Book excerpt: This project addresses the objectives of the Statement of Need number WPSON-17-20 No/Low Global Warming Potential Alternatives to Ozone Depleting Refrigerants. Its goal was to identify low global-warming-potential (GWP), non-flammable refrigerants to replace HFC-134a (GWP=1300) in military environmental control units (ECUs) and to demonstrate their performance. This work is a follow-on to the limited-scope project WP-2740, which used thermodynamic cycle simulation models alone to screen over 100 000 refrigerant blends and identified over 20 candidate HFC-134a replacements. In the present study we narrowed the pool of blend candidates down to three best blends, demonstrated their performance through drop-in tests in a military ECU in environmental chambers over a wide range of operating conditions, and extrapolated the laboratory-measured performance to that of ECUs equipped with modified compressor for each blend to provide the same system capacity while maintaining the isentropic efficiency of the original HFC-134a compressor. The project involved preliminary experimental and analytical tasks in support of the final project task. These included measurements of thermodynamic and transport properties of the novel fluids considered and an update of simulation methods for these properties, fundamental tests exploring the flammability characteristics including calculation methods, fundamental measurements and modeling of forced-convection heat transfer performance, and measurements of cycle performance of candidate blends in a laboratory mini-breadboard heat pump apparatus as the final qualification step of the best blends for full-scale testing in the ECU. The project s conclusion is that R-513A (GWP=573) and a blend we call Tern-1 [R-134a/1234yf/1234ze(E) (49.2/33.9/16.9*), GWP=640] are good replacement blends for HFC-134a offering a similar performance at GWP reduction of 66 % and 51 %, respectively. These fluids do not present any significant application difficulties. If greater reduction in GWP is desirable, R-515B (GWP=344) and CO2 (GWP=1) can be considered but they require further challenging research and developmental work. In the above blend selection, we adopted the ASTM E681 test method as stipulated by ASHRAE Standard 34 for qualifying non-flammability of refrigerants. If military requirements for non-flammability are more stringent than the E681 standard, a smaller reduction of GWP will be possible with qualifying blends.