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12 Defrosting for Air Source Heat Pump

presented in this chapter. First, a review of the studies on frost-suppression measures
for ASHP units is reported, covering changing ambient air parameters at the outdoor
coil inlet, optimizing the structure of an outdoor coil, and other frost-suppression mea-
sures. This is followed by reporting various defrosting methods for ASHP units, with
emphasis on the studies for RCD optimization using experimental and theoretical
approaches. Finally, control strategies to initiate and end a defrosting operation for
an ASHP unit are summarized.

2.2 Frost-suppression measures for ASHP units

As previously discussed, frost formation and accumulation on the surface of the out-
door coil in an ASHP unit is an undesirable phenomenon. Frosting duration accounts
for more than 80% of the operational period in a frosting-defrosting cycle, and thus
frost suppression plays an important role in the optimization of ASHP units. In order to
improve the operating performance of ASHP units, frost-suppression measures have
attracted increasing research attention in recent decades. Previous studies on develop-
ing frost-suppression measures are classified and summarized. There are basically two
types of frost-suppression measures: the external type shown in Fig. 2.1A and the
internal type shown in Fig. 2.1B, respectively.

2.2.1 Changing ambient air parameters at outdoor coil inlet
It is easy to understand that frost formation on the surface of the outdoor coil of an
ASHP unit is closely related to ambient air conditions at which the ASHP unit is oper-
ated, such as air temperature, relative humidity, airflow rate, etc. Consequently, stud-
ies on analyzing ambient air parameters at the outdoor coil inlet of an ASHP unit have
been carried out, with a quantitative investigation of the influence of these parameters

Fig. 2.1 Frost-suppression measures for ASHP units.