Page 263 - Defrosting for Air Source Heat Pump

258 Defrosting for Air Source Heat Pump

experimental values were limited to 7% and 9%, respectively [9]. However, it is still a
difficult problem to accurately detect the frost accumulation for an ASHP unit.
With respect to the time-based defrosting initiation control strategy in application,
the frost distribution on the surface of a multicircuit outdoor coil is always neglected.
In fact, frost is always unevenly distributed due to the complicated and variable ambi-
ent conditions. Two typical types of uneven frosting phenomena for ASHP units were
found, as illustrated in Fig. 9.1. Type 1 is the unequal frost accumulation on the sur-
faces of different circuits. For example, in an experimental study on the performance
of an ASHP unit reported by Wang et al., for a kind of mal-defrost phenomenon appe-
aring in moderate climate conditions, the frost accumulation on the downside is much
more that on the upside of its vertically installed multicircuit outdoor coil [3].As
shown in Fig. 9.1A, a similar phenomenon could also be found in experimental studies
reported by Qu [10],Song [11], and Steiner [12]. Type 2 is the unequal frost accumu-
lation on the windward and leeward sides of the outdoor coil. As known for Type 1
uneven frosting, frost evenness could be quantitatively calculated with the melted
frost collected by water-collecting trays during defrosting. Type 2 results from the
obvious relative humidity difference between the inlet air and outlet air. However, this
type is hardly possible to be further quantitatively described. Furthermore, it is the
total frost accumulation on the circuit that directly affects its defrosting performance.
Therefore, only the Type 1 uneven frosting phenomenon was widely considered in
previous works. This may also be the reason why nearly no attention was paid to
the Type 2 uneven frosting phenomenon. This type of uneven frosting phenomenon
was only mentioned in a recent experimental work reported by Zhang et al. [13],as
shown in Fig. 9.1B. It is reported that the ratios of the frost mass accumulated on the
edge of the windward fins to that on the entire surface were 13.7% (60 min frosting
duration) and 12.5% (120 min frosting duration) for the two heat exchangers used in
this study, respectively. Although the frost accumulations on the two sides of an out-
door coil seem different, their masses were not clearly given. Therefore, only the Type
1 uneven frosting influence on the time-based defrosting initiation control strategy
was investigated here.

9.2 Time-based initiation of defrosting control

As demonstrated, a higher FEC would improve the frosting and defrosting perfor-
mances for an ASHP unit, but how to influence the defrosting performance for differ-
ent frost accumulations at high FECs is still unknown. The uneven frosting
phenomenon should be fully considered when we further develop or optimize the
time-based defrosting initiation control strategy. After this fundamental problem
was qualitatively and quantitatively solved, the preset frosting duration could be given
in a time-based defrosting initiation control strategy [7]. The mal-defrosting problems
could be effectively avoided, and thus potentially large amounts of energy are saved
[14]. Consequently, an experimental study on the optimization of the time-based
defrosting initiation control strategy for an ASHP unit with frost evenly distributed
was designed and carried out. First, the experimental setup was introduced as well