Page 333 - Defrosting for Air Source Heat Pump

328 Defrosting for Air Source Heat Pump

Table 10.9 Experimental results of the four defrosting cases.
Item Parameters Case Case Case Case
D1 D2 D3 D4

1 FEC of outdoor coil 82.6% 79.4% 96.6% 96.6%
2 Total frost accumulation 878 g 1000 g 881 g 969 g
3 Defrosting duration 205 s 197 s 185 s 175 s
4 Total power inputs 124.6 kJ 128.0 kJ 107.6 kJ 117.3 kJ
5 Energy from indoor air 666.1 kJ 673.7 kJ 541.2 kJ 561.5 kJ
6 Condition shown in (A) (B) (C) (D)
Fig. 10.16
7 Results shown in Fig. 10.18 (T1) (T2) (T3) (T4)

(1) Fundamental assumptions
In this work, the following nine fundamental assumptions were first given:

i. Installation, running, and maintenance costs of solenoid valves and the water-collecting
trays in the three-circuit outdoor coil were neglected.
ii. Service life of an ASHP unit was assumed to be 15 years, and the unit price of electricity at
0.9 CNY/kWh [16].
iii. In the whole service life of an ASHP unit, three operating seasons were divided: the
heating season with frost formation, the heating season without frost formation, and
the cooling season.
iv. Durations of three operating seasons were constant, and assumed as the climate conditions
of Beijing (Heating season: Nov. 15–Mar. 15).
v. In the heating season with frost formation, the duration of a frosting/defrosting cycle was
assumed at 70 min.
vi. In a frosting/defrosting cycle, the frosting duration was fixed at 60 min, and two periods of
3–4 min for compressor shutdown were left for its safety, as illustrated in Fig. 10.17 (To
avoid an undesired shutdown for the ASHP unit, the frosting duration was not longer than
60 min in practical application).
vii. Defrosting durations in the four typical cases, system COP, and defrosting efficiency were
constant.
viii. All ambient air parameters, such as air temperature and relative humidity, were constant at
fixed operating seasons.
ix. For the cases without valves installed, their FECs were fixed at 75.7%. For the cases with
valves installed, their FECs were fixed at 96.6%.

(2) Frosting assumptions

To calculate the running cost at the frosting stage, the system COP and indoor heat
supplied should be obtained. Therefore, the following assumptions were also given.
i. Durations of the heating season with frost formation and the heating season without frost
formation were both assumed to be 60 days/year.