Experimental study of defrosting control method based on image processing technology for air source heat pumps

Abstract Air source heat pump (ASHP) technology is widely accepted as a heating and cooling source for the merits of energy saving and environmental protection. However, the mal-defrosting phenomenon caused by indirect-measurement-based defrosting control method is a prominent concern. To avoid mal-defrosting phenomena, Temperature-Humidity-Image (T-H-I) method, a novel defrosting control method based on image processing technology was proposed in this paper. Frost visual characteristics were quantified by image preprocessing, grayscale conversion and multi-threshold segmentation technology. Non-frosting zone, moderate frosting zone and severe frosting zone were distinguished. The frosting coefficient P was introduced to evaluate the degree of frosting. Optimal criterion judgment for defrosting initiation (P1 = 0.3) and termination (P2 = 0.05) was determined and its applicability was verified under various environmental conditions. Both the performance of the commonly used Temperature-Time (T-T) method and the T-H-I method were investigated. The results show that 'belated defrosting' and 'unnecessary defrosting' phenomena are inevitably found under the T-T method due to the lack of direct control information, while the T-H-I method could detect the formation of frost layer directly, thus making more reasonable defrosting decisions. The proposed T-H-I method demonstrates obvious advantages in avoiding mal-defrosting phenomena and therefore has a promising application prospect in ASHP technology.