The EPA provides consumers salient guidelines to consider when purchasing alternative refrigerants. For instance, the EPA evaluates an alternative refrigerant's ozone-depleting potential, global warming potential, toxicity and flammability and, accordingly, puts restrictions on the uses of alternative refrigerants, by which consumers are legally obligated to abide. In addition, the EPA recommends that consumers compare the costs of alternative refrigerants to the costs of other substitutes. Consumers should determine whether the alternative refrigerant is compatible with the system with which they will be using it, and whether using the refrigerant will void warranties on their equipment. Consider all ten of the EPA's questions before purchasing your refrigerant.
Few substances are suitable for use as refrigerants by virtue of their thermodynamic properties, and of these substances, most are eliminated from consideration due to their flammability, toxicity, or potential to damage the ozone layer. For instance, of the methane derivatives identified by Thomas Midgeley, R32, the only remotely practical methane identified was flammable. Industry's response to the paucity of suitable refrigerants has been to blend the available refrigerants, and occasionally add hydrocarbons. R417A, for instance, is a blend that gives slightly better efficiency and glide than R407C.
Natural refrigerants divide into hydrocarbons, ammonia, and carbon dioxide. Hydrocarbons, such as propane (potentially of use in car and window air conditioning) and isobutane (highly successful in domestic refrigerators), make excellent refrigerants, in fully sealed systems of relatively low charge that limit their flammability. Ammonia, increasingly used in air conditioning systems, is especially useful as a refrigerant because ammonia leaks, accompanied by their penetrating smell, are easily detected. Carbon dioxide is used in freeze-drying systems, as a secondary refrigerant to avoid pumping chilled water or glycol and, potentially, as a direct refrigerant; depending on the availability of suitable refrigerating compressors.
Certain refrigerating systems offer an alternative to the practically reversed Rankine system, used in most refrigerating systems. While these systems do not extend beyond certain niche markets, they are highly practical. These systems include acoustic refrigeration, which uses sound intensities to cool objects, magnetic refrigeration, which extracts energy from already cold systems by de-magnetizing them, and water vapor compression, which makes use of the extremely high heat of evaporation of water vapor to obtain efficient refrigeration capabilities.