Page 279 - Defrosting for Air Source Heat Pump

Defrosting control strategy 273

performance for an ASHP unit at different FECs with melted frost locally drained was
carried out [15]. After the melted frost was taken away by the water-collecting trays
during defrosting, the defrosting duration was shortened by 11.2% and the defrosting
efficiency increased by 5.7%, as the FEC increased from 79.4% to 96.6%. Clearly, it is
meaningful to improve the FEC during the frosting stage and locally drain the melted
frost during the defrosting stage. To further clarify the negative effect of melted frost,
a semiempirical modeling study on the defrosting performance for an ASHP with local
drainage of melted frost from its three-circuit outdoor coil was developed [16]. Based
on the validated model, three study cases of varying heat supply to the outdoor unit
were further investigated, demonstrating the optimization of the ASHP unit by
decreasing the defrosting energy to 96.4% and reducing the defrosting duration by
7s [17]. Clearly, when we optimize the time-based initiation defrosting control strat-
egy, the melted frost effects could not be neglected.
Although it was demonstrated that a higher FEC would improve both the frosting
and defrosting performances for an ASHP unit, how to influence the defrosting per-
formance for different frost accumulations at high FECs with the melted frost locally
drained is still unknown. After this fundamental problem was qualitatively and quan-
titatively solved, the preset frosting duration could be given in a time-based initiation
defrosting control strategy. The mal-defrosting problems could also be effectively
avoided, and thus potentially large amounts of energy saved. Therefore, in this paper,
an experimental study on time-based defrosting initiation control strategy optimiza-
tion for an ASHP unit with frost evenly distributed and melted frost locally drained is
carried out. First, the experimental setup was introduced. Then, the experimental pro-
cedures and five continuous cases were designed. After these were observed, the mea-
sured and calculated results were given and the system energy and stability
performances and initiation defrosting indexes were comparatively analyzed and dis-
cussed. Finally, a conclusion was given. It is expected that this work will be used to
optimize the control strategy for intelligent heat pumps as well as the ultimate goal of
building energy savings.

9.3.1 Experimental cases
To optimize the time-based initiation defrosting control strategy, a series of experi-
mental works using the experimental ASHP unit was carried out. In order to obtain
meaningful experimental results, first it was necessary to ensure that the frost accu-
mulations on the surface of the outdoor coil were different. It could be reached with
two methods. One is changing the relative humidity of the outdoor air in the exper-
iment because water vapor in the outdoor air is the source of frost. The other one is
changing the duration of the frosting process. Here, to shorten the frosting duration in
the experiments, the second one was finally taken, with the air relative humidity kept
at 90 3% during frosting. In real application, a time-based RCD is always started
after frosting for 60–90 min, due to the relative humidity of the ambient air always
being much lower, at 40%–80%. In addition, when the frosting duration is longer,
the COP of the system would suddenly decrease. Therefore, in this study, the frosting
durations were designed to be a little shorter, at 50–70 min.