Page 37 - Defrosting for Air Source Heat Pump

Previous related work: A review 29

2.4.1.2 PCM-TES-based RCD

During reverse cycle defrosting, the indoor air fan in an ASHP unit is normally
switched off to avoid blowing cold air directly to a heated indoor space, affecting
the thermal comfort of the occupants [90]. A defrosting operation may also occur
at night when the ambient air temperature is lower than that during the day; sleeping
thermal comfort may also be negatively affected [91–93]. The energy available from
the indoor coil is basically that stored in the coil metal, but there is an insignificant
amount of energy available from the indoor air because of a negligibly small airside
convective heat coefficient resulting from a deenergized indoor air fan during
defrosting. Consequently, low-pressure cut off or wet compression may take place,
which may cause the ASHP unit to shut down and possibly damage the compressor.
To avoid these aforementioned problems, the technologies of TES and phase change
materials (PCM) may be applied due to the advantage of high-density energy storage
[94]. First, DX40 was used as a thermal storage material [95] in a heat source tank for
defrosting, and a higher defrosting efficiency was reached after using PCM-TES.
Then, inorganic PCM such as CaCl 2 6H 2 O was used in an ASHP unit [96, 97].As
shown, this PCM-TES-based defrosting method could help achieve improved indoor
thermal comfort with a shorter defrosting period and a higher indoor supply air tem-
perature during reverse cycle defrosting. The same conclusions were given in similar
studies [98, 99]. As summarized in Table 2.7, PCM-TES-based RCD was widely stud-
ied in 2000–2017.

2.4.1.3 Airflow and refrigerant distribution adjustment

Mal-distribution of refrigerant or airflow might result in uneven defrosting, thus
degrading the defrosting performance. Aganda et al. [103] compared the predicted
and experimental heat transfer performances for a finned tube outdoor coil, and found
that airflow mal-distribution reduced the performances of an evaporator circuit [104].
With refrigerant flow controlled by a TEV, the worst-performing circuit affected the
performance of the entire outdoor coil by as much as 35%. Kim et al. [105, 106] exper-
imentally and numerically investigated a hybrid-individual degree of superheat con-
trol method for refrigerant flow balancing in a multicircuit evaporator: upstream vs
downstream flow control. The results showed that the upstream refrigerant flow con-
trol consistently outperformed the downstream refrigerant flow control, and recovered
most of the loss in cooling capacity and COP due to nonuniform airflow distribution.
Based on these conclusions, they utilized the model to further evaluate the effects of
uneven air and refrigerant flow distributions and the benefits of upstream hybrid con-
trol during defrosting for an ASHP unit [106]. Hence, adjusting airflow and/or refrig-
erant distribution could improve defrosting performance.

2.4.1.4 Sensible heat defrosting method
To avoid adverse shock and “oil rush,” which were commonly seen in conventional
RCD operations, a sensible heat-defrosting method was proposed and numerically
investigated by Liang et al. [86], by using a self-organizing fuzzy controller in an