Page 208 - Defrosting for Air Source Heat Pump

202 Defrosting for Air Source Heat Pump

frost accumulation on this circuit was obviously less, as listed in Table 7.1. Moreover,
from Figs. 7.6 and 7.7, it could be concluded that the fin temperature was mainly
affected by the frost accumulations on each circuit, especially when the refrigerant
was evenly distributed.
Fig. 7.8 presents the measured refrigerant volumetric flow rate during defrosting in
two cases. At the start of the defrosting operation, the compressor discharge pressure
increased suddenly, and the internal diameter of EEV was very small. Moreover, frost
melting consumed a lot of energy, with a lot of refrigerant changing phases from the
gas state to the liquid or two-phase state. This might be the reason why the measured
refrigerant mass flow rate kept fluctuating severely from 0 to 70 s, especially during
the first 40 s. When the defrosting process came into the water layer vaporizing stage
described in Chapter 4, the compressor suction and discharge pressures increased,
with the refrigerant volumetric flow rate changing from increasing to decreasing.
As shown in Fig. 7.8, their peak values came out at 156 s into defrosting in Case 2
and 171 s in Case 1, respectively. This is another parameter to demonstrate the neg-
ative effects of uneven refrigerant distribution on defrosting performance for an ASHP
unit with a vertically installed multicircuit outdoor coil.
Fig. 7.9 shows the variation of the measured temperatures of the surrounding air
and the melted frost collected in the water-collecting Cylinder C in two cases. Before
the water was collected, the temperature of the surrounding air was measured. When
the frost was melted and flowed into the water-collecting Cylinder C, the temperature
of the melted frost was measured. Therefore, as shown in Fig. 7.9, the melted frost
flowed into Cylinder C at 140 s into defrosting in Case 1, and 120 s in Case 2. It is
about 20 s earlier in Case 2 than in Case 1. This phenomenon could also demonstrate
the negative effects of uneven refrigerant distribution on system defrosting perfor-
mance directly. It is obvious that the temperature of melted frost was very low, at
around 0°C. Those temperature values measuring below 0°C are because of the mea-
surement errors of the thermocouple, as listed in Table 3.5.

7.2.3 Energy analysis and discussions
The energy used for RCD comes from three sources: the power input to the compres-
sor, the power input to the indoor air fan, and the thermal energy from the indoor air.
All the energy supplied is used to heat the outdoor coil metal, melt frost, heat the
melted frost and the residual water, heat the cold ambient air, and evaporate the
retained water on the surface of the outdoor coil. As shown in Table 7.2, the energy
supply and consumption during RCD in two cases were calculated, with the calculated
relative standard errors listed in Table 3.5. Here, the total energy used for defrosting
was 648.9 kJ in Case 1, but 576.3 kJ in Case 2, or 11.2% less. The main difference
comes from the energy from the indoor air, with a value of 72.6 kJ difference between
two cases.
Defrosting efficiency can be used to evaluate the performance of a defrosting oper-
ation. As described in Chapter 3, 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