Page 258 - Defrosting for Air Source Heat Pump

Energy transfer during defrosting 253

ambient air. The metal energy storage of the outdoor coil accounts for about 5%.
Decreasing the proportion of energy consumed on heating the ambient air could effec-
tively improve defrosting efficiency. (3) The effect of metal energy storage on
defrosting performance was quantitatively evaluated. As concluded, after the outdoor
coil was enlarged by 50%, from a two working circuit to a three working circuit, the
metal energy storage effects changed from 0.44% to 3.67%. The percentages of
energy consumed on vaporizing retained water and melting frost were both increased.
(4) A higher defrosting efficiency was reached, from 47.13% in a two-working-circuit
case to 58.79% in a three-working-circuit case, with 11.66% improvement. The law of
energy conversion and the effect of metal energy storage could guide the design opti-
mization of the two coils and save energy for ASHP units.

8.4 Discussion on effect of melted frost and thermal
comfort

8.4.1 Effect of melted frost
To analyze the difference between with and without melted frost downward flowing
during defrosting as well as adjusting the number of working circuits in the outdoor
coil, the experimental results in the four cases in the previous sections were summa-
rized in Table 8.10. As seen in this table, the total energy supplies in the four cases are
different. After the water-collecting trays were installed between circuits in Cases 3
and 4, the energy consumptions were clearly reduced. For two-working-circuit cases,
Cases 1 and 3, the energy consumption decreased from 613.2 to 516.7 kJ, with a
reduction of 96.5 kJ. When it comes to three-working-circuit cases, Cases 2 and 4,
the energy consumption reduced about 109.7 kJ, from 761.4 to 651.7 kJ. Their reduc-
tion ratios are similar at 15.7% and 14.4%, respectively. The defrosting efficiency for
two-working-circuit cases, Cases 1 and 3, improved from 42.26% to 47.13%, with an
increase of 4.87% after the trays were installed. For three-working-circuit cases, Cases
2 and 4, the defrosting efficiency improved from 48.34% to 58.79%, with an increase
of 10.45% after the trays were installed. That means that the negative effects of melted
frost on a two working circuit and a three working circuit are different, with the latter
higher. These results also meet the conclusions in Chapter 3. As indicated, after the
working-circuit number increased from two to three, the negative effects of melted
frost improved. The MES effects in Cases 1–4 were calculated at 0.33%, 0.44%,
2.18%, and 3.67%, respectively. Finally, the MES effects on defrosting perfor-
mance were calculated at 2.51% by Cases 1 and 3, and at 3.23% by Cases 2 and 4.
That means that the MES effects on defrosting performance for an ASHP unit would
be increased, from 2.51% to 3.23%, after the melted frost was taken away during
defrosting by installing water-collecting trays under each circuit in the multicircuit
outdoor coil. It is meaningful for the structural optimization of ASHP units as well
as their energy savings with a heating mode at a low-temperature and high-humidity
environment.