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:
- Minimize/eliminate flammability:Military specifications require a flammability class designation of 1 (“no flame propagation”). For the estimation of flammability classification, a novel method was developed combining two flammability metrics: the adiabatic flame temperature and the ratio of the number fluorine atoms to (fluorine + hydrogen) atoms in the molecule.
- MinimizeGWP: TheGWP of a blend is defined as themass-fraction-weightedGWP of the blend’s components. It is easily calculated once GWP values of the components and their composition in the blend are known.
- Maximize the coefficient of performance (COP): The COP characterizes the efficiency of the system. The larger the COP, the better the system efficiency. To estimate the COP, two cycle simulation models of different simulation detail were used. A simple model was used in the initial screening, and a more detailed model was used for the final assessment of the selected “best” blends.
- Match the volumetric capacity (Qvol) of the baseline HFC-134a system: Qvol of an air-conditioning system is a figure of merit that captures the size of the compressor. The larger Qvol, the smaller the compressor needs to be for a given cooling capacity. As with COP, the Qvol were obtained from the two cycle simulation models of different sophistication.
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.