Defrosting start control strategy optimization for an air source heat pump unit with the frost accumulation and melted frost downwards flowing considered

When an air source heat pump (ASHP) unit operates at reverse cycle defrosting mode, it is always started with a pre-set timer, corresponding to a fixed mass of frost accumulation on the surface of a multi-circuit outdoor coil. It is difficult to quantitatively give the exact duration to start a defrosting operation due to the complicated and dynamic frosting conditions, resulting in a series of mal-defrosting phenomenon. The uneven frost distribution condition also affects the aforementioned time-based defrosting start control strategy. In this paper, defrosting performance of a specially made experimental ASHP unit with different frost accumulations evenly distributed were comparatively investigated. Basing on the fluctuations of system operation parameters, as well as its energy performance, a suitable frost accumulation to start defrosting was finally suggested. The defrosting efficiency could reach its peak at 46.05% when the frost accumulation is at 930 g. Meanwhile, the time-based defrosting start method was demonstrated to be optimized, with the frost accumulation and melted frost downwards flowing considered. Contributions of this study are expected to be used for adjusting the control strategy of an ASHP unit, and thus promote energy saving in sustainable cities and society.