Review and Evaluation of Hydrogen and Air Heat Exchangers for Different Thermal Requirements in Fuel Cell-Powered Electric Aircraft Propulsion

Kurzfassung

Strategic initiatives like Flightpath 2050 and Waypoint 2050 aim to reduce the aviation sector's environmental impact, targeting a drastic reduction in emissions by 2050. Fuel cells as main provider of electric energy for the electrically-driven propulsion system are seen as a promising solution due to their high efficiency and low emission. Lower Temperature Polymer Electrolyte Membrane Fuel Cells (LTPEMFC) and Solid Oxide Fuel Cells (SOFC) are particularly promising, yet integrating these systems presents challenges, notably in terms of overall system weight and thermal management. Effective thermal management is critical for preheating the fuel, such as hydrogen, cooling the compressed cathode air, and rejecting the waste heat out of the fuel cell system and electronics components to enable an efficient and optimal operation of the aircraft. This requires a comprehensive thermal management system involving components like compressors, air ducts and mainly heat exchangers (HEXs). Furthermore, electric machines and power electronics in the electric drive train also require thermal management, especially for superconducting concepts. HEXs have to be designed for optimal performance, low-cost operation, and lightweight, as they have a high impact on aircraft weight and overall efficiency. Conventional air-cooled HEXs would become unfeasible or unreasonably large and heavy due to the required extensive heat transfer surface area for the mentioned application. Current research deals with the challenge of designing HEXs with reduced weight, size, aerodynamic drag, required pumping power; and increased compactness relative to the state-of-the-art design without compromising the thermal performance and durability. This paper offers a comprehensive exploration of HEX with temperatures of -200°C for cryogenic cooling; 100°C for LTPEMFC; till 800°C for a commercial electric aircraft's SOFC fuel cell-powered propulsion system, aiming to identify optimal solutions. It outlines the design procedures, operational parameters, and distinctive characteristics of various HEX configurations, including shell and tube, double pipe, plate-fin, extended surfaces, regenerator, microchannel, and Triple Periodic Minimal Surface (TPMS). Focusing on mitigating thermal resistance and enhancing compactness, the utilization of fins, particularly on the gas side, is elucidated, accompanied by detailed guidelines on major fin design. Evaluation criteria are derived from aviation-specific requirements, encompassing key metrics such as mass, volume, pumping power, heat transfer efficiency, cost, leakage, fouling, durability, material compatibility, and operational parameter range. Utilizing the evaluation criteria, the identified heat exchangers undergo quantitative evaluation through a weighted point rating. This rating enables a systematic comparison and prioritization of alternatives, ensuring transparency, consistency, and reproducibility in decision-making processes regarding heat exchanger selection. The evaluation results indicate significant potential for using extended surface heat exchangers, such as plate-fin, tube-fin, and Triply Periodic Minimal Surfaces (TPMS) like gyroid structures, in heat exchanger applications for electrified aircraft. The evaluation covered various operating temperatures relevant to fuel cell applications, assessing heat exchangers for air at 100°C and up to 800°C, and for hydrogen at 100°C and up to 600°C, including liquid hydrogen. This comprehensive assessment ensures that the selected heat exchangers meet the diverse thermal management requirements of electrified aircraft. For the air side, key factors include pumping power, compactness, and heat transfer efficiency. Conversely, for the hydrogen side, primary considerations are leakage prevention and material compatibility due to the risks of hydrogen embrittlement and related safety concerns. In conclusion, this research underscores the crucial role of heat exchanger designs in meeting the rigorous performance requirements of sustainable electric aviation technologies. Through meticulous analysis of various configurations and the implementation of a structured evaluation methodology, this work significantly contributes to the preliminary design of thermal management solutions for electric propulsion systems