Page 236 - Defrosting for Air Source Heat Pump

Energy transfer during defrosting 231

Finally, experimental work was carried out at the two experimental cases. At
frosting mode, there were three working circuits, Circuits 1–3, in the two cases.
However, at defrosting mode, only Circuits 1and2 workedinCase1,but three
circuits all worked in Case 2. The differences of the working circuits for the outdoor
coil in the two experimental cases are illustrated in Fig. 8.4, and the system oper-
ation differences are summarized in Table 8.3. The total metal masses of the out-
door coil in the two cases are 2020 and 3030 g, respectively. Consequently, the
system defrosting performances at different MES could be comparatively and
quantitatively analyzed.
(3) MES calculations

The metal temperatures of the indoor and outdoor coils change with the temperature var-
iations of the inner flowing refrigerant. At the beginning of a defrosting operation, the
metal temperature of the indoor coil was high, where a lot of heat was stored in the metal.
At defrosting, the warm indoor air transferred thermal energy to the cold refrigerant
across the tube and fin of the indoor coil, which resulted in the metal temperature decreas-
ing. Thus, the heat stored in the metal was taken away by the cold refrigerant. Meanwhile,
some energy taken by the hot refrigerant was used to heat the cold outdoor coil metal,
then to heat the frost and the retained water on its surface. The heat transferred around the
metal of the indoor and outdoor coils in the system, Q Me , was evaluated by:

t d
Z
X
Q Me ¼ P Me dt ¼ P Me Δt (8.9)
0
where t d is the defrosting duration. The rate of heat supply from the indoor coil metal,
P Me , was evaluated by:

ΔT Me
P Me ¼ c PMe m Cu + m Al Þ (8.10)
ð
Δt
where m Cu and m Al are the total masses of copper and aluminum of the coils. ΔT Me is
the average temperature difference of the indoor coil metal, and was evaluated by:

(8.11)
ΔT Me ¼ T 0 T t
1
T 0 ¼ ð T in,0 T out,0 Þ (8.12)
2
1
T t ¼ ð T in,t T out,t Þ (8.13)
2
T 0 and T t are the average temperatures of the indoor coil metal at the start and end of
the defrosting operation, T in,0 and T out,0 are the inlet and outlet tube surface temper-
atures of the indoor coil at the start of the defrosting operation, and T in, t and T out, t are
the inlet and outlet tube surface temperatures of the indoor coil at the end of the
defrosting operation, respectively. In Eq. (8.8), c PMe , the average specific heat of cop-
per and aluminum, could be evaluated by: