This project targets the objectives outlined within the Statement of Need WPSON-23-C1 “Low Global Warming Potential Alternatives Refrigerants for Military Use” concerning the development of alternative refrigerants and solutions to replace Hydrofluorocarbon (HFC)-134a and/or HFO1234yf in air-conditioning systems. A holistic approach encompassing thermodynamic property modeling for new candidate refrigerants (and their mixtures), component sizing, and a fundamental understanding of military-style threats (e.g., multi-scale combustion mechanics, energetic materials, etc.) is of paramount importance. This approach develops a better understanding of alternative refrigerants (especially their thermodynamic properties) and system architectures for mobile air-conditioning (MAC) that will meet military requirements and future low-Global Warming Potential (GWP) limits. The property, component, and system models developed in this work will enable further optimization of the MAC systems for mission profiles in non-temperate climates (polar, desert, etc.). This project leverages expertise in the heating, ventilating, and air conditioning refrigeration field at the Ray W. Herrick Laboratories, the interdisciplinary research on energetic materials within the Purdue Energetics Research Center at Purdue University and ongoing collaboration with the U.S. Army, as well as the extensive knowledge on thermophysical properties of National Institute of Standard and Technology (NIST). Moreover, the extensive research findings on low-GWP alternatives to HFC-134a obtained in the SERDP funded project WP-2740 (NIST, 2018) and the follow-on project WP19-1385 (NIST, 2021) have been considered while defining the research objectives of the present work.

The technical approach of this project is based on four major hypothesis-driven tasks that include a combination of numerical and experimental methods: (1) thermodynamic modeling and screening for MAC applications including various climate conditions; mixture properties measurements and development of predictive schemes; (2) extensive multi-scale flammability testing to develop a comprehensive flammability chart and gain deeper understanding of flammability behaviors of candidate low-GWP mixture refrigerants under military-style threats; the flammability studies will cover control tests based on standards, detonation and combustion studies, and military relevant studies; (3) MAC system performance testing with a pair of psychrometric chambers and load-based testing capabilities to emulate real-word operations; (4) detailed system modeling to conduct numerical soft-optimization and hard-optimizations.

The results from this project will enhance the knowledge of MAC systems for military applications to meet further low-GWP refrigerant requirements as well as maintaining performance and efficiency. Moreover, the design tools that will be developed in this effort can be employed in other air-conditioning systems of interest to the Department of Defense. The thermodynamic screening will provide a deeper understanding of non-flammable blend candidates with 750<GWP100yr<150 and GWP100yr<150 that will enable mid-term drop-in solutions through soft-optimizations as well as long-term next-generation systems. The modeling results will be supported by mixture property measurements and in-system performance testing. The multi-scale flammability studies will quantify the risks associated with mid- and long-term low-GWP refrigerant alternatives for MAC applications suitable for military applications and in various scenarios. The holistic approach outlined will link fundamental understanding of thermophysical properties of refrigerants with system performance while ensuring safety and survivability of the warfighters.