There has been a lot of work in the last decade on using carbon dioxide as a refrigerant. It is inexpensive, non-flammable, non-toxic, compatible with most materials, relatively friendly to the environment compared to fluorinated refrigerants, and has the potential to be quite efficient. There is one large obstacle to using CO2 as a refrigerant: its critical point. The critical point is the highest pressure and temperature where the refrigerant can still condense. At and above the critical point there is no distinction between gas and liquid, so no condensation or evaporation can take place. The very top of the hump in a refrigerant enthalpy diagram is the critical point. A normal refrigeration system is a subcritical system because all the system components operate at pressures and temperatures below the critical point. (See Unit 18 in Fundamentals ofHVACR for more details on refrigeration enthalpy diagrams.) The problem is that critical temperature for CO2 is 88°F. The condenser saturation temperature for most refrigeration systems is above 88°F, especially for air cooled systems that must use hot outdoor air for cooling the condenser. This prevents the use of CO2 in a “normal” refrigeration system which uses an evaporator to absorb heat and a condenser to reject heat.
One application of CO2 refrigerant is called a transcritical system because part of the system operates above the critical point and part of the system operates below the critical point. Subcritical, supercritical, and transcritical all describe a refrigeration system’s operating pressures and temperatures relative to the critical point of the refrigerant in the system. A typical “normal” refrigeration system is a subcritical system because everything happens below the critical point. In a supercritical system, all the components operate at pressures and temperatures above the critical point. A supercritical system operates on gas compression and expansion with no change of state. All heat transfer occurs by the gas changing temperature. A transcritical system operates both above and below the critical point. Heat is absorbed in an evaporator where liquid evaporates to a gas, but heat rejection takes place above the critical point, so there is no condensation in what normally would be considered the condenser. The refrigerant does not condense back to a liquid until after the pressure is reduced. For more details on transcritical refrigeration systems, see Unit 85 Commercial Refrigeration Systems in Fundamentals of HVACR, 2nd edition.