COMPARATIVE ASSESSMENT OF COMPACT AIR HEAT EXCHANGERS FOR FUEL CELL-POWERED ELECTRIC AIRCRAFT

Kurzfassung

The development of all-electric regional aircraft faces significant challenges in meeting the emissions goals set for 2050 and beyond. Hydrogen-powered polymer electrolyte membrane fuel cells (PEMFC) are seen as a key enabling technology for an all-electric aircraft. In a PEMFC-powered all-electric aircraft, the electric motors drive the propellers and the required electrical energy is generated on board in PEM stacks by consuming hydrogen gas and air. Although a PEMFC propulsion system would produce lower emissions than a conventional gas turbine engine, regional flights with 70-90 passengers are not yet feasible. The mass-specific power density of the PEMFC system is not sufficient for regional aircraft applications and research is needed to make the system lighter and more efficient. The development of lightweight solutions for PEMFC system sub-components, in particular for the thermal management unit and its heat exchangers (HEXs), is essential for the development of a viable and competitive commercial aircraft. By implementing compact HEXs and reducing the power requirements for their air supply, the overall efficiency and power density of the thermal management unit can be improved compared to a conventional HEX design. This paper presents the design and performance evaluation of compact HEXs selected for two types of nacelle-integrated propulsion topologies. Three different types of compact air-liquid cross-flow HEXs are modeled and performance results are calculated for take-off flight conditions. For the axial configuration, a conventional Offset-Fin-Plate HEX with a rectangular block shape is selected. For the annular configuration, a very compact Fin-Plate HEX and a triple periodic minimum surface (TPMS) Gyroid-Plate HEX are developed. For the requirements of 591 kW heat rejection for PEMFC cooling and 15 K temperature drop in the coolant flow, the conjugate heat transfer on symmetrical HEX unit cells is calculated using CFD simulations. The results show that the Gyroid-Plate HEX is the best candidate in terms of high heat transfer, overall efficiency, thermal performance, gravimetric power density and low required heat transfer area; but requires design optimization to reduce pressure losses in its air channels. Among Offset-Fin-Plate and Fin-Plate HEX, the latter emerged as preferable design in terms of combined thermal-hydraulic performance, heat transfer area and gravimetric power density. From the results, design optimizations and strategies to integrate the thermal management of LTPEM in nacelle-bound propulsion topology are derived, which will be considered in future work.