Determination of defrosting start time in an air-to-air heat pump system by frost volume calculation method

•A method for the determination of defrosting start time is proposed.•Frost volume and blocking ratio are calculated from experimental data.•Frost volume calculation method is verified by image processing of the evaporator.•This method accurately determines the timing of deterioration of the system. Naturally, frost forms at fin-tube evaporator surface when refrigerant below 0 °C flows inside the heat exchanger. Frost tends to block the evaporator, hinders efficient heat exchange and degrades the performance of the system. The determination of the start time for defrosting is one of the essential issues that most of air heat pumps are confronted with because it is directly connected to energy consumption. Many types of sensors can be installed for frost detection but they are usually not cost-effective or related logic is not so physically presented. In this study, a novel frost volume calculation method is suggested in order to determine the start time for defrosting in an air-to-air heat pump system. The detailed explanation of the method is presented and verified with experimental data. With this method, the start timing of defrosting can be appropriately judged. At the time of defrosting, the heating capacity decreased from 16.2 to 39.3% and the COP reduced from 12.3 to 26.4% compared with maximum values. The blocking ratio from image processing at the time was between 0.83 and 0.97.