Carbon dioxide R744 (CO₂) as an alternative refrigerant and secondary fluid

CO₂ has a long tradition in the refrigeration technology reaching far into the 19th century. It has no ozone depleting potential, a negligible direct global warming potential (GWP = 1), is chemically inactive, non-flammable and not toxic in the classical sense. Therefore, CO₂ is not subjected to the stringent containment demands of e.g. HFCs (F-Gas Regulation), flammable or toxic refrigerants. However, compared to HFCs the lower critical value in air has to be considered. For closed rooms, this may require special safety and detection systems.

CO₂ is also low in cost and doesn't require recovery and disposal. In addition, it has a very high volumetric refrigerating capacity: depending on operating conditions, approx. 5 to 8 times as high as R22 and NH₃.

Above all, the safety relevant characteristics were an essential reason for the initial widespread use. The main focus for applications were e.g. marine refrigeration systems. With the introduction of the "(H)CFC Safety Refrigerants", CO₂ became less popular and had nearly disappeared by the 1950s.

Main reasons are its relatively unfavourable thermodynamic characteristics for usual applications in refrigeration and air conditioning.

The discharge pressure with CO₂ is extremely high, and the critical temperature at 31°C (74 bar) very low. Depending on the heat sink temperature at the high pressure side, transcritical operations with pressures far beyond 100 bar are required. Under these conditions, energy efficiency is often lower than in the classic vapour compression process (with condensation), therefore the indirect global warming effect is higher.

Nonetheless, there is a range of applications in which CO₂ can be used very economically and with favourable eco-efficiency. These include subcritical cascade plants, but also transcritical systems, in which the temperature glide on the high pressure side can be used advantageously, or the system conditions permit subcritical operation for long periods. It should further be noted that the heat transfer coefficients of CO₂ are considerably higher than of other refrigerants – with the potential of very low temperature differences in evaporators, condensers, and gas coolers. Moreover, the necessary pipe dimensions are very small, and the influence of the pressure drop is comparably low. In addition, when used as a secondary fluid, the energy demand for circulation pumps is extremely low.

In the following section, a few examples of subcritical systems and resulting design criteria are described. An additional section provides details on transcritical applications.