Page 76 - Defrosting for Air Source Heat Pump

Uneven defrosting on the outdoor coil in an ASHP 67

Defrosting efficiency can be used to evaluate the performance of a defrosting oper-
ation. It is defined as the ratio of the actual amount of energy consumption required to
both melt the accumulated frost and vaporize the retained melted frost to the total
amount of energy available from an outdoor coil during an entire defrosting operation,
as follows:

E m + E v
η ¼ (3.1)
d
Q com + Q fan + Q air
where E m and E v are the total heat used for melting frost and vaporizing the retained
water, respectively, and they are evaluated by:

E m ¼ M f L sf (3.2)
E v ¼ M v L v (3.3)
where M f and M v are the total mass of the frost formed on the outdoor coil and the mass
of vaporized melted frost, respectively, and L sf and L v the latent heat of frost melting
and latent heat of evaporation of water, respectively. Also in Eq. (3.1), Q com , Q fan , and
Q air are the energy consumptions by the compressor and supply fan, and the thermal
energy from the indoor air during defrosting, respectively.
In this section, the defrosting efficiencies calculated for the three cases were
43.5%, 50.6%, and 56.7%, respectively. Similar to the previous section, it was
also demonstrated that allowing melted frost to freely flow down due to gravity would
lead to more energy consumption during defrosting. There can be two reasons for
more energy use: (a) evaporating some of the melted frost as mentioned earlier;
(b) when a defrosting process was prolonged, the fin surface in an up-circuit could
be dry already while that in a down-circuit was still wet, if not still covered with frost.
Therefore, thermal energy would be used for just heating the ambient cold air, which
was highly undesirable. Furthermore, the study results also suggested that the use of
water-collecting trays was effective in mitigating the negative effects.

3.3.4 Comparison analysis
The experimental results by using two-circuit and three-circuit outdoor coils are com-
paratively analyzed, with their differences summarized in Table 3.7. As seen, after the
outdoor coil was changed from a two circuit to a three circuit, the defrosting duration
shorted after the trays used was increased from 16 to 18 s, with the rate decreased from
15.8% to 7.5%. From the view of indoor thermal comfort, the three-circuit outdoor
coil improvement is more obvious. Although the differences of the melted frost col-
lected in two series are similar, at 5.3% for two circuit and 5.0% for three circuit, the
energy consumed savings are nearly the same, at 10.3% and 10.4%, respectively.
Finally, after the defrosting efficiency was compared, the improved values for them
are 10.5% for a two-circuit outdoor coil and 13.2% for a three-circuit outdoor coil,
respectively. The energy performance also demonstrated that the elimination of the
melted frost effects is more obvious after the working circuit number increased from
two to three.