A novel defrosting control strategy with image processing technique and fractal theory

Temperature-time (T-T) defrosting control strategy is widely used in traditional air-source heat pump (ASHP) units. Its fatal problem that urgently need to be addressed is mal-defrosting, which happens frequently in heating season and causes extra energy loss. In order to accurately control the defrosting time and avoid the mal-defrosting problem, a novel Temperature-Humidity-Image (T-H-I) defrosting control strategy is proposed to improve the defrosting process. With the aid of image processing technology, T-H-I defrosting control strategy applies fractal analysis can accurately and effectively perform defrosting operation. Through the experimental investigation on frost formation characteristics, T-H-I defrosting control strategy operates defrosting when the frost thickness reaches 0.726 mm and the fractal dimension reaches 2.839, while it terminates when the fractal dimension decreases to 2.324. Comparative experiments of ASHP unit with traditional temperature-time (T-T) method and the proposed T-H-I method are conducted. Compared with the traditional T-T method, 'delayed defrosting' and 'unnecessary defrosting' are avoided effectively with the T-H-I defrosting control strategy, and the energy consumption of defrosting can be reduced by 10.6% and 22.3%, respectively. In view of its high accuracy in defrosting and effectiveness in energy saving, the proposed T-H-I method has a promising prospect for future application.