Page 30 - Defrosting for Air Source Heat Pump

Previous related work: A review 23

units, the heating source should come from waste heat, such as the heat recovered from
exhausted indoor air or waste hot water. However, the application of this
frost-suppression measure is also limited due to the disadvantages of high running cost
and inconvenience.
Although the energy consumed for frost suppression is carried over by the refrig-
erant circulated in an ASHP system, it is actually input into the system. This measure
is essentially different from external measures such as increasing the inlet air temper-
ature or decreasing the inlet air RH, where the energy is carried away by the inlet air.
As listed in Table 2.3, the evaluation results of 11 classical frost-suppression measures
are given. Nearly all the measures would increase the initial cost and/or the running
cost, except the two measures of adjusting the fin and tube geometry and the fin type.
Adjustment and optimization would increase the system complexity and decrease sys-
tem stability. Additional thermal energy is needed for the measures of preheating the
inlet air and employing an external heat source. Four measures require floor space for
additional equipment, such as a desiccant bed for reducing inlet air humidity and an air
jet or ultrasonic vibration facility to destroy frost formation or growth. Among all the
measures, reducing the inlet air humidity and preheating the inlet air have the best
frost-suppression effect. Among all measures listed, preheating the inlet air with waste
heat and putting a coating treatment on the fin surface with new coating materials are
highly recommended, due to both having the highest evaluation index in Table 2.3.

2.3 Defrosting methods for ASHP units

As discussed earlier, the presence of frost on the surface of the outdoor coil in an
ASHP unit would deteriorate its operating performance, energy efficiency, reliability,
and lifespan. While the use of frost-suppression measures can delay or reduce frost
formation or growth, these measures can be expensive or consume additional energy,
and there will still be frost to be removed. Therefore, periodic defrosting becomes nec-
essary for guaranteeing the satisfactory operation of ASHP units. To distinguish frost
suppression from defrosting, the differences between the two are summarized in
Table 2.4.
In this section, various defrosting methods are reviewed based on the assumption of
normal frosting operation, where no frost-suppression measures are implemented dur-
ing heating/frosting. Generally speaking, there are five types of defrosting methods:
(1) compressor shutdown defrosting, (2) electric heating defrosting, (3) hot water
spraying defrosting, (4) hot gas bypass defrosting, and (5) reverse cycle defrosting.
In order to clearly show the differences for the five defrosting methods.

2.3.1 Compressor shutdown
For the compressor shutdown defrosting (CSDD) method, ambient air is used as the
heat source for defrosting. Therefore, it is normally applied to where the ambient air
temperature is not lower than 1°C. When defrosting is needed, the compressor is shut
down but the outdoor coil air fan continues to move the ambient air at >1°C to pass