Recent research on indirect cooling systems using natural refrigerants has become increasingly common. One such solution is the gravity-induced circulation loop. The paper provides model considerations of the configuration of an indirect propane refrigeration system equipped with a circulation loop using carbon dioxide as a heat transfer fluid. Close attention has been paid to the analytical modelling of the carbon dioxide circulation loop operation. The model was formulated to determine the optimum height of the liquid downcomer based on the determination of flow resistance and heat transfer rate in evaporation and condensation processes. A validation of the proposed analytical model against the available literature on two-phase flow structure predictions and thermal performance predictions was performed. The effect of the change in the refrigeration capacity of the system on the coefficient of performance COP of the entire indirect system was analysed for the first time. The analysis was performed for three different carbon dioxide evaporation temperatures for the system’s refrigeration capacity, ranging from 0.5 to 10 kW. It has been proven that the system efficiency increases by up to 23% with an increase in the refrigeration capacity of the system. An increase in evaporation temperature in the circulation loop from −20 °C to 0 °C improves the COP of the entire indirect refrigeration system by approximately 50%. The above findings indicate that indirect cooling systems using naturally circulated CO₂ as a heat transfer fluid should be designed for operation at maximum refrigeration capacity.

This paper describes the procedure of design and manufacture of a micro-ejector proposed for miniature ejection refrigeration systems. It describes the procedure of design, fabrication, and experimentation on supersonic micro-ejectors and makes the case for isobutane as a working fluid for such systems. It was demonstrated that it is possible to design and fabricate a micro-ejector with a cooling capacity of approximately 3 W. The discussed micro-ejector was driven by a heat source with temperature below 60 °C. The evaporation temperature was approximately 15 °C. For these operating parameters, the reported entrainment ratio was approximately 0.20. The difficulties in fabricating the micro-ejector due to its small dimensions are discussed in the paper. Additionally, the potential difficulties and solutions related to ensuring and maintaining stable operation of the testing stand are presented. The performance of the proposed system is demonstrated and discussed, including relations between mass entrainment ratio, compression ratio, cooling capacity, and temperature.