Carbon dioxide (CO2) is perhaps best well known as dry ice, which is solidified CO2. When exposed to the atmosphere, it directly changes its state to gas, skipping the liquid state in a phase change called sublimation. Historically, dry ice was used as temperature controlled food storage, before the advent of electric refrigeration.

Alexander Twining first proposed CO2 (or today’s R-744, according to the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Refrigerant Designation Standard 34) as a refrigerant in his 1850 patent. And as with many innovations, it took other inventors to develop a system to properly implement Twining's idea. The first CO2 compression refrigeration system in Europe was built by C. Linde in 1881. Franz Windhausen, of Brunswick, Germany, recieved a patent for the CO2 compressor in 1886. Later, British company J. & E. Hall bought the patent from Windhausen and started to build ship refrigeration machines using CO2 as a refrigerant. The first of such a refrigeration system was installed in the steamer Highland Chief in 1890 for the transportation of frozen meat.

However, the later entry of synthetic halogenated refrigerants in the market commenced the decline of CO2 refrigeration systems from 1931 onward. This decline can be attributed to many reasons. First, the inefficiency of CO2 systems at higher ambient temperatures was unattractive for the ships sailing in warm climates. Lack of technological developments by the CO2 compressor manufacturers to adhere to the trend was the second reason. The third was the marketing and promotion of freons as "wonder gases" and ideal refrigerants.

Freons enjoyed domination as the most common refrigerant, that is until ozone layer depletion became a worldwide environmental concern in the latter half of the 20th century. Under the Montreal Protocol, an international environmental treaty, many countries agreed to phase out ozone-depleting substances, notably freons. Another environmental treaty, the Kyoto Protocol, which entered into force in 2005, upholds the United Nations Framework Convention on Climate Change by committing industrialized countries to limit and reduce greenhouse gas (GHG) emissions in accordance with agreed upon individual targets.

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Environmental regulations have curtailed the use of synthetic refrigerants, which has renewed interest in CO2 as an alternative, a technology that was arguably stalled for decades in the shadow of freons.

Properties of CO2

Figure 1. CO2 phase diagram.Figure 1. CO2 phase diagram.

Figure 1 shows the phase diagram (pressure-temperature diagram) of CO2. At the critical point, both liquid and vapor have the same density. The critical point of CO2 is 31° C and 73.7 bar(a). This means that beyond this state point, the CO2 is in the supercritical state.

The supercritical state is such that the substance is neither gas nor liquid. The triple point of CO2 occurs at -56.5° C and 5.17 bar(a). At the triple point, all three phases, namely solid, liquid and vapor, co-exist.

Table 1. Comparison of CO2 refrigerant with other common refrigerants.Table 1. Comparison of CO2 refrigerant with other common refrigerants.

CO2 is compared in Table 1 with other common refrigerants. The global warming potential (GWP) of CO2 is the lowest compared to other refrigerants. It has the ozone depletion potential (ODP) of 0, compared to 0.05 of R22 freon.

Despite having the lowest critical temperature, CO2 has the highest critical pressure. Hence CO2 is a very high-pressure refrigerant. This can also be confirmed by the fact that CO2 has the highest saturation pressure for the temperature of -20° C (19.7 bar) and 30° C (72.14 bar), when compared to other refrigerants for the same temperature.

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A few general properties of CO2 make it ideal for use as a refrigerant in vapor compression refrigeration systems. There is an abundance of CO2 in the atmosphere. As a refrigerant, it is less costly compared to other refrigerants. It is not necessary to reclaim the CO2 during maintenance, saving time and money. CO2 is non-flammable and non-toxic. Therefore, it poses no safety or regulatory problems, other than high working pressure. Already known and widely used lubricating oils, such as polyolester (POE), are compatible with CO2. (Note: The oils with the ISO grades higher than 80 are more suitable for CO2.)

CO2 is compatible with most materials used by the refrigeration industry. A high volumetric refrigeration capacity reduces the size of the components compared to other commonly used refrigerants. (Volumetric capacity of CO2 is 22600 kJ/m3 at 0° C, which is 2860 kJ/m3 for R134a at 0° C.)

Subcritical and transcritical refrigeration cycles

Figure 2. Pressure-enthalpy (p-h) diagram of CO2 (subcritical and transcritical refrigeration cycles).Figure 2. Pressure-enthalpy (p-h) diagram of CO2 (subcritical and transcritical refrigeration cycles).

Figure 2, shows the p-h diagram of CO2. Shown in blue, 1-2-3-4 is the vapor compression refrigeration cycle. Process 1-2 is the isentropic compression, 2-3 is the isobaric condensation, 3-4 is the isenthalpic expansion and 4-1 is isobaric evaporation. The complete cycle occurs below the critical point and is called the subcritical refrigeration cycle.

On the other hand, the refrigeration cycle A-B-C-D shown in the orange color spans both subcritical and supercritical regions, and is termed the transcritical refrigeration cycle. All the processes of the transcritical cycle are similar to the subcritical cycle, except for the fact that the gas at high temperature does not convert to liquid during isobaric heat rejection. Therefore, the heat rejection process B-C is called gas cooling against condensation in the subcritical cycle. The component in which heat rejection takes place is called the gas cooler.

The transcritical cycle is a unique feature of CO2 refrigerants, because no other common refrigerant is used in the supercritical region.

Design considerations

There are many differences to the system design, as well as the commissioning of a CO2 refrigeration system. The design must consider safety precautions for the higher working pressure of CO2. This gas poses a high risk of freeze burns. There is a chance of solid CO2 formation during charging the liquid CO2 into the evacuated system. The charging equipment, pressure gauges, hoses, valves and other components must be rated for the use of CO2 as a working fluid. The technicians must be trained to handle the CO2 as a refrigerant.

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CO2 is fast replacing freon refrigeration systems in European countries. CO2 rack systems are extensively being used for supermarket refrigeration. It is also finding popularity in high-temperature heat pumps. CO2 is being considered as the refrigerant of the future. As the demand further increases, the cost of CO2 refrigeration systems is bound to decrease, which is currently on the high side compared to conventional refrigeration systems. Engineers are working continuously toward making CO2 refrigeration systems more efficient and cost-competitive, such that it will become a viable alternative to the current refrigerants, even in transcritical operation.