**12. Summary**

Refrigerants are the key substances used in all the conventional refrigeration and air conditioning systems. Refrigerants rub though out the system and removes heat by changing its phases during the course of operation. Use of refrigerants evolved from the day of first refrigeration and air conditioning equipment. Air, carbon dioxide, ammonia, sulfur dioxide were used as refrigerants during the early day air conditioning systems. But because of the need in very low temperature applications and human comfort conditions, different artificial refrigerants were came into use. Significant artificial refrigerants are chloro flouro carbons (CFC), Hydro flouro carbons (HFC). Though the temperature produced by these refrigerants are very good for wide applications in residences and industries. But they contribute indirectly to the ozone layer damage and global warming.

To ensure the safety of environment, use of CFCs are banned and to be phased out completely as per the guidelines by Montreal Protocol. Even though, phasing out of CFCs and HFCs are not happening in developing countries, and so the threat to the environment continues. This chapter, introduces the use of phase change materials) PCM) as a potential refrigerant in air conditioning systems. Since phase change materials are thrown as waste byproducts from the petroleum refineries, use of PCMs in air conditioning systems will be additional contribution for the safety of environment.
