Computational studies of mixtures of refrigerants, ionic liquids and deep eutectic solvents for absorption refrigeration systems and absorption power cycle

Abstract

We used computational tools to study several working fluid mixtures for single-effect absorption refrigeration systems and absorption power cycles. In both cases we assumed the required heat was supplied by waste or solar sources at low temperatures (T < 100 °C). Firstly, we studied an absorption refrigeration cycle where the working fluid mixtures resulted from combining a widely used hydrofluorocarbon (HFC) refrigerant, R134a, with three common deep eutectic solvents (DESs) formed by mixing choline chloride (hydrogen bond acceptor, HBA) with either urea, glycerol or ethylene glycol as the hydrogen bond donor (HBD) species. In another study, we performed analysis of absorption refrigeration cycles that used three modern refrigerants R245fa, R1234zeE and HFO-1336mzzE mixed with two common DESs, choline chloride with either ethylene glycol or levulinic acid as the HBD. In both studies, the COSMOtherm/TmoleX software package was used in combination with refrigerant data from NIST/REFPROP, to perform a thermodynamic evaluation of absorption refrigeration cycles using the proposed working fluid mixtures. Afterwards, classical molecular dynamics (MD) simulations of the three mixtures were performed to gain insights of these systems at the molecular level. We evaluated a number of properties for these systems, including local density profiles, Henry's law constant of refrigerants, radial distribution functions g(r) and diffusion coefficients. Following those studies, we introduced a systematic method to screen a very large number of ionic liquid (IL) candidates, as well as 17 common DESs and 10 modern fluorinated refrigerants, to be used as working fluid mixtures in absorption refrigeration systems. From this screening process, we obtained a table of 60 IL/DES-refrigerant candidate systems ranked in terms of their computed cycle efficiency. Lastly, we performed MD simulations of a candidate working fluid mixture for absorption power cycles, consisting of the refrigerant R134a mixed with the IL 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([HMIm][Tf2N]). A thorough understanding of the fundamentals at both macro- and molecular level of ILs and different working fluids will allow more power production specially from waste heat, which could lead to decrease in pollution and human exposure. Henry's law constants of R134a in the IL were initially determined as part of this study, which is currently in progress.