Page 118 - Defrosting for Air Source Heat Pump

110 Defrosting for Air Source Heat Pump

one-third for possible energy savings after the tube surface temperature at the exit on
Circuit 1 reached 24°C. However, because the total durations of the defrosting oper-
ation were longer than those in Study Case 2, the total energy use in Study Case 3 was
more than that in Study Case 2, and slightly lower than the experimental value.
To clearly show the differences of heat supply, Fig. 4.29 shows the heat supply for
defrosting after 172 s into defrosting in the previous experimental study and the three
study cases. It is obvious that the heat supply for defrosting in Study Case 2 was the
shortest, at 33.8 kJ. At the same time, the three study cases all could decrease the heat
supply for defrosting because the value in the previous experimental study was the
biggest, at 73.2 kJ. The heat supply mostly comes from the thermal energy of indoor
air, accounting for more than 86% of the total heat supply. The least part is the heat
supply from the compressor, at only around 1%. In addition, the energy consumption
during defrosting in the previous experimental study and the three study cases is
shown in Fig. 4.30. It could be found that the energy consumption on frost melting
for different study cases was the same, at 329.0 kJ. This is because the frost accumu-
lation on the surface of the outdoor coil was assumed to be the same as that in the
previous experimental study. The differences totally come from the energy consump-
tion on vaporizing. The biggest value of energy consumption on vaporizing is 34.2 kJ
in the previous experimental study. And the smallest one is only 5.5 kJ, or 16.1% of
the value in the previous experimental study. Figs. 4.29 and 4.30 show that the heat
supply for defrosting and energy consumption in Study Case 2 were both the shortest.
This further confirmed that system performance could be improved most by fully clos-
ing the modulating valve on the top circuit when its defrosting terminated in the three
study cases for alleviating uneven defrosting.
In this section, a modeling study on alleviating uneven defrosting for a vertical
three-circuit outdoor coil in an ASHP unit during RCD was undertaken. The following
conclusions could be received: (1) Three study cases were included and the study
results suggested that the best operating defrosting performances in terms of
defrosting durations and energy use were achieved in Study Case 2. In this section,
defrosting energy use could be decreased to 94.6% as well as a reduction of 7 s in
defrosting duration by fully closing the modulating valve on the top circuit when
its defrosting terminated. (2) It is expected that with more refrigerant circuits in an
outdoor coil in an ASHP, the method of fully closing the modulating valves on the
top circuit when its defrosting is terminated will yield a better defrosting performance
for the ASHP unit, as predicted by the modeling study reported in this section. (3) In
this modeling study, frost accumulation on the surface of each circuit and the refrig-
erant distributed into each circuit in the multicircuit outdoor coil both were assumed to
be even. However, when the frost accumulation and refrigerant distribution were
uneven, the performances of the three study cases would be different. Therefore, a
model should be further developed with the considerations of frost accumulation
and refrigerant distribution for an ASHP unit with a multicircuit outdoor coil. (4)
Compared with the previous experimental study, although the heat supply and energy
consumption in Study Case 3 were both decreased by 4.6 kJ, its defrosting duration
was extended by 1 s. Therefore, in consideration of the indoor thermal comfort
requirements, this type of control strategy was not suggested.