Page 42 - Defrosting for Air Source Heat Pump

32 Defrosting for Air Source Heat Pump

Table 2.8 Evaluation results of four defrosting enhancement methods
Initial Running System Defrosting Evaluation
Methods cost a cost a complexity b effect b index b

Basic " ! ! 2 2
component
optimization
PCM-TES- " ! " 3 3
based RCD
Airflow and " " ! 2 2
refrigerant
distribution
adjustment
Sensible " ! " 1 1
heat
defrosting
method

a
", increased; !, unchanged.
b
3, the best; 1, the worst.
ASHP unit. For this defrosting method, the four-way valve did not act when the ASHP
unit changed its operation mode from heating to defrosting. Therefore, it is similar to
the hot gas bypass defrosting method. Refrigerant was discharged from the compres-
sor and passed through a hot gas solenoid valve and a discharge conduit. After being
throttled by the expansion valve, the refrigerant with high temperature and low pres-
sure flowed into the outdoor coil and exchanged heat with the outer frost layer. After
that, the refrigerant passed through the accumulator and was sent back to the compres-
sor for recompression. In this way, the cycle continued. However, in order to ensure
the normal operation of the unit, it should be guaranteed that there would not be any
condensation of the refrigerant. Moreover, the defrosting duration would be prolonged
because the only energy supply for defrosting is from the electricity input to the com-
pressor, and thus the indoor thermal comfort is adversely affected [90]. Consequently,
it is more suitable for industry applications.
The evaluation results of four improvement methods are listed in Table 2.8 by con-
sidering and comparing the initial cost, running cost, complexity, and defrosting
effects. As seen, the PCM-TES-based RCD is the most recommended method to
enhance defrosting for an ASHP unit. The sensible heat defrosting method, on the con-
trary, is the lowest rated.

2.4.2 Theoretical studies
As a nonlinear, moving-boundary, variable-density, multidimensional, phase-
continuously changing, dynamic heat- and mass-transfer process, defrosting is a com-
mon and complicated physical phenomenon. During a defrosting operation, the frost