Page 10 - Defrosting for Air Source Heat Pump

6 Defrosting for Air Source Heat Pump

of melted frost on defrosting performance. In addition, when the circuit number in an outdoor
coil is changed, the variations in the effects due to downward flowing have also been studied.
2. Development, validation, and application of a defrosting model based on the multicircuit
outdoor coil with the melted frost flowing downward over its surface in an ASHP unit.
Although defrosting models are reported in the literature, most of them are based on an entire
ASHP system, and thus cannot be directly applied to defrosting with downward-flowing
melted frost. Based on a multicircuit outdoor coil, a new semiempirical defrosting model
has been developed, taking into account the melted frost flowing downward over the surface
of a multicircuit outdoor coil. This model is validated with experimental data, and thus
applied to optimizing defrosting control.
3. Alleviation of the uneven defrosting and the investigation of the influence of uneven frosting
on frosting and defrosting in an ASHP unit having a multicircuit outdoor coil. When the
defrosting process for different circuits is terminated at a different time, there would be some
thermal energy waste in heating the ambient air during defrosting. Therefore, such an uneven
defrosting should be avoided. An experimental study on the alleviation of the uneven
defrosting has been carried out by changing the installation of the outdoor coil from verti-
cally to horizontally. In addition, unevenly distributed frost on a different circuit’s surface
leads to different thermal insulation levels during frosting and thermal loads during
defrosting. Experimental studies on the influences of uneven frosting on both frosting
and defrosting processes have been carried out.
4. Investigation on the influences of uneven refrigerant distribution into each circuit on the
RCD performance in an ASHP unit having a vertically installed multicircuit outdoor coil.
Similar to the uneven distribution of frost on the surfaces of different circuits, the refrigerant
would also be unevenly distributed into each circuit during frosting and defrosting. For a
vertically installed multicircuit outdoor coil, this may be due to the gravity force effect at
a refrigerant distributor and the tube internal resistance in each circuit. During frosting,
the uneven refrigerant distribution can be coupled with that of the inlet air of an outdoor coil,
thus affecting the frost distribution on each circuit. During defrosting, even and uneven
refrigerant distribution statuses should be experimentally investigated, with the located
defrosting performance analyzed.
5. Investigation on the mechanism of heat transfer during RCD in an ASHP unit, and the eval-
uation of the effect of thermal energy stored in the metal of indoor and outdoor coils on
defrosting performance. There are different types of heat supply and energy consumption
during reverse cycle defrosting, and the reason for a long defrosting duration is the insuffi-
cient energy supply. Therefore, to optimize defrosting performance and shorten the
defrosting duration, the mechanism of energy transfer through the refrigerant has been
experimentally studied and analyzed. In addition, certain thermal energy is stored in the
metal of indoor and outdoor coils during defrosting, and the sum of the two should vary dur-
ing defrosting. Therefore, its influence on defrosting should be further experimentally
investigated.
6. Optimization of the defrosting initiation and termination control strategies during RCD for
an ASHP unit, taking account of with and without the melted frost flowing downward along
the surface of the multicircuit outdoor coil. The mal-defrost phenomenon appears during
frosting and defrosting, resulting in low capacity and efficiency during frosting and a low
defrosting efficiency and longer defrosting duration, respectively. Therefore, for an ASHP
unit having a multicircuit outdoor coil, the optimization for the defrosting initiation and ter-
mination control strategies should be carried out. When the melted frost flowing downward
along the surface of an outdoor coil is considered, the strategies could be more complicated.