R744 (carbon dioxide, CO₂)
Also known as: carbon dioxide, CO2, R-744, transcritical CO₂
Short definitionA non-flammable natural refrigerant with a GWP of 1. It works at very high pressure (about 35 bar at 0 °C) and so needs its own equipment; it is replacing HFCs in supermarket and large warehouse systems.
Carbon dioxide is one of the oldest refrigerants; it came back after the 2000s with the restrictions on greenhouse gases. It does not burn, is non-toxic except at high concentration, and is cheap. Its high heat-carrying capacity keeps pipe diameters small. It is also the reference of the GWP scale: the value of every other refrigerant is given relative to CO₂.
Its critical temperature is 31 °C. In hot weather it cannot condense in the usual way and the system runs in the transcritical region at 90–120 bar. The compressor, pipes, valves and controller are selected specifically for CO₂; it cannot be built from HFC system materials. In cold climates its efficiency beats HFC systems; in hot climates parallel compression, an ejector or an adiabatic gas cooler is added. Ammonia/CO₂ cascade is common in large stores.