Section 01 Alternatives to HCFCs in the Refrigeration and Air Conditioning Sector 28 are offering components and systems for CO2 making it possible to design a wider range of applications. CO2 is introduced in different applications both as single stage “supercritical” refrigerant, part of a cascade with a second refrigerant and as secondary fluid in indirect systems with a different refrigerant in the compression cycle. The challenge and what is going to define the future market share for CO2 is determining at what cost it will be possible to achieve competitive energy efficiency in field conditions. The special characteristics of CO2 and the need to design the systems with consideration to the local conditions to achieve competitive COP will (even more than for conventional technologies) make it necessary to evaluate solutions application by application. Discussion is ongoing if supercritical CO2 system will be energy efficient in warm climates. Special training is required to design, build and service systems for CO2 and high pressures. This includes not only dealing with the high pressures but also the technical know how to make the system energy efficient. CarbCarb on dio xide has a negligible GWP but will operate under significant higher pressure than the traditional refrigerants and cannot be used in existing equipment. As CO2 has a low “critical point” the behaviour will be different to traditional refrigeration systems. Above 31°C the system will work in “supercritical” (sometimes called “trans-critical”) conditions, i.e. traditional condensing will not take place. There are several applications where CO2 can be considered as a commercially-available alternative although the price level is often higher than that of conventional technologies. The ongoing discussion is in which applications the energy efficiency will be comparable with the one achieved in other alternative technologies. In the commercial refrigeration sector, much of the development is focused on this refrigerant due to the negligible direct GWP. The applications with the longest experience are where CO2 is used as secondary fluid in indirect systems and at low temperature industrial/commercial refrigeration in “cascade system” with ammonia (or HFC) in the high stage. A significant number of supercritical CO2 systems have been installed and several reports on the coefficient of performance (COP) of these systems have been published. The statements on efficiency vary depending on the source, and further development and more documentation from commercial installations is needed before actual COP, cost and reliability in different systems and conditions are established. It should be noted that the relative energy efficiency versus other technologies will depend on the climate where the system operates (warm climate will have a more negative effect on COP in supercritical CO2 than in conventional HFC systems). In Japan, CO2 heat pumps with supercritical operation for domestic hot water have found a large market (their development and introduction have been promoted with the help of subsidies from government and utility companies). This technology has also been introduced in Refrigerant inside: CO2Gas condenser Fig. 1.8 Supercritical CO2 system with gas cooler operating at pressures 2 to 3 times that of conventional systems.