Analysis of defrost control logic for air-source heat pumps using image and cycle data
Frost accumulation on the outdoor heat exchanger is a major issue for air-source heat pumps (ASHPs) operating in winter, as it blocks airflow and reduces heating capacity. Periodic defrosting is required to restore performance, but conventional time-and temperature-based defrost logic often initiates defrosting too early or too late because it does not directly measure frost accumulation. This study experimentally evaluated the limitations of conventional defrost logic using a 12 kW ASHP under various outdoor temperature and humidity conditions. A next-generation defrost initiation method was then developed by combining an image-based frosting index, the block ratio (BR), with cycle operating data. This integrated approach enables a more accurate assessment of frost formation. Results show that initiating defrosting immediately when BR reaches 1 minimizes unnecessary compressor operation. While the heating operating ratio remained nearly unchanged, the proposed logic improved heating capacity by up to 1.8% and COP by up to 3.8%. Overall, the BR-based strategy effectively reduces energy waste and enhances system efficiency.