Field Test on the Interactions Between "Cold and Wet Island Effect" and Frosting-Defrosting Phenomenon for Air-Source Heat Pump Array

With the expansion of air-source heat pump (ASHP) heating systems, ASHP arrays have become the primary arrangements for medium-to large-scale heating projects. However, the "cold island effect" and "wet island effect" are likely to form at the center of the ASHP arrays. This results in frequent frosting and defrosting in the heat pump units. Conversely, the frequent frosting and defrosting also influence the "cold and wet island effect" in the ASHP array. To evaluate the mutual influences between the "cold and wet island effects" and the phenomena of frosting and defrosting, this study performed an investigation based on a demonstration project using ASHP array in Weihai, Shandong Province. The study established a field-testing platform and conducted field measurements. Typical day-testing results indicate that under the influence of the "cold and wet island effect", the defrosting frequency of ASHP unit at the center of the array increased by 225% compared with those on the periphery. Frequent defrosting reduced the intensity of the cold island effect of the central units by 4.3% and increased the wet island effect intensity by 3.9%. However, the effect was not significant. Additionally, the nominal heating capacity loss coefficient during the frosting-defrosting cycle for the central units increased by 59.3%, and their average heating capacity and COP decreased by 31.6% and 34.0%, respectively, compared with the peripheral units. This indicates a significant performance degradation in the central units.