Integrated CO2 systems for supermarkets: field measurements and assessment for alternative solutions in hot climate

In Europe CO2 transcritical booster systems have become a trusted solution in the field of commercial refrigeration. The main challenge for these systems is to become appealing in warm climates. Beside the various technological solutions that can be introduced, an interesting possibility is to fulfill all the thermal needs (refrigeration, heating and air-conditioning) with one single and compact unit. The present paper focuses on the monitoring of an integrated CO2 transcritical booster system, serving a 1500 m(2) supermarket located in Italy, able to satisfy both cooling and heating demands using one single unit. The analysis of the monitoring data shows that the unit performance is strongly influenced by the high gas cooler discharge temperature in the summer and by the implemented control strategy for heat recovery. While the parallel compression can increase the system capacity and also entirely satisfy the air-conditioning demand, the system efficiency is still strongly penalized at high external temperatures. In wintertime, the system is forced to work in transcritical condition and the use of the external evaporator allows to entirely meet the space heating and sanitary hot water demands of the supermarket. A steady-state model of the considered refrigeration system is developed and validated against the collected field measurements. The model is then used to assess some solutions to improve the performance of the integrated CO2 booster unit. The simulated configurations considered overfed evaporators, vapour ejectors and evaporative gas cooler. To face the challenge of improving the performance of CO2 system in warm climates, simulations have been run for the cities of Milan and Tunis. The results of the simulations show that vapour ejectors and evaporative gas cooler offer a good potential to reduce the electrical power consumption during the warmest periods, while overfed evaporators increase the performance almost independently from the external temperature. The combined adoption of overfed evaporators along with ejectors and evaporative gas cooler can increase up to 18% the yearly average COP in both locations compared to the original booster system configuration. For existing systems the adoption of overfed freezers and evaporative gas cooler along with the increase of the evaporating pressure at the external evaporator are considered as possible solutions to increase the performance with limited changes.