Page 155 - Defrosting for Air Source Heat Pump

148 Defrosting for Air Source Heat Pump

Table 5.10 Energy and defrosting efficiency calculations in the two cases
Item Parameter Case 1 Case 2 Unit

1 The power input to compressor fan 112.6 98.8 kJ
2 The power input to indoor air fan 1.7 1.3 kJ
3 The power input to outdoor air fan 0 0 kJ
4 The energy from the indoor air 583.6 425.9 kJ
5 Total energy supply during defrosting 697.9 526.0 kJ
6 Energy consumption on melting frost 320.0 305.9 kJ
7 Energy consumption on vaporizing the retained 24.4 17.1 kJ
water
8 Total energy consumption for defrosting 344.4 323.0 kJ
9 Defrosting efficiency 49.4% 61.4% –

effects of surface tension on defrosting performance were further demonstrated,
and the optimization by structure adjustment or fin surface treatment to eliminate
the surface tension of the melted frost could decrease the energy waste fundamentally.
For an ASHP unit with a vertical multicircuit outdoor coil, the RCD efficiency
could be improved by installing water-collecting trays between circuits. However,
there was still some melted frost on the downside surface of each circuit due to
surface tension. That melted frost consumed energy during defrosting, and thus
had negative effects on the system defrosting performance. Therefore, to quantita-
tively study the negative effects due to surface tension, the effects are enlarged as
enlarging the area of the downside surface of the outdoor coil, by changing the
vertical three-circuit outdoor coil into horizontally installed. Finally, a comparative
experimental study with the surface tension kept and destroyed was undertaken and
the relative results analyzed. The following conclusions could be reached from this
section.
(1) For a horizontal multicircuit outdoor coil, it is proved that the defrosting process
of each circuit reached the preset termination temperature (24°C in this section) at the
same time. The condition that a circuit terminated its defrosting and was waiting for
the others could be avoided. The energy consumption on heating the cold ambient air
due to uneven defrosting could be saved [21]. Therefore, the same as installing water-
collecting trays between circuits, the negative effects of downward-flowing melted
frost due to gravity on defrosting performance for an ASHP unit with a vertical mul-
ticircuit outdoor coil could also be eliminated by the coil being horizontally installed,
as suggested in Reference [13]. (2) Compared with the melted frost remaining on the
downside surface of each circuit due to surface tension in Case 1, the defrosting dura-
tion was shortened from 186 to 167 s, or 19 s less, and the length of time it took the fin
surface temperatures to reach 24°C was shortened from 188 to 172s, or 16 s less, when
the melted frost was cleaned off due to the surface tension being manually destroyed
in Case 2. The defrosting performances are obviously improved by decreasing