Page 292 - Defrosting for Air Source Heat Pump

286 Defrosting for Air Source Heat Pump

always delays the temperature increase for the lowest circuit. Consequently, the tem-
perature sensor is usually placed on the lowest liquid-line circuit of a vertically
installed multicircuit outdoor coil, as shown in Fig. 9.28B.
In the open literature, an RCD operation will be terminated once a preset temper-
ature, or DTT, is reached for T e , as shown in Fig. 9.28. It was thought that when the
tube surface temperature at the exit of the lowest circuit reaches DTT, all the frost has
been melted off and the retained water vaporized. It is obvious that when the DTT is
set higher, the defrosting duration would be prolonged. Not only is more energy con-
sumed on heating the ambient air, but also the occupants’ thermal comfort is adversely
affected [19]. Because the low temperature for the ambient air always comes out at
night, sleep thermal comfort is always degraded due to frequent defrosting operations
of an ASHP unit. On the contrary, if the DTT is set lower, the frost may be melted
off from the outdoor coil surface, but much residual water would be left there. As pre-
viously mentioned, the residual water would adversely degrade the system perfor-
mance during the next frosting/defrosting cycle [20]. However, in the reported
°
RCD experimental studies, temperatures at a wide range of 10–35 C were used as
the preset DTTs. Without fixed temperature value or any relatively detailed explana-
tion is given.
Although researchers tried their best to improve the defrosting performance of
ASHP units at serve cold climate, previous attention was paid to improving the frost
evenness values of the outdoor coil, the fin type, the structure, the installation style
optimization, the fin surface coating or microtreatment, the refrigerant distribution
control strategy exploring, additional heating source searching, etc. With respect to
RCD termination, especially for DTT, a scarcity of related research is reported. How-
ever, for an ASHP unit with a multicircuit outdoor coil, it is a fundamental and mean-
ingful problem to find a suitable DTT or its range. Consequently, in this paper, an
experimental investigation on RCD operation for an ASHP unit with a three-circuit
outdoor coil was conducted. First, the detailed description of an experimental ASHP
unit and the experimental conditions are introduced. This is followed by presenting the
experimental method and data analysis results as well as giving conclusions. This
study reports on a method to find a suitable DTT for RCD, and thereby makes con-
tributions for the control strategy optimization for ASHP units.

9.4.1 Methodology
As shown in Fig. 9.29, the methodology used in this study is illustrated in the flow
chart. In order to carry out this study, collecting the RCD experimental data is the first
step. Clearly, an ASHP unit should be first selected, with a vertically installed mul-
ticircuit outdoor coil included. Because the frost formation is essentially a transient
process with heat and mass transfer that depends on six primary parameters, air tem-
perature, air relative humidity, air velocity, air cleanliness, metal surface temperature,
and its property, it is difficult to monitor all those parameters for practical air condi-
tioners or refrigerators. Therefore, the defrosting operation is often executed inaccu-
rately. In addition, the mal-distribution of air and refrigerant into the circuits would
make the frost accumulations on the surface of each circuit unequal. The phenomenon