Experimental & numerical analysis of DHW transient production using CO2 heat pump

• Transient performance of transcritical CO 2 heat pump during DHW production cycles is analysed. • 1D model based on FEM is proposed to simulate stratification in the DHW storage tank. • Transient simulations of realistic DHW usage using EN16147 Standard are benchmarked. • An experimental setup of a CO 2 heat pump and an instrumented DHW storage tank was used. Domestic hot water (DHW) production is a major contributor to energy use in buildings, highlighting the need for efficient and sustainable technologies. This study investigates the performance of a transcritical CO 2 heat pump for DHW production under the European Standard EN16147, which provides a rigorous framework for evaluating system efficiency under realistic demand profiles. The novelty of this work lies in the combined experimental assessment and the development of a validated one-dimensional finite-difference model to simulate thermal stratification in the storage tank. The experimental setup consisted of a full-scale CO 2 water-to-water heat pump coupled to a stratified storage tank, tested under transient extraction and reheating cycles. Results show that the system achieved a global coefficient of performance (COP) of 2.89, with instantaneous peaks exceeding 8 during extraction events due to stratification effects. The numerical model reproduced the temperature evolution with good agreement, confirming its ability to capture transient tank behaviour. Simulations showed that the reduction of the water flow rate at the gas cooler from 1100 to 300 kg·h −1 results in an increase of the overall COP of the system over 12 %. These findings demonstrate both the efficiency of CO 2 heat pumps for DHW applications and the value of the model as a predictive tool for optimizing tank design and operating strategies.