CO2 gets a second life

Certainly there has been a move towards considering and using CO₂ as a refrigerant in supermarket systems in the last several years. It has seen a tremendous increase in use in Europe, and to a lesser degree it has also seen an increase in Canada and the U.S. I admit that I have not been too enamoured of CO₂, until recently when I was motivated to do some in depth research on the subject for a contractor presentation.
Given the fact that global warming, I mean climate change, is not a topic that will leave the public eye any time soon, it is timely to discuss the merits of CO₂ as a refrigerant and how it differs from the refrigerants we are used to using.
As a preface to the CO₂ discussion, let’s review how we got to the point that a refrigerant, which fell out of favour in the 1960s, has been given a second life.
CO₂ first became popular as a refrigerant in the mid to late 19th century. One of the marketed benefits of CO₂, as compared to other contemporary refrigerants (sulfur dioxide (SO2) or ammonia (NH3)), was that it was non-flammable and non-toxic. In addition to widespread use in the shipping industry, CO₂ saw use in some commercial applications as well. It reached its heyday in the 1930s. The 1950s saw the introduction of CFCs (R-12, R-22 and R-502) into the marketplace, which was a dagger to the heart of CO₂. The 1960s saw the end of CO₂ as a viable refrigerant option.
Of course, as great as CFCs were from a practical standpoint they had a high ODP (Ozone Depletion Potential). Scientists realized that CFCs were contributing to the depletion of the ozone layer, which resulted in the Montreal Protocol. It went into effect in 1987, and established phase-out dates for CFCs, with the lower ODP HCFCs to be phased out at a later date. CFCs were initially replaced with HCFCs (R-22) and later HFCs.     
Interestingly enough, while HCFCs were less threatening to the ozone layer, in the 1990s these low ODP refrigerants were targeted as being contributors to global warming–aka high GWP (global warming potential) refrigerants. The Kyoto Protocol was created in 1997 and it set reduction targets for greenhouse gases, which included HFCs. CFCs and HCFCs were not included in the Kyoto Protocol, as they were already covered in the Montreal Protocol.
Both of these protocols allow all participating countries to control their respective reductions of refrigerants to meet their compliance obligations. In the U.S., the EPA has issued regulations under the Clean Air Act to phase out the production and importation of CFCs and HCFCs.
Which brings us full circle, back to CO₂ as it has a rather benign GWP of one. In actuality, GWP is a relative measure of the amount of heat a  greenhouse gas traps in the atmosphere. The GWP number is a comparison between the amount of heat trapped by a certain mass of the gas in question to the amount of heat trapped by a similar mass of  carbon dioxide. So, R-22’s GWP of 1810 means that a given mass of R-22 has 1810 times the effect on global warming than an equal mass of CO₂  does. For comparison, see Figure 1 for GWP values of common refrigerants in use today.
Looking at CO₂ through the lens of these drivers reveals it to be quite favourable: GWP = 1, ODP = 0. Under certain conditions it can be efficient, it is very safe, it is quite inexpensive and it is readily available. In addition, some of the challenges that led to CO₂’s demise in the 1960s have been overcome with advances in technology.
There is no question that a CO₂ system is going to be different than the typical refrigeration system that most supermarket technicians are familiar with. Let’s take a look at the similarities and the differences between a CO₂ system and the typical HCFC/HFC system.

Similarities:
1 Refrigeration is the achievement of a temperature below that of the immediate surroundings. Regardless of the type of refrigerant in use, we are still attempting to lower the temperature of the immediate surroundings.
2 Refrigeration is not cooling, but rather transferring heat from the refrigerated space to the refrigerating medium (the refrigerant). The result is a lower temperature in the space. It is still a simple heat transfer process.
3 Regardless of the refrigerant, we are using a fluid that has a saturation temperature (boiling point) that is somewhere between 5F to 20F lower than the space temperature. Whatever the refrigerant, if we are attempting to keep frozen vegetables at -10F, the refrigerant saturation temperature in the evaporator will likely be approximately -20F.

There are some very obvious differences between CO₂ and HCFCs/HFCs. But, let’s not get too caught up in those differences. After all, we are still refrigerating. We are still using a saturated liquid as the medium that we are transferring heat too. We are still applying the same basic principles that are the basis for every vapour-compression cycle used to refrigerate.
Next issue we will discuss the differences, and explore the common CO₂ systems seen in the modern supermarket.

Dave Demma holds a degree in refrigeration engineering and worked as a journeyman refrigeration technician before moving into the manufacturing sector where he regularly trains contractor and engineering groups. He can be reached at ddemma@uri.com.