Experimental study on frost growth and dynamic performance of air source heat pump system

The effect of the frost growth and frost morphology on the performance of an air source heat pump was investigated experimentally. The frost thickness. frost accumulation and the dynamic performance of the heat PUMP were measured. It is found that the frost growth can be divided into three stages according to the frost morphology. In the initial stage, condensed water freezes and forms a transparent thin ice layer on the fins and tubes firstly, then the granular ices appear and grow gradually on the ice layer, and the column-shaped ice crystals are formed at last. The growth rate of the frost thickness, the heating capacity and COP of the heat pump increase with the frosting time significantly until the column-shaped frost layer is formed. In the second stage, the column-shaped ice crystals grow in its radius rather than in its length, and the frost thickness growth rate decreases or remains to be constant. However, the heating capacity and COP of the heat pump are only slightly affected by frosting on the outdoor coils. In the third stage, the ice crystals mainly grow in its length, and become gradually of all acerose-shaped one, finally a fluffy frost layer is formed. The frost thickness growth rate is about 2-4 times of that in the second stage. The drops per minute in the heating capacity and COP are increased by several times of those in the second stage. In additions it is found that the frost growth rate and the drop in the performance of the heat pump are highest when the outdoor air temperature is about 0 degrees C with various relative humidity. The experimental results are in agreement with the corresponding simulation data except in the third frosting stage. (C) 2008 Elsevier Ltd. All results reserved.