Experimental and theoretical investigation on defrosting characteristics of a multi-split air source heat pump with vapor injection

During the defrosting process of an air source heat pump with vapor injection (ASHPVI), the defrosting duration lasts rather long due to shortage of heat sources for defrost. In theory, vapor injection during defrosting can shorten defrosting duration due to the increase of discharge refrigerant and power input of compressor. However, the injection electronic expansion valve is always turned off during defrosting because vapor injection technology is mainly used for heating mode. To shorten the defrosting duration of ASHPVIs, a test rig using a multi-split air source heat pump with vapor injection was established. Its defrosting performance was investigated through experiment and theoretical analysis. Results showed that the injection ratio was 0–10.08% during defrosting experiment, which shortened the defrosting duration by 7.75%. The defrosting efficiency reached up to 54.11% at the outdoor temperature of −7.7 °C. In addition, the effect of vapor injection on defrosting speed was mainly at the evaporating melted frost stage. The results of preliminary experiments showed a little refrigerant could be injected into compressor at defrosting initial stage and melting frost stage, but the amount of vapor injection increased obviously and accelerated defrosting speed at evaporating melted frost stage owing to high injection pressure difference.