REFRIGERANTS | CO 2 There is no prospect of a regulatory change that could phase out or prevent the use of CO2 and supply is plentiful P T Diagram of CO2 The thermodynamic properties of CO2 versus other working fluids At atmospheric pressure, solid CO2 transforms directly to gas something you might recognise from the dry-ice effect used in theatres and at concerts. One of the complications with CO2 as a working fluid is that the reverse is also true, and unskilled technicians can cause solid CO2 to deposit if the pressure is not controlled properly during a service. The critical point occurs at 31C (see Figure 2). Above this point, the CO2 is a supercritical fluid. This means there is no phase change when heat is removed from the transcritical fluid we call this gas cooling. In a heat pump system, supercritical CO2 will not condense until the pressure has dropped below the critical pressure. No other commonly used working fluid has such a low critical temperature. The advantages this offers are discussed later in the article. CO2 has a GWP of 1, which is very low compared with other working fluids, such as R410A (HFC) or R448A (HFO) see Table 1 on page 70. Every system will lose some of its gas each year from service activities or from leakage between components. Manufacturers and those working with plant take considerable precautions to prevent this, but a 15% loss is not unusual. This loss of gas leads to fugitive emissions that must be accounted for in carbon footprint calculations. An example comparison is given in Table 1. F-Gas regulations have been steadily reducing the quantity of high-GWP gases available and some of the worst are now banned. This trend is expected to continue to reduce fugitive emissions and emissions created during the manufacture of synthetic gases. Not only are they being regulated out, but there have also been significant price rises in some cases by more than 300% as supplies become restricted. There is no prospect of a regulatory change that could phase out or prevent Figure 1: Pressure-temperature diagram of CO2 Log p-h diagram of CO2 (31oC/73.8 bar g) (-56.6oC/5.2 bar g) Figure 2: Log pressure-temperature diagram of CO2 the use of CO2 and supply is plentiful. Therefore, there is no stranded-asset or cost-increase risk associated with its adoption as a working fluid. It should be noted that some working fluids for example, hydrofluoroolefins (HFOs) are blends of different chemicals. Losses from HFO blends can result in the need to replace the entire volume, as it is impossible to determine which component chemicals have been lost. This results in a much higher loss of high-GWP fluid. This does not apply to CO2. CO2 is non-flammable and non-toxic CO2 is non-corrosive, non-toxic and non-flammable. It is a stable molecule that does not decompose, either in the system or when accidentally released. In comparison, a recent study by the University of New South Wales, in Sydney, Australia, suggests that elevated levels of high-GWP HFC-23 (R23) in the atmosphere could be linked to the uptake of HFO-1234ze, which produces R23 as it decomposes in the atmosphere. R23 has a GWP of 14800, making it among the worst gases for climate change. CO2 is an asphyxiant in large concentrations, so the use of detectors in confined spaces is normal. As the gas is heavier than air, it drops to the floor, where detectors should be placed. The best placement for a CO2 heat pump is outside, where the gas disperses naturally in the unlikely event of a severe leak. 68 November 2022 www.cibsejournal.com CIBSE Nov 22 pp67-68, 70, 72 CO2 paper.indd 68 21/10/2022 18:56