Optimization of Heating Stability During Defrosting in Modular Small-Capacity Air Source Heat Pump Systems
Compared with large-capacity air source heat pump (ASHP) units, modular small-capacity ASHP systems are designed to enhance the stability of water supply temperature under low-temperature and high-humidity conditions by miniaturizing heating units and enabling independent defrost control. However, autonomous defrosting of individual outdoor units can lead to concentrated and periodic group defrosting events, causing significant fluctuations in water supply temperature. This study conducted an 86-day field experiment on a modular ASHP heating system in a cold region of China to analyze the relationship between system defrosting behavior and heating stability. Results showed that when ambient humidity exceeded 80% and compressor frequency was above 70Hz, the system exhibited periodic group defrosting at approximately 2- hour intervals, with a maximum simultaneous defrosting ratio (SDR) of 60% and corresponding temperature drops of 3~4K. Statistical analysis indicated that maintaining the SDR below 25% in high-density defrosting periods or under 60% for instantaneous defrosting events can effectively limit water temperature fluctuations within 3K. Based on these findings, this paper identifies the need for distributed group defrost control in modular ASHP systems and proposes a conceptual framework to limit simultaneous defrosting ratios and eliminate periodic synchronization. Future group-defrost control strategies should incorporate time-staggered scheduling, module-level coordination, and predictive algorithms to achieve distributed defrosting behavior and enhance overall heating resilience of modular ASHP systems.