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efficiency comparison. As demonstrated, the most suitable DTT for the experimental
°
ASHP unit with the three-circuit outdoor coil is at the range of 20–25 C, or around
°
22 C. At this period, the average fin surface temperature is at the range of
° °
17.2–23.2 C, or around 18.5 C. The special points in the measured tube surface tem-
perature at the exit of the outdoor coil and the measured refrigerant volumetric flow
rate are obvious. It is demonstrated that the two parameters can work as the defrosting
control indexes in the future experimental study. From the view of only at the
defrosting stage and in an entire frosting/defrosting cycle, the best DTTs for reverse
cycle frosting were comprehensively discussed based on their energy performances.

9.5 Concluding remarks

In this chapter, experimental studies on the time-based defrosting initiation control
strategies and the DTT for an ASHP unit with a multicircuit outdoor coil are presented,
with the following conclusions: (1) The optimization of the time-based defrosting ini-
tiation control strategy for an ASHP unit with different frost accumulations evenly
distributed on the surface of the outdoor coil was experimentally investigated, with
melted frost locally drained or not. (2) Frost accumulation increases as the frosting
duration is prolonged, but without a positive proportional relationship. The defrosting
duration is not at a positive proportional relationship with frost accumulation. (3) In
view of system stability and indoor thermal comfort, the system performance would
be degraded when frost accumulation was more than 930 and 933 g for conditions of
melted frost locally drained or not, respectively, in this study. The defrosting effi-
ciency also reached its peak at this frost accumulation value. As demonstrated, frost
accumulation is the most fundamental reference parameter for frosting termination in
an ASHP unit. (4) A methodology for confirming a suitable DTT for RCD was pro-
posed and experimentally examined. This method was based on the most energy sav-
ing for the ASHP unit, and realized by using the tube surface temperature curve as the
baseline. Here, the suitable DTTs were also validated by a defrosting efficiency com-
parison. It is a fundamental study for optimizing defrosting control strategies for
ASHP units.

References

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for the air source heat pump under two typical mal-defrost phenomena. Appl Energy
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[2] Jha SK, Bilalovic J, Jha A, Patel N, Zhang H. Renewable energy: present research and
future scope of Artificial Intelligence. Renew Sustain Energy Rev 2017;77:297–317.
[3] Wang W, Feng YC, Zhu JH, Li LT, Guo QC, Lu WP. Performances of air source heat
pump system for a kind of mal-defrost phenomenon appearing in moderate climate con-
ditions. Appl Energy 2013;112:1138–45.
[4] Iragorry J, Tao YX. Frost temperature relations for defrosting sensing system. J Heat
Transf 2005;127:344–9.