Theoretical and Experimental Investigation of Methods for Enhancing the Energy Efficiency of CO2 Reverse Cycles

The increasing need for sustainable refrigeration and heat pump tech nologies has driven significant research interest towards the use of carbon dioxide (CO2) as a refrigerant. Despite its environmental advantages, the transcritical behaviour of CO2 systems often leads to reduced efficiency, especially under warm climatic conditions. This doctoral research investi gates multiple strategies to enhance the energy performance, operational f lexibility, and economic viability of CO2-based systems across different applications and configurations. The first part of the thesis explores the use of CO2-based mixtures as a means to improve the performance of conventional refrigeration cycles. Both theoretical and experimental analyses demonstrate that doping CO2 with small fractions (5–10%) of low-GWP fluids can lead to COP improve ments of up to 12%, along with reduced operating pressures. Further inves tigations on CO2/hydrocarbon mixtures for medium-andhigh-temperature heat pumps reveal potential efficiency enhancements and opportunities for subcritical operation, although safety and flammability constraints require careful consideration. Subsequent studies address integrated CO2 systems for commercial applications, focusing on combined refrigeration and space heating. Experi mental monitoring and model validation highlight the benefits of control strategies employing a Dedicated Mechanical Subcooler (DMS) and aux iliary evaporator, achieving energy savings up to 20.6%. Additionally, an innovative ejector capacity control technique based on a thermoelectric sub cooler is developed and tested, demonstrating up to 4.46% COP improve ment and enhanced operational stability. Finally, an energy and economic assessment of advanced CO2 architectures across various European climates confirms the strong potential of liquid and vapor ejector systems, as well as integrated mechanical subcoolers, with payback times as low as 0.8 years. Overall, the research confirms that CO2, when combined with advanced design concepts, hybrid configurations, and optimized control method ologies, represents a highly competitive and environmentally sustainable solution for the next generation of refrigeration and heat pump systems.