Cycle-Model Assessment of Working Fluids for a Low-Pressure CO2 Climate Control System 2000-01-0578

A low-pressure CO₂-based climate-control system has the environmental benefits of CO₂ refrigerant but avoids the extremely high pressures of the transcritical CO₂ cycle. In the new cycle, a liquid “cofluid” is circulated in tandem with the CO₂, with absorption and desorption of CO₂ from solution replacing condensation/gas cooling and evaporation of pure CO₂. This work compares the theoretical performance of the cycle using two candidate cofluids: N-methyl-2-pyrrolidone and acetone. The optimal coefficient of performance (COP) and refrigeration capacity are discussed in terms of characteristics of the CO₂-cofluid mixture. Thermodynamic functions are determined either from an activity coefficient model or using the Soave equation of state, with close agreement between the two approaches. Reductions in COP due to nonideal compressor and heat exchangers are also estimated.