Page 33 - Defrosting for Air Source Heat Pump

Previous related work: A review 25

Table 2.4 The differences between frost suppression and defrosting
Frost
No. Aspects suppression Defrosting

1 Operation initiation When frost After frost accumulates
appears
2 Operation termination Never Periodic
3 During operation Heating mode Heating mode stopped
continued
4 Operation effects Not all frost All frost removed
removed
5 Evaluation index COP Defrosting efficiency;
defrosting duration
6 Duration fraction in a frosting- More than 80% Less than 20%
defrosting cycle

through the outdoor coil to melt the frost. Ameen et al. experimentally investigated the
defrosting performance of an ASHP unit using warm air under controlled conditions in
an air-conditioned wind tunnel [12]. The effectiveness of the compressor shutdown
defrosting method was also demonstrated by Shang [80], by experimentally investi-
gating the effect to prestart an outdoor coil fan of an ASHP unit on defrosting perfor-
mance. Its advantages would include low initial cost, no reconstruction work, and easy
control, making it widely applied. However, because the energy for defrosting comes
from ambient air, it would take a long time for a defrosting operation to complete,
during which the indoor thermal comfort can be degraded.

2.3.2 Electric heating
Electric heating defrosting (EHD) usually involves electrically heating the surface of
an outdoor coil to melt off frost. Kim et al. [81], Bansal et al. [82], and Ozkan et al. [83]
conducted comparative studies using different types of defrosting heaters, but no
quantitative defrosting results or frosting conditions were provided. Melo et al.
[84] carried out a series of experiments using a purposely built testing apparatus.
Among three types of heaters, the highest heating efficiency of approximately 48%
was obtained with a glass tube heater. A calrod heater seemed to be the mostly appro-
priate not only because of its efficiency, which was compatible with that of the other
heaters, but also due to its low cost and easy installation. Using air bypass circulation
and electric heaters, Yin et al. [85] comparatively and experimentally studied a new
cold storage method with different defrosting heaters and air circulation modes. When
using the new method, the defrosting duration was shortened by 62.1% and the
defrosting energy consumption reduced by 61.0%. The defrosting efficiency was
increased to 77.6%, which was 2.93 times that using a conventional electric heating
defrosting method. However, the additional electrical energy required to melt frost is
high-quality energy. Meanwhile, an ASHP unit is out of operation during defrosting,