Page 246 - Defrosting for Air Source Heat Pump

Energy transfer during defrosting 241

coil, respectively. It is obvious that the E MES in Case 1 was positive. It was calculated
at 0.33%, as listed in Table 8.6. Meanwhile, the value of the MES effect in Case 2 was
calculated at 2.18%, which also agreed well with Fig. 8.14. That means the MES had
a negative effect in Case 2, after the working circuit number increased 50% from two
to three circuits.
Basing on the two cases with frost evenly accumulated on the surface of two-
working-circuit and three-working-circuit outdoor coils, an experimental study on
the energy transfer process during RCD in an air source heat pump unit has been car-
ried out and reported, with the following conclusions: (1) Four types of heating supply
and five types of energy consumption were quantitatively analyzed. As observed, the
heating supply of the indoor air thermal energy contributed about 80% of the total
energy usage for defrosting, with more than 40% of the energy wasted in heating
the ambient air. (2) The effect of metal energy storage on defrosting performance
was quantitatively evaluated. As concluded, after the outdoor coil was enlarged
50% from two working circuits to three working circuits, the metal energy storage
effects changed from positive (0.33%) to negative ( 2.18%). (3) The percentages
of energy consumed on vaporizing the retained water and melting the frost were both
increased. Defrosting efficiency was increased about 6.08%, from 42.26% in the two-
working-circuit case to 48.34% in the three-working-circuit case. (4) The law of
energy transfer process and the effect of metal energy storage could guide the design
optimization of two coils and promote energy saving for air source heat pump units.
The effect of metal energy storage on frosting performance should also be further
quantitatively evaluated.

8.3 Defrosting with local drainage of the melted frost

The space heating used energy is changed to be consumed on melting frost and vapor-
izing water. Not only was the indoor space heating interrupted, but also the thermal
comfort level was adversely affected [11]. The energy conversion process directly
affects the defrosting performance, which is a key problem for the application of
ASHP units. Therefore, to improve the defrosting performance, various experimental
studies were conducted. Noticeably, the MES values of the indoor and outdoor coils

Table 8.6 Defrosting efficiency and MES effect in two cases
Item Parameter Case 1 Case 2 Unit
1 Energy consumed on melting frost 237.1 329 kJ
2 Energy consumed on vaporizing retained 22.0 39.1 kJ
water
3 Energy from indoor coil 37.1 34.2 kJ
4 Energy consumed in outdoor coil 35.0 50.9 kJ
5 Total energy supply 613.2 761.4 kJ
6 Defrosting efficiency 42.26% 48.34% –
7 MES effect 0.33% 2.18% –