(137g) Modeling a CO2 Refrigeration Cycle Using an Ionic Liquid Co-Fluid

A number of popular vapor-compression refrigerants, which have been used successfully for decades, have come under environmental challenge for their potential to deplete stratospheric ozone and/or to contribute to global climate change.  While current refrigerants have little or no ozone depletion potential, many of the materials used still have a quite significant global warming potential (GWP) relative to carbon dioxide (for which GWP = 1 by definition).  CO₂ itself is in fact an attractive refrigerant, and has been suggested and used as an environmentally benign replacement for high GWP refrigerants.  Vapor-compression cycles based on CO₂, however, have very high operating pressures and limited coefficients of performance.  Pressures can be reduced and efficiency increased by circulating CO₂ in tandem with a liquid (a “co-fluid”) into which the CO₂ can be absorbed.   The resulting co-fluid cycle is analogous to the vapor-compression cycle with condensation replaced by absorption and evaporation replaced by desorption [1].  Ideal co-fluids will have low volatility, good chemical and thermal stability, and a tunable strength of CO₂ absorption.  Previous work was based on physical absorption into organic liquids, providing only a limited range of absorption strength. 

Recently, aprotic heterocyclic anion (AHA) ionic liquids (ILs) have been proposed for use as co-fluids, due to their extremely low volatility, stability over a wide temperature range, and capacity for CO₂ absorption.  Furthermore, since these materials absorb CO₂ both chemically and physically, their affinity for CO₂ can be finely tuned through appropriate chemical functionalizations.  To analyze their performance in a refrigeration system, we have developed a co-fluid cycle model that incorporates both physical and chemical absorption of CO₂ by ILs.  The model describes the compression, expansion, and heat-exchange processes in terms of basic physical and chemical principles and predicts optimal operating pressures and resulting coefficients of performance.  Principal inputs are the enthalpies and entropies of solution as may be derived from experimental absorption isotherms or predicted by ab initio quantum chemical calculations.  By identifying desirable ranges for the chemical properties of the co-fluid, the model provides theoretical guidance for the design of suitable ILs.

[1] See citations in Mozurkewich et al, Int. J. Refrig. 25, 1123 (2002).