Comparative analysis of optimal performance of a transcritical R744 and a high subcooling subcritical R290 heat pump for the production of sanitary hot water.

Transcritical CO2 cycle has been used for sanitary hot water production due to its good performance working with high water temperature glides. Moreover recent studies have demonstrated that subcritical cycles can achieve similar performance in this kind of application. Nevertheless in both cases the performance depends critically in the optimal control of cycle internal variables like the gas cooler pressure in the transcritical cycle or the subcooling in the subcritical approach. This paper presents the model results of a subcritical Propane and transcritical CO2 heat pump that have been validated with experimental results. Based on this model a theoretical study analyzing the sensitivity of COP to subcooling in the Propane cycle has been developed. Finally, the obtained results has been compared with the sensitivity of COP to gas cooler pressure of a R744 heat pump showing that it is feasible to implement a control strategy in order to maximize COP in the high subcooling subcritical heat pump cycle.