Impact and quantification of various individual thermodynamic improvements for transcritical R744 supermarket refrigeration systems based on advanced exergy analysis

Abstract

Climate change mitigation actions have led transcritical R744 refrigeration systems to take center stage in supermarket applications worldwide. Various measures aimed at making these solutions highly performing in any climate context, in fact, have been developed in the last few years. However, their benefits have been evaluated and quantified almost uniquely with the aid of conventional tools, e.g. energy and environmental assessments. Therefore, in this work the most power thermodynamic tool for such a purpose, i.e. the advanced exergy analysis, was applied to the state-of-the-art transcritical R744 supermarket refrigeration systems for the first time ever. Three advanced solutions, namely R744 unit with parallel compression and a R744 system using parallel compression and overfed evaporators with and without two-phase ejectors, were selected in addition to two baselines, being conventional and improved basic R744 units. The investigation was based on the outdoor temperature of 40 °C and operating conditions derived from field measurements. The results obtained suggested that the conventional basic R744 unit features total irreversibilities of 194.49 kW and total unavoidable irreversibilities of 81.96 kW, whereas total irreversibilities and total unavoidable irreversibilities respectively amounted to 168.80 kW and 81.44 kW for the improved basic R744 system. As parallel compression was considered, potential reductions from 143.30 kW down to 67.20 kW, i.e. approximately 18% lower in comparison with the conventional basic unit and the improved basic system, were assed. The use of overfed evaporators was found to lead to potential decrements in total irreversibilities down to 63.79 kW, which were around 5% lower in comparison with parallel compression and 22% lower than with the conventional basic unit and the improved basic system. In addition, the outcomes associated with advanced exergy analysis implementation revealed the need to adopt two-phase ejectors as well as more efficient parallel compressors. The first measure would permit decreasing the total irreversibilities potentially from 115.04 kW down to 44.35 kW, resulting in decrement by about 46% compared to conventional and improved basic R744 systems, 34% in relation to the unit with parallel compressors and 30.5% over the solution with parallel compressors and overfed evaporators. Also, if 10% more efficient parallel compressors had been available, total irreversibilities, total unavoidable irreversibilities and total avoidable irreversibilities of the solution with two-phase ejectors would have been further reducible by 9.3%, 2.6% and 13.5%, respectively. It could be concluded that ejector-based transcritical R744 supermarket refrigeration systems possibly with high performing parallel compressors are capable of providing great thermodynamic performance at high sink temperatures. In addition, the vapour ejectors were found to be responsible for 22.9% of the total avoidable inefficiencies. These irreversibilities could be decremented by almost uniquely enhancing the performance of the ejectors themselves. However, unlike the results derived from conventional energy and environmental analyses, the outcomes of the advanced exergy methodology did not reveal a significant contribution on the part of the overfed evaporators to the thermodynamic performance enhancement of transcritical R744 supermarket refrigerating plants.