Experimental study on defrosting start control strategy for ASHPs

In applications, the defrosting operation of an air source heat pump (ASHP) unit is always started with a pre-set time, corresponding to a fixed mass of frost accumulation, and thus resulting in series of mal-defrosting phenomenon. The exact defrosting initiation time is hard to be quantitatively given due to the dynamic and uneven frosting conditions. For an ASHP unit with a multi-circuit outdoor coil, when the melted frost was locally drained during defrosting, the optimization of start defrosting control strategy becomes more complicated. Here, this fundamental problem was experimentally investigated, with frost evenly distributed on the surface of the outdoor coil at the start of defrosting. Defrosting performance of the experimental ASHP unit at different frost accumulations were then comparatively analyzed, with the melted frost local drainage specially considered. These physical parameters include the temperature of tube surface and melted frost, compressor suction and discharge pressures and their difference, thermal energy taken from indoor air and electricity inputs on compressor and fans during defrosting, etc. Results suggest that, the defrosting efficiency reached its peak at 46.05% when frost accumulation was at 930 g. After the melted frost was locally taken away during defrosting, it reached its peak at 51.80% when frost accumulation was at 933 g. Thus, the time-based start defrosting control strategy was demonstrated to be fundamentally optimized with this method. Contributions of this study could be used for adjusting the control strategy of ASHP units, which are valuable to energy saving in industrial applications.