Cold island effect in air-source heat pump arrays: experimental characterization and analysis of influencing factors

Abstract

To support the low-carbon transformation of heating systems, deploying arrays of efficient and eco-friendly air-source heat pump (ASHP) units represents a promising strategy. However, ambient wind may be insufficient to carry away the discharged cold air when ASHP units are closely arranged. This can lead to cold air recirculation, inducing a cold island effect (CIE) that deteriorates units’ heating performance. Despite its relevance, CIE has not been thoroughly characterized, and the influence of contributing factors remains unquantified. Considering above, the objective of this paper is to investigate the intensity and spatial extent of CIE through a set of field experiments. Using multiple regression analysis, the impact levels of environmental and operational parameters on CIE were determined. The negative effects of CIE were then comprehensively evaluated. Results show that the CIE’s center tended to be located near the center of turned-on units. Compared to the cold island intensity of units (CII-U) that is calculated as the temperature difference between ambient air and the inlet air of individual units, the cold island intensity of array (CII-A), which reflects the temperature difference between ambient air and air within the array, was more appropriate to characterize the CIE across the entire ASHP array. The CII-A increased with rising ambient temperature and turned-on ratio of units, with the ambient temperature exerting a greater influence. However, no significant correlation was observed between ambient humidity and CII-A. An increase in CII-A from 0.14 ℃ to 7.67 ℃ corresponded to an expansion of the CIE’s spatial extent from 0.63 m to 3.22 m. When the CII-U increased from 0 ℃ to 7 ℃, the ASHP unit’s heating capacity was reduced by 72.17 kW, and the COP diminished by 57 %. Additionally, Unit 2 at the center of array had a 27 % lower defrost frequency than Unit 1 at the periphery, which experienced a more severe CIE.