Carbon dioxide, or CO2, is one of the candidates for the gradual replacement of other refrigerants such as fluorinated gases. Known as R-744, CO2 is a gaseous, colourless, odourless gas naturally present in the Earth’s atmosphere.
It is not a new refrigerant, nor has it only been used today. It was already in use at the end of the 19th century at the height of the industrial revolution, and was used in mass until the introduction of fluorinated gases, which required lower pressures in refrigeration systems, during the thirties of the 20th century.

Over recent years, following confirmation of the impact that fluorinated refrigerants have on the ozone layer, CO2 has once again emerged as a refrigerant of the future.

Theoretically, the critical temperature for CO2 is 31°C, and the critical pressure is 73 bar. CO2 systems operate differently depending on whether they work above or below the critical point. Therefore, refrigeration systems with CO2 can be divided into;

Subcritical systems:
In these systems, the CO2 changes from a gaseous to a liquid state when it loses heat, as is usual in refrigeration systems that use fluorinated gases. The CO2 evaporates as it absorbs heat, is compressed, and is condensed, changing from a gaseous to a liquid state as it loses heat.

Transcritical systems:
In these systems, the CO2 does not change its state (gas) and is not condensed. This is a result of higher discharge pressures which require pressure control systems and special installations with design pressures of 120 bar.

Disadvantages of CO2 as a refrigerant

Appliances which use CO2 as a refrigerant gas must withstand higher operational pressures. They therefore require more complex safety systems, more sophisticated equipment and installations, and more specialised staff.

In the event of a leak, it displaces oxygen because it is heavier than air. Therefore, it is important to have means of extraction, as well as leak detectors installed in the facility. It should be remembered that it is a colourless, odourless gas which is difficult to detect.

Advantages of CO2 as a refrigerant

Its impact on the ozone layer is considered to be non-existent. Its O.D.P. (Ozone Depletion Potential) is 0. Similarly, its contribution to the greenhouse effect is very low, with a GWP (Global Warming Potential) of 1. As a refrigerant, CO2 is the environmentally-friendly option.

Refrigeration systems with CO2 need smaller compressors compared to traditional systems with HFCs (fluorinated gases), given that they require a lower mass flow rate of refrigerant to reach the same refrigeration powers.

The current price of refrigerant is much lower than for fluorinated gases, and thanks to its O.D.P. and G.W.P., it is free from taxes.

CO2 is not catalogued as either a toxic or a flammable refrigerant gas. In the event of a leak, it does not contaminate the product, and the presence of CO2 can be removed by simply airing the zone.

Its thermal transmission coefficient (C.O.P.) is high (it has an excellent thermal exchange rate in evaporators and condensers), and pressure losses are very low, which translates into a more efficient refrigeration system.

It allows operation with exterior temperatures as low as -20°C.

The new European Union F-Gas regulation sets the prohibition of the use of fluorinated gases in refrigeration facilities above 40 kW, in both commerce and industry, for 2022.

Marc Ortí