Page 34 - Defrosting for Air Source Heat Pump

26 Defrosting for Air Source Heat Pump

which results in the interruption of indoor air heating. Therefore, this method is lim-
ited in the civilian application domain such as household air conditioning and residual
ASHP units.

2.3.3 Hot water spraying
The hot water spraying defrosting (HWSD) method can be applied where hot water for
defrosting is available. During defrosting, the indoor and outdoor fans are turned off,
with the hot water spraying onto the outdoor coil. Thus, frost could be melted and the
melted frost flows away with the water. However, only limited reported studies can be
identified, including a patent from Tanker and Abdel-Wahed’s experimental study on
applying the hot water spraying defrosting method to a horizontal flat-plate surface
[12]. Obviously, the availability of hot water limits the application, especially for
low cost and continuous hot water supply. In addition, at the termination of the hot
water spraying defrosting, some water may be retained on the coil surface due to sur-
face tension. The retained water would degrade the operating performance of an
ASHP unit when operated at the heating mode. Hence, this defrosting method is
not widely applied.

2.3.4 Hot gas bypass
Hot gas bypass defrosting (HGBD) is mainly applied to industrial ASHP units. The
superheated refrigerant vapor discharged from the compressor is directed into an
evaporator, or outdoor coil, bypassing a condenser and an expansion device. The
latent heat of the condensation of refrigerant vapor is used as the heat source; however,
the sensible heat of highly superheated refrigerant vapor may also be used [86]. On the
basis of hot gas bypass defrosting, Fu et al. [87] divided an outdoor coil into two parts,
a front part and a rear part, which were used as an evaporator and a condenser, respec-
tively, during defrosting. It was indicated that energy was used more efficiently, and
thus the defrosting duration was shorter and the defrosting loss less than those using
reverse cycle defrosting. A novel dual hot gas bypass defrosting method was also
developed to remove frost from the outdoor coil of an ASHP unit [88], showing that
the proposed method could overcome the main disadvantages for RCD and hot gas
bypass defrosting. However, the defrosting duration is always very long because
the energy use for defrosting comes from the power input to the compressor. In addi-
tion, it is easy for the compressor to suck in liquid during a hot gas bypass defrosting
process due to insufficient energy supply, which impacts badly on the safety of the
compressor [27]. Finally, the hot gas bypass defrosting method is mostly used in
industry units.

2.3.5 Reverse cycle
When an ASHP unit is operated at RCD mode, its outdoor coil acts as a condenser and
its indoor coil as an evaporator [75, 76]. During defrosting, the normal operation cycle
during heating for an ASHP unit is reversed by using a four-way valve, and hot gas is