The goal of this study was to identify nonflammable, nontoxic, low global-warming-potential (GWP) replacements for HFC-134a in an air-conditioning system that would maintain the energy efficiency and capacity. A prior exhaustive work demonstrated that all single-component refrigerants that could serve—from the performance stand-point—as a replacement for HFC-134a are at least mildly flammable. For this reason, this study undertook an evaluation of binary and ternary refrigerant blends to explore the possibility of formulating a nonflammable blend that would satisfy the requirements of military systems.
The quest for suitable HFC-134a replacements undertaken in this study relied on an exhaustive search and evaluation of two- and three-component blends among a slate of 13 single-component refrigerants. All possible combinations of the 13 fluids were considered. A 0.04 mole fraction composition interval was applied resulting in 100,387 blends.
The selection of the “best” blends was a multi-parameter optimization process with four main objectives:
The study identified 22 refrigerant blends, 14 of which have GWP ranging from 633 to 870 and were estimated to be “nonflammable”; the remaining eight have GWP ranging from 8 to 573 and were estimated to be “borderline flammable. The COP of these blends were from 0.6 % to 2.0 % below that for HFC-134a. The Qvol varied in a larger range from -10 % to + 3.1 %. In general, the data showed a trend of COP increasing as the GWP increased.
The study yielded a novel method for estimating flammability of single-component refrigerants and refrigerant blends. This method was applied in the study in the blend selection process.
It can be concluded that it is possible to implement a nonflammable HFC-134a replacement blend with about 50 % lower GWP than that of HFC-134a. Selection of the optimal blend requires experimental validation of representative equipment under controlled conditions.
The study explored available options for replacing HFC-134a using state-of-the-art prediction methods for refrigerant thermophysical properties, flammability classification, and vapor-compression cycle performance.
For selection of the optimal low-GWP blend to be applied in the field, the present results require experimental verification.