Page 77 - Defrosting for Air Source Heat Pump

68 Defrosting for Air Source Heat Pump

Table 3.7 Differences between two-circuit and three-circuit outdoor coil experiments
Two-circuit Three-circuit
Item Parameter outdoor coil outdoor coil

1 Total case number 2 3
2 Defrosting duration without trays 152 s 186 s
3 Defrosting duration with trays 136 s 168 s
4 Reduction in defrosting duration 16 s (15.8%) 18 s (7.5%)
after using trays
5 Total melted frost collected 620 g 921 g
without trays
6 Total melted frost collected with 655 g 969 g
trays
7 Melted frost collected difference 35 g (5.3%) 48 g (5.0%)
8 Total energy consumed without 576 kJ 727 kJ
trays
9 Total energy consumed with trays 517 kJ 652 kJ
10 Total energy saved after trays used 59 kJ (10.3%) 75 kJ (10.4%)
11 Defrosting efficiency with trays 52.5% 43.5%
12 Defrosting efficiency without trays 63.0% 56.7%
13 Defrosting efficiency improved 10.5% 13.2%
after trays used

3.4 Concluding remarks

The experimental results and corresponding quantitative analysis reported in this
chapter demonstrated the negative effects of allowing melted frost to flow downward
due to gravity over the airside surface of experimental vertical two-circuit and three-
circuit outdoor coils in an ASHP unit on defrosting performance during RCD: a longer
defrosting duration and more energy consumption. Furthermore, the study results
also suggested that the use of water-collecting trays was effective in mitigating the
negative effects.
For two-circuit and three-circuit outdoor coils, the increases in defrosting effi-
ciency values are different, at 10.5% and 13.2%, respectively. That means the energy
performance could be improved more obviously for a three-circuit outdoor coil
after using the water-collecting trays. Additionally, for a three-circuit outdoor coil,
a greater reduction in defrosting duration could be achieved. This further demonstrates
that a better thermal comfort performance may be achieved when using a three-circuit
outdoor coil. However, the circuit number in an outdoor coil is always limited by its
physical dimensions. Also, when a larger number of circuits is used, or when the heat
exchanger area of an outdoor coil is fixed and the circuit number increased, the effects
of melted frost flowing downward should be further studied.
On the other hand, further mathematical modeling studies on the heat and mass
transfer mechanisms when the melted frost from the upper circuits flows over the
frosted surface of the lower circuits in a vertical multicircuit outdoor coil have also
been carried out, and the study results will be reported in the next chapter separately.