Page 257 - Defrosting for Air Source Heat Pump

252 Defrosting for Air Source Heat Pump

neglected in previous calculations. In conclusion, most of the energy for defrosting
came from the indoor air, and the ratio of MES would be different when the number
of working circuits was changed.
Fig. 8.24 shows the heat consumption during defrosting in the two cases. As seen,
there are the following five consumptions: (1) heating the ambient air, (2) heating the
melted frost, (3) heating the outdoor coil metal, (4) vaporizing the retained water, and
(5) melting the frost. Obviously, the energy consumed on heating the ambient air took
a big percentage, at 46.33% in Case 1 and 33.10% in Case 2, respectively. Their dif-
ferences mainly result from different total areas of the outdoor coils. The percentage of
energy consumed on melting frost took 43.99% in Case 1 and 53.70% in Case 2, due to
different frost accumulations at the start of defrosting. Around 10% of the energy was
consumed on heating the retained water and outdoor coil metal and vaporizing the
retained water. Compared with Case 1, the energy consumed on heating the outdoor
coil metal in Case 2 was increased. It would degrade the MES effect on defrosting
performance. However, the ratios of energy consumed on melting frost and vaporizing
retained water were also increased in Case 1. Therefore, a higher defrosting efficiency
was expected.
To further quantitatively study the effect of MES, Fig. 8.25 shows the MES of the
indoor coil and outdoor coil during defrosting. The defrosting efficiency and MES
effects were also calculated and listed in Table 8.9. As seen in Fig. 8.25, the E MES
values in the two cases were negative. They were calculated at 0.44% in Case 1
and 3.67% in Case 2, respectively. This agreed well with Fig. 8.25, in which the area
shadow in Fig. 8.25B was much bigger than that in Fig. 8.25A. Therefore, after the
two-working-circuit outdoor coil was changed to a three-working-circuit coil, the neg-
ative effects of MES on defrosting performance increased.
In conclusion, the following conclusions could be reached from this section: (1)
Four types of heating supply were quantitatively analyzed. As indicated, the heating
supply of the indoor air thermal energy contributed about 80% of the total energy
usage for defrosting, with 15% energy from the compressor inputs. The total energy
from the metal energy storage of the indoor coil and the input to the indoor air fan costs
only about 5%. (2) Five types of energy consumption were divided and listed. During
defrosting, nearly 90% of the energy was consumed on melting frost and heating

Table 8.9 Defrosting efficiency and MES effect in the two cases
Item Parameter Case 1 Case 2 Unit

1 Energy consumed on melting frost 239.5 360.7 kJ
2 Energy consumed on vaporizing retained 17.1 34.2 kJ
water
3 Energy from indoor coil 27.7 20.0 kJ
4 Energy consumed in outdoor coil 30.1 44.7 kJ
5 Total energy supply 516.7 651.7 kJ
6 Defrosting efficiency 47.13% 58.79% –
7 MES effect 0.44% 3.67% –