Using CO2, or R-744, as a refrigerant is not a new concept, but it has not had widespread commercial use since the 1930s. The use of CO2 as a refrigerant was replaced in 1930 with a new synthetic refrigerant, developed by DuPont and General Motors, known as Freon. Freon is a chlorofluorocarbon (CFC) refrigerant and was used extensively as an automobile and commercial refrigerant until it was banned in 1996, due to CFCs leading to the depletion of the ozone layer. Today the most common refrigerants are hydrofluorocarbons (HFCs), such as R-404, R-410A, R-507A, and R-134A, which do not damage the ozone layer. However, HFCs have a global warming potential (GWP) thousands of times larger than CO2. Due to the low GWP of CO2 compared to HFCs, there is a revitalized interest in CO2 as a refrigerant.
Current CO2 refrigeration systems generally fall under three different configurations: transcritical, cascade, or secondary loop systems.
Transcritical CO2 systems operate with CO2 as the only working fluid. “Transcritical” refers to the CO2 operating as a supercritical gas at temperatures greater than 88°F, and a subcritical liquid-vapor fluid at lower temperatures. Supercritical CO2 gas is unable to condense into a liquid, and a gas cooler is used instead of a traditional condenser. With lower outside air temperatures, the CO2 operates as a subcritical fluid, which operates in the same way any traditional refrigerant does. To achieve the desired refrigeration temperatures, CO2 systems operate at very high pressures – as high as 1,500 psi at the discharge of the compressor and roughly 400–500 psi on the suction side for the refrigerated temperatures desired.
A CO2 cascade system consists of two refrigeration systems with a heat exchanger acting both as an evaporator for the top system and condenser for the lower system. A traditional HFC refrigerant is used for the top system, while CO2 is the refrigerant for the lower system. With this configuration, CO2 is always a subcritical fluid regardless of the outside temperature.
Finally, secondary loop systems provide cooling with a recirculating chilled liquid loop. A traditional HFC refrigerant is used in a chiller to cool liquid CO2. The CO2 is then pumped to end uses, providing cooling.
These three configurations have advantages and disadvantages. Click here to read more about the advantages and disadvantages of CO2 refrigeration systems.