Page 228 - Defrosting for Air Source Heat Pump

8
Energy transfer during defrosting

8.1 Introduction

In recent years, various frost-retarding measures have been investigated to improve
the operating performance of an ASHP unit. While the use of frost-retarding measures
can only delay frost formation or growth, such measures are always expensive or con-
sume additional energy, and there is still frost that must be removed even after apply-
ing the measures. Periodic defrosting therefore becomes necessary for guaranteeing
the satisfactory operation of an ASHP unit. As mentioned in Chapter 2, defrosting
may be realized by a number of methods, and the most widely used standard defrosting
method for ASHP units is reverse cycle defrosting (RCD).
As shown in Fig. 8.1, when an ASHP unit changes from heating mode to RCD mode,
its outdoor coil changes to act as a condenser and its indoor coil as an evaporator. The
energy that should have been used for space heating is consumed to melt frost and
vaporize melted frost. Not only is the indoor space heating interrupted, but also the
indoor thermal comfort level may be adversely affected. Usually, the ambient air tem-
perature is low at night, therefore, sleep thermal comfort can be degraded due to fre-
quent defrosting operations of an ASHP unit when it is used for space heating in a
sleeping environment. As a transient and nonlinear heat and mass transfer process,
the energy transfer during defrosting directly affects defrosting performance. Hence,
to improve the defrosting performance of ASHP units, many experimental studies have
been carried out and reported, such as (1) changing the installation method of the out-
door coil, (2) adjusting the refrigerant distribution, (3) eliminating uneven defrosting,
(4) improving FECs, and (5) applying PCM-TES to defrosting. Among these studies,
two indexes—defrosting duration and defrosting efficiency—were simply used to eval-
uate the defrosting performance. However, the dynamic and complicated energy trans-
fer process has not been studied in great detail.
It is easy to understand that the energy transfer process can be evaluated using a
numerical method. Notably, Cole built a defrosting model for large commercial
freezers, and reported the heat and mass transfer and fluid flow mechanisms.
The resultant refrigeration loads due to defrosting were theoretically estimated [1].
Thereafter, Krakow et al. reported an idealized RCD model for an evaporator [2,
3]. Two evaluation indexes, system performance coefficient and defrosting efficiency
(presented as coil efficiency), were separately defined. Furthermore, the fact that the
refrigerant stored in a receiver for a transient system was acting as an energy source
during defrosting was demonstrated [3].
Although many follow-up modeling studies were reported, attention was most
paid to the system performance improvement by refrigerant distribution [4, 5] or
component optimization [6, 7]. A detailed energy transfer process during a single
RCD operation is not given. A previous experimental study tried to analyze the
defrosting heat supplies and energy consumption during defrosting for an ASHP
Defrosting for Air Source Heat Pump. https://doi.org/10.1016/B978-0-08-102517-8.00008-4
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