Experimental analysis of a CO2 heat pump for instantaneous domestic hot water production

The performance of a CO2 (R744) heat pump (HP) depends strongly on the application and on the environmental operating conditions. In particular, the CO2 HP becomes competitive compared to traditional heat pumps using halogenated refrigerants in applications characterized by a high hot water demand (hospitals, sport centres, etc.). This paper deals with the experimental analysis of an air-to-water commercial R744 HP tested in ENEA laboratories for instantaneous production of domestic hot water (DHW). The results show that HP efficiency is strongly affected by the inlet water temperature at the gas cooler. A test equipment capable of maintaining the ambient temperature at a desired value (between -10 °C and +20 °C), and managing water flow and water temperature (between 15 °C and 45 °C) is used to evaluate the machine COP as a function of the boundary conditions (inlet gas cooler water temperature, water flow rate, external ambient temperature). The analysis shows the considerable potential of R744 heat pumps for the instantaneous production of DHW, combining a high performance and relevant energy saving. As well, it points out that, to maximize the COP it is necessary to combine the HP with a high stratification storage system, which ensures a suited inlet water temperature, and with a correct value of water flow.