Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) have been used as refrigerants since the 1930s. Because of their ozone depleting effect and the phaseout of the production of these chemicals (production of Class I ODSs was banned as of January 1, 1996), development of alternative refrigerants and refrigeration and air conditioning processes are becoming increasingly important.

Air conditioning and refrigeration use the principle of vapor compression to achieve a cooling effect. This process has long relied on CFCs and HCFCs as the refrigerant materials of choice for use in the vapor compression process. The discovery of their probable effect on the ozone layer has resulted in the development of alternative processes, as well as development of new refrigerants.

The first substitute refrigerants for CFCs and HCFCs have been developed and are known as hydrofluorocarbons (HFCs), since they do not contain any chlorine atoms, HFCs are already beginning to be used. Due to the concern for future regulation of HFCs for global warming, other processes are being looked at to replace them in the long-term.

Applications for:

• Vapor compression using hydrocarbons, ammonia, carbon dioxide, or water:
  • Ammonia - refrigerated warehouses and industrial processes;
  • Hydrocarbons - industrial applications and more recently small appliances;
  • Water - above 0^o C applications only, such as air conditioning;
  • Carbon dioxide - stationary air conditioning and potentially automobile air conditioning in the future; Being used in small appliances in many parts of the world, but not in the U.S.
• Evaporative cooling (direct and indirect):
  • Residential and industrial air conditioning systems
• Gas expansion:
  • Transport of perishable substances
• Absorption: Industrial processes with excess waste heat but also needing refrigeration, gas fired systems are often used in remote areas where electrical costs are high or the supply of electricity will not meet demand, often used in conjunction with electrically powered vapor compression systems to reduce peak load power demands.
• Stirling Cycle: Practical only for small applications
• Air (Joule) Cycle: Not practical in many applications due to high power requirements
• Thermoelectric Cooling: Small applications, not economically viable in most larger applications due to its low efficiency, often used in specialty applications where low noise or high reliability is desirable e.g. on submarines
• Magnetic Cooling: Without cost considerations and very low temperature requirements

• Vapor compression using hydrocarbons, ammonia, carbon dioxide, or water - zero ozone depletion potential (ODP), zero global warming potential (GWP) (except negligible for carbon dioxide and hydrocarbons), widely available, and good thermal properties.
• Evaporative cooling (direct and indirect) - zero ODP and GWP, high efficiency in dry climates, provides humidity, improves indoor air quality, high air flow rates, commercially available, life cycle is cost effective, adaptable to various energy sources.
• Gas expansion - zero ODP and GWP, simple mechanical design, and low capital costs
• Absorption - zero ODP and GWP, can use waste heat, reliable (few moving parts), commercially available, most economically viable when waste heat is available.
• Adsorption - zero ODP and GWP, energy efficient, can use waste heat.
• Stirling Cycle - zero GWP, can be used over wide temperature range, theoretically high efficiency.
• Air (Joule) Cycle - zero ODP and GWP, non-toxic, non-flammable, low installation and maintenance costs.
• Thermoelectric Cooling - zero GWP, immediately available, high reliability, small, no moving parts, wide cooling range (-100 to +125 degrees C).
• Magnetic Cooling - zero ODP and GWP.
• Thermoacoustic Cooling - zero ODP and GWP, no moving parts.

• Vapor compression using hydrocarbons, ammonia, carbon dioxide, or water - Ammonia and hydrocarbons are flammable, ammonia is toxic, and water and carbon dioxide systems are generally bigger and more expensive.
• Evaporative cooling (direct and indirect) - high equipment costs and service requirements; usually works poorly in high humidity climates, new techniques such as indirect evaporative cooling and use of desiccants are expanding evaporative cooling into more humid climates; retrofits difficult for existing vapor compression systems.
• Gas expansion - low efficiency, high refrigerant costs, limited applications.
• Absorption - less efficient than vapor compression, Lithium Bromide (Li Br) can be toxic.
• Adsorption -low cooling efficiency, large equipment, high cost, not available in short term.
• Stirling Cycle - low demonstrated efficiency, significant materials development required.
• Air (Joule) Cycle - low efficiency, high power requirements.
• Thermoelectric Cooling - low efficiency, not efficient enough for large applications.
• Magnetic Cooling - very high costs, low efficiency, superconducting materials required, high magnetic fields require shielding.
• Thermoacoustic Cooling - low efficiency, still requires long term development.