Monitoring of a CO2 commercial refrigeration plant to evaluate the effect of a dedicated mechanical subcooler

Since the phase out of high-GWP refrigerants started, it has been necessary to find an environmental-friendly alternative to reduce direct emissions to the atmosphere. At the same time, a high efficiency of refrigeration systems must be ensured, in order to limit also indirect greenhouse gas emissions from the electricity consumption. In commercial refrigeration, CO2 is a refrigerant which can maintain limited the detrimental emissions of a refrigeration plant, and simultaneously can guarantee a high level of safety when used as working fluid. But while the environmental impact of refrigerant leakage and safety risks are easily minimized by the use of CO2, thanks to the fact that it has a GWP of 1, is non-flammable and non-toxic, achieving high energy efficiency of the plants is not trivial. This is mainly due to the fact that the refrigeration cycle becomes transcritical at relatively low heat rejection temperature. Several strategies can be implemented in order to increase the energy efficiency of CO2 refrigeration systems. Among most effective techniques, ejectors [1], overfed evaporators [2] and parallel compression [3] allow to reach an energy performance comparable, or even higher, to that achieved using traditional fluids [4]. In addition, CO2 refrigeration plants can also be profitably integrated with space heating, domestic hot water and air conditioning systems, to the limit case of an all-in-one system which can provide all the thermal loads of a commercial building with a high energy efficiency, as demonstrated by Karampour et al. [5]. Another way which can be used to increase the performance of a CO2 refrigeration plant is the use of a Dedicated Mechanical Subcooler (DMS). It consists of a small refrigeration unit which cools down the refrigerant at the outlet to the condenser/gas cooler of the main plant. As shown, theoretically in D’Agaro et al. [6] and experimentally in Llopis et al. [7], it can be a viable solution to increase energy efficiency of a commercial refrigeration unit. However, there is still a lack of real application monitoring [8]. In this work, the preliminary results of a real commercial CO2 plant are presented, with the aim to confirm through field data that the use of a DMS can achieve a decrease of energy consumption of a real commercial plant.