Heat pump operating with NH3 or CO2 - A comparative study.

The paper deals with a dual purpose system that operates as heat pump and refrigerator for heating and air conditioning. Due to the large temperature differences between the two costumers (one supplied with heat and the other with refrigeration) the system operates in two stages. A comparative study of the conduct of different schematics of the system when operating with NH3 or CO2 was performed. The exergetic analysis was used to point out which are the causes of inefficiencies when operating with each one of the two natural refrigerants. Observing that CO2 causes lower exergy destructions when functioning at low temperatures and also that CO2 can be used directly in the cooling chamber (while NH3 needs an intermediary agent) a new schematic is proposed operating in cascade with NH3 in the upper stage and CO2 in the lower one. The efficiency of the cascade system is similar to the two-stage NH3 system but in the first case the installation takes advantage of the fact that CO2 in the cold space is harmless in case of accidentally leakages.