Environmental Project, 951 – Comparisons of Energy Consumption for Refrigeration in Supermarkets

1 Introduction

In connection with the phasing out of CFC and HCFC refrigerants it is being discussed whether the refrigerant in new systems should be HFC or natural substances. The problem with HFC refrigerants is their very high greenhouse effect (GWP), which in future may lead to a phasing out of this group of refrigerants also. In Denmark their use is prohibited in small and large systems from January 1^st 2007. Natural refrigerants do not have this drawback but are often either flammable (HC refrigerants) or toxic (ammonia). An alternative, which is neither flammable nor classified as toxic, is carbon-dioxide (CO₂). CO₂ does, however, have some drawbacks. Firstly, the critical temperature is very low (31°C). This has a negative effect on efficiency at one stage operation when ambient temperatures are close to or above the critical temperature. Secondly, the triple point pressure is greater than the atmospheric pressure (5.18 bar). This puts special demands on installations of safety valves, since any blow-off of liquid will result in solid CO₂ being formed. A system that does not suffer from the first disadvantage is a cascade system with CO₂ in the low-temperature circuit and, e.g., propane or propylene in the high-temperature circuit. The high-temperature circuit can be very compact with a small amount of refrigerant, which can minimise the danger of fire.

The present investigation has compared the energy consumptions of two conventional systems with R404A with two newly developed cascade systems with CO₂ in the low-temperature circuit and R404A in the high-temperature circuit. Because of the location it has not been possible to get permission to use propane/propylene in the high-temperature circuit. Therefore, R404A has been used. The amount of R404A used is, however, very small compared to that used in conventional systems.