Page 142 - Defrosting for Air Source Heat Pump

Investigation of effect on uneven defrosting performance 135

Table 5.7 Energy performance analysis for the three experimental cases
Item Parameter Case 1 Case 2 Case 3 Unit

1 The power input to compressor 111.0 112.6 140.3 kJ
2 The power input to indoor air fan 6.3 1.7 1.6 kJ
3 The power input to outdoor air fan 0 0 7.2 kJ
4 The energy from the indoor air 609.9 538.6 663.0 kJ
5 Total energy supply during defrosting 727.1 697.9 812.0 kJ
6 Energy consumption on melting frost 307.6 323.3 313.0 kJ
7 Energy consumption on vaporizing 55.0 48.9 66.0 kJ
the retained water
8 Total energy consumption for 362.6 372.2 379.0 kJ
defrosting
9 Defrosting efficiency 43.5% 53.3% 46.7% –

In this section, to decrease the flow path of downward-flowing melted frost due to
gravity, the multicircuit outdoor coil was changed to be horizontally installed. In addi-
tion, the operation of reversing the outdoor air fan and blowing the remaining water
away was undertaken to decrease the total mass of the remaining water. Finally, the
following conclusions were reached: (1) When a vertically installed multicircuit out-
door coil in an ASHP unit was changed to horizontally installed, the defrosting effi-
ciency increased from 43.5% to 53.3%, or at an increasing value of 9.8%. Meanwhile,
the negative effects of melted frost downward flowing due to gravity were eliminated.
The positive effects of a horizontally installed multicircuit outdoor coil on defrosting
performance for an ASHP unit are obvious. (2) For an ASHP unit with a horizontal
multicircuit outdoor coil, when the outdoor air fan was used to blow the melted frost
during defrosting, the total mass of the retained water collected obviously decreased,
from 566 g to 344 g, or 222 g less. However, the defrosting efficiency was not
increased, but decreased from 53.3% to 46.7% due to the heat transfer being enhanced
between the hot coil and the cold wind. Therefore, destroying the surface tension to
enhance the melted frost’s local drainage as well as an outdoor coil structure adjust-
ment and fin surface treatment may be a better choice. (3) To evaluate the energy con-
sumption on heating the ambient air due to waiting for the other circuit to terminate its
defrosting, the DEC was innovatively defined and first used here. The experimental
results demonstrated that, to improve the defrosting performance, the DEC should
also be improved. Therefore, there would be less energy consumed on heating the
ambient air to wait for the other circuit’s defrosting termination. Furthermore, the
DEC was suggested to be used as an index to evaluate the defrosting performance
of an ASHP unit with a multicircuit outdoor coil. (4) During an operational cycle
of frosting-defrosting for an ASHP unit, the frosting duration is always much longer
than the defrosting duration. Consequently, frosting studies account for more impor-
tant parts in studies of system performance improvement. Therefore, system frosting
performance should also be studied for an ASHP unit with a horizontally installed out-
door coil before a traditional vertical outdoor coil is changed to being horizontally
installed.