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28 Defrosting for Air Source Heat Pump

comprehensive evaluation index and are not widely applied. Hot water spraying
defrosting is limited by its inconvenience, discontinuity, and high cost for the hot
water supply. Hot gas bypass defrosting has a good operational stability and defrosting
effect. However, more electric energy is needed than that for reverse cycle defrosting.
Overall, the comprehensive evaluation index for RCD is the highest.

2.4 Improvements for reverse cycle defrosting

Currently, the most widely used standard defrosting method is reverse cycle
defrosting. The RCD system is simple and can be easily controlled, but an RCD oper-
ation is a complex process involving spatial and time variations of the temperatures of
the refrigerant, metal, and air as well as many other indeterminate factors resulting
from transient cycling, which may last for only a few minutes [27]. Also, an energy
balance on the airside of an outdoor coil is complex due to the fact that the energy
extracted from the hot refrigerant gas is utilized in five different ways. As previously
discussed, defrosting an ASHP unit consumes energy and causes undesirable fluctu-
ations of the indoor air temperature and other operational problems, such as low-
pressure cut off or wet compression. Therefore, using both experimental and numer-
ical approaches, extensive research work has been carried out to improve the operating
performance of ASHP units during reverse cycle defrosting.

2.4.1 Experimental studies
2.4.1.1 Basic component optimization
Due to the operational characteristics of reverse cycle defrosting, investigations on the
optimization of the basic component in a refrigerant cycle have been conducted. First,
a thermal expansion valve (TEV) was used to experimentally investigate the transient
RCD performance for a nominal 3-ton residential ASHP unit. It was found that the
accumulator of the ASHP unit and the TEV impacted significantly on the unit’s
dynamic responses. With either a scroll or a reciprocating compressor, the cycle per-
formances during RCD for an ASHP unit were further experimentally compared [12].
Second, the effects of an accumulator in the suction line on the frosting/defrosting
performance were investigated, showing that the removal of the accumulator pro-
duced a 10% reduction in defrosting duration but a 25% reduction in the integrated
cyclic COP [27]. Also, using a refrigerant charge compensator instead of an accumu-
lator led to an increase in refrigerant flow rate and higher suction and discharge pres-
sures of the compressor in an ASHP unit during its defrosting [89]. The defrosting
effect was improved by the addition of a compensator with an increased circulation.
However, the effect of basic component optimization is limited, and other enhance-
ment methods such as using a thermal energy storage (TES) system in an ASHP unit
are being studied.