Transcritical CO2 systems have advanced considerably and are being used in many settings all over the world.
The transcritical CO2 refrigeration (R744) cycle was invented by Gustav Lorentzen as an alternative to synthetic refrigerants in the early 1990s. Since then, it has been used widely in Europe and Japan, and is being increasingly adopted throughout the world.
Sahu et al. (2017) describe R744 as a refrigerant with excellent thermodynamic and heat transfer properties. It is non-toxic and non-flammable, though it poses the risk of asphyxiation in small closed areas.
CO2 offers excellent thermophysical and heat transfer properties. Further, CO2 is cheap, easily available, non-toxic, non-flammable, which makes [it] a wise choice over other natural refrigerants.” — Sahu et al. (2017)
A transcritical CO2 system uses only R744 and can work above the critical point. The term critical point does not refer to a danger point but to the point above which it exists as a supercritical fluid.
For CO2, the critical point is at 31°C (88°F), which is lower than that of other commonly used refrigerants. A system using R744 operates in transcritical mode when the condensing temperature exceeds 31° C. At this point, no distinction can be made between the refrigerant as a fluid or a vapour. Due to this, the condenser acts as a gas cooler.
A transcritical system’s efficiency can decrease when it operates above in warm temperatures above the critical point. However, components such as an adiabatic condenser, a parallel compressor, an ejector or a subcooling system can allow the system to operate efficiency in warm climates.
Emerson and Danfoss summarise the properties of the refrigerant on their websites, including R744 leading to higher operation pressures of the system due to its thermodynamic characteristics.
The term “critical point” has been the cause of misunderstandings as the word “critical” today often is used in the sense of “dangerous” or “serious.” — Danfoss
Gullo et al. (2017) compared the energy efficiency of R404A and R744 systems using the latest technology. The researchers found that transcritical CO2 refrigeration systems require 12 to 37% less energy than a state-of-the-art R404A system, even in Mediterranean climates. Emerson argues that simple comparisons of R744 with other refrigerants are difficult, due to the difference in design that stem from the lower critical point and the higher pressures used by these systems.
Fourie (2014) explains that R744, can be used in many application and climates, but drop-in replacements are not possible. Some of the applications include:- Heat Pumps (for domestic hot water heating, commercialised in Japan under the brand name "ecocute")
- Commercial refrigeration (the most popular application so far, including integration with air conditioning and heat recovery systems)
- Industrial refrigeration (in systems up to 4 MW, such as food processing, wineries, ice rinks, etc.)
- Mobile air conditioning
See the articles below to find out more about specific applications.
IGA Market Central opts for CO2 transcritical
Henningsen to install CO2 system in existing Pa. plant
China's second transcritical CO2 store opens in Beijing
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