In 1931, dichlorodifluoromethane, CFC-12, was commercially produced. Only ammonia remained in use, and its application was confined to large industrial systems. In recent years, though, natural refrigerants have gained considerable attention as alternative refrigerants for mobile air conditioners in Europe. Although natural refrigerants were used extensively in the early years of refrigeration technology, a number of technical and safety challenges caused them to be readily abandoned when CFCs became available. These challenges still exist today for air, water, ammonia, hydrocarbons, and carbon dioxide.
Air is used extensively as a refrigerant in aircraft cooling.
Its advantages are that open systems require fewer heat exchangers, aircraft have compressed air available already, and systems tend to be low in weight. Its efficiency, however, is quite poor. Nonetheless, German railways have installed air cycles in the latest generation of high-speed trains, because of weight concerns and, most importantly, because maintenance time is very short. There is no cumbersome and time-consuming refrigerant reclamation and no system evacuation is required. Water has the potential to be a very efficient refrigerant, but because it requires operation in a deep vacuum, its vapor density is quite low.