Page 245 - Defrosting for Air Source Heat Pump

240 Defrosting for Air Source Heat Pump

But the total value of frost accumulation in Case 2 was more than that in Case 1. There-
fore, during defrosting, more melted frost was downward flowing along the surface of
the outdoor coil in Case 2. This increased the flowing rate of the melted frost, and thus
the temperature was lower. However, as shown in Fig. 8.11, the temperature variations
of the ambient air around each circuit of the outdoor coil in the two cases are totally
different. The temperature in Case 1 was always lower than 4.5°C, but the peak value
was nearly at 9.5°C in Case 2. It is obvious that the ambient air was warmed a lot in
Case 2. Therefore, more energy would be wasted during defrosting in Case 2, which
also prolonged the defrosting duration and degraded the defrosting efficiency.
Fig. 8.12 summarizes the heat supplies from the four fields during defrosting: (1)
thermal energy of the indoor air, (2) MES of the indoor coil, (3) input to the indoor air
fan, and (4) input to the compressor. The total heat supply was about 613 kJ for Case 1
and 761 kJ for Case 2, 24% higher. This results from different total frost accumula-
tions in the two cases. As illustrated, the indoor air accounts for the highest ratio in the
two cases, at 77.9% in Case 1 and 80.1% in Case 2, respectively. Energy supplies from
the indoor coil metal for the two cases were 37 and 34 kJ, accounting for 6.04% and
4.50%, respectively. The ratio differences of energy input to the compressor and
indoor air fan were very small, less than 1%. This is obvious and easily understood
because the two values increased as time progressed. This may be the reason why
it was 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. 8.13 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
the biggest percentage, at 52.04% in Case 1 and 44.97% in Case 2, respectively. Their
differences mainly result from different total areas of the outdoor coil. The percentage
of energy consumed on melting frost took 38.7% in Case 1 and 43.2% in Case 2, due to
different frost accumulations at the start of defrosting. Around 20% of the energy was
consumed on heating the retained water and the 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. 8.14 shows the MES of the
indoor coil and outdoor coil during defrosting. Defrosting efficiency and MES effects
were also calculated and listed in Table 8.6. As seen in Fig. 8.14, the energy is rep-
resented by the area of shadow. The total energy used for defrosting can be expressed
by,

E MES ¼ E S1 E S2 + E S3 E S4 (8.17)
in which E S1 and E S3 are the net energy transferred from the indoor coil to the outdoor
coil, and E S2 and E S4 are the net energy transferred from the outdoor coil to the indoor