This project addressed the objectives of the Statement of Need number WPSON-17-02 for “No/Low Global Warming Potential Alternatives to Ozone Depleting Refrigerants.” The goal of this Statement of Need is to identify low global-warming-potential (GWP), nonflammable refrigerants to replace 1,1,1,2-Tetrafluoroethane (HFC-134a) (GWP = 1300) in military equipment.
This work is a follow-on to the SERDP limited-scope project WP-2740, carried out at the National Institute of Standards and Technology, which used thermodynamic cycle simulation models to screen over 100,000 refrigerant blends and identified over 20 candidate HFC-134a replacements. The present study has narrowed down the pool of blend candidates to three “best” blends, demonstrated their performance through “drop-in” tests in a military environmental control unit (ECU) in environmental chambers over a wide range of operating conditions. Through simulation, the laboratory-measured performance was extrapolated to that of ECUs equipped with a compressor modified 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 called Tern-1 [R134a/1234yf/1234ze(E) (49.2/33.9/16.9* ), GWP = 640] are good replacement blends for HFC-134a offering a similar capacity and coefficient of 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) can be considered but it requires further challenging research and developmental work.
The GWP reduction also depends on the stringency of the military requirements for “nonflammability.” At the time of execution of the project, military requirements for nonflammability were not yet established. For this reason, the project team used the American Society for Testing and Materials E681 test method as stipulated by American Society of Heating Refrigerating and Air-Conditioning Engineers Standard 34 for qualifying non-flammability. If military requirements for "non-flammability" are more stringent than the E681 standard, a smaller reduction of GWP will be possible with qualifying blends.