Optimisation énergétique d’un groupe frigorifique au CO2 transcritique

Carbon dioxide (CO2, R744) is a promising alternative for refrigeration systems, as it does not contribute to ozone depletion and has little impact on global warming compared with other conventional refrigerants such as HFCs and HFOs. In addition, it is neither toxic, flammable nor corrosive. However, CO2 refrigeration systems have limited performance due to their low critical temperature (31.1°C), which can be lower than the outside temperature. In addition, CO2 critical pressure is very high compared with that of other refrigerants. For these two reasons, the CO2 thermodynamic systems often work in transcritical conditions and at very high pressure (over 73 bar). The aim of this thesis is to study different solutions for improving the performance of single-stage CO2 refrigeration systems (one single evaporation stage) in the production of positive cold for supermarket refrigerated display cases or for air-conditioning applications. It focuses on improvements related to the architecture of the refrigeration system, which means the use of specific components such as internal heat exchangers, ejectors, parallel compressors, etc. First, a literature review was conducted on various configurations proposed in existing literature. This was followed by an experimental study to assess the performance of these configurations, including the addition of heat exchangers, a parallel compressor, and a liquid multi-ejector. These components were tested on an industrial-scale controlled test bench with a cooling capacity of 40 kW. The experimental results highlighted the advantages of operating with reduced superheat (1.8 °C) rather than positive superheat (8 °C). Notably, there was a significant performance difference between using a parallel compressor and a flash gas valve. The use of a flash gas valve resulted in an increase in the Coefficient of Performance (COP) of approximately 6 % during transcritical operation with an 8 °C superheat. Conversely, a parallel compressor provided an improvement of approximately 16 %. With reduced superheat, these improvements were approximately 10 % and 18 %, respectively. Furthermore, the addition of a subcooler, an economizer, two internal heat exchangers, or a single-phase ejector all exhibited improvements compared to the reference system. However, there were no significant performance differences among these modifications. The thesis also shows that the choice of equipment must consider the climatic conditions in which the machine will be installed. Based on experimental data, a correlation of COP as a function of Carnot COP and cooling load was developed for each configuration in order to carry out an annual performance study. Four French cities with different temperature profiles were chosen for the study: Nancy, Marseille, Paris and Brest. By choosing the most efficient configuration for each city, it is possible to improve the annual energy performance of the refrigeration plant by around 7.5 %.