Simulated performance and cofluid dependence of a CO2-cofluid refrigeration cycle with wet compression

Recent experiments demonstrate the viability of a low-pressure CO2-cofluid compression refrigeration cycle in which CO2 and a non-volatile cofluid are circulated in tandem and co-compressed in a compliant scroll compressor. This work explores the theoretical performance limitations of such a cycle operating under environmental conditions representative of automotive air conditioning and studies the dependence of this performance on the properties of the CO2-cofluid mixture. The vapor–liquid equilibrium and thermodynamic properties of the mixture are described using a previously reported activity-coefficient model. A coupled system of physically based equations that allows for consideration of both ideal and real hardware components is used to represent the system hardware and its interaction with the environment. The system efficiency is analyzed in terms of entropy generation rates in the various hardware components; entropy generation in the internal heat exchanger—a component required to achieve sufficiently low cooling temperatures—strongly influences overall system efficiency. The vapor pressure of the CO2-cofluid mixture and the heat of solution of CO2 in cofluid have large and somewhat independent contributions to the system performance: lower saturation pressure lowers the optimal operating pressures at fixed CO2 loading, while increasingly negative heat of solution contributes to higher specific refrigeration capacity and efficiency.