Page 277 - Defrosting for Air Source Heat Pump

Defrosting control strategy 271

Fig. 9.15 Energy supply and effective energy consumption in the five experimental cases.

respectively. And from Case 1 to Case 5, the total effective energy consumptions are
294.7, 343.4, 356.7, 368.1, and 389.1 kJ. Most energy comes from the indoor air ther-
mal energy, which accounts for more than 80%. And most of the energy is consumed
in melting the frost, which takes more than 80%. As the frost accumulation increased,
the energy supply during defrosting increased correspondingly. However, the effec-
tive energy consumption is not changed as the frost accumulation is increasing. There-
fore, it is demonstrated that the highest defrosting efficiency in the five cases is not
Case 1 or Case 5. That means there is the most-suitable frost accumulation for this
experimental ASHP unit, which can best improve the defrosting performance.
In order to analyze the relationship between frost accumulation and defrosting per-
formance, the defrosting efficiency was calculated in the five cases. As shown in
Fig. 9.16, the frost accumulation increases as the increase of the frosting duration,
and reached its highest value at 1051 g at 70 min in Case 5. Clearly, the increasing
rate is not constant as time, which is much higher at 50–55 min. On the other hand,
although the defrosting duration is prolonged as the frost accumulation increases, the
defrosting efficiency reached its highest value at 46.05% in Case 2. That means that
after the frost accumulation is more than 930 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. In addi-
tion, compared with the results in Case 2, the system defrosting duration was pro-
longed by 29 s, or 16.2%, and the defrosting efficiency was decreased by 2.36%,
or 5.13%, in Case 5. Furthermore, when the frosting performance decrease is consid-
ered in Case 5, the frost accumulation is highly recommended at 930 g. Therefore, it is
suggested as the reference value for frosting termination.
In conclusion, an experimental study on the optimization of a time-based defrosting
initiation control strategy for an ASHP unit with different frost accumulations that
were evenly distributed on the surface of the outdoor coil was conducted. The