Multidisciplinary Optimization of a Turbofan Engine Integrated with Solid Oxide Fuel Cells Under Mass and Volume Constraints for Civil Low-Carbon Aircraft

Mass and volume penalties associated with fuel cell integration have hindered the manufacturing and integration of hybrid power systems into civil aircraft. To address this issue, a novel scheme, a turbofan engine integrated with solid oxide fuel cells, is proposed in this paper, which has the advantage of high thermal efficiency and propulsion efficiency, where the electricity produced by the solid oxide fuel cell (SOFC) is used to drive the ducted fan. Then, a multidimensional model that accounts for mass, volume, and thermodynamic performance is established. The equivalence ratio and split ratio have a significantly stronger influence on the volume and mass ratios than the current density. These two parameters directly determine the power ratio between the fuel cell and the gas turbine. An increase in the power ratio leads to a simultaneous rise in both the volume and mass ratios; however, the overall efficiency cannot be continuously improved, with its maximum value being approximately 63% without mass and volume constraints. During the multi-objective optimization process, the weight and volume of the hybrid engine have a linear relationship with the overall efficiency when the efficiency is lower than 50%. However, both mass and volume exhibit exponential increasing trends as the overall efficiency is over 50%. This is caused by the nonlinear change in the fuel cell reaction area; as polarization loss decreases, the thermal efficiency of the hybrid system improves. The maximum overall efficiency is taken as the optimization objective, with the constraints that the mass fraction of the fuel cell does not exceed 0.4 of the total engine mass and that the volume fraction of the fuel cell stack is less than 0.2 of the gas turbine engine volume. Compared with the CFM56 3C1, the thermal efficiency for the novel hybrid engine is increased by 21%, and its propulsion efficiency is increased by 42%.