Page 125 - Defrosting for Air Source Heat Pump

118 Defrosting for Air Source Heat Pump

In this section, to solve the previous contradictory problem and study the heat trans-
fer performance of a horizontally installed heat exchanger, an experimental study on
RCD performance for an ASHP unit with a horizontal multicircuit outdoor coil has
been carried out. First, the ASHP unit under experiment is presented, followed by
the experimental procedures and conditions. Thereafter, the experimental cases and
their results are given. The defrosting durations and energy consumptions for each
case study are measured and discussed, with a conclusion given at the end.

5.2.1 Experimental work
5.2.1.1 Experimental ASHP unit
An experimental ASHP unit was specifically established for carrying out the exper-
imental work reported in this section. Similar to the experimental setup used in
previous sections, it was also modified from a commercially available 6.5 kW
heating-capacity variable speed ASHP unit and was installed in the same existing
environmental chamber, with a simulated indoor heated space and a simulated outdoor
frosting space. As previously introduced, the sizes of both the indoor and outdoor
spaces were each measured at 3.8 m (L) 3.8 m (W) 2.8 m (H). Fig. 3.1 shows
the schematics of the ASHP unit installed in the environmental chamber. The exper-
imental ASHP unit was a split-type one consisting of a swing-type compressor, an
accumulator, a four-way valve, an electronic expansion valve, an indoor coil, and
an outdoor coil. Additionally, to maintain the suitable experimental conditions in both
the indoor and outdoor spaces, there was a separate A/C system in the environmental
chamber, and the same LGUs were used to simulate thermal loads in the experiment.
Finally, during normal heating (frosting) operation, a frosting environment in the out-
door space was maintained by running the experimental ASHP unit and LGUs
together while an indoor heated environment was maintained by the experimental
ASHP unit and the existing A/C system.
Different from the previous experimental setup in Chapter 3, the outdoor coil was
specially designed and made for this study, as shown in Fig. 5.2. There were three
individual and parallel refrigerant circuits and the airside surface areas corresponding
to each of the three circuits were equal. There were four wind boards installed on the
two air sides of the outdoor coil, which were used to prevent the air passing the outdoor
coil through separations between circuits. The outdoor coil was horizontally installed,
and in each circuit an SV and an MV were fixed, with their locations shown in Fig. 5.2.
In order to easily describe the process of frost melting, the topside and downside of the
primary vertically installed three-circuit outdoor coil were named Side A and Side B,
as shown in Figs. 5.1 and 5.2. Side C was the inlet air side of the outdoor coil, where
the frost will be formed and accumulated.
A 700 mm 750 mm water-collecting tray made of PVC placed under the outdoor
coil was added to the experiment rig, and a 2000 mL water-collecting cylinder made
of PVC was connected to the tray. Both of them would be used for collecting and mea-
suring the melted frost. At the same time, in the experiments, the retained water on the
surface of the fins, especially on the downside of each circuit, was absorbed by