An experimental study on achieving even-frosting for an air source heat pump using a novel dual-fan outdoor coil

Many related studies suggested that a fixed airflow direction usually led to that the most frost would only deposit on the windward side of finned tube heat exchangers (FTHXs) of the traditional single fan outdoor coils in air source heat pumps (ASHPs), result in more frequent defrosting operations and a lower operating efficiency of ASHPs. In this experimental study, to achieve evener frosting on outdoor coils, a novel outdoor coil having two identical fans was proposed. The two fans, Fan A and Fan B, may operate alternately to reverse airflow direction based on a pre-set value of time interval for alternate operation (t_TIAO). Feasibility and stability of the novel dual-fan outdoor coil to achieve even frosting on both windward and leeward sides of the FTHX were experimentally studied via seven experimental cases including both single and multiple frosting-defrosting operation cycles. The experimental results demonstrated that when using the novel dual-fan outdoor coil in Cases 3, 4 and 7, the heating/frosting duration was lengthened by 23.6%, 34.5% and 46.4%, and the averaged output heating capacity (q¯) was increased by 5.5%, 9.5% and 9.6 %, and averaged COP (COP¯) by 6.7%, 10.5% and 11.1% respectively, and the defrosting frequency was decreased by 33.3% in Case 7,when compared to using the traditional single-fan outdoor coil. Besides, as t_TIAO values were decreased from 15 mins to 10 mins for both fans, and to 6 mins for Fan A and 8 mins for Fan B, heating/frosting operation duration was increased by 8.9% and 19.4%, respectively, at a higher q¯ and COP¯, as compared to those using the traditional single-fan outdoor coil. The experimental results clearly demonstrated that the use of the novel dual-fan outdoor coil can help achieve evener frosting on both sides of the FTHX, and hence a longer heating operation duration and a higher operating efficiency and output heating capacity.