Experimental characterization of the coupling and heating performance of a CO<inf>2</inf> water-to-water heat pump and a water storage tank for domestic hot water production system

© 2022 The AuthorsThis work presents an experimental study of the dynamic performance of a CO2 water-to-water heat pump in a domestic hot water production system. A facility was developed and used to characterize the time evolution of the COP of this heat pump, the heating and stratification processes of the hot-water storage tank, and the global COP of the system. Results showed that, when heating the water storage tank, strategies based on promoting stratification to reach Ri ~40, such as the use of vertical tank filling velocities v ~ 10-4 m·s¿1 with low water flow rates between the tank and the heat pump gas cooler, permits an increase of ~12.4% in the system global COP and a reduction of ~16% of the compressor energy consumption compared to other strategies. However, strategies based on considering higher water flow rates (i.e. Ri ~1) increase the thermal energy available in the tank (~6% when flow rate and v increases a factor 3.6) but enhance the water mixing and extend the heating time which reduces the global COP of the system. Besides, an increase of the evaporator inlet water temperature from 5 °C to 20 °C increases the system global COP by 59% and reduces the heating time ~40%.