The ideal refrigerant has good thermodynamic properties, is unreactive chemically, and safe. The desired thermodynamic properties are a boiling point somewhat below the target temperature, a high heat of vaporization, a moderate density in liquid form, a relatively high density in gaseous form, and a high critical temperature. Since boiling point and gas density are affected by pressure, refrigerants may be made more suitable for a particular application by choice of operating pressure. These properties are ideally met by the chlorofluorocarbons. Corrosion properties are a matter of materials compatibility with the mechanical components: compressor, piping, evaporator, and condenser. Safety considerations include toxicity and flammability.
Until concerns about depletion of the ozone layer arose in the 1980s, the most widely used refrigerants were the halomethanes R-12 and R-22, with R-12 being more common in automotive air conditioning. R-134a and certain blends have tended to replace the earlier compounds.
Most cars have a label stating the type of refrigerant used
Following the ban on chlorofluorocarbons (CFCs) and hydro-chlorofluorocarbons (HCFCs), substances used as substitute refrigerants such as fluorocarbons (FCs) and hydrofluorocarbons (HFCs) have also come under criticism. They are currently subject to prohibition discussions on account of their harmful effect on the climate.
Natural refrigerants such as ammonia, carbon dioxide and non-halogenated hydrocarbons preserve the ozone layer and have no (ammonia) or only a low (carbon dioxide, hydrocarbons) global warming potential (GWP).
Emissions from automotive air-conditioning are a growing concern because of their impact on climate change. From 2011 on, the European Union will phase out refrigerants with a GWP of more than 150 in automotive air conditioning. This will ban potent greenhouse gases such as the refrigerant HFC (R-134a), which has a GWP of 1410. One of the most promising alternatives is the natural refrigerant CO2 (R-744). Carbon dioxide is non-flammable, non-ozone depleting, has a global warming potential of 1, but is toxic and potentially lethal in concentrations above 5% by volume. GM has announced that it will start using Hydrofluoro-olefin (HFO-1234yf) in all of its brands by 2013. This new refrigerant has a GWP rating of 4.
GWP = 100 year warming potential of one kilogram of a gas relative to one kilogram of CO2
An interesting possibility for use as a refrigerant is air. With suitable compression and expansion technology, air can be a practical refrigerant, free of the possibility of environmental contamination or damage, and almost completely harmless to plants and animals. It is however not as efficient as other substances.