Thermodynamic and environmental comparison of R744 booster supermarket refrigeration systems operating in Southern Europe

Supermarket applications are characterized by enormous direct and indirect greenhouse gas emissions. In an attempt to substantially reduce the carbon footprint associated with the food retail industry, carbon dioxide (or R744) seems to be the most suitable long-term working fluid. In fact, R744 possesses a negligible Global Warming Potential (GWP) and advantageous thermo-physical properties, besides being non-toxic, non-flammable, inexpensive and readily available. However, the energy efficiency of the pure R744 (i.e. transcritical R744 or “CO2 only”) systems still needs to be considerably enhanced when it comes to units featuring the basic architecture and operating in warm regions. The reason for this lies in the occurrence of transcritical operating conditions at which very poor performance can be ascribed to the aforementioned solutions. The full deployment on the part of the commercial refrigeration sector in high ambient temperature countries towards “CO2 only” systems is also strongly related to the presence of some remaining non-technological barriers. In this study, the performance of the state-of-the-art arrangements for pure CO2 supermarket applications was theoretically compared with that of the most currently used solutions (i.e. HFC-based systems). The comparison was based on both energy and environmental assessments. Also, the actual improvement potential achievable by a conventional R744 booster refrigeration configuration, as well as some enhancement strategies were evaluated by applying the advanced exergy analysis. All these assessments were carried out by opting for the typical running modes of retail stores located in Southern Europe. In relation to the studies available in the open literature, this work involved at least four relevant and peculiar scientific merits, such as the application of the multi-ejector concept, the evaluation of the performance of CO2 refrigeration systems integrated with the air-conditioning (AC) unit, the selection of various warm climate cities and the implementation of the advanced exergy analysis. It could be concluded that nowadays transcritical R744 ejector supported parallel systems are the most highly efficient and eco-friendly technologies for food retail industry in Mediterranean Europe, being able to successfully replace HFC-based solutions.