Page 229 - Defrosting for Air Source Heat Pump

224 Defrosting for Air Source Heat Pump

Fig. 8.1 Energy transfer process during (A) heating mode and (B) defrosting mode.

unit [8]. It was indicated that the heat supply from the indoor air contributed to
71.8% of the total heat supplied for defrosting while 59.4% of the supplied energy
was used for melting frost. In addition, the condition of even frosting at the start of a
defrosting operation was neglected. Obviously, to optimize an ASHP system, the
dynamic energy transfer during defrosting should be further comprehensively
studied.
During RCD, as shown in Fig. 8.1, the energy stored in the indoor and outdoor coil
metals (MES), E i, MES and E o, MES , changes with the fluctuation of coil temperature.
Although it has been reported that the energy consumption for heating the outdoor
coil metal accounts for 16.5% of the total heat supplied [4], this part of the energy
has always been neglected [7]. Hence, MES was first considered [9, 10] earlier when
studying the effects of the downward flowing of melted frost. As a fundamental prob-
lem, the MES effects on defrosting performance for an ASHP unit are still not clear.
Also, when the MES is changed, the energy transfer process in an ASHP unit, as
expressed by the two equations shown in Fig. 8.1, should be qualitatively and quan-
titatively studied. The condition of melted frost being locally drained away should also
be considered.

8.2 Energy transfer process during defrosting

Understanding the energy transfer process and the MES effect on defrosting
performance are of importance for ASHP units’ application, but studies are scarce
in the open literature. An experimental investigation on the energy transfer process
in an ASHP unit and the effect of MES during its RCD has been carried out and a
comparative and quantitative analysis was conducted using the experimental data.
A tailor-made three-circuit outdoor coil is first introduced and two settings of the
two-working-circuit and three-working-circuit cases are designed. Then, a description
of the experimental setup and experimental procedures are given. This is followed by
reporting the experimental results. Lastly, the evaluations and discussions on defrosting
performance for this experimental ASHP unit are presented, followed by a conclusion.