Performance analysis of an air source heat pump space heating system with variable water temperature difference in a severe cold region

Air source heat pumps (ASHPs) are widely used for clean heating in cold climates. However, previous studies have mainly focused on the heat pump unit itself, while overlooking the power consumption of circulation pumps. Although large water temperature difference (LTD) operation with a reduced flow rate can lower pump power consumption, its overall effects on system performance and ASHP operational stability under severe cold conditions remain insufficiently explored. This study experimentally and numerically investigated the feasibility and benefits of the LTD strategy for an ASHP system in Jiamusi, a severe cold city in China. High-accuracy models were developed and validated for the defrosting-free coefficient of performance (COP) of the ASHP and pump power consumption, with mean relative errors of 2.09% and 1.47%, respectively. The results confirmed that LTD operation can ensure stable and reliable heating without compromising indoor comfort. At outdoor temperatures ranging from −25 °C to −12 °C, the LTD strategy increased the heating capacity by up to 8.32%. At an outdoor temperature of −25 °C and a supply water temperature of 45 °C, compared with a temperature difference of 5 °C, the COP of the ASHP unit decreased by 4.07% and 8.72% at temperature differences of 7 °C and 10 °C, respectively, whereas pump power consumption decreased by 48.27% and 60.47%, respectively. Based on these findings, a staged variable water temperature difference (VTD) strategy was proposed to minimize the total operating energy consumption of the heating system. For Harbin, the energy-saving thresholds for increasing the temperature difference were identified as −16.9 °C (7 °C vs. 5 °C) and −12.4 °C (10 °C vs. 7 °C). Seasonal VTD operation reduced energy consumption by 5.11% and increased the system COP by 5.56% in Harbin, while in the milder climate of Beijing, it reduced energy consumption by 7.81% and increased the system COP by 8.48%, demonstrating strong energy-saving potential in cold regions.