Hydrofluorocarbons, currently used as refrigerants in air-conditioning systems, are potent greenhouse gases, and their contribution to climate change is projected to increase. Future use of the hydrofluorocarbons will be phased down and, thus replacement fluids must be found. Here we show that only a few pure fluids possess the combination of chemical, environmental, thermodynamic, and safety properties necessary for a refrigerant and that these fluids are at least slightly flammable. We search for replacements by applying screening criteria to a comprehensive chemical database. For the fluids passing the thermodynamic and environmental screens (critical temperature and global warming potential), we simulate performance in small air-conditioning systems, including optimization of the heat exchangers. We show that the efficiency-versus-capacity trade-off that exists in an ideal analysis disappears when a more realistic system is considered. The maximum efficiency occurs at a relatively high volumetric refrigeration capacity, but there are few fluids in this range.

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The merits of an alternative refrigerant are established based on many attributes including environmental acceptance, chemical stability in the refrigeration system, low toxicity, flammability, efficiency and volumetric capacity. In an earlier work, these criteria were used to screen a comprehensive database to search for refrigerants with low global warming potentials (GWP). The present paper summarizes the screening process and presents the performance of the ‘best’ replacement fluids for small and medium-sized air-conditioning, heating, and refrigeration applications. In addition to considering cycle calculations based only on thermodynamic properties, a simulation model that included transport properties and optimized heat exchangers was used to assess the performance potentials of the candidate fluids. The need for this more detailed modeling approach is demonstrated for systems relying on forced-convection evaporation and condensation. The study shows that the low-GWP refrigerant options are very limited, particularly for fluids with volumetric capacities similar to those of R 410A or R-404A. The identified fluids with good COP and low toxicity are at least mildly flammable. Refrigerant blends can be used to increase flexibility in choosing tradeoffs between COP, volumetric capacity, flammability, and GWP. The probability of finding ‘ideal’, better-performing low-GWP fluids is minimal.

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