Page 224 - Defrosting for Air Source Heat Pump

218 Defrosting for Air Source Heat Pump

Table 7.7 Different durations, defrosting efficiencies, and relative differences in two
experimental cases
Item Parameter Case 1 Case 2 Difference

1 Duration of tube surface temperature 185 s 173 s 6.5%
of Circuit 3 reaching 24°C
2 Duration of fin surface temperature of 211 s 200 s 5.2%
3 circuits all reaching 24°C
3 Duration of TDOEE reaching its peak 110 s 105 s 4.5%
value
4 Peak value of TDOEE 42.8°C 38.0°C (4.8)
5 Duration of TDIEE reaching its peak 25 s 30 s 20%
value
6 Peak value of TDIEE 43.2°C 40.9°C (2.3)
7 Duration of refrigerant volumetric flow 160 s 160 s 0
rate reaching its peak value
8 Duration of melted frost reaching the 120 s 115 s 4.2%
water collecting cylinder
9 Temperature of the lowest temperature 0.45°C 0.2°C (0.25)
of melted frost collected
10 Total energy supply for defrosting 790.5 kJ 691.6 kJ 12.5%
11 Total energy consumption during 319.9 kJ 331.5 kJ 3.6%
defrosting
12 Defrosting efficiency 40.5% 47.9% 18.3%

Therefore, it could be speculated that the defrosting durations of the tube and fin could
be used to compare the defrosting performances for systems with different defrosting
efficiencies. In addition, for Items 4, 6, and 9, temperature differences were small, espe-
cially the value of melted frost at just 0.25°C. However, the peak value of the TDOEE
difference, 4.8°C, as well as their durations’ difference, 4.5%, could also be considered
comprehensively as one of the system defrosting performance evaluation parameters.
In conclusion, for an ASHP unit with a vertically installed three-circuit outdoor
coil, a comparative experimental study on system RCD performance when the refrig-
erant is evenly distributed into each circuit or not was undertaken, as well as the
melted frost downward flowing during defrosting. After the study results were ana-
lyzed, the following conclusions could be drawn: (1) For an ASHP unit with a mul-
ticircuit outdoor coil, the refrigerant distributed into each circuit would be affected
by the tube internal resistance and gravity during RCD. And the negative coupled
effects of MFDF and URD on the system defrosting performance were qualitatively
confirmed. (2) After the distribution of refrigerant was evenly adjusted, the duration
of the tube surface temperature of Circuit 3 reaching 24°C decreased from 185 to
173 s, or a ratio of 6.5%. Meanwhile, the duration of the fin surface temperature of
three circuits all reaching 24°C decreased from 211 to 200 s, or a ratio of 5.2%.
(3) As compared with the refrigerant being unevenly distributed into each circuit,
for an ASHP unit with a vertically installed multicircuit outdoor coil, the total energy