Page 170 - Defrosting for Air Source Heat Pump

164 Defrosting for Air Source Heat Pump

During frosting, the measured air temperature difference between the indoor coil
inlet and outlet, the tube surface temperatures at the exit of the outdoor coil, and the
average fin surface temperatures at the center of the three circuits during frosting in the
two cases, were all measured and presented in this paper, and energy consumption in
the two cases was calculated. The measured air temperature difference between the
indoor coil inlet and the outlet stands for the energy taken from the indoor air during
frosting. Measured tube surface temperatures at the exit of the outdoor coil and aver-
age fin surface temperatures at the center of the three circuits reflect the energy given
from the refrigerant to the outdoor coil during frosting in the two cases, respectively.
As shown in Fig. 6.13, from 0 s to 2500 s into frosting, the COP kept above 4.2. How-
ever, after the 2500 s point, the COP of the two cases decreased sharply because of
frost accumulation on the airside surface of the outdoor coil. Although the COP dif-
ference from 0 s to 3600 s between the two cases is small, with 4.10 in Case 1 and 4.26
in Case 2, the COP difference in the last 10 min (3000–3600 s) is obvious, with 3.18 in
Case 1 and 4.00 in Case 2. The difference accounted for as high as 20.5%. Moreover,
all the curve trends in Figs. 6.10–6.13 are the same. Therefore, the FEC would pos-
itively affect the COP of the whole system for an ASHP unit with a multicircuit during
frosting. The higher the FEC for a multicircuit outdoor coil, the higher the
system’s COP.
In this section, an experimental study on even frosting for a vertical three-circuit out-
door coil in an ASHP unit during heating mode was undertaken and the following con-
clusions could be reached: (1) There are many factors that could affect the frost
distribution on each circuit’s surface for an ASHP unit with a multicircuit outdoor coil
during frosting, such as the distribution of refrigerant, the distribution of inlet ambient
air, the structure of the outdoor coil, etc. For an ASHP unit with a multicircuit outdoor
coil, all the factors would finally decide its FEC. (2) The measured tube surface temper-
ature,whichmeansthetemperatureoftherefrigerant,attheexitofeachrefrigerantcircuit
during frosting could directly reflect the FEC. Therefore, the tube surface temperature at
the exitofeachrefrigerantcircuitcouldbeusedasthecomprehensiveevaluationindexof
evenfrostingforanASHPunitwithamulticircuitoutdoorcoil.(3)Becausethemeasured
tubesurfacetemperature attheexitofeachrefrigerantcircuitduringfrostingwasdecided
by the refrigerant mass flow rate on each circuit, the FEC, for a multicircuit outdoor
coil, could be adjusted by the opening degree of the valves, which directly adjust the
refrigerant mass flow rate on each circuit. (4) The higher the FEC for an ASHP unit with
a multicircuitoutdoorcoil, the higherthe COP could reach.The tube surface temperature
at the exit of the outdoor coil and the average fin surface temperature at the center of
each circuit during frosting can play a supporting role on analyzing the system’s energy
consumption. (5) For an ASHP unit with a three-circuit outdoor coil, when the FEC
increased from 75.7% to 90.5%, the COP could increase from 4.10 to 4.26 at a 3600 s
frosting process, and increased from 3.18 to 4.00 in its last 600 s.

6.3 Defrosting performances at different FECs

For a multicircuit outdoor coil in an ASHP unit, uneven defrosting might result from
an uneven frosting start, which means the frost accumulations on each circuit’s surface