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94 Defrosting for Air Source Heat Pump

Meanwhile, as shown in Fig. 4.17, the sudden increase point occurs at 150, 155, and
160 s for Circuits 1–3, with their peak value at about 5000, 4700, and 4400 J,
respectively.

4.2.4 Potential uses and limitations
4.2.4.1 Potential uses of the two models developed
The two experimentally validated semiempirical models can have a number of poten-
tial applications. First, they can be used to quantitatively analyze not only the negative
effects of the downward-flowing melted frost due to gravity, but also the impact of
locally draining away the melted frost using water-collecting trays on RCD perfor-
mance in an ASHP unit. Second, the two models can be used to optimize the design
of an outdoor coil and positioning water-collecting trays. Third, a number of operating
parameters for an ASHP unit, which are difficult to measure experimentally, could be
predicted, for example, the frost melting rate and the temperature of retained water
over the surface of the outdoor coil. Also, with the validated models, the energy con-
sumption on melting frost, vaporizing the melted frost, heating low-temperature ambi-
ent air, and heating the outdoor coil metal during an RCD process could be evaluated.
Finally, the models will help increase awareness and spur efforts in exploring and
maximizing the potential of ASHP units to realize greater defrosting efficiency.

4.2.4.2 Limitations of the modeling work
Although the two models were developed based on energy and mass flow conserva-
tions, there exist a few limitations. These included the 10 listed assumptions, which
were necessary in model developments but introduced errors into the models. How-
ever, the errors were still within acceptable levels. Moreover, certain empirical for-
mulas were used, such as Eq. (4.20), which had their limitations. Furthermore,
experimental data were also used in assisting the model development, thereby making
the two models empirical. Therefore, appropriate modifications might have to be
introduced when the models are to be used for studying ASHP units with different
configurations or operating conditions. Nonetheless, the models developed could ade-
quately describe the defrosting performance for the experimental ASHP unit with
local drainage of the melted frost from its outdoor coil.
In this section, following on an experimental study on draining away locally the
melted frost for an experimental ASHP unit having a three-refrigerant-circuit outdoor
coil using water-collecting trays between circuits, a modeling study on the defrosting
process, at the two experimental settings of with and without the use of water-
collecting trays between circuits, was carried out. Two empirical models,
corresponding to the two settings, were therefore developed. Based on the validated
defrosting Model 1, without trays installed, the model extrapolation was also carried
out, with some modeling results given. The validated models have their limitations,
but also could adequately describe the defrosting performance for the experimental
ASHP unit with local drainage of the melted frost from its outdoor coil.