Thermodynamic and Cycle Models for a Low-Pressure CO2 Refrigeration Cycle

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).