Design point selection of fuel cell propulsion systems under consideration of off-design performance analysis

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Abstract
The design point (DP) selection of a novel gas turbine cathode air supply system (GTCS) for a fuel cell-based aircraft propulsion system is investigated. Previous studies were based solely on on-design considerations regarding the DP selection which led to substantial violations of turbine inlet temperature and component limits during subsequent mission analysis. Therefore, the need for an in-depth analysis taking into account the off-design performance is obvious. In this study, we take into consideration a broad design parameter space of the GTCS based on fuel cell design operating pressure, air excess ratio and turbine inlet temperature. For these design parameters, on-design calculations are executed in a full factorial design of experiments for a single-spool GTCS. Then, off-design analyses of each DP are done in crucial missions such as the reference, design, maximum power, hot day and maximum power hot day. Based on the off design results, the design parameter space is narrowed down to a design envelope from which a final design point is chosen. Next, the operating envelope of this design is investigated in more detail to gain further insight into the feasible combinations of power settings and corresponding operating pressure of the fuel cell stack. Finally, the process is repeated for a 2-spool design. Design points were identified for both a single-spool and a 2-spool system. The latter however yields improvements regarding fuel consumption (-1.8%) due to an improved GTCS efficiency (+4.9%) and component limits such as the surge margin of the low and high pressure compressor.