similar magnitude reduction of carbon emissions. As the number of equipped aircraft grows, the environmental impact of the light aircraft fleet will drop precipitously. Larger aircraft equipped with hybrid SOFC APUs could enable a substantial drop in airport surface carbon emissions due to APU use.
C. Objectives This paper documents the approach to select a concept for a low-cost, high-impact hybrid SOFC-electric
demonstration aircraft to be flight-tested in future development efforts. The ultimate goal is to demonstrate a 50% reduction in fuel cost for an appropriate light aircraft cruise profile (payload, range, speed, and altitude). This demonstrator will gather design and operational data (which will be made available to industry and regulators) to provide a path towards further development and certification.
II. Technical Approach To develop an infrastructure-friendly electric propulsion demonstrator, the authors conducted an extensive
exploration of viable demonstration aircraft candidates, along with the conceptual design and description of the systems necessary for successful demonstration. This was accomplished via a multidisciplinary, risk-driven approach to exploration and design.
A. Demonstrator Decision Criteria The choice of demonstration aircraft is a critical part of identifying the requirements and constraints for the
design of the SOFC-enabled electric propulsion architecture, as well as the overall demonstrator test program and risk matrix. In an effort to keep the demonstration system cost down, the team considered modifications to existing aircraft, rather than purpose-building an all-new SOFC-electric aircraft. This approach also provides the demonstration program with a definitive performance and operations cost baseline for comparison. In selecting a demonstrator baseline, the team considered the following criteria:
Airframe acquisition and NASA processing – This refers to the cost, schedule impact, and uncertainty associated with acquiring the baseline (unmodified) test aircraft, bringing it up to NASA airworthiness standards, and making the appropriate modifications.
Propulsion (motor, propeller, and accessories) – This refers to the cost, schedule impact, and uncertainty associated with the acquisition, testing, and integration of the electric powertrain, including (if necessary) selecting an appropriate alternate propeller. The propeller is an important consideration, as many aircraft use engine oil as the working fluid for hydraulically-actuated constant-speed propellers, and an electric motor replacement would need an alternate means to control propeller RPM or blade pitch, if the baseline aircraft was so equipped.
Power system (SOFC and balance of plant) – This refers to the cost, schedule impact, and uncertainty associated with the design, fabrication, testing, and integration of the hybrid SOFC-electric power system, including all support components (battery, plumbing, electrical lines, protection circuits, packaging and insulation, etc.).
Number of US vs. foreign manufacturers for demonstrator and components – Given that the team will be proposing a flight demonstrator program to NASA, it is important to show that U.S. tax dollars are largely going to domestic institutions. Additionally, working with domestic companies can be preferable should any International Traffic in Arms Regulations (ITAR) or Export Administration Regulations (EAR) technology issues arise during the program.
Mass and volume margin with new motor and power system – This refers to the excess mass and volume available in the aircraft as compared to the expected mass and volume of the electric power and propulsion system. More margin enables greater design flexibility, or use of less expensive components that may not require optimization for the application.
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American Institute of Aeronautics and Astronautics