Carbon dioxide is one of the oldest refrigerants; it lost out to the freons in the 1930s and returned after the 2000s with the greenhouse gas restrictions. It is non-flammable, non-toxic (except at high concentrations), cheap and available everywhere; because its heat-carrying capacity is very high, pipe diameters are small. Its GWP is the reference value of 1: every refrigerant’s GWP is measured against CO₂.
Its difficulty is pressure: because its critical temperature is 31 °C it cannot condense normally in hot weather and works in the "transcritical" region at 90–120 bar; in a cold room even 0 °C evaporating means 34 bar. Compressors, pipes, valves and controllers are designed specifically for CO₂. In return its efficiency beats HFC systems in cold climates; in hot climates the gap is closed with solutions such as parallel compression, ejectors and adiabatic gas coolers.
Today the majority of new refrigeration systems in European supermarkets are transcritical CO₂ racks; in large cold storage warehouses ammonia/CO₂ cascade systems are common. In Turkey too, supermarket chains and logistics warehouses are moving in this direction. The pressure–temperature chart of CO₂ is on our refrigerant page; the −56 °C triple point and 31 °C critical point are visible in the chart.