Page 348 - Defrosting for Air Source Heat Pump

344 Defrosting for Air Source Heat Pump

The third is the qualitative and quantitative confirmation of two influence param-
eters on uneven defrosting performance in a multicircuit outdoor coil. These param-
eters are the distributions of frost accumulation on the surface of each circuit and the
refrigerant inside each circuit. First, the FEC was used to describe the frost distribution
status on the surface of the outdoor coil, and both the frosting and defrosting perfor-
mances of the experimental ASHP unit were tested when the FEC was at different
values, at the range of 70%–100%. Here, the adjustment of FEC value was success-
fully achieved by using the valves installed in each circuit of the outdoor coil. In the
defrosting process, the water-collecting trays were also used to eliminate the effects of
downward-flowing melted frost. As reported in Chapter 6, the frosting and defrosting
performances, with or without the melted frost locally drained, were all improved
when the FEC was at a higher value. Second, the refrigerant’s even distribution status
was adjusted with the valves installed on each circuit by operating the ASHP unit at
defrosting mode while no frost was accumulated on the surface of its outdoor coil.
Meanwhile, when the valves were all fully open, the uneven refrigerant distribution
status was used as a reference case. With the comparative experimental study, the con-
clusion that even refrigerant distribution improves the defrosting performance was
finally determined and quantitatively analyzed in Chapter 7. The two parameters’ dis-
tribution studies are valuable for the control strategy optimization of ASHP units hav-
ing a multicircuit outdoor coil.
The fourth is the reveal of the energy transfer mechanism in an ASHP unit during
its RCD. When an ASHP unit works at RCD at a lower defrosting efficiency or/and a
longer defrosting duration, the insufficient energy supply and wasted energy con-
sumption are the main reasons. Before this work was carried out, nearly no study
clearly and systematically reported the energy transfer mechanism in an ASHP unit
during its RCD. Here, the energy supply was divided into four sections, and the heat
consumption into five sections. The energy transferred in the metal of the two coils,
the indoor and outdoor coils, due to their temperature variation when their roles chan-
ged in the condenser and evaporator, were also considered. Based on experimental
studies, all nine sections were quantitatively analyzed. The effect of the thermal
energy stored in the metal of the two coils on defrosting performance was also eval-
uated. In the aforementioned experimental studies, both the melted frost locally dra-
ined or not were considered. This section is described in Chapter 8.
The fifth is the control strategy optimization for the initiation and termination of
RCD in an ASHP unit with a multicircuit outdoor coil. The time-based defrosting ini-
tiation control strategy for an ASHP unit with different frost accumulations and even
distribution on the surface of the outdoor coil was experimentally investigated, with
melted frost locally drained or not. As found, the frost accumulation increased as the
frosting duration was prolonged, but without a positive proportion relation. The
defrosting duration was not at a positive proportion relationship with frost accumula-
tion either. In view of system stability and indoor thermal comfort, the system perfor-
mance would be degraded when the frost accumulation reached some fixed value, no
matter whether the melted frost was locally drained. In addition, a methodology for
confirming a suitable DTT for RCD in an ASHP unit having a multicircuit outdoor
coil was proposed and experimentally examined. For the optimization of defrosting
initiation and termination control strategies, the defrosting efficiency always worked