Abstract
In the present paper, current states of the art of natural working fluids for heat pumps are overviewed. The coefficients of performance of ammonia and propane are as high as those of HCFC22 and HFC alternatives. Because of their flammability and toxicity, the use of these refrigerants is restricted for limited applications. Carbon dioxide has inherent disadvantage of low COP for air conditioning application. A high-performance water heater, in which carbon dioxide was used for a working fluid, was put on the market. Possibility of other natural refrigerants is discussed.
