Page 103 - Defrosting for Air Source Heat Pump

Modeling study on uneven defrosting 95

4.3 Alleviating uneven defrosting for an ASHP unit

An effective alternative to experimentally investigate the defrosting performance in
an ASHP unit is via a numerical approach and therefore, the last two decades saw
a growing number of modeling studies on defrosting operations [32,33]. Noticeable,
Krakow et al. first developed a hot-gas defrosting model for evaporators [11,12], and
later presented an idealized RCD model for an ASHP unit with a receiver [13,14].
Similar to Krakow, a detailed transient simulation model for hot-gas bypass defrosting
in an air-cooled evaporator was also developed by Dopazo et al. [21]. However, in the
above-mentioned defrosting models, an uneven defrosting phenomenon was not con-
sidered. Thus, the effects of the downward flowing of the melted frost due to gravity
along the surface of a multicircuit outdoor coil on the defrosting performance all were
neglected by assuming either no water retention on the coil surface or a stable water
layer. Only in 2012 when a semiempirical model was developed by Qu et al. were the
negative effects of melted frost considered for the first time [22]. Thereafter, as a fol-
low up to Qu’s study on uneven defrosting, a series of experimental and modeling
studies on the defrosting performance of an ASHP unit when the melted frost was dra-
ined away locally from its three-circuit outdoor coil was carried out by the authors and
separately reported in previous sections.
While the outcomes from these studies demonstrated the effectiveness of locally
draining away the melted frost from a vertical multicircuit outdoor coil, for existing
ASHP units, however, it is hardly possible to install water-collecting trays between
circuits. Nonetheless, for existing ASHP units, it is still possible to vary the heat input
to each refrigerant circuit through varying the refrigerant supply to each circuit. This is
because uneven defrosting was fundamentally caused by different thermal loads
imposed to each circuit due to the downward flowing of the melted frost, when the
supply of refrigerant or heat to each circuit was the same. Consequently, if the heat
to be supplied to each circuit may be varied according to the actual defrosting thermal
load each circuit is to deal with, then the problem of uneven defrosting may be alle-
viated. Modulating valves installed at an inlet refrigerant pipe to each circuit may be
deployed to vary the refrigerant flow, thus the heat input to each circuit. Therefore, in
this section, a modeling study on varying the heat (via refrigerant) supply to each
refrigerant circuit in the three-circuit outdoor coil to alleviate uneven defrosting
was carried out and reported. First, the methodology and three study cases are
explained. Second, the results of the modeling study on defrosting durations and
energy use in the three study cases are presented. Finally, a conclusion is given.

4.3.1 Methodology and study cases
The reported study was carried out using a previously developed and validated semi-
empirical mathematical model at the experimental setting of not using water-
collecting trays between circuits. In this section, a brief description of the previous
experimental study is first introduced, and the validated models are then shown. This
is followed by presenting three study cases in this section. Finally, the assumptions
used for the three study cases will be given.