Page 66 - Defrosting for Air Source Heat Pump

Uneven defrosting on the outdoor coil in an ASHP 57

part of Circuit 2, which was warmer due to the hot refrigerant flowing from top to
bottom within the circuit during defrosting.
Furthermore, the downward flowing of melted frost not only delayed the defrosting
process, but it also led to energy waste. The energy used for RCD comes from three
sources: the input power to the compressor, the input power to the indoor air fan,
and the thermal energy from the indoor air. In the experimental study presented in this
paper, the total energy used was 580 kJ for Case 1, but 517 kJ for Case 2, or 10.3%
less. Therefore, allowing melted frost to freely flow downward due to gravity would
lead to more energy use for RCD. There can be two reasons for more energy use:
(a) evaporating some of the melted frost as mentioned earlier, or (b) when a defrosting
process was prolonged, the surface of the fins in the upper part of Circuit 1 could be
dry already while that of the fins in the lower part of Circuit 2 was still wet or covered
with frost. Therefore, thermal energy would be used for just heating the ambient cold
air, which was highly undesirable.
In conclusion, the experimental work reported in this section quantitatively dem-
onstrated the effects of allowing melted frost to downward flow from up to down over
the airside surface of an outdoor coil in an ASHP unit during RCD. The time duration
for defrosting was prolonged and more energy was consumed during defrosting.
Therefore, allowing melted frost to flow downward due to gravity would impact neg-
atively on the operating performance of a reverse cycle defrost operation for ASHP
units. However, in this section, only two circuits were used and tested. The influence
of downward-flowing melted frost on defrosting performance for a multicircuit out-
door coil should therefore be studied. Additionally, measures to mitigate the negative
impacts should be further considered.

3.3 Three-circuit experimental study

As reported in Section 3.2, the downward flowing of melted frost over a vertical two-
circuit outdoor coil during RCD could adversely affect the defrosting performance
of an ASHP unit by using more energy for defrosting and prolonging the defrosting
process. This was because the downward flowing of the melted frost helped form or
reinforce a water layer between the frost and coil surface, introducing a thermal resis-
tance, and thus reducing the heat transfer between the two. More residual water could
be left on the surface of the bottom circuits and thus more energy would be needed to
dry the residual water on the bottom circuits. Moreover, during a defrosting process,
not only a great deal of energy for melting frost and vaporizing the retained melted
frost off the outdoor coil surface is consumed, but also the occupants’ thermal comfort
may be adversely affected because no heating is provided during defrosting [1].
Therefore, shortening a defrosting period should be one of the control purposes for
ASHP units. For example, Chinese Standard GB/T 7725-2004 specifies that the
defrosting duration for an ASHP unit should not exceed 20% of its total working
hours. However, for an outdoor coil having more than two circuits, the melted frost