During reverse-cycle defrosting of an air-source heat pump

the defrosting time is always long owing to insufficient heat sources. For the quasi-two stage compression air-source heat pump (QTASHP)

owing to its larger outdoor coil operated at lower ambient temperatures

a considerable amount of heat is dissipated from the surface of the outdoor coil during defrosting

such that the long defrosting time problem becomes more severe. To improve the defrosting performance of the QTASHP

defrosting with vapor injection was proposed based on the existing cycle structure. Five groups of experiments were designed to investigate the defrosting performance. The results showed that vapor injection was beneficial for shortening the defrosting duration

decreasing power consumption

and increasing defrosting efficiency. At the optimal opening of the injection electronic expansion valve

the defrosting duration was shortened by 20.61%; the heat absorbed from water and power consumption decreased by 8.74% and 17.98%

respectively; whereas the defrosting efficiency increased by 6.22%.