The selection of an available natural alternative to replace harmful artificial refrigerants depends on the application, which defines the system boundaries and the operation range. In the last ten years, CO2 was introduced as a semi ideal refrigerant due to its environmentally friendly and good performance characteristics. In some applications the unique thermophysical and performance properties of CO2 match the boundary conditions of the system and make it very close to be the ideal working fluid for that specific application.
Since the rediscovery of CO2 as a refrigerant [Lorentzen, 1990], the studies focussed on finding the possible applications of CO2 according to its properties. Commercial refrigeration accounts for almost 17% of worldwide refrigerants consumption, 11% is the percentage in cold storage applications, 28% in total (135576 tons/year) [DOE/AFEAS, 1991]. CO2 as a phase changing secondary refrigerant was studied and applied successfully in Sweden in the applications of supermarkets and cold stores. By the year 2000, 40 plants are running with capacities ranging from 10 to 280 kW; the refrigerants used in the machine room are NH3, R404, and Care50 (ethane/propane). Two and single-stage plants are used for chilling and freezing needs within the supermarkets.
This study present the basics of technologies applied in these specific applications and the main results obtained compared to the old systems with artificial refrigerants.
Installation and running costs, of the systems and the technical problems appearing during the installation of the CO2 systems are discussed. Another important issue discussed in this study is the possibility of converting the old plants to CO2 secondary systems.
The operation and the analysis of the existing plants proved that CO2 could be successfully used as an alternative for the artificial refrigerants within the application of refrigeration in supermarkets. In addition to the obvious environmental advantages by using such systems, an extra economical achievement was observed during the installation and running of the plants.
IEA Heat Pump Centre , 2000.
IEA Annex 27 Workshop: Selected Issues on CO2 as Working Fluid in Compression Systems. 2000. Trondheim, Norway