Page 237 - Defrosting for Air Source Heat Pump

232 Defrosting for Air Source Heat Pump

m Cu c Cu + m Al c Al
c PMe ¼ (8.14)
m Cu + m Al
Using the Eqs. (8.9)–(8.14), the MES values of the indoor coil and outdoor coil during
RCD were obtained.
In Eq. (8.2), Q i, MES is the energy discharged from the metal of the indoor coil dur-
ing defrosting, and Q o, MES is the energy stored in the metal of the outdoor coil. The
two parameters were calculated by the following equations,

(8.15)
Q i,MES ¼ Q i,MES 0 Q i,MES t
Q o,MES ¼ Q o,MES t Q o,MES 0 (8.16)
where Q i, MES 0 and Q o, MES 0 are the energy stored at the metal of the indoor and
outdoor coils at the start of defrosting, and Q i, MES t and Q o, MES t are the energy
stored at the metal of the indoor and outdoor coils at the end of defrosting, respec-
tively. The four parameters could be calculated with the Eq. (8.9).
When the energy discharged from the metal of the indoor coil is higher than the
energy stored in the metal of the outdoor coil during defrosting, there is η m > 0. It
means that the metal energy storage has a positive effect on system defrosting perfor-
mance. On the contrary, the metal energy storage has a negative effect on system
defrosting performance when the energy stored in the metal of the indoor coil is less
than that of the outdoor coil.

8.2.2 Results and analysis
Experimental results in the two cases are listed in Table 8.5. At the start of defrosting,
the total masses of frost accumulation were 710 g in Case 1 and 952 g in Case 2, with
their average values at 355 and 317.3 g for each circuit in the two cases, respectively.
That means that more frost accumulated on a circuit’s surface during the frosting stage
in Case 1. During defrosting, the total masses of melted frost collected were 620 g in
Case 1 and 921 g in Case 2, with their average values at 310 and 307 g for each circuit,
respectively. It is easy to understand that larger frost accumulation means more melted
frost is collected. As weighted and calculated, the total masses of retained water were
81 g in Case 1 and 88 g in Case 2, with their average values at 40.5 and 29.3 g for each
circuit, respectively. As calculated, more frost was vaporized for each circuit in Case
1. Therefore, it is demonstrated that the defrosting efficiency in Case 1 is higher in the
two cases due to more vaporization energy for a circuit being efficiently used.
Fig. 8.5 presents eight photographs showing the airside surface conditions of the
outdoor coil during defrosting in the two cases. Obviously, there are no water-
collecting trays installed between circuits. Therefore, the two cases can reflect the
defrosting conditions in a traditional ASHP unit, with the melted frost flowing down-
ward freely along the outdoor coil surface. Fig. 8.5(1A–1D) are in Case 1, with the two
working circuit, and Fig. 8.5(2A–2D) are in Case 2, with the three working circuit,
respectively. As observed from Fig. 8.5(1A and 2A), the surface conditions at the start
of defrosting for each circuit in the two cases were visually the same, which agreed