Optimizing working fluids for advancing industrial heating performance of compression-absorption cascade heat pump

Independent cascade hybrid heat pump (ICHHP) can bridge the large temperature gap between low-grade air sources and high-temperature industrial demands. However, the working fluids used in previous studies are just considered sufficient as long as they perform their functional role. They may not always be the most suitable choices in different situations, and the maximum efficiency of ICHHP has not been achieved. Ionic liquids (ILs) as alternative absorbents have shown the potential to enhance ICHHP performance with their unique ability to tailor thermal properties by freely combining anions and cations. However, the scarcity of IL thermodynamic properties data, underscored by limited vapor-liquid equilibrium experiments, has impeded the full exploitation of this inherent advantage. Obviously, the lack of dedicated research on screening optimal working fluids limits ICHHP performance. To address the identified limitations, the optimal fluid is recommended in this paper by comprehensively evaluating the performance among 100 fluid combinations under different working conditions. The results indicate that R161 is the best choice for the compression subloop. For the absorption subloop, the performance improvement is more sensitive to the anionic species, with the order of influence generally being [OAC]− > [Br]− > [OMS]− > [TFA]−. Specifically, H2O/[EMIM][OAC]—R161 stands out, with maximum improvements in coefficient of performance (COP) and exergy coefficient of performance of 9.4% and 5.6%, respectively, compared to other candidates. Furthermore, it doubles the COP relative to the reference fluid H2O/LiBr—R134a. Consequently, H2O/[EMIM][OAC]—R161 is the superior working fluid, significantly advancing industrial heating efficiency for ICHHP in large temperature lift conditions.