Page 156 - Defrosting for Air Source Heat Pump

Investigation of effect on uneven defrosting performance 149

their durations at the ratio of 10.2% and 8.6%, respectively. (3) Experimental results
show that the total energy used for defrosting was 697.9 kJ in Case 1, but 526.0 kJ in
Case 2, or 24.6% less. Total energy consumption for defrosting for the two cases was
344.4 kJ and 323.0 kJ, respectively, or a 6.2% difference. Most energy came from the
indoor air, and was used on melting frost. Their defrosting efficiencies were calculated
at 49.4% and 61.4%, with about 12.0% higher in Case 2. (4) As shown in Fig. 5.15A,
for a vertical three-circuit outdoor coil with separations to install water-collecting
trays, the total area of the downside surface of the outdoor coil is three times the area
of Side B, or about 0.264 times the area of Side C shown in Fig. 5.15D. It means that
the defrosting efficiency could be improved by about 0.264 12.0%, or 3.2%, when
the remaining melted frost at the downside of each circuit was drained away during
RCD. (5) For an ASHP unit with a vertical three-circuit outdoor coil, installing water-
collecting trays between circuits could improve the defrosting efficiency from 43.5%
to 56.7%, or about 13.2% [11]. Consequently, for an ASHP unit with a vertically
installed three-circuit outdoor coil, the defrosting efficiency is supposed to be
improved about 16.4% after downward-flowing melted frost is locally drained by
water-collecting trays installed between circuits and the remaining water is cleaned
off by destroying the surface tension.

5.4 Concluding remarks

In this chapter, the following conclusions could be reached. (1) The uneven defrosting
phenomenon could be eliminated by horizontally installing a multicircuit outdoor coil.
After the uneven defrosting was avoided, the defrosting performance of the ASHP unit
would be improved with less energy consumption and a shortened defrosting duration.
(2) It was experimentally demonstrated that the residual water retained on the
downside of the circuit due to surface tension would have negative effects on
defrosting performance for an ASHP unit. After the residual water was wiped off man-
ually, the optimization of the defrosting performance was quantitatively investigated.
(3) After installing horizontally a vertically installed multicircuit outdoor coil, the
uneven defrosting problem was alleviated. However, before we consider this modifi-
cation, the frosting or heating performance of the ASHP unit should be further tested.
This is because the defrosting period accounts for a smaller proportion of time in a
frosting-defrosting cycle. (4) As mentioned in Chapter 2, after the circuit number
was increased from a two circuit to a three circuit for a multicircuit outdoor coil in
an ASHP unit, the negative effects of the melted frost were increased due to a larger
increase of defrosting efficiency after the water-collecting trays were installed. How-
ever, the number of circuits is always limited by the dimension of the structure of an
outdoor coil. Here, the negative effects of the residual water retained on the downside
of the circuit due to surface tension also limits the number of circuits for an outdoor
coil having a fixed heat exchanger area.