Page 44 - Defrosting for Air Source Heat Pump

34 Defrosting for Air Source Heat Pump

2.4.2.2 Defrosting model outdoor coil only

Although the aforementioned defrosting models [12, 107, 108] were developed and
used in studying defrosting performance, none of them considered the negative effects
of the downward flowing of melted frost due to gravity along the surface of an outdoor
coil on defrosting performance, by either assuming a stable water layer or no water
retention on the coil surface. In 2012, Qu et al. [109] reported on a modeling analysis
where a semiempirical model for the defrosting on the airside of a four-circuit outdoor
coil in an ASHP unit was developed. The negative effects of melted frost on defrosting
performance were considered and quantitatively studied using this model, which was
different from the aforementioned defrosting modeling studies. It was further
predicted that if the melted frost could be drained away locally, the defrosting effi-
ciency for the ASHP unit could be increased by up to 18.3%. A similar energy con-
sumption ratio for defrosting was also reported by Dong et al. [110] in a study on the
energy consumption analysis for vaporizing the melted frost and heating the ambient
air during RCD in an ASHP unit. To clearly understand the modeling study reported
by Qu et al., the main equations used in the two models are listed in Table 2.9.
Being the most popular defrosting method for ASHP units, RCD has attracted
much research attention. A series of related experimental investigations was under-
taken, covering optimizing the original component, the use of PCM-TES-based
reverse cycle defrosting, adjusting the air and refrigerant distribution, and the sensible
heat defrosting method. Among all the experimental studies on RCD optimization, the
PCM-TES-based RCD was the most strongly recommended, with its advantages of
easy and low-cost installation and a better defrosting effect. However, both frosting
and defrosting performances should be examined when an ASHP unit is optimized
with any defrosting enhancement methods.
On the other hand, numerical studies on RCD were conducted, with defrosting
models developed. For the system-based defrosting model, component optimization
and the effects on defrosting efficiency were the hot issues. However, in outdoor coil
only defrosting models, the defrosting process was always described in greater detail.
For example, a defrosting process was divided into only two stages, premelting and
melting [12], in a system-based defrosting model while it was divided into three stages
in Qu’s defrosting model [109].

Table 2.9 Main energy equations used in two defrosting modeling studies [109]

Stage Description Main equation for energy balance
First Frost melting dM w, j T w, jÞ
ð
q j ¼ L sf m f, j + c p
stage without water dt
flow
Second Frost melting dT w, j
q j + c p m w, j 1 T w, j 1 ¼ L sf m f, j + c p M w,max
stage with water dt

flow + c p m w, j T w, j + h c,w T w, j T a A f a
Third Water layer dM w, j T w, jÞ
ð
q j ¼ c p + h c,w T w, j T a A w a + h c,d T r, j T a A d a + m v, j L v
stage vaporizing dt