Page 347 - Defrosting for Air Source Heat Pump

11
Conclusions and future work

11.1 Conclusions of the present work

The present research work is devoted to the research, analysis, and methods of frosting
and defrosting for ASHP units, with an emphasis on an ASHP unit having a multi-
circuit outdoor coil. The following six achievements are obtained.
The first is the qualitative and quantitative evaluation of the effect of downward-
flowing melted frost due to gravity along the surface of a multicircuit outdoor coil in
an ASHP unit on its defrosting performance, based on experimental studies using a
specially designed ASHP unit with a vertically installed multicircuit outdoor coil.
Water collecting trays were designed and installed under each circuit when necessary,
and thus the said effect could be investigated with comparative study cases. Based on
the experimental results, a set of uneven defrosting models, with the melted frost
locally drained considered or not, were developed and validated. The negative effect
due to the downward-flowing melted frost on defrosting performance further demon-
strated, as well as many difficult and impossible measured parameters predicted with
the validated models, for example, the temperature of downward-flowing melted frost
and the variation of thermal resistance of water film. These defrosting models are
based on experimental conditions and thus have some limitations, but also high accu-
racy. The defrosting models are based on the outdoor coil, but not the system, which is
also their advantage. It is also the first set of defrosting models that considered the
thermal energy stored in the metal of the outdoor coil during RCD. Because the
models have the aforementioned advantages, they were further used to study the per-
formances of three given defrosting control strategies. And finally, one was suggested
to alleviate the uneven defrosting for an ASHP unit having a multicircuit outdoor coil.
Both the defrosting efficiency and defrosting duration were used in the evaluation.
The conclusions are reported in Chapters 3 and 4.
The second is the realization of even defrosting for an ASHP unit with a multicircuit
outdoor coil. In this study, to alleviate uneven defrosting by eliminating the effect of
downward-flowing melted frost, the vertically installed outdoor coil was horizontally
placed. It was determined that the uneven defrosting could be alleviated by changing
the installation style of the multicircuit outdoor coil. As expected, the defrosting perfor-
mance could be improved when even defrosting was realized by defrosting efficiency
increased and defrosting duration shorted. Meanwhile, retained water was found on the
downside of the horizontally installed multicircuit outdoor coil, and the effect of the
retained water on the downward surface of the coil due to surface tension was further
experimentally evaluated. With this method, the effect of retained water due to surface
tension on the downside surface of each circuit during defrosting for a vertically
installed multicircuit outdoor coil could be further calculated. Additionally, the
defrosting evenness status was also evaluated with the defrosting terminations of each
circuit, with the named of DEC. This section is reported in Chapter 5.
Defrosting for Air Source Heat Pump. https://doi.org/10.1016/B978-0-08-102517-8.00011-4
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