REFRIGERANTS | CO 2 THE CASE FOR CO2 HEAT PUMPS Gbor Bszrmnyi, from Clade, argues that CO2 should be the refrigerant of choice for heat pumps S imple comparisons between CO2 and other working fluids, such as hydrofluorocarbons (HFCs), can be misleading because CO2s low critical temperature either requires differences in system design such as the use of cascade systems or the application of a transcritical system. As a result, like-for-like comparisons are not easy to make. This article will attempt to set out the reasons why CO2 should be the working fluid of choice for heat pumps in many applications, giving comparisons where possible. The vapour-compression cycle, as used in heat pumps and refrigerators, has an intertwined history, with refrigeration being the dominant service provided to date. As a result, the terms working fluid and refrigeration cycle have become commonplace. In order that this article reflects the changing market, working fluid and heat pump cycle will be used. The history of natural refrigerants Clades Rowan CO2 heat pump CO2 was one of the very first working fluids to be used in a heat pump cycle. Subsequently, synthetic working fluids designed specifically for refrigeration were invented and employed. These synthetic fluids have very high global warming potential (GWP; a measure of the effect on global warming, measured in CO2 equivalence) and have high embedded carbon in production, but they are ideal for refrigeration. Over the past 10 years, CO2 has come back as the working fluid of choice for many applications because it has excellent heat-transfer properties, and engineering standards have advanced sufficiently to make it safe and economic. The first CO2-specific parts were developed in the 1990s, but it took several years for them to become commonly available. CO2 technologies are now considered as standard, and component availability is much wider and developing in line with the market. There are two key values for a working fluid that give rise to its behaviour: the critical point and the triple point (see Figure 1). The triple point is the only temperature and pressure at which all three phases will exist. It is unique to a substance and can be used to identify it. The critical point is the highest temperature and pressure at which a pure material can exist in vapour/ liquid equilibrium. When compared with other working fluids, CO2 has a high triple point and a low critical point, as shown opposite. www.cibsejournal.com November 2022 67 CIBSE Nov 22 pp67-68, 70, 72 CO2 paper.indd 67 21/10/2022 16:32