Page 290 - Defrosting for Air Source Heat Pump

284 Defrosting for Air Source Heat Pump

Fig. 9.27 Analysis on frost accumulation and defrosting efficiency.

Fig. 9.27 presents the analysis results on frost accumulation and defrosting effi-
ciency. The frost accumulation increased with the frosting duration, and reached its
highest value at 1074 g at 70 min in Case 5. Clearly, the increasing rate was not con-
stant as time, which was much higher at 50–55 min. Although the defrosting duration
was prolonged as the increase of frost accumulation, the defrosting efficiency reached
its highest value at 51.80% in Case 2. That means that after the frost accumulation is
more than 933 g, the defrosting performance becomes worse and worse as the frost
accumulation increases. Consequently, the most suitable frost accumulation for good
defrosting performance comes out around Case 2. Compared with Case 2, the
defrosting duration was prolonged by 29 s, or 16.2%, and the defrosting efficiency
decreased by 2.53%, or 5.14%, in Case 5. Although the defrosting efficiency goes
up when the frost duration is longer, as shown in Fig. 9.25, the COP of the system
during the frosting stage would adversely degrade due to more frost accumulation.
Therefore, in this study, the frost accumulation is highly recommended at 933 g for
defrosting initiation control. It is also suggested as the reference value for the frosting
termination control strategy.
In conclusion, an experimental study on time-based defrosting initiation control
strategy optimization for an ASHP unit with frost evenly distributed and melted frost
locally drained was conducted, and the study results were analyzed and conclusions
were reached. To make the frost evenly accumulate on the surface of the multicircuit
outdoor coil, an experimental setup was designed and carried out. In the three-circuit
outdoor coil, three water-collecting trays were installed under each circuit during
defrosting to experimentally validate the frost distribution status. Second, frost accu-
mulation increased as the frosting duration was prolonged, although it is not at a pos-
itive proportional relationship. After 50 min into frosting, the frost accumulation
increased quickly at first, and then much more slowly. In the same interval of