An experimental study on improving defrosting performances of air source heat pump unit based on hot-gas bypass method

A novel hot-gas bypass defrosting method for an air source heat pump (ASHP) unit has been presented in a previous work, which can satisfy defrosting and uninterrupted heating simultaneously. To further improve the novel hot-gas bypass defrosting performances, the effects of electronic expansion valve (EEV) on both the defrosting and uninterrupted heating performances are experimentally investigated. The results showed that the defrosting period and consumption decreased by 64.7% from 510 to 180 s and 19.7% from 1133.8 to 910.2 kJ when the opening of a 500-step EEV increased from 120-step to 480-step. Although the defrosting period for EEV opening of 480-step was the shortest, the period of blowing cold air was the longest which lasted for around 60 s. Besides, the percentage of consumed energy amount used for defrosting and the average heating capacity during defrosting period ranged from 65.2%~81.7% and 1178 ~ 903 W, respectively, with five EEV openings. The maximum average COP was 0.57 with EEV opening of 120-step, while the minimum value was 0.36 with EEV opening of 280-step. The results can help to optimize the defrosting control strategy of the ASHP units using the novel hot-gas bypass defrosting method and facilitate the development of ASHP technology.