Carbon Dioxide, R744 — the new old refrigerant
CO2 is experiencing a massive resurgence world-wide after its «re-invention» by Gustav Lorentzen in the early ’90’s. CO2 was one of the most widely used refrigerants alongside ammonia in the early 20th century, but was displaced by the early CFC refrigerants, and practically disappeared from use as refrigerant by the 1950’s. Hence much of the technical understanding relating to its use as refrigerant has been lost, and research is underway on many fronts to regain an understanding for the potential of this abundant gas as refrigerant.
It is ironic that CO2, which is the most common greenhouse gas and much vilified in the media as the main cause of global warming, should in fact be regarded as an environmentally benign refrigerant. However, as the Global Warming Potential values for HFC refrigerants are several thousand times higher than for CO2, leakage of CO2 gas from a refrigeration system is consequently several thousand times less damaging to the environment than the release of any HFC gas.
This fact alone, along with the low cost and excellent thermodynamic properties of CO2 is driving its renaissance throughout the industry.
The temperature-pressure chart for CO2 (above) illustrates the various phases of CO2 that can be encountered in a refrigeration system. At low pressures and temperatures CO2 can occur as a solid, whilst at high temperatures above 31.2 ºC CO2 enters the transcritical phase, in which the substance has characteristics of both liquid (very dense) and gas (compressible). Two singular points where three phases can occur at the same combination of pressure and temperature exist: The triple point, at which vapour, dry-ice and liquid can co-exist, and the critical point, at which liquid, vapour and transcritical fluid coexist.
Efficiency of CO2 refrigeration systems
CO2 lends itself to energy and cost-efficient refrigeration systems. Whilst the detailed technical explanation goes beyond this overview, CO2 has several intrinsic properties that are very favourable, including high density (which results in small suction lines), shallow temperature/pressure curve (which results in small suction line losses) and excellent thermodynamic properties (which results in small heat exchangers and close approach temperatures).
The following graphs for medium and low temperature refrigeration were prepared by Enex, a leading Italian supplier of transcritical CO2 systems for supermarket refrigeration, to illustrate the energy benefit of these systems, particularly at low ambient temperatures. At temperatures below 20% (for instance, most parts of New Zealand exhibit these temperatures for >80% of the year) both medium and low temperature transcritical CO2 systems have substantial energy benefits.
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