Page 247 - Defrosting for Air Source Heat Pump

242 Defrosting for Air Source Heat Pump

were quantitatively analyzed. As indicated, the energy consumptions for heating the
outdoor coil metal accounted for 16.5% of the total defrosting energy consumption
[12]. And thus, the MES was considered in the previous defrosting model develop-
ment, as introduced in Chapter 4. The MES effects on defrosting performance for
an ASHP unit should be identified. It is meaningful to quantitatively study the energy
transfer mechanism shown in Fig. 8.1.
On the other hand, the negative effects of melted frost on defrosting were demon-
strated in the aforementioned experimental studies. It was demonstrated that much
thermal energy was consumed during defrosting when the melted frost flowed down-
ward along the surface of the multicircuit outdoor coil, or was kept on the downside of
the outdoor coil due to surface tension. After water-collecting trays were installed
between circuits in two-circuit and three-circuit outdoor coils, the total defrosting
energy consumption could be decreased by 10.3% and 10.4%, respectively. Clearly,
when the energy conversion process was analyzed, the condition of the melted frost
locally drained should not be neglected. It is a fundamental parameter that affects the
optimization of ASHP units.
Understanding the melted frost and MES effects on defrosting performance are of
importance for the application ASHP units, but energy conversion studies are scarce in
the open literature. An experimental investigation on the energy conversion process in
an ASHP unit during defrosting with the melted frost locally drained has been carried
out. In this section, the experimental setup used is totally the same as that used in the
previous section. All information about the environmental chamber, the outdoor coil
and indoor coil, and the DAS system are detailed in Chapter 3. Here, two settings of
the two-working-circuit and the three-working-circuit cases are first designed. Basing
on the experimental results, the evaluations and discussions on defrosting perfor-
mance of this experimental ASHP unit are presented, and the effect of MES is com-
paratively and quantitatively analyzed. The conclusions of this study are finally given.

8.3.1 Experimental cases
To qualitatively and quantitatively investigate the energy conversion process in an
ASHP unit during RCD with melted frost locally drained, a series of experiments
should be carried out. Basing on these experimental results, all types of energy sup-
plies and consumptions during defrosting could be calculated. Meanwhile, the effect
of MES on defrosting performance could be studied. First, an ASHP unit was selected
in which a three-circuit outdoor coil was tailor-made. Then, two typical experimental
conditions were designed, with two and three circuits of the outdoor coil used. To
avoid uneven frosting influence, the frost was adjusted to be evenly accumulated
on the surface of each circuit. Their FECs were kept at higher than 90%. Here, the
tube surface temperature worked as the controlling index of frost accumulation dis-
tribution during frosting. Then, the energy supplies and consumptions in different
fields were calculated. Finally, the defrosting performance was evaluated by the
defrosting efficiency and MES effect on defrosting. The equations were listed in
the previous section.
A series of experimental works using the experimental ASHP unit was carried
out to investigate the effect of MES on its RCD in an ASHP unit with melted frost