Page 80 - Defrosting for Air Source Heat Pump

72 Defrosting for Air Source Heat Pump

frost melting, air presence, retained water vaporizing, and dry heating. However,
although the above-mentioned defrosting models were developed and used in study-
ing defrosting performance, the effects of the downward flowing of melted frost due to
gravity along the surface of an outdoor coil on the defrosting performance were
neglected, by assuming either no water retention on the coil surface or a stable
water layer.
In 2012, Qu et al. [22] reported on a modeling analysis where a semiempirical
model for the defrosting on the airside of a four-circuit outdoor coil in an ASHP unit
was developed. In this model, the negative effects of melted frost on defrosting per-
formance were quantitatively studied. It was further predicted that if the melted frost
could be drained away locally, the defrosting efficiency for the ASHP unit could be
increased by up to 18.3%. Similar results were also reported by Dong et al. [23] in a
study on the energy consumption for vaporizing the melted frost and heating the ambi-
ent air during RCD for an ASHP unit. To remove the melted frost as soon as possible,
it was further suggested that a vertically installed outdoor coil be installed horizon-
tally, as the flow path for the melted frost was shortened [23]. However, in Qu’s model
[22], the energy used to heat the metal in an outdoor coil during defrosting was
neglected. In fact, this part of energy consumption accounted for as much as
16.5% of the total defrosting energy used during RCD [23].
As a follow-up, a series of experimental studies was carried out on the defrosting
performance when the melted frost was drained away locally during RCD in an ASHP
unit with a two-circuit and a three-circuit outdoor coil; this was separately reported in
Chapter 3. In these studies, a tailor-made experimental multicircuit outdoor coil was
used by placing water-collecting trays under each circuit. Comparative experiments
with and without the use of water-collecting trays between circuits were carried
out and the experimental results suggested that the use of the water-collecting trays
helped shorten the defrosting duration by 9.2% and reduce the defrosting energy
use by 10.4%.
To enable further quantitative analysis on the effects of locally draining away the
melted frost on RCD performance in an ASHP unit, a related mathematical modeling
study on defrosting performance when using water-collecting trays was considered
necessary. Therefore, a modeling study of the defrosting process taking place in
the tailor-made three-circuit experimental outdoor coil, at two experimental settings
of with and without the use of water-collecting trays between circuits, was carried out
and is presented in this chapter. Two semiempirical mathematical models,
corresponding to the two settings, were developed. In this chapter, first the detailed
development of the two semiempirical models is presented. This is followed by
reporting the experimental validations of the two models using the experimental data
previously introduced. Then, some parameters that are hard to measure could be
numerically predicted. Finally, detailed discussions on the potential uses of the two
models developed and the limitations of the modeling work reported are included.
Moreover, the models will be used to predict some control methods, and thus the
defrosting performance for an ASHP unit with a multicircuit outdoor coil is expected
to be fundamentally and efficiently improved.