Staged Return Water Temperature Control for Air-Source Heat Pumps with Phase-Change Storage: Experimental Enhancement of COP and Indoor Temperature Stability

In the practical operation of air-source heat pump heating systems coupled with phase-change energy storage tanks, wide fluctuations in outdoor temperatures often cause issues such as excessive heating, frequent unit start–stops, and low operational efficiency. Traditional start–stop control strategies struggle to balance heating quality with system energy savings. To enhance the system’s energy efficiency across all operating conditions and improve the stability of indoor temperatures, this study introduces a straightforward and easy-to-implement return water temperature zone control strategy. Using physical reference points, a three-zone control approach for return water temperature was created, which integrates outdoor temperature feedback along with combined indoor temperature adjustments. The proposed strategy’s effectiveness was confirmed through comparative experiments that split the heating season into two parts: one employing traditional control and the other using the zone control method. The results show that, compared to empirical start–stop control, the segmented control strategy increased the system’s average coefficient of performance (COP) from 3.06 to 3.11, representing a 1.63% improvement; reduced indoor temperature deviation from 1.4 °C to 1.2 °C, a 14.2% decrease; and narrowed the amplitude of extreme temperature deviations from 7.9 °C to 3.9 °C, a 50.6% reduction. Total electricity consumption for the entire heating season was approximately 4191 kWh. These findings indicate that the proposed control strategy effectively improves system energy efficiency and indoor temperature stability while meeting heating demands. It significantly suppresses excessive heating during transitional seasons and enhances heating reliability under extreme low-temperature conditions. This study involves low retrofitting costs and balances both energy-saving and comfort objectives, providing a practical, engineering-ready solution for the intelligent control of air-source heat pump heating systems.