This part of the study investigates the performance of HFC refrigeration systems for supermarkets and compares the performance with alternative CO2 trans-critical solutions. The investigated HFC system solutions are typical in supermarkets in Sweden. The analysis in this study is based on field measurements which were carried out in three supermarkets in Sweden. The results are compared to the findings from Part I of this study where five CO2 trans-critical systems were analyzed.
Using the field measurements, low and medium temperature level cooling demands and COP’s are calculated for five-minute intervals, filtered and averaged to monthly values. The different refrigeration systems are made comparable by looking at the different COP’s versus condensing temperatures. The field measurement analysis is combined with theoretical modelling where the annual energy use of the HFC and CO2 trans-critical refrigeration systems is calculated.
Comparing the field measurement and modelling results of COP’s for HFC and CO2systems, the new CO2 systems have higher total COP than HFC systems for outdoor temperatures lower than about 24 °C. The modelling is used to calculate the annual energy use of HFC and new CO2 system in an average size supermarket in Stockholm, new CO2 systems use about 20% less energy than a typical HFC system.
The detailed analysis done in this study (Part I and Part II) proves that new CO2 trans-critical refrigeration systems are more energy efficient solutions for supermarkets than typical HFC systems in Sweden.
The three HFC systems analyzed in this paper are variations of the same technical solution which is presented in a simple schematic in Fig. 1. The system consists of two parallel refrigeration units; one serves the medium temperature level (ML) cabinets and the other serves the low temperature level cabinets; i.e. freezers (FR). Cooling is provided to the ML cabinets by a heat transfer fluid (referred to as brine in this paper) in indirect loop arrangement, while direct expansion (DX) is applied on the freezers. The liquid after the condenser is sub-cooled on both ML and FR units with the use of a separate heat exchanger; denoted as sub-cooler. The two refrigeration units are not completely isolated, since the FR unit is sub-cooled by the brine at the ML level.