Field measurements of supermarket refrigeration systems. Part I: Analysis of CO2 trans-critical refrigeration systems

• Five Swedish supermarkets using three CO2 refrigeration solutions are analyzed.
• A trend of improvement in energy efficiency has been observed.
• Parallel CO2 system with indirect heat rejection offers the lowest energy efficiency.
• CO2 trans-critical booster with gas removal achieves up to 35% COP improvement.

This study investigates the refrigeration performance of three CO2 trans-critical solutions based on field measurements. The measurements are carried out in five supermarkets in Sweden. Using the field measurements, low and medium temperature level cooling capacities and COP's are calculated for ten-minute intervals, filtered and averaged to monthly values. The results indicate that the systems using trans-critical booster system with gas removal from the intermediate vessel have relatively the highest total COP. The reasons are higher evaporation temperatures, lower internal and external superheat and higher total efficiency of booster compressors. Another important factor is gas removal from the intermediate vessel which leads to higher COP of low temperature level. Comparing the older and newer installed systems, a trend in energy efficiency improvement has been seen. The study shows this improvement originates from both changes in the system design (e.g. two stage expansion) and components efficiency improvement (e.g. higher total efficiency of compressors - lower internal superheat and higher evaporation temperatures of cabinets).