Carbon dioxide (CO2)-based refrigeration systems have been proposed as environmentally benign alternatives to current automotive air conditioners. The CO2 vapor-compression system requires very high operating pressures and complicated control strategies. Recent experimental results indicate that operating pressures comparable to those of current automotive air conditioners can be attained by the inclusion of a secondary carrier fluid (a “co-fluid”), with solution and desolution of the CO2 from the co-fluid substituting for condensation and vaporization of pure CO2.
In this work, modeling tools have been developed to optimize the CO2/co-fluid cycle, including the selection of a co-fluid, the CO2/co-fluid ratio (the “loading”), and the operating conditions. The modeling tools are based on thermodynamic functions (e.g. enthalpy and entropy), which are shown to depend on the heat capacities, densities, and isothermal compressibilities of pure CO2 and of the co-fluid, as well as on the heat of solution of the two. The methodology is demonstrated for a CO2/N-methyl-2 pyrrolidone (NMP) mixture.
An ideal refrigeration cycle model was constructed for the CO2/co-fluid system, using the thermodynamic functions as inputs. The optimal cycles and coefficients of performance (COP) were evaluated for the CO2/NMP mixture under three different environmental operating conditions. The results demonstrate the theoretical feasibility of a CO2/co-fluid refrigeration system with a calculated efficiency intermediate between those calculated for R-134a and transcritical CO2 (assuming ideal hardware for all three).