Carbon dioxide as a natural refrigerant

In the beginnings of mechanical refrigeration, at the end of the nineteenth century, carbon dioxide was one of the first refrigerants to be used in compression-type refrigerating machines, later gaining widespread application mainly onboard refrigerated ships, but common in other sectors of refrigeration as well. It was only immediately after World War II that CO2 was rapidly eclipsed as a refrigerant, due to the advent of the synthesised halogenated working fluids, addressed as safe and ideal refrigerants at that time. Because of the stratospheric Ozone depletion environmental issue, CFC and HCFC working fluids are now in the process of being phased out of use under the Montreal Protocol. The Global Warming environmental issue casts concern over the use of the new HFC fluids as substitute refrigerants, because of their high GWP values, which make them subject to regulations under the Kyoto Protocol. In this mixed situation, CO2 is being revisited as a fully environmentally friendly and safe refrigerant. An intense research activity on its prospective applications is underway in many research establishments in Europe, Japan and North America, and important results have already been reached in exploiting the peculiar characteristics of this high-pressure fluid operated with a transcritical cycle. In some applications CO2 systems have already been commercialised; this applies to heat pump water heaters, as a brine in indirect systems and in the low temperature stage of cascade systems. The paper critically analyses the prospects for the future return of CO2 as a working fluid, or sometimes as a brine with change of phase, in important application areas. These include air conditioning and heat pump systems in the residential and commercial sectors, commercial and transport refrigeration and mobile air conditioning.