| Currently, the most widely used standard defrosting method for air source heat pumps (ASHPs) is reverse cycle defrost. In order to improve reverse cycle defrosting performance, and thus minimize or even eliminate the operational problems, it is necessary to develop a novel thermal energy storage (TES) based reverse cycle defrosting method for ASHPs to address the fundamental problem of inadequate heat available for defrosting through TES technology, and study the defrosting performance on the surface of multi-circuit outdoor coil.;This thesis begins with reporting the development of a novel TES-based reverse cycle defrosting method using phase change material (PCM). The experimental results suggested that the use of the novel TES-based reverse cycle defrosting method would result in: a shorter defrosting duration, a smoother transition from defrosting to space heating resumption, an enhanced operational reliability, reduced energy consumption and improved occupants' indoor thermal comfort. The thesis then reports on a study on the airside defrosting performance of an on outdoor coil having four parallel circuits in the experimental ASHP unit, with a particular focus on studying the impact of allowing melted frost to flow downwards freely due to gravity along the coil surface on defrosting performance, using both experimental and modeling analysis approaches. It was observed that defrosting was quicker on the airside of the upper circuits than that on the lower circuits in the four-circuit outdoor coil. A semi-empirical defrosting model for the four-circuit outdoor coil of the experimental ASHP unit, the first of its kind, was developed. Using the validated model, the negative effects of downwards flowing of the melted frost along the surface of a multi-circuit outdoor coil on defrosting performance were quantitatively analyzed and are reported. Finally, an experimental investigation on reverse cycle defrosting operation for the experimental ASHP unit when using an electronic expansion valve (EEV) as a refrigerant flow throttle regulator is reported. The experimental results revealed that when the EEV was regulated by the DS controller during defrosting, a higher defrosting efficiency and less heat wastage would be resulted in. |
