Page 197 - Defrosting for Air Source Heat Pump

Frosting evenness coefficient 191

In this section, a comparative experimental study on the defrosting performance of
an ASHP unit with a vertically installed multicircuit outdoor coil at different FECs
with the melted frost locally drained was undertaken and the study results were con-
cluded. The negative effects of uneven frosting at the start of a defrosting operation on
defrosting performance were first confirmed, and the performance would be better
when it starts at a higher FEC with melted frost locally drained. When the FEC
increased from 79.4% to 96.6%, the defrosting duration would be shortened by 23,
from 198 s to 175 s, or about 11.2%. At the same time, the total energy supply during
defrosting could be decreased by about 3.7%, from 781.8 to 678.8 kJ, and the
defrosting efficiency increased by about 5.7%, from 50.7% to 45.0%. In addition,
there are many parameters that could be used to confirm the negative effects on
defrosting performance for a lower FEC at the RCD start. These parameters could
be used to regulate the termination of an RCD operation, especially the fin surface
temperature reaching some preset value, and the temperature of the melted frost
collected reaching its lowest value.

6.5 Concluding remarks

Uneven defrosting always results in a lower defrosting efficiency and a longer
defrosting duration. In Chapter 3, the negative effects of downward-flowing melted
frost along the surface of the outdoor coil were experimentally examined. Further-
more, the negative effects of downward-flowing melted frost on defrosting perfor-
mance were quantitatively analyzed with two-circuit and three-circuit outdoor coils
in an ASHP unit. However, frosting unevenly distributed on the surface of a multi-
circuit outdoor coil is also an important reason for uneven defrosting because the
heating load for each circuit is different at the start of defrosting. In this chapter, to
further quantitatively describe the uneven frosting, the frosting evenness coefficient
was defined as the ratio of the minimum frost accumulation to the maximum one.
Uneven frosting results in a lower COP. The higher the FEC for an ASHP unit with
a multicircuit outdoor coil, the higher the COP could reach. For an ASHP unit with a
three-circuit outdoor coil, when the FEC was increased from 75.7% to 90.5%, the COP
could be increased from 4.10 to 4.26 at a 3600 s frosting process. Meanwhile, uneven
frosting also downgrades the defrosting performance. The defrosting performance
would be better when it starts at a higher FEC, irrespective of melted frost local
drainage. For a traditional ASHP unit, when the FEC was increased from 82.6% to
96.6%, the defrosting duration would be shortened by 17 s, from 202 to 185 s, and
the defrosting efficiency increased by about 6.7%, from 42.0% to 48.7%. For an ASHP
unit with water-collecting trays installed between circuits, when the FEC was
increased from 79.4% to 96.6%, the defrosting duration would be shortened by
23 s, from 198 to 175 s, and the defrosting efficiency increased by about 5.7%, from
50.7% to 45.0%.
However, more conditions should be further examined. First, the FECs all are very
high in this chapter, at the ranges of 80%–90% and 90%–100%. Hence, the defrosting
performances for the FEC at lower ranges of 50%–60%, 60%–70%, and 70%–80%