Thermodynamic Investigation and Comparison of Absorption Cycles Using Hydrofluoroolefins and Ionic Liquid

To overcome the problems of conventional absorption working fluids, the novel hydrofluoroolefins (HFOs) and ionic liquid (IL) were analyzed. The refrigerants were 2,3,3,3-tetrafluoropropylene (R1234yf) and trans-1,3,3,3-tetrafluoropropene (R1234ze(E)), and the absorbent was 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([hmim][Tf₂N]). The property models of HFO/IL and the thermodynamic models of absorption cycles were built with verified accuracy. Analyses show that the coefficient of performance (COP) of R1234yf/[hmim][Tf₂N] is up to 0.414, while that of R1234ze(E)/[hmim][Tf₂N] is up to 0.498. The better performance of R1234ze(E)/[hmim][Tf₂N] is contributed by its higher solubility and lower saturation pressure. The compression-assisted cycle not only extends the operation range but also improves the COP for both HFO/IL pairs. The minimum generation and evaporation temperatures are reduced by 17 and 11 °C under a compression ratio of 1.5. The COPs of HFO/IL are lower than those of H₂O/LiBr and NH₃/H₂O. More HFO/IL mixtures need to be explored for further performance improvement.