Performance analysis and operation optimization of air-to-water CO₂ heat pump with phase change thermal storage

When CO₂ heat pump is used for space heating, the high return water temperature leads to excessive throttling loss and low efficiency of the heat pump unit. In order to solve this problem, an air-to-water CO₂ heat pump unit combined with phase change thermal storage for space heating was proposed and its feasibility has been analyzed. On the basis of previous research, a simulation analysis was conducted in this work to further analyze and optimize the performance of the combined system. Firstly, a mathematical model of the combined system was established, and the correctness of the model was verified by experimental results. Secondly, the influence of the key parameters, such as the ambient temperature, the inlet water temperature of gas cooler, on the system performance was discussed, and a correlation of the optimal discharge pressure for the system was developed. The results indicated that the COP of CO₂ heat pump unit increased by 13.3% when the ambient temperature rose from −23 °C to −5 °C at the inlet water temperature of 30 °C. Lastly, an optimized control strategy of the combined system was developed under the different operating modes. These results provided a basis in improving and optimizing the performance of the combined system and helping to promote its application.