FIELD MEASUREMENTS AND PERFORMANCE EVALUATION OF CO_2 SUPERMARKET REFRIGERATION SYSTEMS

摘要

This paper studies field measurements and performance analysis of eight supermarkets in Sweden among which five supermarkets use CO_2 and three use HFC as the refrigerant. The differences in system design are discussed and detailed performance evaluation of one of the systems is presented. The performances of all the supermarkets are compared and the reasons for higher or lower energy efficiency are discussed. Some key components of CO_2 systems are evaluated, as well. According to the results, the older CO_2 systems have lower COP than the reference systems. High vapour fraction in low temperature cabinets inlet, relatively high amount of internal and external superheating and 10-15% lower overall efficiency of low temperature compressors compared to the other systems are some of the main reasons for this lower COP. But, the newer CO_2 systems proved to have higher efficiency, or at least, as energy-efficient as the conventional systems. This originates from the modification in the system design and more efficient components. System design modification includes two-stage expansion by receiver and flash gas by-pass. The newer CO_2 components have higher efficiency; medium temperature level compressors have 5-10% higher overall efficiency compared to HFC and older CO_2 compressors and the cabinets run with lower superheating so the system can be controlled to run at higher evaporation temperatures. This study shows that CO_2 systems have comparable COP to advanced conventional systems applied in Sweden.