Page 230 - Defrosting for Air Source Heat Pump

Energy transfer during defrosting 225

8.2.1 Methodology
To qualitatively and quantitatively study the energy transfer process in an ASHP unit
and the effect of MES on RCD, a series of experiments should be carried out. Basing
on these experimental results, all types of energy supplies and consumptions during
defrosting could be calculated. First, an ASHP unit was selected and a three-circuit
outdoor coil was specially made. And then, two typical experimental conditions were
designed, with two-working-circuit and three-working-circuit outdoor coils used,
respectively. To avoid the uneven frosting influence, frost was adjusted to be evenly
accumulated on their surfaces during frosting. The FEC was kept at higher than 90%,
which was defined as the ratio of the minimum frost accumulation among three cir-
cuits to the maximum one, and calculated by the melted frost collected from the water-
collecting cylinders, with the water vaporized into the ambient air neglected. The tube
surface temperature worked as the controlling index of frost accumulation distribution
during frosting. Energy supplies and consumptions in different fields were calculated,
including power consumptions for the compressor, the indoor and outdoor air fans,
etc. Finally, the defrosting performance was evaluated by the two following indexes,
(1) Defrosting efficiency,

Q m + Q v
η ¼ 100% (8.1)
d
E comp + E i, fan + Q i,air
and

(2) MES effect on defrosting,

Q i,MES Q o,MES
η ¼ 100% (8.2)
m
E comp + E i, fan + Q i,air
in which Q m is the energy consumed on melting the accumulated frost, and Q v on
vaporizing the retained melted frost. E comp and E i, fan are the power inputs to the com-
pressor and indoor air fan during defrosting, respectively. Q i, fan is the thermal energy
supply from the indoor air. Q i, MES and Q o, MES are the MES values of the indoor and
outdoor coils, respectively. All the mentioned parameters are shown in Fig. 8.1.
(1) Experimental setup

In this section, the experimental setup used is the same as that reported in Chapter 3.
So, it is briefly introduced here. As shown in Fig. 8.2, the experimental ASHP unit was
installed in an existing environmental chamber having a simulated heated indoor
space (left) and a simulated outdoor frosting space (right). In each space, sensible
and latent LGUs were installed and used to simulate thermal loads. The experimental
conditions were jointly maintained by using a separate DX A/C system in the envi-
ronmental chamber and the two LGUs. During experiments, to slightly adjust the
latent load, two humidifiers were added in the outdoor frosting space. During normal
heating (or frosting) operation, a frosting environment in the outdoor space was