Solid Oxide Fuel Cell APU Feasibility Study for a Long Range Commercial Aircraft Using UTC ITAPS Approach

Summary The objective of this contract effort was to define the functionality and evaluate the propulsion and power system benefits derived from a Solid Oxide Fuel Cell (SOFC) based Auxiliary Power Unit (APU) for a future long range commercial aircraft, and to define the technology gaps to enable such a system. The study employed technologies commensurate with Entry into Service (EIS) in 2015. United Technologies Corporation (UTC) Integrated Total Aircraft Power System (ITAPS) methodologies were used to evaluate system concepts to a conceptual level of fidelity. The technology benefits were captured as reductions of the mission fuel burn and emissions; however, the life cycle cost was not part of the work scope. At the system level, the overall integration of a fuel cell power unit, turbine engine and power subsystems for a long-range commercial transport was evaluated and the associated enabling technologies were identified. The baseline aircraft considered was the Boeing 777-200ER airframe with more electric subsystems, Ultra Efficient Engine Technology (UEET) engines, and an advanced APU with ceramics for increased efficiency. In addition to the baseline architecture, four architectures using an SOFC system to replace the conventional APU were investigated. Architecture-A simply replaced the APU with a 450 kW SOFC—gas turbine (GT) hybrid system, which operated for all phases of the mission, including the flight climb-cruise-descent operation, thereby reducing the engine shaft extractions substantially. Architecture-B comprised greater integration between the SOFC system and aircraft Environmental Control System (ECS), Thermal Management System (TMS), and Electrical Power System (EPS), and let to improved overall system efficiency and hence reduced the mission fuel burn. Architecture-C employed greater integration of the SOFC with aircraft sub-systems and advanced aircraft technologies to improve the benefits and applications of the SOFC system. Architecture-C also utilized proprietary UTC technology to extend the fuel heat sink capability thus enabling recovery of the waste heat from the SOFC to pre-heat the fuel for the propulsion engines. This waste heat recovery further reduced the mission fuel burn. Architecture-D explored the potential for reducing parasitic power losses due to pressurization by running the Architecture-C SOFC system at one atmosphere pressure. System integration is critical to maximize benefits from the SOFC APU for aircraft application. The mission fuel burn savings for Architecture-A, which had minimal system integration, was 0.16 percent. Architecture-B and Architecture-C employed greater system integration and obtained fuel burn benefits of 0.44 and 0.70 percent, respectively. Architecture-D had the highest level of integration and obtained a benefit of 0.77 percent. System integration will also minimize the technology development cost/time. Recognizing that most of the benefits of the SOFC system were realized during ground operations, the potential benefits for a short-range mission were also explored. The preliminary short-range mission study showed that the SOFC system provided more benefits to short-range mission aircraft (about 3 percent mission fuel burn savings for the 500 NM B777-200ER aircraft mission) as compared to the long-range mission aircraft (0.7 percent mission fuel burn benefit). The SOFC APU produced zero emissions, thus eliminating the emissions of the conventional APU during ground operations. The reduction in engine fuel burn (partly due to reduced extractions) also resulted in a reduction in emissions from the engines. For Architecture-C, the engine emissions in flight decreased by 0.23 percent for oxides of nitrogen (NO