Experimental characterization of the coupling and heating performance of a CO2 water-to-water heat pump and a water storage tank for domestic hot water production system

•A reduction of vertical filling velocity a factor ∼ 3 increases the system COP ∼ 12%.•The increase of evaporator inlet water temperature from 5 to 20 °C increases COP 59%.•Richardson number (Ri) is defined based on ṁ, heat pump COP & tank characteristics.•Ri ∼ 40 in the DHW system is recommended to ensure stratification.