The search for new and environmentally being refrigerants to replace the existing CFCs and HCFCs has led to the introduction of HFCs. However, HFCs have a much higher global-warming potential and higher costs than natural refrigerants. These concerns have spurred calls for the investigation of alternatives to HFCs. Some environmentalists would like the refrigeration industry to bypass HFCs and employ natural refrigerants as soon as possible.
Natural refrigerants are working fluids based on molecules that occur in nature. Examples are such substances as air, water, ammonia, hydrocarbons, and carbon dioxide. Nevertheless, the actual fluids used in refrigeration systems may very well be synthesized and will not necessarily be extracted from nature. Ammonia, for instance, is synthesized in large quantities, and hydrocarbons undergo an extensive chemical processing procedure. Still, the cost of these fluids is much lower than that of HFC refrigerants, and they do not affect the environment in an unknown way. Also, the amount of fluid produced is negligible compared with the amount available in nature.
Studies of natural refrigerants are already underway. For example, Annex 22 of the International Energy Agency implemented a three-year project, Compression Systems with Natural Working Fluids, in 1995. Air, water, ammonia, hydrocarbons, and carbon dioxide have a low or zero direct global-warming potential and zero ozone-depletion potential (ODP), as shown in the table at the top of page 98.