In the old days, the carbon dioxide (CO2) gas, also known as R744 by international designation, was widely used in refrigeration machines, especially for seagoing systems; but after the Second World War, it became completely abandoned because of the advent in the market of the Chloro-Fluoro-Carbon (CFC), the halogenated synthetic refrigerants.
The 1988 is the year in which the TIME magazine does not celebrate “The man of the Year” on its cover but suggests the “Planet of the Year: Endangered Earth”, endangered by the ozone depletion and by pollution. Being just a little bit romantic, we can imagine that the same year a Norwegian professor, called Gustav Lorentzen had reinvented the use of carbon dioxide as refrigerant.
Cover of the TIME magazine, 1988. www.time.com
In the 1994, the scientific journal “International Journal of Refrigeration” publishes an article by the same prof. Lorentzen entitled: “Revival of carbon dioxide”, which meant taht the CO2 was coming back in fashion. From that time to the present days both the academic and the industrial research and innovation process have led to the development of CO2 refrigeration machines, which can be applied in many fields of the refrigeration and air conditioning: from the automotive market to the commercial refrigeration systems.
From the chemical point of view, the carbon dioxide is colourless, odourless and is also heavier than air. With a Global Warming Potential equal to 1, CO2 is actually the reference value for comparing other refrigerant’s direct impact on global warming. Carbon dioxide carries an A1 safety classification, indicating that it has low toxicity and is non-flammable. With a long atmospheric lifetime, CO2 does not lead to any by-product formation or decay products with serious environmental impact. Moreover, it is largely available and is very cheap, if obtained from industrial waste.
The figure shows the p, h diagram for the carbon dioxide. This refrigerant presents a low critical temperature (Tc= 31.06 °C) and a critical pressure of pc= 73.84 bar. Considering an idealized refrigeration cycle between +30 °C / -20 °C, which is characterized by isobaric two-phase processes and by an isentropic compression, it is possible to compare the carbon dioxide with other two synthetic refrigerants: R410A and R134a.
The condensation pressure at which the CO2 condenser should work is greater than 70 bar, while for the other two refrigerants, it is lower than 20 bar for R410A and lower than 8 bar for R134a.
Generally speaking, refrigeration cycles operating with CO2, present way higher pressure levels when compared to the other refrigerants at constant values of the external heat sink temperatures.
This implies that all the components of the refrigeration cycle, from the piping to the compressor, have to be designed to resist very high pressure levels, sometimes exceeding 100 bar.
In addition, low critical temperature entails low values of the Coefficient of Performance (COP), as reported in the table. Furthermore, the average air temperature of summer in the temperate climate zones is very close, or sometimes higher, than the critical temperature of the R744. Therefore, the rejection of the heat load in the ambient cannot be carried out through the condensation of the working fluid and this process has to be substituted by a single phase heat transfer process, where the CO2, as supercritical gas, is gradually cooled.
The condenser will be substituted by a gas-cooler, the thermodynamic trasformation is reported in the figure below by the 2-3 and 2’-3’ transformations.
Transcritical carbon dioxide cycle.
The refrigeration cycle reported in the above figure is commonly used in water cooled heat pumps for civil applications. The gas cooling process with CO2 in transcritical conditions requires a particular attention while designing the gas-cooler heat exchanger, because, the thermo-physical properties of the carbon dioxide can suddenly change near the critical point
Considering the data reported in the table above, it is worthy to notice that the high value of the CO2 volumetric cooling effect, permits to reduce the overall sizes of the many components of the systems, as compared with similar machines operating with traditional fluids. Furthermore the low volumetric flow rate, due to the high density of both the suction vapor and of the latent heat of vaporization, also allows for reducing the tube sizes as compared to the traditional solutions.
Finally, the possible application of the carbon dioxide essentially depends, from the technological development of rational systems, which control the energy consumptions, especially in the case of transcritical cycles. Even though the CO2 gas a null direct impact on global warming, its low efficiency can dramatically increase the indirect impact on global warming (simply because of the energy consumption) so much that its important characteristics, to be a natural fluid, can just be ignored.
- Cavallini, D. Del Col, L. Doretti, C. Zilio, I fluidi frigorigeni processi di sostituzione e nuove frontiere tecnologiche, 2007, n. 26, Progetto Novimpresa, AREA Science Park, Trieste, Italia.
- SHECCO, Guide 2012: Natural Refrigerants Market Growth for Europe, 2012, Shecco Publications